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Victoria University Antarctic Research Expedition Science and Logistics Reports 1979-80: VUWAE 24

Marine seismic surveying in McMurdo Sound (B.A. Sissons)

page 38

Marine seismic surveying in McMurdo Sound (B.A. Sissons)

The present survey was intended to extend marine seismic coverage to: i) the western side of McMurdo Sound, in order to examine the nature of any faulting or other structures in the boundary zone between the Trans Antarctic Mountains and the Ross Sea: ii) to examine the continuity of the basin structure to the north side of Ross Island; iii) to extend seismic coverage northwards to link with previous work at Terra Nova Bay; iv) to obtain reconnaissance results around Beaufort and Franklin Islands and also across the sould into Granite Harbour. With the limited time available it was not possible to achieve many of our objectives. However, a moderate amount of good data was obtained in this year's survey, which will form a good basis for future work intended in this region.

Seismic operations commenced at 0530 UT, 7 February, and continued until 0600, 11 February, except for a seven hour break on 8 February to return to McMurdo station. Several piston cores were also taken by the Project S207 team during the survey, some of which coincided with downtimes of K-12 seismic system. K-12's larger compressor was damaged due to oil pressure failure before the start of surveying, and they were therefore obliged to use their backup airgun system and smaller compressor throughout the survey. This meant that maintenance downtime was much greater than would otherwise have been the case. However, the K-12 backup system in general operated reliably.

Standard operation procedure was as follows: three siesmic arrays and a magnetometer fish were trailed astern of the fantail, while the airgun was launched and towed from the portside fantail winch. The signal from one array was filtered and fed to the K-12 EPC recorder to give an immediate analog display of the seismic coverage, while a second array was fed without filtering into a FM tape recorder for later processing and playback. The third array was also directly recorded to provide a trigger signal corresponding to gun fire instant. The magnetic field was recorded in analog form on two chart recorders. Three disposable sonobuoys were launched to obtain seismic refraction data. Two worked well, and one transmitted data which can probably be reprocessed to give a usable record. A continuous PDR watch was also maintained by members of the S207 project team and the marine science technicians during seismic operations.

The approximate coordinates of end points of seismic lines, and the positions of sonobuoy drops are listed below. Deviations from straight course and breaks in seismic records along these lines Occurred due to ice conditions, equipment downtime and coring station stops.

Track Lines
From To
Line 1 −77°41′, 165°53′ −77°46′, 164°50′
Line 2 −77°46′, 164°47′ −77°38′, 164°23′
Line 3 −77°39′, 164°23′ −77°35′, 165°06′
Line 4 −77°32′, 164°53′ −77°36′, 164°13′
Line 5 −77°36′, 164°17′ −77°38′, 164°25′
Line 6 −77°37′, 164°25′ −77°22′, 166°19′
Line 7 −77°24′, 166°30′ −77°30′, 166°3′
Line 8 −77°32′, 166°6′ −77°15′, 166°0′
Sonobuoy 1 −77°21′, 166°7′
Line 9 −77°7′, 167°5′ −77°19′, 169°18′
Sonobuoy 2 −77°17′, 167°43′
Line 10 −77°19′, 168°14′ −77°22′, 167°35′
Line 11 −77°22′, 167°35′ −77°12′, 167°53′
Line 12 −77°12′, 167°53′ −76°53′, 167°4′
Line 13 −76°51′, 167°4′ −76°55′, 163°16′
Line 14 −76°53′, 163°16′ −76°7′, 167°1′
Sonobuoy 3 −76°52′, 163°20′
page 39

A reasonable amount of useful seismic data, and a good bathymetric record were obtained. The Beaufort Island - Granite Harbour leg in particular gave a good seismic record. Gently dipping strata are evident and are truncated at or near the seabed by an unconformity. The unconformity is thought to be the same as that observed elsewhere in McMurdo Sound and the Ross Sea.