Experience with the issue of seismic surveying over basalts

Abstract

In some of Australia?s underground coal mining areas, near-surface basalt layers create problems for seismic reflection surveying. Affected mines have less confidence in their geological conditions and mine safety and productivity may be compromised. In order to gain insights and obtain potential solutions to this important problem, we have investigated seismic reflection and borehole vertical seismic profiling data from surveys at the North Goonyella Mine and the Moranbah South coal mine lease, which are both located in the Bowen Basin of Queensland. At North Goonyella, seismic velocities determined from the VSP survey indicate that the basalt is not fresh and up-going reflections from layers below the basalt are clearly seen. For the seismic reflection survey, advanced seismic data processing techniques such as pre-stack depth migration were able to improve the continuity of the coal seam reflections across the zone affected by the basalt. No special acquisition parameters were required. At Moranbah South, the problem with the basalts proved to be intractable. The basalts are hard and fresh, their total thickness is up 40 m, their width reaches 360 m and unconsolidated sediments lie between individual flows. The target coal seams are at depths less than 310 m and neither long-offset data acquisition nor prestack depth migration were able to produce satisfactory results. In general, the main issues for seismic surveying in basalt covered areas are (i) the generation of complex downgoing and up-going wavefields which are due to the strong impedance contrasts between the basalt and the surrounding strata, and (ii) the generation of incoherent scattered waves from inhomogeneities within the basalts and their rough margins. We have studied and confirmed these seismic phenomena through computer modelling.