Resolving overburden characteristics via shear wave propagation

Abstract

In shallow site investigations information is often required on both the thickness of overburden material and its physical properties prior to drilling the site. While conventional compressional wave methods may resolve the former, shear wave refraction shooting can also provide information on the sediment's shear deformation characteristics. Furthermore, in some instances the shear wave is more suited to resolving depth to bedrock due to its characteristically large velocity contrast at a sediment/rock interface and thus shear wave methods should be given due consideration when planning any geophysical site survey. Data from a research exercise on a potential opencast coal site clearly illustrate the case for shear wave refraction shooting. However, the accuracy of bedrock depth determination is shown to be dependent on interpretation procedures, the input of a shear wave velocity gradient function for the overburden being essential to the interpretation. Given the availability of shear wave velocity data other engineering applications become apparent. In reconnaissance surveys such as the above they may be used in assessing the rippability of the overburden, while in more comprehensive site investigations, the geophysically-derived parameters from crosshole shooting may be used in the design of structural foundations.