Seismic Techniques for Exploration

The first step in the exploration for oil or natural gas usually involves a seismic survey of the onshore or offshore area.Currently, the technique in most common use for sub-surface structure delineation is seismic wave reflection. In the simplest case, a seismic source propagates an acoustic wave from the surface of sea or land and successive reflections from the interfaces of sub-surface strata are detected by geophones (on land) or hydrophones (at sea).In practice, arrays of multiple sources and detectors are used.The sources are arranged so that waves incident at differing angles at a single reflecting sub-surface point are picked up by specific detectors.A procedure is thus provided for obtaining many signals from one point which can then be superimposed and digitally-processed to give averaged information reduced in noise and wave interference by a factor proportional to the square root of the number of traces superimposed.This is the elementary principle behind the Common Depth Point or CDP stack.

Such surveys are predominantly compression wave surveys and these have reached such a level of sophistication in digital processing, particularly for three-dimensional structure mapping, that it is difficult to see how they could be improved upon.Nevertheless, there has been considerable interest in the past decade or so in the use of shear wave reflection to provide more-detailed information on sub-surface structure.Unlike compression waves, which are longitudinal in nature, shear waves are transverse waves and have very special properties.They are slower by ca. 40% than the equivalent compression or P- wave and can be both horizontally (SH) and vertically (SV) polarized. Compression waves will readily propagate through fluids, but transverse waves are unable to do so.The simultaneous use of compression and shear waves in reflection studies therefore provides an option for a fluid-sensitive detection system for land-based exploration at least.A number of applications of combined compression and shear wave propagation have been used in the field, ranging from systems for the direct detection of hydrocarbons to the use of shear wave polarization for the characterization of seismic anisotropy in rocks and hence the identification of cracks and cavities in sedimentary reservoirs.

An important further application of seismic reflection is that of mapping the sub-surface structure of an identified reservoir. Here the technique of vertical seismic profiling (VSP) has found particular application.The usual procedure is to drill one or more deep boreholes in the reservoir and equip these with strings of geophones set at pre-determined depths.The source is fired at the ground surface, usually in a shallow borehole, and trains of downward-propagating and upward-reflecting waves are registered on the detectors.In this way, a picture of the three-dimensional structure of the extent of the reservoir can be built up.The use of a source capable of generating simultaneously compression waves and shear waves lends an extra dimension to VSP in that more information is provided on the characteristics of the reservoir rocks themselves.Needless to say, special seismic sources are required to produce seismic waves of sufficiently high shear wave energy to be useful.Melvin and Clarke (Proc. Roy. Soc. London A 450, 351 (1995)) have described the application of a shock wave seismic source which generates such high shear wave energies to vertical seismic profiling in the study of rock structures down to ca.3000 metres depth.