Estimation of static corrections for shear‐wave profiling using the dispersion properties of Love waves

Abstract

When processing shear‐wave data it is often difficult to compute the static corrections using the same methods as for P-waves, because of the low velocities involved and of the interference with residual P-waves. A method especially tailored for static corrections of SH-waves is presented, which makes use of the dispersion property of Love waves. Assuming that the weathered zone of thickness H acts as a wave guide for the surface waves, their phase velocity ranges from [Formula: see text] at infinite frequency to [Formula: see text] at zero frequency, where [Formula: see text] and [Formula: see text] are the shear‐wave velocities into and below the weathered zone. For intermediate frequencies the phase velocity of Love waves is a known function of H, [Formula: see text] and [Formula: see text]. If the phase velocity of Love waves is measured on field records for a number of distinct frequencies, it is therefore possible to compute H, [Formula: see text], [Formula: see text], and the static corrections. In contrast with conventional techniques which require first arrivals to be picked manually, the computation of static corrections using the dispersion of Love waves is essentially based on velocity analysis efficiently performed by digital computers. Three field examples were conducted. The first of these was a noise analysis using 256 geophone positions ranging from 3 to 258 m. The two other examples are CDP lines obtained with polarized SH sources. The geophone group extension is 20 m and the distance between groups is 10 m. In all these examples, the upper layer thickness and velocity values resulting from the Love wave dispersion appeared to be in good agreement with those computed from the conventional methods.