The purpose of this paper is to discuss field and interpretive techniques which permit, in favorable cases, the quite accurate determination of seismic interval velocities prior to drilling. A simple but accurate formula is developed for the quick calculation of interval velocities from “average velocities” determined by the known [Formula: see text] technique. To secure accuracy a careful study of multiple reflections is necessary and this is discussed. Although the principal objective in determining velocities is to allow an accurate structural interpretation to be made from seismic reflection data, an important secondary objective is to get some lithological information. This is obtained through a correlation of velocities with rock type and depth.
The main procedures used by L. Cagniard to calculate seismic pulse motion have been carried through for the case of a point source in an infinite medium. This relatively simple case serves as a basis for a simplified exposition of Cagniard’s method. By using this case, we avoid cluttering the exposition with a large collection of algebraic details that tend to befog the main issues.
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.
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