Geotomography for the delineation of coal seam structure

Abstract

The in-seam and surface reflection methods of seismic delineation of disruptions to coal seams have recently been extended by the use of seismic sources and receivers located in boreholes. Both reflection and transmission (tomographic) borehole seismic data, as well as borehole EM attenuation data, can assist in forming improved pictures of coal seam structure in the vertical plane. This paper examines the problems associated with imaging of geological features via tomography. Computer simulations of crosshole and surface-to-borehole (offset VSP) seismic surveys are used to illustrate the effect on tomographic images of the parameters of the survey geometry, particularly the range of depths sampled by in-hole sources and receivers, the distance between boreholes and the in-hole and surface sampling density. We find that it is essential to include surface-to-borehole data in addition to crosshole data to be able to see lateral seam disruptions. We illustrate how the recovery of tomographic images from field data can be improved by constraining them during processing, using velocity information and information about the survey geometry. Finally, we address the question of picking travel times from transmission seismograms recorded in a sedimentary environment and illustrate the necessity of accounting for waveform distortions due to the complex propagation conditions in order to extract travel times from a transmission data set that are suitable for tomographic reconstruction.