Using surface waves for studying the shallow subsurface

Surface waves are an important source of information about the shallow subsurface. Dispersion of surface waves is closely related to the structure and properties of the subsurface material and, in particular, to shear wave velocity Vs. The vertical distribution of the Vs can be estimated on the basis of the dispersion analysis of different kinds of surface waves contained in conventional (P-wave) seismic records. On land data, dispersion curves of Rayleigh waves can be usually identified in a wide range of frequencies, whereas data from shallow-water marine surveys usually contain a low-velocity, low-frequency wave train identified with Scholte waves, with the dispersion curves located in a narrow frequency range. The Vs estimation procedure includes identification and picking of the dispersion curves of the surface waves, followed by inversion of the curves; the inversion results in a stepwise curve defining the vertical distribution of the Vs. Application of the method to the data of both land and marine surveys enables us to obtain estimates of the Vs down to depths of 40-50 m. The advantages and limitations of the surface wave method for Vs estimation, as compared to the S-wave refraction technique, are discussed. Surface waves can also be effectively used for detecting and mapping various subsurface inhomogeneities (such as voids, fracture and fault zones, etc.). Since strong lateral variations of Vs related to the inhomogeneities are expressed as clear anomalies in the corresponding dispersion patterns, tracing the dispersion anomalies along seismic profiles can be used for detecting the inhomogeneous regions. In order to facilitate continuous tracing of the dispersion patterns, special types of seismic sections, based on stacking of surface waves in frequency domain, can be utilized. A brief discussion of the stacking procedure is followed by a number of examples illustrating its application.