Imaging Coal Seam Structures under Basalt Cover

In Australian coal mining areas, basalt flows can occur at or near the ground surface. These flows are usually of Tertiary age and are thickest where they have flowed down pre-existing river channels. Multiple flows may be present with unconsolidated sediments lying between. The basalts may be weathered, especially at their margins but when fresh, they are extremely hard. Seismic P-wave velocities in fresh basalts can be in excess of 6 km/s.

It has long been recognised that where basalt flows occur, seismic reflection exploration for underlying coal seams is problematic. At best, weak reflection data is obtained but in many instances, the reflection surveys totally fail to map the coal seams. This inability to map the coal seams under basalt is not just a failing of the seismic reflection method, it can also lead to a failure in a mining company's exploration program and a failure to map geological structures, which can have serious impacts on mine safety and productivity.

Near-surface basalts also present huge problems to the seismic exploration programs undertaken for petroleum exploration. There have been many studies into the reasons for these problems and possible solutions. In this project, the work from the petroleum industry has been thoroughly reviewed and supplemented by our own theoretical study investigating the propagation of seismic waves through geological scenarios typical of those found at Australian coal mines. In addition, results of seismic reflection surveys and borehole vertical seismic profiling surveys from the North Goonyella and Moranbah South coal mine leases have been analysed in an attempt to provide insights and solutions to this important problem.

In general, the main issues for a seismic survey in a basalt covered area are (i) the generation of complex down-going and up-going wavefields which are due to the strong impedance contrast between the basalt and the surrounding strata, and (ii) the generation of incoherent scattered waves from the points of inhomogeneity within the basalts and from their rough margins. We have studied and confirmed these seismic phenomena through computer modelling, as part of this project.