Seismic refraction surveys for Canterbury groundwater research

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Broadbent, M. 1978 Seismic refraction surveys for Canterbury groundwater research. Wellington: Geophysics Division, DSIR. Report / Geophysics Division 131 66 p.

Abstract: Seismic refraction measurements made in the Rakaia Gorge near the road bridges showed that the velocity of propagation of compressional waves through the water saturated gravels in the gorge ranged from between 1.9 and 2.3 km/s. Velocities in the rhyolite, in which the gorge is there incised, varied between 2.6 and 2.9 km/s. The rhyolites was not everywhere fully water saturated. The river channel passing under the eastern bridge contained a maximum thickness of gravels of about 30 m, whereas the corresponding figure for the western channel was 13 m. The cross sectional area of the gravel deposits transverse to the river at the narrowest parts of the east and west channels were 1400 and 300 m2 respectively. Seismic refraction measurements were made during 1975 at 27 sites on the gravels of the Canterbury Plain between the Waimakariri and Rakaia Rivers. The measurements were interpreted in terms of vertical distributions of seismic velocity. Published literature suggests that the observced velocity variations are likely to be mainly due to changes in a) sediment porosity, b) the fractional saturation of the sediment's pores with water, and c) the stress on the sediment due to gravitational loading. Known depths to water in wells have been used to develop criteria which can be used at some sites to identify the velocity changes likely to mark the water table. However predictions of water table depth derived using these criteria do not agree closely with predictions based purely on well water levels. Water table depths can probably often be predicted in the area studied to within 13 metres by making use of the depth estimated to a velocity exceeding 1.95 km/s. The observed velocities seem to vary systematically both vertically and horizontally. Populations of velocities measured for outcropping Springston and Burnham formation gravels (means 0.93 and 1.24 km/s respectively) are significantly different. The elevations of the top of the deepest seismic layers detected and the water table seem to be related. The observed distribution of velocity can be explained by the existence of an infilled former valley cut in the old gravels, or by the present distribution of water in the gravels. Gravel porosities have been estimated to vary between 20 and 35%. The seismic refraction method has proved to be only a crude reconnaissance tool in the area investigated (auth)