Radiation patterns of a shear‐wave vibrator in near‐surface shale

Abstract

Two shallow, surface‐to‐borehole field tests were conducted to investigate the radiation patterns of a shear‐wave vibrator in near‐surface shale. In the in‐line direction, for which baseplate motion is transverse to the source‐geophone raypath, the vibrator produces a broadband, high‐amplitude signal of the SH type and virtually no signal of the SV or P types. The amplitude of the SH radiation is approximately constant as a function of polar angle, but it falls off as the cosine of the azimuthal angle measured with respect to the in‐line direction. In the broadside direction, the vibrator produces a strong, uncontaminated signal of the SV type. The radiation pattern of the SV signature is lobate in the polar plane and peaks at an incident angle of 30 to 35 degrees. In the horizontal plane, the SV radiation varies as the sine of the azimuthal angle measured with respect to the in‐line direction. The field tests were conducted on a Permian shale in Oklahoma and an Eocene shale in East Texas. Comparison between SH and SV wave surfaces indicates that both shales behave approximately like transversely isotropic solids with a vertical axis of symmetry. Theoretical wave surfaces are fit to the observed data by using elastic moduli determined from the field measurements and by making the assumption that the near‐surface shale at each field site is a single, homogeneous, inherently transversely isotropic material. The analyses indicate that horizontal SH group velocity is about 30 percent greater than horizontal SV group velocity in the Permian shale and about 25 percent greater in the Eocene shale.