SH-wave generation by a vertical force applied close to a pit

Shear-wave seismic surveys are usually carried out with SH waves. We would not normally expect to generate SH waves by applying a vertical force at the free surface because of the axial symmetry. However, during a VSP experiment in 1988, we unexpectedly observed the predominant generation of SH waves. This experience led us to devise experiments for solving its mechanism. We found that the phenomenon was directly connected with low rigidity zones in the ground. A typical case of a low rigidity zone is a pit, whether empty or filled with soft soil. For this reason, we call this phenomenon the pit effect. The radiation characteristics of such pits were also investigated. One possible application of the pit effect is to make use of it as an SH-wave source. A detailed explanation of most aspects of the field experiments described in this paper has been given in recent Japanese papers (Kitsunezaki and Goto 1992; Kitsunezaki and Okuzumi 1992; Kitsunezaki et al, 1992).