CRITICAL‐ANGLE EFFECTS IN SEISMIC EXPLORATION*

Abstract

ABSTRACTIn the case where a medium of velocity α1 is underlain by a medium of higher velocity α2, no propagation of compressional elastic wave energy into the lower medium takes place at angles of incidence exceeding a certain critical value. This so‐called critical angle –which is a measure of the actual velocity‐contrast –is furthermore marked by a sharp increase in the amount of reflected compressional energy. An investigation has been made to find out whether this critical‐angle effect might be usable as a velocity‐contrast indicator in seismic exploration.Model experiments confirmed the expectation that this effect should be manifest at the surface under ideal conditions.Two small‐scale field set‐ups showed that the effect is actually measurable in the field (in one case after applying corrections for planting variations).Finally a seismic line along the shore of the North Sea provided the data for contrast analysis over a considerable distance along the base of the Tertiary. Comparison of the measured changes in contrast and subcrop velocities obtained at those locations where subsurface data from independent sources were available, showed good agreement.It is concluded that critical‐angle shooting may be used to indicate subcrop changes along a reflector, or even to obtain the same sub‐stratum velocity as is sought in conventional refraction work, at considerably shorter operating distances.