Applicability of seismic reflection sounding in regions of Precambrian geology

Abstract

The paper discusses the possibilities for observing reflections from discontinuities within crystalline rocks, at depths of interest for drilling and mine-working. It is concluded that the impedance contrasts of most mafic rocks and high-density ores against felsic rocks such as granites, granodiorites and gneisses are sufficient for the generation of observable reflections. Layers are detectable if thicker than 20 m. A transition layer less than, say, 30 m thick between different rock types gives a reflection comparable to that of a sharp boundary. A major problem in interpreting the records will be the irregularity of the rock structures. It will probably be necessary to process and interpret not only reflected but also diffracted waves. The data-processing method known as automatic migration could be used to locate and delineate diffracting bodies with complex shapes. This requires the use of digital recording in the field survey. Synthetic seismograms are computed for a reflection survey over a spherical inclusion of high-density ore in granite. Waves diffracted by a single inclusion with a diameter of a few tens of meters lying at a depth of 500 m are marginally detectable at the surface, the apparent coefficient of reflection being 5–7%.