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Immediate report of Victoria University of Wellington Antarctic Expedition 1989-90: VUWAE 34

Seismic Refraction Survey at Upper Erebus Hut

Seismic Refraction Survey at Upper Erebus Hut

As a pilot study of the shallow velocity structure of Erebus, needed to complete a study of the velocity structure of the erupting magma column, two refraction lines were run between the upper hut and Nausea Knob, using the Nimbus 12 channel stacking seismograph. The seismograph was kept at operating temperature (0-30 degrees C) by the space heater in the hut, and the line was run over the near horizontal snow free ground between the hut and Nausea Knob. The first 100 m of the line was on Camp Plow, which is actually a slump, separated from the undisturbed cone by a line of ice towers (fumaroles) extending from the main crater rim to the Tramways.

Line 1 was 180 m long, with geophones at 30 m spacing on Camp Flow, and 10 m spacing over the 80 m length on the undisturbed cone. Stacked sledgehammer blows were used at each end and in the middle on the cone side of the fumarole line. On the cone, velocity was 3000 +/−300 m/s, and on the slump it was 2100 +/− 300 m/s. Onsets were much better on the cone than on the slump.

Line 2 was 330 m long, and extended further towards Nausea Knob, because all geophone spacings were 30 m. Sledgehammer blows proved inadequate, and up to 5 shots of 1.5m of red detonating cord, laid in a circle on the ground were stacked per record. The Nimbus blaster in its recently overhauled state was unable to trigger the Nimbus through 330 m of cable. Triggering was by tying the shot instant cable around the det. cord so that it open circuited during the shot Fair records were obtained on the Nimbus screen using 100 Hz bandpass littering, but printer noise marred the printout. Also the only stacked record of 5 shots was ruined by the shot instant cable re-shorting momentarily after the 5th shot, retriggering the Nimbus during maximum amplitudes. We had no spare det. cord to repeat it.

One direct comparison between stacked hammer blows and a single shot of det. cord was made on line 1, and showed that det. cord and lowpass filtering gave the best result. However to get good arrivals over 330m distance the effective charge weights will have to be increased to about 30 stacks of 1.5m lengths of red det. cord per record. This would best be done by using a heavier grade det. cord, because appreciable trigger-induced: system noise appears when too many shots are stacked. For the 1990/91 survey, when spreads up to 1 km long are planned, the shots will have to be even larger in effect by up to 10 times and 1.5 kg charges of Gelignite in 1 m deep shot holes are planned.