Some shallow seismic reflections

Abstract

The seismic reflection method has been applied mainly in the exploration for oil and gas, with targets in the depth range 500-10000 m. For shallow depths between, say, 10 and 200 m there are several possible applications for seismic reflection surveys on land: groundwater exploration, site investigation for civil engineering construction, identification of shallow sources of geothermal energy, delineation of aquifers for seasonal storage of hot water, and exploration for coal and other minerals at strippable depths. However, the use of the seismic reflection method for these applications has been limited. A fundamental difficulty with shallow seismic reflection surveying on land is that for receivers close to the source there can be substantial interference from refracted arrivals and surface waves. This is illustrated in Fig. 1 where these other wave types generated by the shot mask any reflectcd energy which may be present at travel times less than 100 ms. For most purposes economic considerations preclude the obvious way out of the difficulty. The shallower the target, the cheaper is the cast of drilling boreholes, so it is not usually worthwhile to drill deep shotholes to improve seismic reflection data quality, Either a surface or near-surtace source must be used, or the seismic reflection method might as weil be abandoned in favour of drilling boreholes only. Many companies and institutions must have acquired shallow seismic reflection data in recent years, yet little of it has appeared in the published literature. Consequently it is frustratingly difficult to assess the relationship between shallow geology and seismie reflection data quality. Engineers responsible for shallow exploration projects need same guidance on this relationship if they are to give serious consideration to the use of the seismic reflection method. In this article we combine some surprisingly good data obtained using a surface source in the eastern Netherlands with some more typical data obtained using explosive detonators in northern England, make a comment on the use of the explosive sources in shallow reflection work, and review the geological factors which influence data quality.