Improving separation of shallow S-wave reflections by source-response deconvolution of raw vibrograms

Seismic shear (S) waves are important in geotechnical engineering, as shear waves address directly the small-strain rigidity of the soil. Moreover, in recent years it has been found that the amplitude of shear-wave reflections provides information of the soil strength distribution (Ghose and Goudswaard, 2000). In soft, water-saturated soils shear waves offer smaller wavelength and hence much higher resolution than the compressional (P) waves. Recently the strength of shear-wave reflection seismic has been enhanced by the introduction of a small, electromagnetic vibratory source that can generate relatively high frequency shear waves (Ghose et al., 1996). One common problem in shallow, high-resolution reflection seismic is the reverberating nature of the reflection events. This severely limits the separation between two successive reflections, posing serious problem to interpretation. Such reverberations are commonly caused by ringing nature of the source wavelet. Because of the high resolution offered by the high-frequency shear waves generated by the vibrator, the problem of lacking separation is more critical in shear-wave vibrator data than in P-wave data. For an impulsive source data, reverberation can be removed by spiking deconvolution. Spiking deconvolution requires that the seismic wavelet is minimum phase, but on a cross-correlated vibroseis trace, the seismic wavelet does not meet the minimum-phase requirement necessary for spiking deconvolution (because on a cross-correlated vibroseis trace, the seismic wavelet is the convolution of the zero-phase Klauder wavelet with the component minimum-phase wavelets due to the effects of recording instruments, coupling, attenuation, ghosts, reverberations, and other multiple reflections), and hence the final result of spiking deconvolution is less than optimal. In this paper, we have evaluated on a shear-wave reflection dataset obtained on soft soil, the effect of source response deconvolution on the separation of the very shallow reflection events.