REFLECTION SEISMIC CRUSTAL STUDIES

Abstract

Recordings taken in a region having a very thin sedimentary section (less than 100 m thick) with normal reflection prospecting equipment, using 100 to 300 kg of explosive in holes less than 10 m deep, with geophone spreads 580 m long set from two to 15 km from the shot, show events most easily interpretable as reflections. If the reflection interpretation is accepted and multiple reflections are too weak to be observed, then there are many reflectors of low dip between 8‐ and 35‐km depth in this region. The strongest and most consistent group of events in the 22 recordings arrives at about 8 sec and corresponds to a depth agreeing approximately with the crustal depth obtained by refraction methods across this region. It is emphasized that the reflection view and the refraction view may be essentially different; the latter being insensitive to low‐velocity layers and to thin, high‐velocity layers such as sills might present, whereas the former is insensitive to a gradual transition over a kilometer of depth which may occur at the crustal base. Thus the possibility exists that the reflection and refraction techniques may give wholly or partially independent views of crustal structure. The inherent inaccuracy of head‐wave methods appears to drive us toward the reflection techniques to advance our knowledge on this problem in the future. This approach is still very difficult except under favorable circumstances.