Mapping dykes using surface and downhole seismic methods

Seismic methods are commonly used for the detection of faults in the exploration and production of coal. Surface seismic methods are not used in industry for the delineation of vertical structure, such as the imaging of dykes. This is because seismic waves transmit from the surface down to horizontal reflection surfaces, and reflect back up to the surface. Consequently, where sub-vertical structure such as dykes occurs, the surface seismic method fails.

Seismic methods can use different source-receiver geometries. Their ability to image dykes may therefore depend upon on the geometry used, the dyke thickness and the seismic wave propagation mode in relation to dyke composition and internal structure. Surface seismic methods have difficulty distinguishing between faults or fractures and very thin dykes (l–2m in thickness) when a dyke’s thickness is less than the seismic wavelength. Borehole seismic methods have to be used to detect the presence of dykes and avoid these problems.

This paper presents some results from a research project that is attempting to use both surface and downhole seismic methods to detect dykes. The paper shows how surface seismic methods imaged a sub-vertical dyke of some 40 m thickness and its associated faulting. The alternative approach of using downhole seismic sources and receivers (single borehole seismic profiling), showed that dyke sides can also be successfully imaged at depth. In the future it should be possible to produce an image of both sides of a dyke, in its correct orientation, using existing boreholes.