Simultaneous reflection - refraction acquisition

Abstract

The deeper geological targets of exploration projects are not always detected by seismic reflection. This may be due to weak or progressive impedance contrast or multiple reflections from the overburden. In some cases, seismic refraction can provide deeper information and complement reflection results. When a vibrating source is used, refraction data can be obtained together with conventional reflection data by using simultaneous vibrating techniques. Similar techniques were described by Garotta (1983) for simultaneous recording of several reflection lines, which is especially attractive for 3D acquisition, and by Garotta (1985) for simultaneous recording of conventional and shallow reflection data. Simultaneous recording of several offset VSPs was also described by Naville (1984). In simultaneous reflection and refraction profiling, refraction data acquisition must not in any respect adversely affect the quality of the reflection data, and the production of the conventional reflection crew must not be reduced by more than a few percent. The reflection recording spread can be used for refraction as: 1. 8 to 10 Hz geophones have low enough responses to record refracted energy; 2. refraction events from deeper strata usually have wavelengths of several hundred metres, and thus are not affected by the most comonly used geophane patterns, of some tens of metres; 3. modern reflection spreads are most of ten 4-6 km long, which is a reasonabIe length for a refraction spread; 4. although the usual reflection trace spacing is shorter than for refraction recording, this is not really a drawback.