R/V Maurice Ewing 9512

Cruise Report

Controlled source electromagnetic and seismic investigation of the Valu Fa Ridge, Lau Basin, SW Pacific

24th November - 23rd December 1995

Suva, Fiji - Auckland, New Zealand

Christine Peirce Department of Geological Sciences University of Durham Science Laboratories South Road Durham DH1 3LE UK

Christine.Peirce@durham.ac.uk
 
Martin Sinha Bullard Laboratories Department of Earth Sciences University of Cambridge Madingley Rise Madingley Road Cambridge CB3 0EZ UK sinha@esc.cam.ac.uk
Steven Constable IGPP Scripps Institution of Oceanography University of California San Diego La Jolla California 92093-0225 USA steven_constable@igppqm.ucsd.edu

February, 1996

Summary

The objective of this cruise was to investigate the physical state and properties of the crust and upper mantle and the processes of magma delivery and emplacement beneath the Valu Fa Ridge, by conducting an integrated geophysical experiment using a variety of techniques. The most important techniques used were controlled source seismic - both wide-angle, using digital ocean bottom seismometers (DOBS) and sonobuoys, and single channel normal incidence profiling; and electromagnetics using a controlled source method (CSEM). Additional geophysical datasets collected consisted of gravity, magnetometry, XBT and swath bathymetry. The targets chosen for this study were the Central and Northern Valu Fa Ridges (CVFR and NVFR) centred on approximately 22° 26.4'S 176° 41.8'W and 22° 11.4'S 176° 37.5'W respectively, where previous work has shown the existence of a robust, but segmented magma chamber (figure 1).

During the cruise we conducted the first ever integrated geophysical experiment on a back-arc spreading ridge to combine seismic and electromagnetic methods. The seismic experiment comprised of two parts; each consisting of six Durham DOBS and six sonobuoy deployments, plus simultaneous single channel normal incidence data collection. A total of 5220 airgun shots were fired along lines arranged both along and across-axis, forming a 3-D grid. No seismic instruments were lost.

The EM experiment was also composed of two parts. Firstly we deployed an array of six sea bottom instruments from Scripps and Cambridge, three of which recorded controlled source electromagnetic transmissions from the Cambridge deep-tow transmitter (DASI) along a single off-axis tow line. This, while also providing useful data, acted as a test of the entire EM system. For the main CSEM experiment, twelve instruments were deployed and ten of these successfully recorded DASI transmissions from seven tows along five axis-parallel and across-axis tow lines. During the CSEM experiments a number of problems were experienced with the DASI system, which resulted in repeated instrument recoveries and major on-deck repairs. Despite this, we were able to complete the most extensive survey ever of this type, collecting a total of over 575 hours of two-channel data, from 68.5 hours of transmissions at three different frequencies along 156 km of transmitter tow lines. One Cambridge EM instrument was lost.

The subsidiary gravity, magnetic and swath bathymetry data collected covers an area of some 3225 km² surrounding the CVFR. A complete track chart of EW9512 is shown in figure 1.

Very preliminary observations from the seismic data include evidence from the DOBS wide-angle record sections of a shadow zone associated with each ridge axis and the overlapping spreading centre (OSC). A bright reflection event is also observed beneath each ridge on the single channel normal incidence seismic data. These observations are consistent with a zone of low velocity and/or high attenuation at mid-crust level, as would be expected in the presence of a mid-crustal melt body.

A NERC-funded Ph.D. student (I.M. Turner) is working on the seismic dataset in Durham, while the EM data analysis will be conducted jointly by Scripps and Cambridge. All underway data, including the swath bathymetry, will be made available to the coastal state (Kingdom of Tonga) and SOPAC.

Lau Basin

1. Introduction and Cruise Objectives

1.1 Introduction

The Valu Fa Ridge is an active back-arc spreading centre in the Lau Basin, SW Pacific, with an intermediate spreading rate of 60 mm/yr. In 1988 a dense grid of seismic lines were collected over a 35 km long morphological segment of the ridge system, the Central Valu Fa Ridge, and a small overlapping spreading centre (Sinha, 1988). This experiment showed the whole of the ridge system to be underlain by a robust magma chamber (Collier & Sinha, 1990: 1992a,b). We planned to return to the same site to: (1) investigate the physical properties of the crust beneath the ridge axis by CSEM and wide-angle seismic methods; and (2) investigate the distribution of seismic velocities, and by implication melt fraction, beneath a magmatically robust OSC.

Over the last few years, a number of detailed seismic studies have been conducted on ridges, most notably on the East Pacific Rise (EPR) between 9° and 13° N (Harding et al., 1989; Detrick et al., 1987; Vera et al,, 1990; Kent et al., 1990; Toomey et al., 1990; Burnett et al., 1989, Caress et al., 1992). These have revealed a detailed picture of a fast spreading centre - including the fine structure (both laterally and vertically) of the uppermost crust related to the presence of faulting, sheet and pillow volcanism, and porosity structure in the hydrothermal regions; and seismic low velocity zones and seismic reflections due to a region of partial melt beneath the ridge axis. These studies provided the first unequivocal in situ constraints on the dimensions, physical state and geometry of the crustal melt reservoir beneath a ridge. Some of these studies (e.g. Toomey et al., 1990: Collier and Sinha, 1992a,b; Kent et al., 1993) also provide information about the along-strike variability and segmentation of crustal magmatic processes. These investigations support the mounting evidence from high resolution bathymetric/morphological studies (e.g. Macdonald et al., 1984, 1992; Wiedicke & Collier, 1993) and from petrological studies (e.g. Langmuir et al., 1986) for along-strike segmentation on a number of scales, including unexpectedly short length-scales of the order of 10 to 15 km.

The interpretation by Collier & Sinha of the 1988 seismic reflection data from the Valu Fa Ridge (cruise CD34/88) revealed some unexpected features in the along-strike variability of the magma chamber geometry and properties. Our reason for returning to the Lau Basin was to investigate these features further using complementary geophysical techniques. The key observations which lead to the main issues to be addressed by data collected during this cruise are summarised below.

1) A bright reflector, which was shown to be coincident with a velocity inversion at a depth of 3.2 + 0.2 km below the seafloor, was observed on every one of the 40 across-axis profiles. The reflector was also seen as a continuous event for at least 10 km on a profile along the ridge axis. Collier and Sinha interpreted this reflector as being the roof of a crustal magma chamber. This is one of only a few places in the world (the others being the EPR at 9° - 13° N and 16°S, the East Scotia Ridge and the Reykjanes Ridge at 57°N (Sinha et al., 1996) where the presence of a magma chamber has been proved to exist beneath a spreading centre.

2) The reflection coefficient of the chamber roof was estimated to be between -0.34 and -0.65. This requires the melt at the top of the chamber to have a P-wave velocity less than 2.7 kms-1 if the interface is planar or less than 3.8 kms-1 if the interface is layered. This is a much higher reflection coefficient than that of the magma chamber reflector beneath the EPR (estimated to be just -0.2 - Barth et al., 1987; Harding et al., 1989). This makes the Valu Fa Ridge the best target for research into the physical properties of a robust magma chamber currently known.

3) The width of the magma chamber reflector was shown to be between 0.6 and 4 km. The narrowness of the chamber is in agreement with geophysical work at the EPR (Detrick et al., 1987; Harding et al., 1989; Kent et al., 1990; Toomey et al., 1990).

4) The 35 km long morphological segment surveyed during CD34/88 was shown to be further subdivided into three sections, each 8 to 12 km long, with distinct bathymetric, morphological, magnetic, gravitational and magma chamber reflector (width and brightness) characteristics. These observations were interpreted as implying that each segment had different magmatic properties, driven by variations in melt supply along axis. Two of the three regions are centred on small devals, which as they form bathymetric highs, are thought to be local foci of extrusion. Segmentation of volcanic activity on a similar length scale has been previously suggested at the EPR. However, devals have previously been thought of as at the extremities of melt injection sites rather than at their centres (Mutter et al., 1988; Macdonald et al., 1987).

5) The widest magma chamber reflector occurred beneath the overlapping spreading centre. Magma was imaged beneath both ridges and, in places, beneath the overlap basin. This observation, together with bathymetric, petrological and hydrothermal considerations, implies that the currently most magmatically vigorous part of the ridge is the OSC. Models of accretion based on morphological observations at the EPR suggest that OSCs should be regions with a low overall magmatic budget. This is clearly not the case at the Valu Fa Ridge.

In 1989 Sinha at Cambridge, working in collaboration with Constable and Cox at Scripps Institution of Oceanography, conducted the first controlled source electromagnetic sounding experiment directly on a ridge axis. This experiment, centred on the EPR at 13° 10' N, involved the deployment of seafloor electric field instruments from both Cambridge and Scripps and the use of the Cambridge deep-towed controlled source instrument. That experiment (Evans et al., 1991; 1993) showed, among other new results, that the crust beneath that part of the EPR contains no melt in an interconnected, conductive phase other than that associated with the small melt lens which produces the axial magma chamber (AMC) reflection (which is very weak or absent at 13° N) - confirming the view that the East Pacific Rise at this location is currently in a state of relative magmatic quiescence. The implication is that the cycles of magmatic/tectonic activity operating at ridges are linked to a strong modulation in the rate of supply of melt into the crust - with the crust beneath the axis, even at a fast-spreading ridge, containing only very little (if any) melt during the least magmatically active phase.

In October 1993, Sinha led a another cruise this time to the Reykjanes Ridge near 57° N to carry out a second ridge CSEM experiment (Sinha et al., 1994). This was again done in collaboration with the Scripps group, with the additional collaboration of White and Heinson (Flinders University, Australia) to enable a magneto-telluric sounding experiment to be included. In addition, the Reykjanes Ridge cruise benefited from the collaborative involvement of Peirce (Co-Chief Scientist, University of Durham) who added a seismic component to the work, using Durham and Cambridge DOBSs, airguns, explosives and multichannel seismic reflection profiling. Analysis of the seismic data from the CD81/93 cruise shows that the chosen AVR is magmatically active and is underlain by a significant crustal low velocity zone - the first time one has been unequivocally imaged using seismic techniques beneath a slow spreading ridge. The EM data analysis also supports the existence of a crustal melt body at this location (Sinha et al., 1996). Now that these techniques are proven in their ability to distinguish melt and non-melt crust beneath spreading centres, we intended to use them at the Valu Fa Ridge to better constrain the properties of the magma chamber reflectors already known to exist here.

The Valu Fa Ridge, and the Lau Basin in general, have been the target of a number of multi-disciplinary studies over the last decade. During three cruises by R/V Sonne between 1985 and 1987, a large number of rock and mineralisation samples were collected, and the ridge was surveyed by underwater cameras and swath bathymetry (Jenner et al., 1987; von Stackelberg et al., 1988; Frenzel et al., 1990; Davis et al., 1990). Based on this work, a morphological segmentation of the ridge was recognised which sub-divides it, in the area surveyed, into three distinct segments which von Stackelberg et al. (1988) named the Southern, Central and Northern Valu Fa Ridges (SVFR, CVFR and NVFR). Diving by Nautile on the northern part of the CVFR in 1989 discovered one of the most active hydrothermal fields known (the 'Vai Lili' field at 22° 13' S), with black smokers emitting fluids with temperatures up to 400°C (The Nautilau Group, 1990; Fouquet et al., 1991). GLORIA data were collected over much of the central part of the basin in 1988 (Parson et al., 1990), leading to the first clear definition of the pattern of seafloor spreading here and of its relation to the Valu Fa spreading centre. In the winter of 1990, ODP drilling (leg 135) occurred - the main objective of which was to determine the evolution of the basin. Owing to the extent and quality of existing UK involvement in Lau Basin studies, this region was selected by the BRIDGE programme as one of its four areas of the global ridge system for detailed study. Our programme of research was funded by BRIDGE.

1.2 Scientific Objectives

The first objective of this cruise was to obtain controlled source electromagnetic sounding data from the southern end of the Central Valu Fa Ridge. This dataset would complement the existing seismic reflection images of this area, and provide improved constraints on the nature, dimensions and physical properties of the crustal melt reservoir flanking and underlying the axial magma chamber reflector. It would also provide constraints on the distribution of porosity in the upper crust - the source zone for hydrothermal circulation systems, and the site of thermal and chemical exchanges between sea water and lithosphere. The CSEM profiles would consist of a line of LEMUR and ELF receivers deployed across the axis of the southern CVFR where the brightest and widest (apart from the OSC) axial magma chamber reflector was previously imaged. The transmitter (DASI) would be towed along five profiles. Three of these lines were to be ridge parallel; one running along the axis, one offset 5 km off-axis to the west and another in an off-axis basin some 10 km to the east. Two further lines run across-axis. We planned to operate DASI at frequencies of between 0.25 and 8 Hz and aimed to obtain maximum spatial coverage at a small number of carefully chosen frequencies. We also planned to arrange transmitter positions and orientations so as to generate both ridge-parallel and ridge-normal polarisation patterns (Unsworth et al., 1993). The CSEM experiment would be navigated using RVS's OCEANO long-baseline acoustic navigation system, tied to absolute co-ordinates by GPS; and hence we could use acoustic methods to accurately locate the final positions of EM instruments on the seabed.

Secondly we planned to shoot a wide-angle seismic profile across the CVFR, coincident with the CSEM profile, deploying six Durham DOBSs across-axis. Using the Ewing's large volume airgun array we planned to collect densely spaced wide-angle and single channel reflection data from an ~80 km across-axis profile, and from additional profiles running northwards along the axis of the CVFR and parallel to the axis at distances westwards from it of 10, 20 and 40 km. Although the 1988 cruise provided excellent seismic reflection images, and seismic velocity profiles through the upper crust (down to the AMC reflector on axis and down to about 4 km depth off-axis) using sonobuoys, prior to the cruise we could only infer seismic velocities within the low velocity zone from the amplitudes of reflections. In addition we had no information on the depth to the Moho in this area. The DOBS data from this part of our proposed experiment will provide an across-axis image of the seismic velocity structure down into the upper mantle and enable us to measure crustal thickness.

Thirdly we planned to shoot another wide-angle seismic profile across the overlapping spreading centre, where we believe the greatest accumulation of melt to be present. We planned to deploy six DOBS along an ~80 km line, including one on each spreading axis limb of the OSC, and a third in the overlap basin. A fourth instrument would be deployed off-axis to the ESE and a fifth and sixth off-axis to the WNW. On both across-axis seismic lines, shooting would extend approximately 40 km off-axis to the WNW and 40 km to the ESE on to the edge of the Tonga Platform, so as to image unrifted island-arc crust that predates seafloor spreading at the VFR at both ends of the profiles. Two additional lines of airgun shots would be fired northwards, one along the Northern Valu Fa Ridge, and the other ~2-5 km off-axis to the east parallel to and extending the CVFR axial line.

Specific aspects of the structure that we plan to investigate using all of the recorded data are:

(i) how does the porosity of the upper crust vary across the VFR?;

(ii) how does the electrical resistivity of the middle and lower crust vary across the VFR, and how does this correlate with the seismic velocity structure and with intra-crustal seismic reflectors seen in the 1988 dataset?

(iii) given both the seismic and electromagnetic data, what can we infer about the across-ridge temperature structure and the nature of the molten and partially molten regions?

(iv) how does seismic crustal structure vary across the OSC?

(v) what can this tell us about the nature of magma injection, migration and accumulation in the crust beneath this spreading centre offset?

(vi) what can we learn from all of the above about the process of construction and evolution of oceanic crust at the CVFR? In particular we shall use the OSC data to test the interpretation of Collier and Sinha that the OSC segment of the spreading centre is particularly magmatically vigorous - a proposition which has profound implications for the nature of the relationship between the along-axis variations in magma supply from the mantle, and the morphological expression of ridge segmentation.

2. Work Carried Out and Data Collected

2.1 The Wide-Angle Seismic Experiment

The Ewing 9512 seismic experiment was divided into two sections - Seismic South and Seismic North - named in relation to their location within the work area. The Seismic South experiment (figure 2) was shot first with six DOBSs deployed at intervals along an ~80 km across-axis line (see tables 1 and 2) and the Ewing's large volume, 20 gun array (see figure 3). The line was located along a ridge perpendicular profile centred on the CVFR where the brightest melt reflector was observed in the CD34/88 reflection data. Combined with the across-axis line, a further profile was shot along the CVFR and two off-axis lines shot to the west. Along each of these lines sonobuoys were deployed not only to provide detailed upper crustal velocity information, but also to reverse ridge-parallel lines which only had one DOBS located at the southern end.

Each of the DOBSs was fitted with a 3-component geophone package, in addition to a hydrophone, in order to maximise the chances of making high quality recordings of S-waves. Airgun shots provide closely spaced traces (~200 m) which maximise trace-to-trace coherence of phases. This allows the recognition of late arriving and low amplitude P-phases and detailed travel time and amplitude modelling of P-wave velocity structure. It was also an objective of this experiment to study S-wave structure. The S-wave velocity structure and, equally importantly, any evidence of S-wave shadowing due to high attenuation, will provide important constraints on the physical state of crustal rocks. The unsedimented seabed near the ridge axis results in significant conversion of P- to S-wave energy at the water-rock interface. These phases are readily observable on oceanic wide-angle seismic sections.

The DOBSs were programmed to record at an 80 s interval (i.e. alternate shots) in a windowed mode, mainly to enable collection of four channel data at 200 sps - resulting in a trace spacing of ~200 m at a surveying speed of 4.9 knots. Each DOBS also recorded shots fired along all other lines of the southern survey, hence providing 3D ray coverage of the CVFR (see table 3). The experiment resulted in 4680 seismograms per DOBS. Single channel normal incidence data were also recorded along each line using a spare DOBS datalogger to record the raw streamer output, as well as sonobuoy signals.

Data quality is extremely high, with low levels of background noise. Some interesting features can be noted on the first record sections to be constructed. These features include shadow zones and microearthquakes (figure 4 and figure 5).

With all remote seabed seismometers, shot timing is an extremely important factor in constructing final wide-angle record sections. All DOBS internal clocks were synchronised to the Ewing's TrueTime clock and were checked for drift on recovery. All shipboard navigation, gravity, magnetic and bathymetry data were also logged with respect to the TrueTime clock. However, as the shot firing system on the Ewing is designed for multichannel normal incidence data collection, it was not a simple task to fire the array exactly on the appropriate second on which each DOBS started recording in its windowed mode. Handshaking between the three computers in the system incorporated a 40 msec delay between sending the firing pulse and the array actually firing. If allowed to "drift" at this rate unchecked, this would have eventually resulted in the seismic signals of interest being delayed out of the corresponding DOBS recording window, and hence valuable data being lost. To attempt to remedy this situation a "randomiser" was used to keep the shot instant to less than 1 s after the start of each window. As the time of each shot instant was recorded in a file this information could then be incorporated during data processing and the sections reconstructed to take account of this variation. This effectively random shot instant was the only significant problem encountered throughout shot firing except for the sonobuoys which will be discussed in the next section.

All seabed instrumentation was deployed and recovered using a 10 kHz acoustic system and dunking 'dolphin' transducer, without problem or instrument loss.

The Seismic North experiment was conducted in a similar manner to Seismic South except that the ~80 km seismic line was centred on the overlapping spreading centre, between the Central and Northern Valu Fa Ridges (figure 2 and table 1). This time the grid of additional lines consisted of three located off-axis to the west, one along the NVFR and another along the CVFR. The latter extended some 25 km north past the end of this segment, to investigate the structure of the crust beneath the spreading tip. A shorter across-CVFR axis line was also shot slightly to the south. This will help to map the extent of the OSC melt body and its relation to that beneath the CVFR axis. Once again sonobuoys were deployed to increase the near surface resolution and reverse some lines. A total of 5760 seismograms per DOBS were recorded and again each instrument was recovered without loss. Data quality is again extremely high (figure 6).

In total, 62640 seismograms were recorded along eleven 2D lines, equivalent to ~1 Gbyte of DOBS data. These data will be analysed using 2-dimensional seismic interpretation methods, based on forward modelling of travel times, amplitudes and wave forms. All the Durham DOBSs were fitted with 3-component geophone as well as hydrophones, allowing the possibility of recognising and interpreting shear wave arrivals - which, if observed, will significantly enhance the constraints on crustal physical properties. Since the DOBSs also effectively recorded a 3D dataset it will also be possible to analyse delay times associated with ray paths running obliquely to the trend of the ridge.

Seismic determinations of crustal thickness and its variation along and across the axis will be related to gravity observations. Sinha (1995) has shown that variations of about 20 mGal in the mantle Bouguer gravity anomaly exist along the axis, between the southern end of the CVFR and the OSC, and recent studies of gravity anomalies on the mid-Atlantic ridge have indicated that mantle Bouguer anomaly variations are due primarily to crustal thickness variations. The seismic and gravity data collected during this cruise, together with the CD34/88 gravity data, will allow this to be tested in an intermediate spreading rate back-arc setting.

2.2 The Seismic Reflection Experiment

As previously mentioned, seismic reflection data were recorded using a spare DOBS datalogger and a single channel streamer, as there was no other recording system to digitally record this data which seemed quite surprising. Data were recorded along all of the seismic lines during wide-angle shooting, with the full airgun array and a 100 m shot interval (i.e. 40 s). The reflection data will be processed at Durham and used to provide information on the near-surface structure and sediment thickness. An example of the raw data is shown in figure 7. Even on the raw and rather crude sections a reflection event corresponding to a sub-axis magma chamber can clearly be seen.

Of the numerous sonobuoys deployed during shooting, 12 provided useful data (see table 1 for successful sonobuoy locations) of which one, supplied by Durham (a Dowty Marine type SSQ906A(D)), provided quite high quality data for over 3 hours. Initially, the sonobuoys were deployed using one of the two launchers on B-deck. The Ewing had been supplied with a batch of sonobuoys by the U.S. Navy. Unfortunately, the majority of these were of the same channel (#10) which meant that they could only be deployed singly, and we had to wait until one timed out before deploying another - limiting data coverage. Also many of these sonobuoys were apparently defective, not deploying their hydrophone or floatation device when deployed. This forced us to adopt a procedure in which each sonobuoy had its floatation and hydrophone manually released by an operator who, while holding it all together, then crawled out along one of the airgun booms and dropped it into the water. This only proved a viable solution in our case as we did not experience any heavy seas during shot firing. Sonobuoy signals were received using an aerial on the main mast and an ICOM receiver located in the main laboratory. Receiver output was digitised and recorded using a spare DOBS datalogger. Paper play outs were also made using an EPC graphic recorder. Example sonobuoy data are shown in figure 8.

2.3 The Controlled Source Electromagnetic Experiment

The controlled source electromagnetic (CSEM) sounding method has been described by Chave & Cox (1982), Cox et al. (1986), and Webb et al. (1985); while the equipment that has been developed at Cambridge to make use of the technique is described by Sinha et al. (1990). For this programme we used the Cambridge deep-towed controlled source (DASI); 4 Cambridge and 7 Scripps seafloor electric field recorders, each equipped with an orthogonal pair of 12 m horizontal electric dipole antennas (LEMURs and ELFs respectively); and 2 Scripps long-wire electric field recorders, each equipped with a single, 300 m horizontal electric dipole antenna (LEMs). Each LEM antenna carried two parallel pairs of electrodes, recorded independently, to provide data redundancy.

The overall CSEM programme was made up of three independent experiments, though all three investigated a single across-axis transect coincident with the main line of the Seismic South experiment. The first two experiments were designed primarily as tests of the CSEM system, while the third was the main data-collecting experiment. The tests were considered essential for two reasons. Firstly, the complex deep-towed controlled source system, DASI, had not previously been used aboard the Maurice Ewing. Mains supply voltages aboard the Lamont vessel differ from those aboard the UK vessels from which DASI had previously been deployed (460 Vrms 60 Hz 3 phase, cf. 415 Vrms 50 Hz 3 phase), requiring modifications to the 12 kVA input stage of DASI's ship-board power supply that could only be partially tested using European mains voltages before the equipment was shipped. The system puts a heavy load - up to 2000 Vrms and 10 kVA at 256 Hz - on the vessel's deep-tow cable and its associated terminations and slip rings. This is greatly in excess of any electrical load that had been applied to the Maurice Ewing's deep-tow cable previously. It was therefore essential to fully test the DASI system as installed on the Ewing before investing large amounts of ship time in deploying an extensive array of sea bottom receivers. Secondly, the Cambridge LEMUR instruments had been largely redesigned, including the provision of new high-gain amplifiers, high capacity dataloggers and pressure cases since their previous use at sea in 1993. Again, testing of this system was considered essential before we committed ourselves to a full-scale deployment.

CSEM Experiment 1 was our first, and unsuccessful, attempt at testing the new systems. For it, we deployed a pair of Cambridge LEMURs (12 and 14) in a sedimented basin 10 km E of the CVFR axis (figure 9). Three Scripps ELFs (Pele, Rhonda and Trevor) were then deployed along the main CSEM profile, from the edge of the basin to the CVFR axis. DASI was launched (deployment 1) at the northern end of a ridge-parallel profile located in the off-axis basin, passing through the locations of the two LEMURs. Initial DASI tests went well, and the deep-tow was lowered to within 200 m of the seabed. Transmissions were started at full power and at a frequency of 1 Hz, but within a few minutes the ship-board power supply system abruptly became overloaded and had to be shut down after suffering significant damage. DASI was recovered, and the time needed to carry out repairs was used for acoustically surveying the positions of the instruments on the seabed. DASI was re-launched (deployment 2) in the same position as deployment 1, and again the initial tests went well. However, as before, attempts to run the transmitter at full power close to the seabed were unsuccessful. This time it was clear that some part of the high-voltage circuit was short-circuiting to earth when under pressure. DASI was recovered for further investigations, and CSEM Experiment 1 was abandoned. Since the ELFs had sufficient recording capacity to run for many days, they were left on the seabed. However the LEMURs, operating at a higher sampling rate, would run out of recording capacity before another DASI deployment was possible. We therefore attempted to recover them. LEMUR 14 was recovered without difficulty, but LEMUR 12 would not release and was lost. The only positive outcomes of CSEM Experiment 1 were firstly that the piggy-back source field datalogger on DASI had worked well for the short periods during which it was transmitting; and secondly that analysis of the records from LEMUR 14 showed that it too had worked well. An additional ELF, Noddy, was deployed at the end of this experiment, to record future DASI tows.

After completing the first seismic experiment (Seismic South), we carried out CSEM Experiment 2. This was again a test experiment, using the four ELFs now on the seabed and towing DASI along the same profile as that planned for experiment 1. DASI deployment 3 was short-lived. Despite extensive, trouble-free testing of the system on deck, the short circuit problem recurred when DASI was tested at full power just 200 m below the sea surface. DASI was recovered, stripped down, tested exhaustively and reassembled. In the meantime, the slip ring and swivel systems on both inboard and outboard ends of the 0.680" cable were replaced. Eventually when we felt that we had removed all possible sources of the short circuit, we deployed DASI for the fourth time. This time, all went well and during DASI deployment 4 we successfully transmitted along 17 km of ridge-parallel profile at a frequency of 1 Hz (figure 9). After completing this DASI tow we recovered ELF Pele, which was fitted with a smaller disk drive and therefore had a shorter recording time than the other three ELFs.

CSEM Experiment 3, the main deployment, was carried out after we had completed the second seismic experiment (Seismic North). For Experiment 3 we first deployed two Scripps long-wire receivers, Opus and Kermit, at distances of 8 and 15 km respectively off-axis to the WNW of the CVFR. Both were deployed with their 300 m dipole antennas aligned parallel to the ridge axis. Although the deployment of this type of instrument - which need to be deep-towed to within a few tens of metres of the seabed before being released - is expensive in terms of ship time, the extra sensitivity due to the length of the antennas allows signals from the controlled source to be recorded at ranges of at least several tens of kilometres - greatly extending the maximum offset of an experiment, and hence extending the sensitivity of the experiment to resistivity structure downwards into the uppermost mantle. We then deployed all the remaining short-arm receivers in an array along and across the CVFR axis (figure 10). One of the remaining LEMURs (No. 13) was found to have developed an electronic fault that could not be rectified. Also, the pressure case for one of the ELFs (Lolita) was found to be damaged. We therefore deployed Lolita's recording system in LEMUR 13's mechanical hardware. This hybrid instrument was designated LOLEMUR.

For CSEM Experiment 3, we also deployed an array of sea bottom acoustic transponders to provide long-base line acoustic positioning of the DASI deep-tow package. A relay transponder was attached to the DASI tow cable, 200 m above the vehicle itself. Transponders 1 to 3 were OCEANO units supplied by NERC RVS. Transponders 4 and 5 were SIO units. All of the LEMURs (including LOLEMUR) also carried OCEANO transponders, making a total of 9 transponders in the sea bottom acoustic navigation net. Direct and relay ranges to all visible transponders were logged throughout CSEM Experiment 3, to constrain the source-receiver geometry with an accuracy of 10 to 50 m.

For this experiment we defined 5 tow lines for DASI transmissions. Tow line 1 is the same as that used for Experiment 2, runs through the positions of Pele (deployment 2) and LOLEMUR, and is located 10 km E of the axis. Tow line 2 runs along the axis of the CVFR, through the positions of Ulysses and LEMUR 14. Tow line 3 is located 5 km W of the axis, and runs through the positions of LEMUR 11 and Quail. Tow line 4 runs orthogonally to the CVFR axis, from WNW to ESE, through the positions of Kermit, Quail, Noddy, LEMUR 14, Trevor, Rhonda, LEMUR 15 and Pele 2. Tow line 5 runs parallel to tow line 4, and 5 km to the south of it, on the E side of the CVFR axis only. This combination of tow lines provides both ridge-parallel and ridge-normal source polarisations, to maximise the experiment's sensitivity to two-dimensional structure.

The first DASI deployment of this experiment - deployment 5 - resulted in successful transmissions along tow lines 3 (at 1 Hz) and 4 (at 8 Hz). At the end of the second of these tow lines, the DASI transmitting dipole streamer was found to be damaged at its forward end, close to where it attaches to the tow vehicle. The damage was due to wear and tear caused by the continual vertical movements of the DASI vehicle, arising from the pitching of the surface ship. The necessary repair work was started, and in the meantime the available ship time was used to start a gravity survey to the south of the main work area. During this survey, ELF Quail was spotted floating on the surface, having prematurely released one of its bottom weights. It was safely recovered, carefully checked, and re-deployed in its original position. After completion of repairs to its array, DASI was launched again (deployment 6) for a profile along tow line 2 at 8 Hz. Unfortunately, after only 73 minutes of transmissions, the streamer failed again. DASI was recovered for further repairs. Finally, DASI was launched for the 7th and last deployment, and this time we were able to complete tow line 2 at 8 Hz, tow line 1 (again) at 0.25 Hz, tow line 1 (for the third time) at 8 Hz, and tow line 5 at 1 Hz initially, reduced to 0.25 for most of the tow.

After the end of DASI transmissions on experiment 3, DASI, all 12 sea bottom receivers and all 5 acoustic navigation transponders were recovered without difficulty. Attempts were made to recover LEMUR 12 (lost during experiment 1) by dragging, but these proved unsuccessful. Two of the recovered instruments had failed to record - ELF Trevor due to a faulty disk, and LEMUR 15 due to a faulty internal connector. LEMUR 14 recorded only DASI deployments 5 and 6 - a clock problem prevented it from recording deployment 7. Nonetheless we ended up, despite all the difficulties, with an excellent CSEM dataset, and one that is larger by a factor of about 8 than that from any previous experiment of this type. The ten working receivers obtained high quality, 2-channel recordings totalling 575 hours 20 minutes of data from 68 hours 34 minutes of DASI transmissions along 158 km of tow lines. An example of the recorded data is shown in figure 11.

We shall interpret the EM data jointly with the new and existing (1988) seismic data, and using a combination of 1D inversions and 1D and 2D forward modelling. We shall use the seismic data to locate boundaries or regions of steep gradients in physical properties of the crust, and to identify other regions where physical properties change more slowly. Incorporation of such boundaries as a priori elements in the CSEM interpretation, constrained independently by the higher resolution seismic models, will allow us to concentrate during CSEM modelling and inversion on defining the resistivities of particular regions, so as to obtain the best possible constraints (by combining electrical resistivities and seismic velocities) on the physical state and properties of different parts of the crust and uppermost mantle beneath the Central Valu Fa Ridge.

2.4 Additional Datasets

The primary objective of the cruise was to carry out the seismic and electromagnetic experiments outlined above. However in the course of the cruise we were also able to collect significant amounts of swath bathymetry data. High quality bathymetry data will be important for the interpretation of both the seismic and the electromagnetic data - since both seismic and EM responses are substantially affected by seafloor topography. A summary track chart showing ship tracks for which swath bathymetry data were logged and subsequently processed and gridded during the cruise is shown in figure 12. The tracks provide almost 100% coverage over an area of approximately 3225 km² The gridded data are shown in figure 13. Gravity data were collected throughout the cruise (see figure 14) together with total field magnetics data along the lines shown in figure 15. In addition we collected bathymetry, gravity and magnetics data for most of the transit from Suva to the work area. All of these data will be made available to the coastal state (Kingdom of Tonga), SOPAC, and ultimately to the wider ridge community for inclusion in regional studies.

Water column data were collected during the cruise using a sound velocity meter and expendable bathythermographs (XBTs). The sound velocity meter dip was carried out immediately after we reached the work area, using RVS's Plessey sound velocity profiler at 22° 25.90'S 176° 35.92'W (see table 7). Accurate knowledge of the water column sound velocity structure is essential for many aspects of the work carried out during this cruise, including long-base line acoustic navigation of the DASI deep-tow vehicle, accurate positioning of the DOBSs and EM instrumentation on the seafloor and corrections to the swath bathymetry data for travel time variations and ray bending. The velocity profile was supplemented by 12 XBT deployments (see table 8). The most intriguing of these appears to show the presence of a hydrothermal plume (see figure associated with table 8). A temperature anomaly of about 5°C is present in the bottommost 200 m of the water column. The seafloor is at 2000 m depth, and these data may indicate that the XBT passed through the buoyant part of a plume. This site is near the axis of the CVFR, close to the southern limit of our swath bathymetry survey. Despite deploying three further XBTs near this location, no further anomalies were observed.

Two particular datasets were collected in addition to the originally planned scientific programme. Shortly after leaving Suva, we conducted a short (2.5 hours) swath survey of an inshore area of potential economic interest, in response to a request from SOPAC (see figure 16). The small amount of data from this work will be added by SOPAC to other tracks collected by swath-equipped vessels using Suva, in response to similar 'ship-of-opportunity' requests. Secondly, the significant amount of down-time of the DASI controlled source system during the CSEM component of the experiment left a number of short periods during which the ship could be used for other purposes while repairs were carried out. Much of this time was used for essential tasks such as acoustic calibration of the positions of instruments and transponders on the seabed, or filling in gaps in bathymetric coverage. However we used one period of DASI down-time to start collecting gravimetric and magnetometric tracks along a series of profiles which extend the coverage southwards towards the southern end of the SVFR. At the end of the cruise, immediately before departing for Auckland, we were able to use a small amount of spare time to continue this survey further southwards. As a result, we have a series of eight across-axis profiles, each 70 to 80 km long, of gravity, magnetics and single-beam bathymetry which extend coverage southwards to near 23°S, beyond the southern propagating tip of the ridge. We shall use these data to investigate the structure of this propagating rift system and its interaction with previously unrifted, island-arc lithosphere to the south.

3. Cruise Narrative

The duration of the cruise was 29 days 1.25 hours. Of this, 1 day 10.5 hours were spent on passage from Suva to the work area (although for some of this time we were collecting geophysical data outside Suva harbour for SOPAC); and 3 days 14.75 hours were spent at the end of the cruise on passage to Auckland - leaving a total of 24 days on station in the work area. No time was lost due to bad weather or major equipment failure - when the latter occurred during EM surveying the time was usefully employed collecting across-axis gravity and magnetics data further south, completing swath coverage along DASI tow lines or acoustically navigating in seafloor instruments. None of these datasets would have otherwise been collected. In our original ship time application submitted to NERC in November 1993, we requested a minimum of 23 days on station to complete the highest priority components of the project, and a maximum of 25 days to complete the entire scientific programme. Hence 24 days represented a more than acceptable compromise. A summary of the events that took place appears below. All times are in GMT and dates given by the appropriate Julian day as local time was GMT+12. Reference Julian days: 327 - 24th November; 357 - 23rd December.

24th November, 1995.

327

22:45 Depart Suva. Transit to start point of swath survey outside Suva harbour for SOPAC.

328

02:30 End SOPAC swath survey and begin passage to work area.

07:19 - Tested DASI power supply in Lazarette.

07:40

329

05:48 Deployed magnetometer ready for arrival at start point of swath survey of planned seismic lines.

09:13 Commenced swath survey of seismic lines.

330

03:06 Recovered magnetometer after completion of swath survey.

03:59 Hove to and commenced sound velocity dip.

04:33 Deployed XBT 1

06:28 Completed sound velocity dip.

09:02 Conducted wire test of 0.680" deep-tow cable down to 2000 m and re-spooled.

10:45 Completed wire test.

11:12 Power failure in main lab and bridge disrupted onboard computing/datalogging systems.

12:30 Deployed ELF Pele I for CSEM experiment test tow.

16:01 Deployed LEMUR 12.

18:18 Deployed LEMUR 14.

19:34 Commenced swath survey at northern part of survey along-axis survey area.

331

07:47 Deployed ELF Rhonda.

11:36 Deployed ELF Trevor.

12:51 Commenced swath survey, conducted during remaining hours of darkness.

18:30 Finished swath survey, ready to deploy DASI in the early hours of daylight.

19:03 Deployed DASI streamer.

21:50 DASI deployed (experiment 1, deployment 1) - commencing lowering instrument to 50 m above seabed for test tow.

23:35 Beginning DASI transmissions at 50 m above seabed.

23:46 DASI power supply blew up.

332

00:00 Pulled DASI up to 300 m above seabed.

04:32 Begin DASI recovery.

06:20 DASI on deck, streamer being towed at 1-2 knots.

08:27 Commencing swath survey at 2 knots while acoustically navigating seabed positions of deployed EM instruments and effecting repairs to DASI systems.

333

09:05 Completed repairs to DASI systems and commenced deck tests prior to deployment 2.

10:48 DASI deployed for CSEM Experiment 1 (test tow), DASI deployment 2.

14:41 DASI close to seabed, turning transmissions to full power. Short-circuit in system. Begin to recover for inspection.

19:03 DASI back on deck. Testing circuits for HT leakage.

20:31 Begin recovery of DASI streamer.

20:40 Streamer onboard.

22:39 Attempting recovery of LEMUR 12.

23:20 LEMUR 12 abandoned. Failed to release.

334

00:23 Released LEMUR 14.

01:25 LEMUR 14 on surface.

01:49 LEMUR 14 recovered. End of CSEM 1.

05:26 Deployed ELF Noddy for CSEM experiment 2.

08:31 - Attempting to release LEMUR 12 again, but failed to release.

08:39

12:11 Deployed DOBS 1 for seismic experiment Seismic South.

14:36 Deployed DOBS 2.

16:27 Deployed DOBS 3.

18:21 Deployed DOBS 4.

20:03 Deployed DOBS 5.

22:51 Deployed DOBS 6.

335

00:25 Deployed magnetometer, airgun array and single channel streamer.

01:04 Problems firing airgun array at exact 40 s time mark. Circled at start of line to resolve problem.

01:15 All airguns in water and firing but still not synchronised.

04:42 First synchronised shot Seismic South.

05:00 Guns firing on shooting track.

05:06 Deployed sonobuoy 1.

11:24 Deployed sonobuoy 2.

14:13 Deployed sonobuoy 3.

15:40 Deployed sonobuoy 4.

336

00:28 Deployed sonobuoy 5 which failed so immediately deployed sonobuoy 6.

02:07 End shooting Seismic South, start recovery of airguns, magnetometer and single channel streamer.

05:08 DOBS 1 released.

06:12 DOBS 1 on deck.

07:58 DOBS 2 released.

09:14 DOBS 2 on deck.

10:12 DOBS 3 released.

11:30 DOBS 3 on deck.

12:17 DOBS 4 released.

13:12 DOBS 4 on deck.

14:04 DOBS 5 released.

15:23 DOBS 5 on deck.

16:52 DOBS 6 released.

17:51 DOBS 6 on deck. End of experiment Seismic South.

20:00 Commencing transit to start point of CSEM Experiment 2.

23:10 Deployed XBT 2.

337

02:44 Start of CSEM Experiment 2, deployment 3, tow line 1. Commenced deploying DASI streamer.

05:40 DASI and acoustic nav. fish deployed.

06:15 DASI back on deck for minor adjustments after testing to full power at 200 m depth. Attempting to track down a short in the system. Stripped all HT connections down, tested and reassembled.

22:34 DASI reassembled. Beginning HT tests on deck.

22:39 Beginning a 12 min., full power test on deck. Successfully completed.

23:10 DASI deployed again (experiment 2, deployment 4, tow line 1).

338 02:08 Shallow water tests of DASI successfully completed to full power, attaching acoustic

transponder.

06:09 At start of DASI tow line 1.

07:41 DASI flying above seabed and "stuffing out the current" [pers. comm.

L.M. MacGregor, 1995].

18:06 End of DASI tow line 1 for experiment 2, deployment 4. Start hailing in wire.

20:06 DASI on deck.

20:24 Acoustic nav. fish on deck.

21:38 DASI streamer recovered.

22:22 Released ELF Pele I.

339

01:00 ELF Pele I on deck. End of CSEM Experiment 2. Transiting to first DOBS deployment position for seismic experiment Seismic North.

04:24 Deployed DOBS 6.

06:25 Deployed DOBS 5.

08:24 Deployed DOBS 4.

09:33 Deployed DOBS 3.

10:54 Deployed DOBS 2.

12:44 Deployed DOBS 1 and commenced deployment of airgun array, magnetometer and single channel streamer.

20:00 First shot Seismic North.

21:34 Deployed sonobuoy 1.

340

02:38 Deployed sonobuoy 2. Failed.

02:54 Deployed sonobuoy 2 (again).

05:01 Deployed sonobuoy 3.

08:18 Deployed sonobuoy 4.

11:00 Deployed sonobuoy 5.

11:46 Deployed sonobuoy 6.

341

04:32 Last shot Seismic North. Recovered magnetometer, airgun array and single channel streamer.

06:16 DOBS 6 released.

07:24 DOBS 6 on deck.

08:50 DOBS 5 released.

09:54 DOBS 5 on deck.

11:19 DOBS 4 released.

11:43 DOBS 5 released.

12:05 DOBS 4 on deck.

12:21 DOBS 2 released.

12:38 DOBS 3 on deck.

13:10 DOBS 2 on deck.

14:20 DOBS 1 released.

15:14 DOBS 1 on deck. End of seismic experiment Seismic North. Transiting to first EM instrument deployment position.

18:41 Commenced deploying LEM Kermit for Main CSEM Experiment 3.

18:44 Streaming Kermit antenna.

20:04 LEM Kermit in water.

342

00:43 Kermit released from end of 0.680" cable.

02:40 0.680" cable recovered and commencing acoustic survey of Kermit deployment position.

04:35 Commenced deployment of LEM Opus.

05:06 LEM Opus in water.

10:22 LEM Opus released.

12:15 0.680" cable recovered.

12:30 Commencing acoustic survey of Opus deployment position.

16:00 Commencing swath bathymetry survey of EM instrument positions and tow lines.

19:08 Deployed acoustic nav. fish at LEMUR 11 deployment position.

19:46 LEMUR 11 deployed.

22:32 LEMUR 14 deployed.

343

01:27 LEMUR 15 deployed.

03:13 ELF Pele II deployed.

07:53 Acoustic nav. transponder 1 deployed.

10:05 Transponder 2 deployed.

11:39 Transponder 3 deployed.

13:38 Transponder 4 deployed.

14:58 Transponder 5 deployed.

15:20 Recovered acoustic nav. fish.

18:22 Deployed ELF Quail I.

21:35 XBT 4 deployed but failed.

21:47 Deployed ELF Ulysses.

21:51 XBT 5 deployed and transit to LOLEMUR deployment position.

344

00:20 Deployed acoustic nav. fish at LOLEMUR deployment position.

01:39 LOLEMUR deployed.

02:40 Recovered acoustic nav. fish for transit to start of DASI tow line.

04:37 Acoustic nav. fish deployed at DASI deployment position.

06:09 Commenced deploying DASI streamer, CSEM Experiment 3, deployment 5, tow line 3.

07:35 DASI streamer deployed.

10:24 DASI deck tests successfully completed.

11:17 DASI deployed.

11:35 Acoustic transponder attached to 0.680" cable 200 m above DASI.

11:50 Lowering DASI to seabed.

13:28 Commenced DASI transmissions on tow line 3 at 1 Hz.

23:11 End of tow line 3. Start turn and transit to start of tow line 4, CSEM Experiment 3, deployment 5.

345

07:59 Start of tow line 4 at 8 Hz.

20:30 End tow line 4.

20:51 Deployed XBT 7.

23:00 Transponder removed from 0.680" cable.

23:22 DASI on deck.

23:41 DASI streamer onboard for repairs.

23:50 Acoustic nav. fish recovered.

346

00:36 Commenced gravity survey with associated magnetics, 3.5 kHz and swath data collection.

02:30 Commenced G1G2 gravity profile.

06:40 Completed G1G2 gravity profile.

07:17 Commenced G3G4 gravity profile.

10:08 Strobe light identified on horizon from surface instrument.

10:11 Magnetometer recovered and gravity profile interrupted to go and investigate.

10:56 ELF Quail I recovered.

11:25 Magnetometer re-deployed.

12:10 Turning to re-join gravity profile G3G4 before cut-off point.

13:04 Completed G3G4 gravity profile. Transiting to re-deploy ELF Quail II.

17:50 Recovered magnetometer.

18:41 Deployed ELF Quail II.

20:42 Commenced DASI CSEM Experiment 3, deployment 6, tow line 2.

20:56 DASI streamer in water.

21:27 DASI in water.

21:46 Bringing DASI back on deck for adjustments to towing system.

22:29 DASI re-deployed.

22:52 Transponder attached at 200 m up 0.680" cable.

22:57 Acoustic nav. fish deployed.

347

00:02 Commencing DASI transmissions at 8 Hz on DASI tow line 2 (experiment 3, deployment 6).

01:15 Streamer damaged, commencing recovery.

02:39 Deployed XBT 8.

03:20 DASI recovered and streamer being towed while connections are repaired.

10:23 DASI repaired and reassembled, beginning deck tests.

10:40 Deck tests completed successfully.

10:49 DASI back in water (CSEM Experiment 3, deployment 7, tow line 2).

11:06 Transponder attached to 0.680" cable at 200 m, paying out wire.

12:30 DASI transmissions on CSEM Experiment 3, deployment 7, tow line 2 commencing at 8 Hz.

21:18 End of DASI tow line 2 (experiment 3, deployment 7).

21:55 XBT 9 deployed.

348

01:50 Commencing DASI tow line 1 at ¼ Hz.

02:32 Acoustic nav. fish failed. Commencing repair.

05:29 Water in Lazarette. HT turned right down.

05:30 HT turned back up after tarpaulin installed in Lazarette to keep sea water of PSU.

12:00 Completed DASI tow line 1 (CSEM Experiment 3, deployment 7).

15:19 Commencing DASI tow line 1 at 8 Hz.

349

01:22 Completed DASI tow line 1 (CSEM Experiment 3, deployment 7).

02:05 Commenced DASI tow line 5 at 1 Hz (CSEM Experiment 3, deployment 7).

03:55 DASI frequency changed to ¼ Hz.

08:06 Completed DASI tow line 5 (CSEM Experiment 3, deployment 7). End of DASI transmissions. Instrument recovery commences.

09:39 Recovering transponder from 0.680" cable.

10:10 DASI on deck.

10:19 DASI deck tests completed successfully.

11:09 DASI streamer onboard.

12:30 Released LEM Opus.

13:42 LEM Opus on deck.

14:34 Released LEM Kermit.

15:33 LEM Kermit on deck.

16:12 ELF Quail II released.

17:29 ELF Quail II on deck.

18:15 ELF Noddy released.

19:13 ELF Noddy on deck.

19:40 ELF Trevor released.

20:54 ELF Trevor on deck.

21:01 Acoustic nav. fish deployed for LEMUR recovery.

21:20 LEMUR 14 released.

22:15 Acoustic nav. fish recovered.

22:30 LEMUR 14 on deck.

23:12 Released transponder 3.

23:53 Transponder 3 on deck.

350

00:40 Transponder 2 released.

01:12 Transponder 2 on deck.

01:47 LEMUR 11 released.

03:03 LEMUR 11 on deck.

03:52 Transponder 1 released.

04:30 Transponder 1 on deck.

05:31 LOLEMUR released.

06:51 LOLEMUR on deck.

07:40 LEMUR 15 released.

09:00 LEMUR 15 on deck.

09:23 Commenced gravity survey G9G10 with magnetometer out.

16:55 End of gravity survey G9G10. Transit to ELF Pele II.

18:54 Magnetometer recovered.

19:00 ELF Pele II released.

21:00 ELF Pele II on deck.

21:43 Trying to release transponder 5, but experiencing problems with hull 12 kHz transducer, which is unable to transmit on 9 kHz.

22:31 Transponder 5 released.

23:27 Transponder 5 on deck.

351

00:01 ELF Rhonda released.

01:18 ELF Rhonda on deck.

02:00 Transponder 4 released.

03:12 Transponder 4 on deck.

03:49 ELF Ulysses released.

04:57 ELF Ulysses on deck.

05:52 Hove to over LEMUR 12.

06:04 Acoustic nav. fish in water, penultimate attempts at releasing LEMUR 12 acoustically.

06:57 Dredge wire in water, three unsuccessful dredge/drag attempt made.

21:20 Dredge wire recovered.

23:46 Deployed XBT 10.

352

00:18 Deployed XBT 11.

00:46 Deployed XBT 12.

03:16 Dredging wire in water for fourth and final attempt to recover LEMUR 12.

12:40 End of dredging/dragging operations and wire recovered.

12:45 Final attempts at acoustically releasing LEMUR 12. Release on LEMUR 12 disabled before departure.

12:56 Acoustic nav. fish recovered.

13:17 Magnetometer deployed and heading for start of final gravity and magnetic survey.

15:20 Arrived at start way-point G5 for gravity survey en route to Auckland.

353

09:15 Gravity survey completed. Recovered magnetometer and setting course for Auckland. End of science.

357

00:00 Arrived alongside at Capt. Cook Wharf, Auckland.

23rd December, 1995.

4. Mobilisation and Demobilisation

Mobilisation took place over a 5 day period (20 to 24 November 1995) in Suva. It was complicated, from the scientific point of view, by three factors: (i) the large amount of scientific equipment to be embarked, which filled five twenty-foot containers (one from Scripps, one from Durham and 1½ each from Cambridge/RVS); (ii) the need to install and test (particularly for electrical safety) the DASI power supply system in the Lazarette, and to construct out of the parts in which it was shipped, the DASI antenna streamer and wind it onto the multichannel streamer storage drum; and (iii) the late arrival of the containers from the UK. This last aspect was particularly frustrating, since the equipment had all been shipped from the UK early in September. Our containers were originally due in Suva well before the Maurice Ewing's arrival there on Sunday 19th November. However because of sailing cancellations on the part of the shipping lines involved, the containers were delayed. We had hoped to begin mobilisation on Monday 20th, but in the event the Scripps container was delivered to the ship on Tuesday 21st, and the four UK containers were not delivered until Wednesday 22nd. As a result the period between their arrival and the vessel sailing on Friday 24th was one of intense activity. Ship's staff and the scientific party worked very hard to embark all the equipment and carry out the necessary preparations, and it was a considerable achievement that the vessel was able to sail on time. It is possible that at least some of the problems experienced at the start of the cruise during CSEM Experiment 1 might have been avoided, had mobilisation been a less frenetic period. The problem of late delivery of the scientific equipment was something entirely outside the control of either the scientific party or Lamont.

By comparison, demobilisation in Auckland - which took place on December 23rd and 24th - was straightforward. Scientific equipment was disembarked and loaded into four containers for shipment to the USA and UK without any difficulties.

5. Equipment Performance

The dominant aspect of this cruise in terms of equipment was the vast amount that we had on board in order to undertake such a large scale, multi-disciplinary programme. The ship was quite literally full of equipment and people. The rest of this section should therefore be read in the context that all parts of this huge array of equipment that receive no further mention here, simply worked.

5.1 The Seismic Experiment

The seismic experiment was very successful, and the DOBS and acoustic release systems worked well. No instruments were lost and data quality is extremely high. The only problems experienced during the seismic experiments were those of synchronising airgun shot firing to DOBS recording windows, and of the disposable sonobuoys which failed to operate correctly. Both of these are discussed above in Section 2 of this report.

5.2 The Electromagnetic Experiment

The electromagnetic experiment was also successful, though we experienced a number of problems. The most serious of these was the loss of an instrument - LEMUR 12 from Cambridge. The LEMUR responded to acoustic signals and acknowledged the release code, showing that the motor driven cam on the release unit had rotated, but the instrument did not leave the seabed. This instrument was fitted with a rigid bottom weight, and suspicion as to the cause of its loss centred on that component. For all other LEMUR deployments during the cruise, we used chain clumps as bottom weights, and no further problems were encountered. Attempts to recover the recalcitrant LEMUR by dragging using the dredging wire at the end of the cruise, proved unsuccessful.

Several of the fourteen electric field recorders (9 Scripps and 5 Cambridge) that we took with us experienced problems of some kind, though we recovered high quality data from 13 out of a total of 16 deployments. One ELF (Trevor) suffered a disk failure and recorded no data. One LEMUR (15) had a problem with a connector to its ADC interface, and also recorded no data. Another LEMUR (14) recorded data only from the first half of the main CSEM experiment. One of the Scripps pressure cases was found to have a scored O-ring surface, so could not be used. Another LEMUR (13) developed a CPU fault just before deployment, so ELF Lolita was deployed using LEMUR 13's hardware, to make the hybrid instrument LOLEMUR. One ELF (Quail) released a bottom weight prematurely, and floated back to the surface. Fortunately it was sighted, recovered and re-deployed after modifications to its release system, with no loss of data. In total there were 16 deployments and 15 recoveries, of which 13 had recorded high quality electric field data.

The Cambridge DASI deep-towed CSEM transmitter system also caused a number of difficulties. At the start of the cruise, a series of apparent short-circuits of the high-voltage part of the system to earth caused damage to the power supply, and resulted in failure to transmit more than a few minutes of data during deployments 1, 2 and 3. The source of the leakage was never definitively isolated, although the problem was solved eventually by (i) replacing the slip rings on the deep-tow winch and re-terminating the inboard end of the cable, (ii) replacing the swivel system at the outboard end of the cable, and (iii) stripping down and reassembling the DASI system itself. Deployment 1 ended by causing serious damage to the power supply system, which fortunately we had sufficient spares to repair.

The other major source of problems was the connection between the antenna streamer and the DASI vehicle. At the end of every deployment, significant damage was found to have occurred to the flexible cables that provide the electrical connection between the DASI vehicle and the streamer. The two likely causes of the mechanical damage inflicted on these cables are either vortices created at the rear of the vehicle by its passage through the water, or rapid vertical accelerations of the vehicle due to ship heave being transmitted down the deep-tow cable. A combination of these causes is not unlikely. Previous experience with DASI had never resulted in repeated and serious damage on the scale experienced on this cruise, so this problem requires investigation and solution before DASI is used again.

It also became clear that the DASI vehicle was suffering extreme mechanical vibrations during deep-towing. This is most likely due to strumming of the deep-tow cable. It caused both noise on the deep-tow cable, and problems with some electronic components, particularly in the piggy-back logger - at times the vibration was sufficiently intense to cause an interface board to fall off its connectors, and to loosen all the screws holding the lid onto a metal electronics enclosure. Appropriate shock-mounting systems should be investigated for mechanically isolating the DASI from strumming of the cable.

5.3 Ship's Machinery and Other Fitted Equipment

The ship's fitted equipment and machinery functioned efficiently and reliably throughout most of the cruise. However, two aspects should be mentioned at this point for future reference for those intending to use a deep-towed vehicle attached to the 0.680" cable. A set of slip rings were purchased by L-DEO for the 0.680" cable winch which were supposedly rated to a sufficiently high voltage to cope with the 2000 Vrms signal output from the power supply and passed down the cable to the deep-towed DASI. The massive short circuit of the HT system during DASI deployment 1 occurred in two stages. Firstly, a large, short-duration current surge was logged by the power supply monitoring system, though at this stage it survived. This occurred just a few seconds after power was turned up to full operating levels. A short time later, a second current surge blew the output stage of the power supply, before the output fuses had time to blow. The time interval between these surges was close to the rotation period of the winch at the time, and we speculate that the problem may have been due to breakdown of the slip rings at a particular point in their rotation. The entire HT system had previously been tested, without any problems, to considerably above the operating voltage - but the winch was not turning during the tests. Fortunately, amongst the five container loads of equipment dispatched for the cruise, we had brought (just in case!) a set of RVS slip rings which had been used with the DASI system on several previous occasions. These were installed and the inboard cable termination was carefully remade, and thereafter no similar catastrophic current surges were experienced.

The Ewing's 0.680" cable winch is also fitted with a heave compensation mechanism which, as the actual winch and cable had only been used for about 24 hours in its life time, had never been fully adjusted. No matter how much this system was tweaked it could not be made to operate in the smooth way that would have minimised ship heave effects. We were fortunate that the sea state was relatively calm during DASI towing, allowing us to tow close to the bottom (within 30 m on some profiles). Even so, the DASI tow vehicle experienced large and rapid changes in altitude caused by ship motion being transmitted down the cable, and these could be clearly seen on the telemetered 3.5 kHz profiler data from DASI. We believe that these vertical motions are at least partially responsible for the damage to the DASI antenna connections described in 5.2 above, which necessitated premature abandonment of tows 5 and 6, and major, time consuming repairs. If future cruises require use of this system for deep-tow purposes, we recommend that the heave compensation system be investigated thoroughly in advance, especially if a heavy sea is anticipated.

6. Conclusions

This cruise was notable for the quantity and variety of equipment in use, the complexity of the scientific programme, and (from the perspective of habitual North Atlantic sea goers) the benignity of the weather. The outcome was a successful, integrated, geophysical investigation of the Central Valu Fa Ridge and its overlapping region with the Northern Valu Fa Ridge.

The seismic experiment generated a large quantity of high quality data from all its component parts. Initial inspection of the dataset reveals bright reflection events in the unprocessed, single channel reflection data corresponding to the top of the axial melt body. This reflector is observed along the entire CVFR. Shadow zones are observed within the wide-angle dataset corresponding to the locations of these apparent melt bodies, together with numerous microearthquakes. These data will enable us to resolve the physical properties of the on- and off-axis ridge crust down to the mantle in both 2 and 3 dimensions.

The CSEM component, while causing more in the way of instrumental problems during the cruise, also resulted in the successful collection of an unprecedentedly large dataset. This will allow us to investigate in detail the electrical resistivity structure of a magmatically robust section of the CVFR, from the seafloor to the upper few kilometres of the mantle. The combination of seismic and electromagnetic datasets should provide entirely new constraints on the nature of magmatic and hydrothermal processes at crustal levels beneath a spreading ridge.

In addition we collected a considerable quantity of gravity, magnetic and bathymetric data to be used in supplementary studies, and incidentally collected an XBT profile that may indicate a major hydrothermal site. The quantity and quality of data collected, and the achievement of all experimental objectives, allow us to claim the cruise as a success.

The scientific work was enormously assisted by the professionalism and commitment of the ship's officers and crew, and of the L-DEO support staff, who made us Brits (and our strange ways) feel very much at home. The Chief Scientists would like to take this opportunity to express our deepest gratitude to all of the Ewing's crew and scientific support staff - without you this cruise would not have been so successful. We look forward to sailing with you again.

Acknowledgements

We wish to thank the master, officers and crew of the Maurice Ewing and the support staff of Lamont-Doherty Earth Observatory for their efforts in ensuring a successful and enjoyable cruise. We would particularly like to thank the master for his participation and support on deck during every instrument deployment and recovery, no matter the time of day or night - dedication far above and beyond the call of duty. We also thank John Dibernardo for his willingness and ability to weld anything to anything and rapidly reproduce odd-shape brackets apparently out of thin air. Mike Rawson's shore-side pre-cruise organisation was also greatly appreciated.

Ship time was funded by the U.K.'s Natural Environment Research Council (BRIDGE), who also supported the work through research grants (GST/02/1123 and GR3/9414) and research studentship (GT4/95/72/E) awards to Cambridge and Durham Universities. We thank the Royal Navy's Hydrographic Office for supplying the XBT probes.

References

Barth, G.A., J.C. Mutter & R.S. Detrick, 1987. Along-axis variability in East Pacific Rise seismic structure (abstract). EOS Trans. AGU, 68, 1491-1492.

Burnett, M.S., D.W. Caress & J.A. Orcutt 1989. Tomographic image of the East Pacific Rise at 12° 50'N. Nature, 339, 206-208.

Caress, D.W., M.S. Burnett & J.A. Orcutt, 1992. Tomographic image of the axial low-velocity zone at 12° 50'N on the East Pacific Rise. J. Geophys. Res., 97, 9243-9264.

Chave, A.D. & C.S. Cox, 1982. Controlled electromagnetic sources for measuring electrical conductivity beneath the oceans, 1, forward problem and model study. J. Geophys. Res., 87, 5327-5338.

Collier J.S. and M.C. Sinha, 1990. Seismic images of a magma chamber beneath the Lau Basin back-arc spreading centre. Nature, 346, 646-648.

Collier J.S. and M.C. Sinha, 1992a. The Valu Fa Ridge: the pattern of volcanic activity at a back-arc spreading centre. Marine Geology, 104, 243-263.

Collier J.S. and M.C. Sinha, 1992b. Seismic mapping of a magma chamber beneath the Valu Fa Ridge, Lau Basin. J. Geophys. Res., 97, 14031-14053.

Cox, C.S., S.C. Constable, P.D. Chave & S.C. Webb, 1986. Controlled source electromagnetic sounding of the oceanic lithosphere. Nature, 320, 52-54.

Davis, A.S., D.A. Clague, & J.L. Morton, 1990. Volcanic glass compositions from two spreading centres in the Lau Basin, South-west Pacific. Geol. Jahrbuch, B72.

Detrick, R.S., P. Buhl, E. Vera, J. Mutter, J. Orcutt, J. Madsen & T. Brocher, 1987. Multi-channel seismic imaging of a crustal rnagma chamber along the East Pacific Rise. Nature, 326, 35-41.

Evans, R.L., S.C. Constable, M.C. Sinha, & C.S. Cox, 1991. Upper crustal resistivity structure of the East Pacific Rise near 13°N. Geophys. Res. Lett., 18, 1917-1920.

Evans, R.L., M.C. Sinha, S.C. Constable & M.J. Unsworth, 1993. On the electrical nature of the axial melt zone at 13°N on the East Pacific Rise. J. Geophys. Res., (accepted).

Fouquet, Y., U. Von Stackelberg, J.L. Charlou, J.P. Donval, J. Erzinger, J.P. Foucher, P. Herzig, R. Mühe, S. Soakai, M. Wiedicke, & H. Whitechurch, 1991. Hydrothermal activity and metallogenesis in the Lau back-arc basin. Nature, 349, 778-781.

Frenzel, G., R. Mühe, & P. Stoffers, 1990. Petrology of the volcanic rocks from the Lau Basin, south-west Pacific. Geol. Jahrbuch, B72.

Harding, A.J., J.A. Orcutt, M.E. Kappus, E.E. Vera, J.C. Mutter, P. Buhl, R.S. Detrick & T.M. Brocher, 1989. The structure of young oceanic crust at 13° N on the East Pacific Rise from expanding spread profiles. J. Geophys. Res., 94, 12163-12196.

Jenner, G.A., P.A. Cawood, M. Rautenschlein & W.M. White, 1987. Composition of back-arc volcanics, Valu Fa Ridge, Lau Basin: Evidence for a slab-derived component in their mantle source. J. Volc. & Geothermal Res., 32, 209-222.

Kent, G.M., A.J. Harding, & J.A., 1990. Evidence for a smaller magma chamber beneath the East Pacific Rise at 9° 30'N. Nature, 344, 650-653.

Kent, G.M., A.J. Harding, & J.A. Orcutt, 1993. Distribution of magma beneath the East Pacific Rise between the Clipperton Transform and the 9° 17'N deval from forward modelling of common depth point data. J. Geophys. Res., 98, 13945-13.969.

Langmuir, C.H., J.F. Bender & R. Batiza, 1986. Petrological and tectonic segmentation of the East Pacific Rise, 5° 30' - 14° 30' N. Nature, 322, 422-429.

Macdonald, K.C., J.C. Sempere & P.J. Fox, 1984. East Pacific Rise from Siqueiros to Orozco fracture zones: Along-strike continuity of axial neovolcanic zone and structure and evolution of overlapping spreading centres. J. Geophys. Res., 69. 6049-6069.

Macdonald, K.C., J.-C. Sempere, P.J. Fox & R. Tryce, 1987. Tectonic evolution of ridge-axis discontinuities by the meeting, linking, or self-decapitation of neighbouring ridge segments. Geology, 15, 993-997.

Macdonald, K.C. & 12 others, 1992. The East Pacific Rise and its flanks 8 - 18° N: History of segmentation, propagation, and spreading direction based on SeaMARC II and Sea Beam studies. Mar. Geophys. Res., 14, 299-344

Mutter, J.C., G.A. Barth, P. Buhl, R.S. Detrick, J.A. Orcutt & A.J. Harding, 1988. Magma discontinuities across ridge-axis discontinuities on the East Pacific Rise from multichannel seismic images. Nature, 336, 156-158.

The Nautilau Group, 1990. Hydrothermal activity in the Lau Basin. EOS. 71, 678-679.

Parson, L.M., J.A. Pearce, B.J. Murton, R.A. Hodkinson & the RRS Charles Darwin Scientific Party, 1990. Role of ridge jumps and ridge propagation in the tectonic evolution of the Lau back-arc basin, south-west Pacific. Geology, 18, 470-473.

Sinha, M.C., 1988. Cruise report: RRS Charles Darwin 34, Southern Lau Basin. Internal report, Bullard Laboratories, Cambridge University.

Sinha, M.C., C. Peirce and S. Constable, 1994. RRS Charles Darwin cruise report: An integrated geophysical investigation of the axial volcanic region of the Reykjanes Ridge at 57° 45'N. Internal report, Universities of Cambridge and Durham.

Sinha, M.C., 1995. Segmentation and rift propagation at the Valu Fa ridge, Lau Basin: Evidence from gravity data. J. Geophys. Res., 100, B8, 15025-15043.

Toomey, D.R., G.M. Purdy & S.C. Solomon, 1990. The three-dimensional seismic velocity structure of the East Pacific Rise near latitude 9° 30'N. Nature, 347, 639-645.

Unsworth, M.J., B.J. Travis & A.D. Chave, 1993. Electromagnetic induction by a finite electric dipole source over a 2-D earth. Geophysics, 58, 198-214.

Vera, E.E., J.C. Mutter, P. Buhl, J.A. Orcutt, A.J. Harding, M.E. Kappus. R.S. Detrick, & T.M. Brocher, 1990. Structure of 0 to 0.2 - m.yr. old oceanic crust at 9° N on the East Pacific Rise from expanded spread profiles. J. Geophys. Res., 95, 15529-15556.

von Stackelberg, U. & the Shipboard Scientific Party, 1988. Active Hydrothermalism in the Lau back-arc basin (SW Pacific): First results from the SONNE 48 Cruise (1987). Marine Mining. 7, 431-442.

Webb, S.C., S.C. Constable, C.S. Cox & T.K. Deaton, 1985. A seafloor electric field instrument. J. Geomag. Geoelectr., 37, 1115-1129.

Wiedicke, M. & J. Collier, 1993. Morphology of the Valu Fa spreading ridge in the southern Lau Basin. J. Geophys. Res., 98, 11769-11782.

List of Figures

Figure 1. Summary track chart of cruise Ewing 9512, including an expanded view of the work

area.
 

Figure 2. Seismic experiment track chart showing instrument positions.
 

Figure 3. Airgun array.
 

Figure 4a. Example Seismic South record sections.
 

Figure 4b. Example Seismic South record sections.
 

Figure 5. Example Seismic South microearthquake.
 

Figure 6a. Example Seismic North record sections.
 

Figure 6b. Example Seismic North record sections.
 

Figure 7. Example single channel seismic reflection data along and across-axis.
 

Figure 8. Example sonobuoy data.
 

Figure 9. Controlled source EM experiment - transmitter tracks and receiver locations for the CSEM test Experiments 1 and 2.
 

Figure 10. Controlled source EM experiment - transmitter tracks and receiver locations for the main Experiment 3.
 

Figure 11. Example EM data, showing LEMUR time series and amplitude spectra.
 

Figure 12. Swath bathymetry track chart.
 

Figure 13. Gridded swath bathymetry data.
 

Figure 14. Gravity data track lines.
 

Figure 15. Magnetic data track lines.
 

Figure 16. SOPAC swath bathymetry track chart.

List of tables

Table 1. Instrument positions for the seismic experiment.

Table 2. 2D Seismic line locations.

Table 3. Recorded 3D seismic data.

Table 4. EM instrument positions.

Table 5. EM tow line locations.

Table 6. Positions of acoustic navigation transponders.

Table 7. Sound velocity dip data.

Table 8. XBT locations.

Table 9. Gravity survey lines.

Table 10. Shipboard party.





Table 1

Instrument Positions for the Seismic Experiment Seismic North


 Instrum.    Instrum.  Latitude  Latitude  Longitude  Longitude    Water    
   Type       Number    (° S)      (' S)     (° W)      (' W)    Depth (m)  



   DOBS         1         22       14.50      176       28.63       2366    

   DOBS         2         22       11.84      176       36.10       2133    

   DOBS         3         22       11.59      176       36.74       2147    

   DOBS         4         22       11.38      176       37.44       2035    

   DOBS         5         22       07.70      176       47.54       2369    

   DOBS         6         22       03.94      176       57.89       2355    



 Sonobuoy       1         22       18.84      176       16.52        -      

 Sonobuoy       2         22       13.88      176       40.84        -      

 Sonobuoy       3         22       20.32      176       39.39        -      

 Sonobuoy       4         22       05.17      176       33.65        -      

 Sonobuoy       5         22       02.95      176       35.17        -      

 Sonobuoy       6         22       06.51      176       36.17        -      

Seismic South


 Instrum.    Instrum.  Latitude  Latitude  Longitude  Longitude    Water    
   Type       Number    (° S)      (' S)     (° W)      (' W)    Depth (m)  



   DOBS         1         22       18.64      177       03.35       2563    

   DOBS         2         22       23.41      176       49.86       2768    

   DOBS         3         22       25.09      176       45.27       2675    

   DOBS         4         22       26.35      176       41.82       2136    

   DOBS         5         22       28.33      176       36.33       2799    

   DOBS         6         22       32.25      176       25.42       1915    



 Sonobuoy       1         22       34.13      176       20.07        -      

 Sonobuoy       2         22       20.36      176       45.47        -      

 Sonobuoy       3         22       14.49      176       37.26        -      

 Sonobuoy       4         22       21.03      176       39.70        -      

 Sonobuoy       5         22       15.00      177       01.99        -      

 Sonobuoy       6         22       10.85      177       00.47        -      


Table 2

2D Seismic line locations


              From                  To                                       

 Line No.    Lat.      Long.      Lat.       Long.     Leng.       DOBS      
             (° S)      (° W)     (° S)      (° W)     (km)      on line     



 2D data                                                                     

    1       22.3075   177.0650   22.5692    176.3333    80.62     S1S6,      
                                                                   SB1s      

    2       22.3992   176.7817   22.2108    176.7075    22.29                
                                                                   SB2s      

    3       22.2058   176.6958   22.2308    176.6250     7.81                
                                                                    SC       

    4       22.4483   176.7000   21.9925    176.5258    53.73    S4, N2,     
                                                                 SB3+4s,     
                                                                  SB3+4n     

    5       22.0580   176.9625   22.3200    177.0600    30.80     S1, N6     
                                                                   SB6s      

    6       22.0625   176.9750   22.3242    176.2500    82.21      N1N6      
                                                                   SB1n      

    7       22.3250   176.7167   22.1850    176.6625    16.55                
                                                                   SB2n      

    8       22.3375   176.7108   22.3508    176.6725     4.21                
                                                                    SC       

    9       22.1967   176.6267   21.9892    176.5700    23.84       N4       
                                                                SB5n, SB6n   

    10      22.2100   176.6208   22.2225    176.5842     4.04                
                                                                    SC       

    11      22.3675   176.8792   22.1183    176.7875    29.32       N5       

Single channel reflection data was collected along all lines

S - Southern DOBS deployments N - Northern DOBS deployments

SB - Sonobuoy data (south/north experiment) SC - Single channel data only



Table 3

Recorded 3D seismic data


Line No.    1     2     3    4     5     6     7     8    9    10    11   



  DOBS                                                                    

   S1             X     X    X°                                           

   S2             X     X    X     X                                      

   S3             X     X    X     X                                      

   S4             X     X          X                                      

   S5             X     X    X     X                                      

   S6             X     X    X     X                                      



   N1      X¹                X²    X           X     X    X     X     X   

   N2      X¹                X²    X           X     X    X     X     X   

   N3      X¹                X²    X           X     X    X     X     X   

   N4      X¹                X²    X           X     X          X     X   

   N5      X¹                X²    X           X     X    X     X         

   N6      X¹                X²                X     X    X     X     X   

° - southern half of line only S - Seismic South DOBS

¹ - western end of line only N - Seismic North DOBS

² - northern half of line only X - 3D Data recorded from this line



Table 4

EM instrument positions


 Instrum.    Instrum.  Instrum. Name  Latitude Latitude  Longit.   Longit.   Water   
   Type       Number                            (' S)     (° W)    (' W)     Depth   
                                       (° S)                                  (m)    

Test                                                                                 
Expt. 1 &                                                                            
2                                                                                    

ELF             1          Pele I        22      27.92     176      37.52     2802   

ELF             2          Rhonda        22      27.36     176      39.02     2410   

ELF             3          Trevor        22      26.75     176      40.70     2245   

ELF             4          Noddy         22      26.00     176      42.83     2225   

LEMUR           12                       22      24.61     176      35.31     2805   

LEMUR           14                       22      28.36     176      36.38     2806   



Main                                                                                 
Expt. 3                                                                              

LEMUR           11                       22      21.17     176      42.77     2354   

LEMUR           14                       22      26.47     176      41.47     2146   

LEMUR           15                       22      27.94     176      37.64     2797   



ELF             1         Pele II        22      28.30     176      36.41     2750   

ELF             2         Ulysses        22      22.02     176      40.18     1950   

ELF             3         Quail I        22      25.60     176      44.25     2675   

ELF             4         Quail II       22      27.31     176      44.25     2625   



ELF/LEMUR       1         LOLEMUR        22      24.22     176      35.02     2750   



LEM             1          Kermit        22      24.55     176      46.90     2619   

LEM             2           Opus         22      23.55     176      49.55     2776   


Table 5

EM tow line locations


                                              From               To                        

Experiment    Tow     Trans.   Instruments   Lat.     Long.     Lat.     Long.    Length   
     &        Line     Freq.    Recording     (°S)     (°W)     (°S)      (°W)     (km)    
Deploy-ment   No.                                                                          
    No.                                                                                    

  Expt 2       1       1 Hz       Pele I    22.3598  176.569   22.5054  176.618    16.93   
  Deply 4                         Rhonda                                                   
                                  Noddy                                                    

  Expt 3       3       1 Hz        Opus     22.2652  176.684   22.5080  176.762    28.18   
  Deply 5                         Kermit                                                   
                                 Quail I                                                   
                                  Noddy                                                    
                                  Rhonda                                                   
                                 Pele II                                                   
                                 Ulysses                                                   
                                 LOLEMUR                                                   
                                 LEMUR 11                                                  
                                 LEMUR 14                                                  

  Expt 3       4       8 Hz        Opus     22.3971  176.814   22.5083  176.545    30.29   
  Deply 5                         Kermit                                                   
                                 Quail I                                                   
                                  Noddy                                                    
                                  Rhonda                                                   
                                 Pele II                                                   
                                 Ulysses                                                   
                                 LOLEMUR                                                   
                                 LEMUR 11                                                  
                                 LEMUR 14                                                  

  Expt 3       2       8 Hz        Opus     22.3041  176.651   22.3486  176.662    5.07    
  Deply 6                         Kermit                                                   
                                 Quail I                                                   
                                  Noddy                                                    
                                  Rhonda                                                   
                                 Pele II                                                   
                                 Ulysses                                                   
                                 LOLEMUR                                                   
                                 LEMUR 11                                                  

  Expt 3       2       8 Hz        Opus     22.3105  176.650   22.5145  176.711    23.48   
  Deply 7                         Kermit                                                   
                                 Quail I                                                   
                                  Noddy                                                    
                                  Rhonda                                                   
                                 Pele II                                                   
                                 Ulysses                                                   
                                 LOLEMUR                                                   
                                 LEMUR 11                                                  

  Expt 3       1      0.25 Hz      Opus     22.5604  176.637   22.3533  176.567    24.08   
  Deply 7                         Kermit                                                   
                                 Quail I                                                   
                                  Noddy                                                    
                                  Rhonda                                                   
                                 Pele II                                                   
                                 Ulysses                                                   
                                 LOLEMUR                                                   
                                 LEMUR 11                                                  

  Expt 3       1       8 Hz        Opus     22.3285  176.555   22.5083  176.618    20.96   
  Deply 7                         Kermit                                                   
                                 Quail I                                                   
                                  Noddy                                                    
                                  Rhonda                                                   
                                 Pele II                                                   
                                 Ulysses                                                   
                                 LOLEMUR                                                   
                                 LEMUR 11                                                  

  Expt 3       5     0.25 Hz¹      Opus     22.5109  176.619   22.4801  176.732    12.11   
  Dply 7                          Kermit                                                   
                                 Quail I                                                   
                                  Noddy                                                    
                                  Rhonda                                                   
                                 Pele II                                                   
                                 Ulysses                                                   
                                 LOLEMUR                                                   
                                 LEMUR 11                                                  

¹ - first hour of this tow line at 1 Hz



Table 6

Positions of acoustic navigation transponders


Transponder    Name    Latitude  Latitude  Longitude  Longitude    Water    
                         (° S)     (' S)                (' W)    Depth (m)  
                                             (° W)                          

     1          T1        22       19.78      176       40.26       2050    

     2          T2        22       24.14      176       41.56       2030    

     3          T3        22       28.18      176       43.25       2200    

     4          T4        22       25.54      176       36.77       2725    

     5          T5        22       30.12      176       37.97       2250    


Table 7

Sound velocity dip data

location: 22° 25.90'S 176° 35.92'W


 Depth    SV Down    SV Up    SV avg  

  (m)      (m/s)     (m/s)    (m/s)   



   5      1532.90   1533.50  1533.20  

   10     1532.50   1532.70  1532.60  

   20     1531.50   1532.10  1531.80  

   30     1531.50   1531.60  1531.55  

   40     1531.30   1531.60  1531.45  

   50     1530.10   1531.60  1530.85  

   60     1529.40   1530.50  1529.95  

   70     1529.00   1529.70  1529.35  

   80     1528.70   1529.40  1529.05  

   90     1528.40   1528.90  1528.65  

  100     1528.00   1528.50  1528.25  

  150     1525.80   1526.20  1526.00  

  200     1522.70   1523.50  1523.10  

  250     1520.60   1521.10  1520.85  

  300     1517.00   1517.30  1517.15  

  350     1514.20   1516.30  1515.25  

  400     1509.60   1511.10  1510.35  

  450     1504.50   1505.20  1504.85  

  500     1499.00   1499.70  1499.35  

  550     1493.60   1494.70  1494.15  

  600     1489.50   1490.90  1490.20  

  650     1488.30   1488.80  1488.55  

  700     1487.50   1487.70  1487.60  

  750     1486.20   1486.10  1486.15  

  800     1485.60   1485.40  1485.50  

  850     1484.20   1484.50  1484.35  

  900     1482.90   1483.40  1483.15  

  950     1483.00   1483.10  1483.05  

  1000    1481.20   1481.80  1481.50  

  1100    1481.00   1481.10  1481.05  

  1200    1481.70   1481.50  1481.60  

  1300    1482.50   1482.10  1482.30  

  1400    1483.10   1482.90  1483.00  

  1500    1483.90   1483.90  1483.90  

  1600    1485.00   1484.80  1484.90  

  1700    1486.30   1486.10  1486.20  

  1800    1487.60   1487.50  1487.55  

  1900    1489.00   1488.90  1488.95  

  2000    1490.60   1490.50  1490.55  

  2100    1492.20   1492.10  1492.15  

  2200    1493.80   1493.60  1493.70  

  2300    1495.60   1495.30  1495.45  

  2400    1497.36   1497.00  1497.18  

  2500    1498.75   1498.60  1498.68  

  2600    1500.47   1500.30  1500.39  

  2700    1502.05      -     1502.05  


Table 8

XBT Locations


   XBT      Latitude    Latitude   Longitude    Longitude  
              (° S)       (' S)       (° W)       (' W)    



 9512-01       22         25.86        176        36.01    

 9512-02       22         19.88        176        33.43    

 9512-03       22         20.89        176        33.96    

 9512-04       22         26.45        176        47.57    

 9512-05     FAILED         -           -           -      

 9512-06       22         22.51        176        40.75    

 9512-07       22         28.58        176        35.81    

 9512-08       22         22.36        176        40.04    

 9512-09       22         31.32        176        42.17    

 9512-10       22         31.05        176        42.18    

 9512-11       22         31.67        176        42.38    

 9512-12       22         31.45        176        41.92    


Table 9

Gravity and magnetic survey lines

(in addition to general underway data collection throughout the cruise)


                   From                           To                      

    Line         Lat. (° S)      Long. (° W)       Lat. (° S)      Long. (° W)    



G1G2            22 42.00'      176 22.60'     22 28.40'      177 04.80'   

G3G4            22 34.10'      177 05.40'     22 46.70'      176 22.80'   

G5G6            22 47.50'      176 33.00'     22 40.00'      177 07.50'   

G7G8            22 44.50'      177 09.00'     22 50.00'      176 34.00'   

G9G10           22 24.25'      177 03.20'     22 36.75'      176 24.00'   

G11G12          22 53.00'      176 35.00'     22 47.00'      177 09.50'   

G13G14          22 50.00'      177 10.00'     23 00.00'      176 30.00'   

G15G16          23 03.50'      176 31.00'     22 53.75'      177 11.75'   


Table 10

Shipboard Party


M. Sinha                Cambridge (Co-Chief Scientist)                   

C. Peirce               Durham (Co-Chief Scientist)    

S. Constable            Scripps (Co-Chief Scientist)   

D. Navin                Durham                         

I. Turner               Durham                         

D. White                SOC                            

D. Booth                RVS/SOC                        

L. MacGregor            Cambridge                      

M. MacCormack           Cambridge                      

S. Riches               Cambridge                      

P. Carter               Cambridge                      

J. Hobro                Cambridge                      

T. Fatai                Tongan observer                

B. Francis              L-DEO (Science Officer)        

J. Dibernardo           L-DEO (Chief airgunner)        

C. Alvarez              L-DEO (Airgunner)              

P. Olsgard              L-DEO (Airgunner)              

M. Wittreich            L-DEO (Airgunner)              

C. Donaldson            L-DEO (Electronics technician)
                    
S. Budhypramono         L-DEO (Systems Manager)        

I. Young                L-DEO (Master)                 

A. Karlyn               L-DEO (Chief Engineer)