Multi-component seismic surveying for near surface investigations: examples from central Wyoming and southern England

Within the crust, the weathered layer has been shown to contain some of the greatest values of shear wave velocity anisotropy. There are two main causes of this shear wave velocity anisotropy. In all rock types, anisotropy results from structural weaknesses commonly manifest as aligned open fractures. In sedimentary rocks, additional anisotropy can result from mineral or grain particle alignment and layering or bedding. Unconsolidated sediments also show this preferential particle alignment which causes ordered heterogeneity or anisotropy that can be described as transverse isotropy. Both forms of anisotropy can have significant implications not only for the strength of a material but also for the passage of fluids through it. Two case histories are presented that describe these forms of anisotropy in the near surface and the implications for environmental investigations. In central Wyoming, a downhole survey was conducted using 28 borehole locations Ž . instrumented with three-component 3C receivers and surface shear wave impact sources. The sandstone bedrock at the site showed anomalous areas of high shear wave anisotropy that were interpreted to be due to a dominant regional fracture pattern. In southern England, a downhole survey was conducted in two boreholes using an array of 3C receivers and both surface and down hole shear wave sources. At this site, the heavily overconsolidated Oxford Clay showed transverse isotropy due to the strong preferential particle alignment. Both examples illustrate the use of shear wave seismic studies for mapping near surface geological features that may have important impacts for environmental and hydrogeological investigations. q 2000 Elsevier Science B.V. All rights reserved.