Crosshole seismic testing; procedures and pitfalls

Crosshole seismic testing is a valuable technique for determining seismic velocity profiles for critical structure siting investigations. Improper field procedures, however, can result in the acceptance of apparent velocities as true velocities, leading to unconservative design values. Common pitfalls in crosshole seismic testing are (1) the use of source-receiver borehole spacings too large to enable the determination of true formation velocities, (2) the use of in-hole station spacings too large to allow resolution of thin velocity layers of interest, (3) the use of incremental traveltimes between widely spaced receiver boreholes, and (4) the naive assumption that boreholes are vertical and hence parallel. A procedure is presented whereby field programs can be more rationally planned.