Analysis of the 1969 Ashburton River seismic survey

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Atkins, R.A.E.; Hicks, S.R. 1977 Analysis of the 1969 Ashburton River seismic survey. [s.l.]: [s.n.]. Report / Geophysics Division 127 24 p.

Abstract: Seismic refraction profiles and some seismic reflection profiles were surveyed on the Canterbury Plains, near the Ashburton River, during the latter part of 1969. These have been interpreted with the help of some gravity data from the area. Two interpretations of the seismic refractions have given somewhat different results; the discrepancies are believed to be a result of ambiguity of data rather than of errors in interpretation. Quaternary gravels with seismic P-wave velocities from 1 to 3 km/s have been found over most of the area, in places over 700 m thick. They overlie Pliocene sandstones and other sedimentary material (velocity about 3.5 km/s) which dip from the foothills of the Southern Alps to the Ashburton River mouth on the Pacific Coast. This sedimentary sequence is underlain by layers having velocities greater than 4 km/s, which by reference to a nearby borehole (J.D. George - 1) are probably andesite and rhyolite, and the greywacke basement. A fault with a vertical displacement of 300 to 400 m was found under section D near Pudding Hill. Another, with a vertical throw of probably more than 1000 m is thought to exist near Methven. Some gravity data from the area have been interpreted to help understanding of the seismic sections. The major source of the gravity anomalies seems to be the upper layers in the gravels. The effect of the deeper layers seems to be small