Seismic-wave velocities in anisotropic ice: A comparison of measured and calculated values in and around the deep drill hole at Byrd Station, Antarctica

Ultrasonic (28 kHz) velocity logging was carried out to a depth of 1550 meters in the 2164-meter drill hole at Byrd station, Antarctica, in 1969–1970. Smoothed traveltimes from the logger chart were calibrated by taking spot measurements with a calibrated oscilloscope. As a result, velocities are accurate within about ±10 m/sec throughout most of the log. Velocities at depths of <400 meters show an excellent correlation with density. At all depths the correlation with velocities calculated from fabric diagrams is also good, although the calculated velocities show substantially more scatter than those observed. An explanation for a previously unexplained shear-wave arrival observed in refraction shooting has been found. Extrapolating velocities to the bottom of the ice on the basis of fabrics leads to a mean vertical velocity below the compaction zone of 3915 m/sec, which is in satisfactory agreement with estimates from wide-angle reflection shooting at the surface but is about 2½% higher than those commonly used in calculating ice thickness from reflection shooting. A very slow downward increase in the velocity calculated for horizontally propagating waves (0.02 sec−1) has been found to be consistent with both the effect of pressure on the solid ice and the results of detailed examination of refraction velocities. The velocities to a depth of 1000 meters appear to be very closely similar at two sites about 20 km apart near old and new Byrd stations.