High lateral resolution exploration using surface waves from noise records

Abstract

Determination of the shear-wave velocity structure at shallow depths is a common necessity for engineering or environmental projects. To this end, subsoil structure exploration using surface waves is frequently used. Methods such as SASW or MASW determine phase velocity dispersion from surface waves generated by a source recorded on a line of geophones. Using MASW, it is important that the array of receivers be as long as possible to increase the precision at low frequencies. However, this implies that possible lateral variations are disregarded. Hayashi and Suzuki (2004) proposed a different way of stacking gather shots to increase lateral resolution. They combined strategies used in MASW with the CMP (Common Mid-Point) summation current in reflection seismology. In their method (CMPCC), they cross-correlate traces sharing common mid-point locations before determining phase velocity dispersion. Another recent approach to subsoil structure exploration is based on seismic interferometry. It has been shown that cross-correlation of a diffuse field such as seismic noise allows determining the Green's function between two receivers. Thus, a virtual source seismic section may be constructed from the cross-correlation of records of seismic noise obtained in a line of receivers. In this paper, we use seismic interferometry to process seismic noise records obtained in seismic refraction lines of 24 geophones, and analyze the results using CMPCC to increase lateral resolution. Cross-correlation of the noise records allows reconstructing seismic sections with virtual sources at each receiver location. The Rayleigh wave component of the Green's functions is obtained with a high signal-to-noise ratio. Using CMPCC analysis of the virtual source seismic lines, we are able to identify lateral variations of phase velocity inside the seismic line, increasing the resolution obtained previously with those data.