Voyage Plans and Summaries

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Franklin Voyage Summary No. FR08/2000

Title

The Continent-Ocean Transition of the Crust and Mantle across the North West Shelf (Leg 2)

Itinerary

Depart Dampier 0700hrs Friday 22^nd Sep, 2000
Arrive Dampier 0800hrs Monday 25^th Aug, 2000

Principal Investigators

Dr Graham Heinson (Chief Scientist)
Adelaide University
Department of Geology and Geophysics
Adelaide SA 5005
Tel 08 8303 5377
Fax 08 8303 4347
Email graham.heinson@ adelaide.edu.au

Assoc. Prof. Antony White (Project Leader)
Flinders University
School of Earth Sciences
GPO Box 2100, Adelaide SA 5001
Tel 08 8201 2020
Fax 08 8201 2676
Email antony.white@ flinders.edu.au

Scientific Objectives

The north west margin of Australia includes the Pilbara Archaean shield, the Phanerozoic Canning and Canarvon sedimentary basins and the Exmouth Plateau. Geophysical measurements of magnetic and electric fields onshore and offshore provide a means of imaging the crust and mantle structure in terms of electrical conductivity. The principal aims of the project are:

1. to determine crust-mantle structure and anisotropy across the continental margin, from Archaean shield to ocean abyssal plain;

2. to investigate the Canning Basin conductivity anomaly across the margin and its relationship to extensional faults imaged from deep-seismic profiles of the Fitzroy Trough.

Cruise Objectives

Twelve seafloor magnetotelluric (MT) instruments (squares in Figure 1) will be recovered from the seafloor deployments sites established from the first leg of the cruise in August. The same instrumentation was recently recovered off the coast of Eyre Peninsula (1998) and Gulf of Carpentaria (1999) in a similar configuration, so we did not anticipate any logistical difficulties.

Cruise Track

See Figure 1

Results

Twelve low frequency MT instruments were deployed in August 2000 along a transect across the Exmouth Plateau, as detailed in Table 1.

Table 1: Instrument site, name, location, depth and data. In the last column, B stands for magnetic field data and E for electric field data.

All instruments had an acoustic burn-wire release mechanism. We successfully recovered eleven instruments, but EX7 (Horace) was lost. On deployment in August, EX7 stopped responding while sinking at 1200 m depth and we believe that the instrument probably flooded.

Instrument EX9 (Igor) was eventually recovered by dragging a cable on the seabed in 300 m water depth. The acoustics were working, and acknowledged the release code, but the instrument remained on the seabed. We suspect that strong sea-bottom currents may have flipped the instrument upside down soon after deployment, with the lead ballast on top of the instrument, and hence the burn wire corroded, but the lead remained in place. On dragging, the cable presumably knocked the ballast off, and we were able to track EX9 to the surface. Recovery at night was somewhat complicated as the instrument was upside down in the water and had no visual beacons. However, acoustic ranging was maintained and we successfully picked up EX9 late on the 24^th September.

Table 2 below shows data retrieval so far, with an indication of quality.

Table 2: Instrument site, name, depth and data quality. In the last column, B stands for magnetic field data and E for electric field data. Instruments at sites EX2, EX4, EX6 and EX8 have not yet been opened to check data recovery.

Cruise Narrative

Summary

The quality and distribution of geophysical instruments across the Exmouth Plateau will provide us with a means of determining the deep geological structure of this part of the North West Shelf. We anticipate being able to produce a two-dimensional geophysical model of the region and compare this with AGSO seismic data. In addition, the instruments are also sensitive to oceanographic currents across the North West Shelf.

The data will be worked on by the Principal Investigators and we anticipate that one Ph.D. student and a number of Honours students will also benefit from the experiment.

Personnel

Acknowedgements

Cruise FR08/00 on the RV Franklin was a great success. Losing one instrument was a small disappointment, but expected given the number of deployments made and the usual rate of loss of instrumentation in marine geophysics (typically 5%). Recovery of the last instrument at EX9 with the seabed cable and then picking the instrument up at night with no lights and only acoustic contact was superb, and we are very indebted to the skill and willing of the RV Franklin's Master and crew in the operation. Without their efforts, this instrument would have been lost.

As always, the experience and expertise of the Master, Mr Neil Cheshire, and the crew of the RV Franklin were much appreciated. Their cheerful and friendly approach made recovery of instruments smooth and uneventful. The CSIRO personnel (Mr Ron Plaschke (Cruise Manager), Mr Lindsay Pender (Computing) and Mr Erik Madsen (Electonics)) were always helpful and co-operative, and we are very grateful for their guidance throughout the cruise.

This cruise represented the third of a series of marine geophysical investigations of the continental margin around Australia. In April-May 1998, we spent a total of three weeks south of Eyre Peninsula, and in November-December 1999 another three weeks was spent in the Gulf of Carpentaria. In each of the three experiments, the RV Franklin has been extremely suitable for our needs. We are very grateful to the Franklin Steering Committee and CSIRO Marine Research for time granted for the research projects, and the various Masters and crews over the last three years.

Graham Heinson
Chief Scientist.

Figures

Figure 1: Cruise Track from Dampier to the 12 deployed instruments (squares) across the Exmouth Plateau.

Updated: 31/01/03