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

Seismic (Sonic) Velocities (PCF)

Seismic (Sonic) Velocities (PCF)

Routine on-site measurements of sonic velocity, density and chemistry of the core were planned. Due to low and fluctuating temperatures in the Science Hut and the cramped space, only sonic velocities were finally performed. Sonic velocity measurements were undertaken with a PUNDIT, determining the travel time between two transducers of an 82 kHz pulse, as described in Barrett & Froggatt (1978). Only whole, coherent segments of core free from fractures or clasts and at least 50 mm long were chosen. Sampling between transducers and core was improved by washing measuring points on the core with water and use of water as a complant. Grease or oil was avoided to lessen core contamination. Consistent readings about the core axis were obtained in this manner. Measuring was performed as soon as possible after cote recovery, usually as soon as the core was boxed and before initial logging. Except for Core 2, the cores remained unfrozen. Time after recovery was usually less than one hour although delays of up to eight hours occurred.

Velocities measured ranged between 1.78 and 5.10 km.s −1 for sediments and 3.44 to 5.92 km.s −1 for basement clasts of dolerite, granite etc. Clast velocities agree well with the data of Barrett & Froggatt 11978). However, sediment velocities were generally higher than determined from seismic refraction profiles in McMurdo Sound (Barrett 1979, Nor they et al., 1975) and more varied than for DVDP 15 where many of the cores were frozen (Barrett s Treves, 1976). Velocities determined on each core or unit within a core have, initially been averaged (Table 2) and plotted against depth (Figure 3). A steep uniform velocity gradient of V = 19.OH + 1.6 (V = velocity in kms and H = depth in km) is shown to 110 m sub-bottom where there is a marked velocity reversal. A further reversal occurs at 190 m. Reflectors labelled A, K, B & C (Northey et al., 1975) have been tentatively identified at 141 m, 168 m, 180 m and 210 m sub-bottom respectively. The lithology of each reflector is a well cemented medium sandstone up to 0.5m thick with velocities of between 4 to 5 km.s −1, compared to 2.0 - 3.5 km.s −1 for less cemented material adjacent to the reflectors. There is no apparent correlation between these reflectors and lithologic units. There is an overall good correlation, however, between lithology and reversals in the overall velocity gradient.

Sonic velocities determined with the PUNDIT were found to provide quick, easy and reliable measurements on the core. These allowed the actual Velocity profile to be built up as drilling proceeded with consequent readjustment of projected target depths using the available seismic reflection records. Only reflectors A, C, & D were recognised on the reflection profiles beneath MSSTS 1, so once A had been penetrated, adjusted velocities gave projected depths of C at 204 m and D at 270 m sub-bottom. Based on two-way travel times from Track XXII (Northey et al., 1975) depth estimates were revised after each reflector was recognised. Calculated and actual depths are compared in Table 3. General close agreements between depths indicates reliability of average core velocities.

TABLE 2 ft. Sonic Velocity (Km.s−1) averaged for each MSSTS core

TABLE 2 ft. Sonic Velocity (Km.s −1) averaged for each MSSTS core

Figure 3 Plot of sonic velocity against depth for core from MSSTS 1.

Figure 3 Plot of sonic velocity against depth for core from MSSTS 1.

TABLE 3 Calculated and actual depths to recognised reflectors at MSSTS 1

TABLE 3 Calculated and actual depths to recognised reflectors at MSSTS 1