First Break - Volume 3, Issue 11, 1985

Seismic exploration has developed in a symbiotic relationship with the oil industry since the early years of the century, and the overwhelming majority of the literature covering seismic exploration techniques is concerned with exploring the depths and structures relevant to oil production. Implicit in this development has been the use of compressional body wave energy to interrogate the Earth, with shear wave techniques making a valuable contribution over recent years. Throughout the literature the humble Rayleigh and Love waves are given the role of ugly sisters, being designed out, stacked out, muted out, filtered out, or simply ignored in favour of the body wave Cinderella. Only if it achieved the mighty wavelengths of use to global seismology has the surface wave been worthy of serious consideration. In exploration the root cause of this low status is the fact that, in their normal habitat (i.e. the Earth's surface), Rayleigh and Love waves are subject to the high attenuation and rapid variation in character which result from superficial sediments of quickly varying composition and thickness. They also suffer from the unforgivable habit of being dispersive and thus do not present the eye with a straightforward visual correlation from trace to trace along an array. Thus, the charms which these unfortunates could in theory offer the seismic praetitioner (such as P and S velocities as functions of depth through the weathered layer and horizontal distance along the array) are generally left untouched. There is, however, at least one area of exploration in which roles are reversed and it is the body wave which is dealt with as noise. Coal seams offer a low velocity horizon of generally constant thickness sandwiched between dense and well-cemented sediments. Both Rayleigh and Love waves can thrive in this environment, their dispersion characteristics being finely tuned by variations in seam thickness of only a few per cent, and reflections resulting from any structure in the seam (primarily faulting) greater than about one quarter seam thickness in scale. Since it is just this small-scale structure which critically controls the economics of modern mechanised coal mining, and which is fequently well below the resolution threshold of conventional seismic techniques, in-seam seismic methods have grown rapidly in the past decade from feasibility studies to become routine and highly cost-effective tools of mine planning and design. In the V.K. alone, the National Coal Board has carried out in excess of 350 underground seismic surveys in the past five years, and is currently performing well over 100 surveys a year throughout the British coalfields. A thorough review of the theory and practice of in-seam seismic tchniques is given by Buchanan (1983).