Surface and borehole seismic methods to delineate kimberlite pipes in Australia

Abstract

Gravity, magnetic, and electromagnetic methods are primary exploration techniques used in kimberlite exploration to define a kimberlite pipe's location and shape. Sometimes these methods produce nonunique results because of weathering of the upper part of a kimberlite pipe and its considerable inhomogeneity. Complex pipe geometries and associated faulting and fracturing may be very difficult to assess by potential-field methods. Application of seismic methods in kimberlite exploration has been limited. In sedimentary basins, however, kimberlite pipes are likely to be a suitable target for the application of seismic reflection methods because of the difference in elastic properties between the pipe material and the host rock. The drawback of surface seismic methods is that they are not designed for direct detection of vertical bodies such as a pipe. However, a kimberlite pipe may be inferred from seismic data indirectly by observing the termination of reflections against its flanks. Such an approach can produce good results if the sedimentary sequence is of moderate to high reflectivity. Unfortunately, McArthur Basin sediments of the Northern Territory in Australia are quite poor reflectors (Figure 1). The only reflecting horizon would probably be the sandstone/black shale interface at 140 m. Near-surface conditions are also unfavorable for surface seismic because of a thick layer of bull dust, which causes poor geophone coupling. Hence, very poor seismic data quality is typical for this area. In this study, instead of attempting a conventional surface seismic reflection approach, we used modes generated by a kimberlite pipe to precisely determine its location. The single borehole imaging (SBI) method also was used for delineation of the kimberlite pipe shape at depth. By a combined analysis of P - and S -wave data recorded on the surface and in downhole experiments, the pipe's location, shape, …