Efficiency of MASW in detecting near-surface cavities

Abstract

The purpose of this study is to evaluate the efficacy of using multichannel analysis of surface wave (MASW) to detect the near-surface cavities. The methods used in this study include interpreting dispersion curves and amplitude mapping of the multichannel analysis of surface wave technique and interpreting the delay in first arrivals of compressional waves. To test these methods, a seismic survey was conducted above a known near surface cavity in Al-Suman Area, Saudi Arabia. The cause of the cavity is carbonization in the area; there are many cavities similar to this one. The seismic data were collected using a seismograph system with 48 vertical geophones. Both techniques show a tangible result for detecting the cavity. The 2D section of shear wave velocity, which was obtained by inverting the dispersion curves from the MASW technique, leads us to determine the shape of the cavity, as described by a low-velocity zone. Frequency against relative offset is plotted and shows a significant frequency drop in the presence of the cavity, which also provides an indication to the presence of cavity underneath. This interpretation is matched by the interpretation of observed delays in first arrivals of compressional waves. The integration of both P-wave seismic refraction and MASW gives confidence in the result and matches observations of the existing cavity closely.