Statistical multioffset phase analysis for surface-wave processing in laterally varying media

Abstract

Standard procedures for dispersion analysis of surface waves use multichannel wavefield transforms. By using several receivers, such procedures integrate the information along the entire acquisition array. That approach improves data quality and robustness significantly, but its side effects are spatial averaging and loss of lateral resolution. Recently, a new approach was developed to address that issue and maximize lateral resolution. The new method uses multioffset phase analysis to detect and locate sharp lateral variations in velocity. By using the phase analysis approach, the number of usable channels can be maximized, thereby gaining data quality without compromising lateral resolution. In fact, such preliminary data analysis also allows selection of the appropriate traces on which to perform multichannel processing. Such multioffset phase analysis can be enhanced by f-k filtering, which assures the selection of only one wave-propagation mode, and by a statistical analysis that takes advantage of data redundancy of multishot data, usually collected, for example, in land refraction surveys. Moreover, this novel statistical method with f-k filtering can be used to retrieve a dispersion curve, in principle, for each receiver location. The quasi-continuous pseudoimage of shear-wave velocity as a function of offset and frequency allows a characterization of lateral variations in velocity, whether they are sharp or smooth

Geophysical characterization of a small pre-Alpine catchment

“…(1). Starting from these considerations, Vignoli and Cassiani, 2010 proposed an automatic and robust strategy, based on MOPA, to detect and locate sharp lateral velocity variation; later, Vignoli et al (2011) described a surface wave processing to locate and characterize lateral velocity variations, no matter if sharp or smooth.…”

“…Theoretically, the described multi-offset phase analysis that recovers the wavenumber values as a function of the frequency could be used in place of standard frequency-wavenumber analysis (Strobbia and Foti, 2006;Vignoli et al, 2011). The standard approach extracts the dispersion curve from a double Fourier transform of the observed signal: collected data are mapped from time-offset (t − x) domain to frequency-wavenumber (f-k) domain; then, the maxima of the f-k spectrum amplitude single out the phase velocities C R = ω/k(ω) at which excitation can propagate.…”

“…Ideally, a strategy shall be adopted that use the maximum number of adjacent channels with no detriment to the lateral resolution. Recent work based on multi-offset phase analysis (MOPA) has been developed (Strobbia and Foti, 2006;Vignoli and Cassiani, 2010;Vignoli et al, 2011) to detect and locate abrupt lateral shear-wave velocity variations. By means of MOPA, the number of utilizable channels can be maximized without loss of lateral resolution.…”

Numerical Investigation of 3D multichannel analysis of surface wave method

“…Recently, an automatic technique based on multioffset phase analysis was developed to detect and locate sharp lateral variations in shear-wave velocity (Strobia and Foti, 2006). Vignoli et al (2011) improved this method by performing a statistical analysis of the data, and extract more reliable phase-offset curves. Furthermore, the statistical multi-offset phase analysis method can be extended straightforwardly to 3D acquisition geometries.…”

Recent Technologies for Land Seismic Imaging Over a Complex Near Surface Overburden

“…Transition zones present different challenges for both acquisition and processing of data, as matching zero-phase signals from Vibroseis to the quasi-minimum-phase signals from air guns or small dynamite shots. In those cases, methods such as the Multi-Azimuth Surface Wave inversion (MASW) have been proposed and tested with some success , Socco et al 2010, Vignoli et al 2011 to detect shallow structures by S waves.…”