Estimation of Converted Waves Static Corrections Using CMP Cross- Correlation of Surface Waves
Roohollah Askari, Robert James Ferguson, J. Helen Isaac
We enlarge upon the idea of CMP Cross-Correlation of Surface Waves (CCSW) to obtain an S-wave velocity model for the calculation of PS receiver static corrections. In our approach, we cross-correlate each trace of a shot record with a reference trace that is selected from within the shot gather based on high signal to noise ratio. New midpoints that relate to the correlated traces are then calculated. We calculate the phase velocity for each CMP gather, and we convert the resulting dispersion curve to an S-wave model. Our approach is faster than the conventional CCSW because in the conventional CCSW all traces within a CMP gather are cross-correlated with each other. In this study we show that, in order to have a precise estimation of a dispersion curve, we only consider those traces that lie in a spatial window and we found that the optimum window length (aperture) should be close to (one to one and half times) the maximum wavelength in a CMP gather. When the aperture is optimum, we see a high resolution image of each mode within the dispersion curve that avoids modal interferences. We obtain 2D near surface S-wave velocity models for two real data sets. By decimating traces from the first dataset, we show that we can obtain a good trend of S-wave statics relatively similar to those obtained from the original dense array data. This demonstrates that CCSW has a capacity to address static correction of converted waves when geophone spacing is wide. Using the second data set, we show the importance of wavelength-dependent aperture for estimating the phase velocity. We obtain static corrections based on an S-wave velocity model obtained from CCWS and successfully apply them to the data.