Borehole seismic methods for opencast coal exploration

Abstract

Surface seismic techniques lack the resolution to image the top 100m or so of the earth's surface necessary for opencast coal exploration. The work reported in this thesis is the development of borehole seismic methods making use of the closely spaced boreholes that are routinely drilled by British Coal. The first method investigated was to use a tomographic technique to observe any reduction in seismic velocities above old workings, and hence infer the presence of old workings. In order to obtain clear images of the subsurface, it was necessary to interpret the field data for the presence of head waves, and to pick the later arrival direct waves for the tomographic inversions. However, independent data obtained from uphole surveys showed that there was no measurable reduction in the seismic velocity above old workings for strata below the water table, and the tomographic method was abandoned in favour of borehole seismic reflection methods. Fifteen hole-to-surface seismic reflection surveys were acquired using down- hole explosive charges as sources and a linear spread of surface geophones passing through the borehole position as receivers. A complete package of processing software was developed for processing the data, and eight of the surveys are presented in this thesis. The final migrated and stacked sections delineate a washout and faulting at both large and small scales. The vertical resolution of the data is high due to the wideband temporal frequencies in the data, typically up to 300Hz.The hole-to-surface method is compared to the crosshole seismic reflection method, which was developed in parallel by M. J. Findlay. The relative merits of the two techniques are discussed, and suggestions are made to improve the acquisition of the data to make both methods applicable to a wider variety of problems. Although the vertical resolution of the hole-to-surface method is lower than the crosshole method, this could be more than compensated for by extending the hole- to-surface method to three-dimensions, using areal arrays of surface geophones around the borehole.