Shallow VSP work in the Appalachia Coal Basin

Abstract

Most geophysical applications in North American coal exploration have centered around the conventional surface seismic reflection method to provide continuous subsurface coverage for evaluating both good and anomalous coal reserve areas (Ruskey, 1981; Dobecki and Bartel, 1982; Greaves, 1984; Lawton, 1985; Lyatsky and Lawton, 1988; Gochioco and Cotten, 1989; Lawton and Lyatsky, 1989; Gochioco and Kelly, 1990; Gochioco, 1991; Henson and Sexton, 1991). The surface seismic reflection method, however, has inherent resolution limitations because the seismic wavelet must propagate substantial distances through the weathered layer, resulting in rapid attenuation of the desired higher frequencies. Since the depths and thicknesses of coal seams are usually known beforehand, it is imperative that the seismic reflection associated with the target coal seam is absolutely identified in the seismic section to avoid misinterpretations. However, it is common that checkshot data and sonic and density logs are not available to generate synthetic seismograms to assist in the interpretation of coal seismic data. To overcome some of these limitations, the vertical seismic profiling (VSP) technique was tested in a coal exploration program to provide additional information for correlation with surface seismic reflection [or common-depthpoint (CDP)] data and a synthetic seismogram generated from density and sonic logs. VSP has an advantage over the surface seismic reflection method in that the receiver is placed in the borehole beneath the weathered layer, which is responsible for severely attenuating higher signal frequencies. As a result, VSP data tend to have a broader frequency spectrum. The recorded VSP field data also provide better insight into the fundamental properties of reflection and transmission of seismic wavelets in the subsurface near the borehole location because the receiver records both upgoing and downgoing seismic events. In this study, the VSP data are correlated to the CDP seismic data as well as to a synthetic seismogram for comparative analysis of its accuracy