Reciprocal characteristics of surface waves: Detecting near surface anomalies

Abstract

The dispersive nature of surface waves (different frequencies traveling at different velocities) predominately depends on the subsurface shear-wave properties. The near-surface community takes advantage of this phenomenon through a technique known as Multi-Channel Analysis of Surface Waves (MASW), where the dispersion characteristics of surface waves are inverted to shear-wave velocity (VS) profiles. Currently, the inversion is a one-dimensional averaging of the subsurface volume below a spread (source followed by a set of receivers). The greatest challenge lies in model assessment; short of a known embedded target, it is difficult to determine the reliability of the inverted profile. Here, we propose using reciprocity for assessing the inverted model. This simple and intuitive process tests for similarity by cross correlating the dispersion images acquired from two spreads moving in opposite direction and occupying a common profile on the surface. We demonstrate this idea using a 47 meter profile with known targets (two Transite pipes 15 centimeter inside diameter (6 inch I.D.) and buried roughly 1 meter deep). The best reciprocity and target resolution was obtained using a 12 meter spread length. The maximum depth of investigation was 6 meters. Results advocate the use of reciprocity as a viable method of analyzing and optimizing seismic surveys for near subsurface anomaly detection.