Robust quantitative estimation of the seismic attenuation from shallow geotechnical borehole VSP data

Abstract

Estimating seismic attenuation from shallow geotechnical borehole surveys can be a delicate task. Measured data are often collected within the source near-field domain and the classic inverse-distance correction of the geometric spreading (GS) is inappropriate at this scale. We develop a novel approach based on a 3D full-waveform modeling to substitute the inverse-distance correction. It consists of scaling the picked amplitudes using their counterparts obtained from an elastic simulation carried out under conditions mimicking the data acquisition. The seismic attenuation may be inferred from the corrected amplitudes using either a piecewise regression or a ray-based inversion. Numerical experiments involving P- and S-wave synthetic data indicate that our correction better compensates for the GS effect than the inverse-distance correction. For a synthetic example with 5% noise, the Q-factor values derived from amplitude corrected via the proposed approach have a relative error of approximately 10% compared with 40% for the traditional correction. We investigate the effect of the velocity and density uncertainty upon the calculated correction terms and show that our approach is unbiased and stable. Finally, the robustness of our workflow is assessed on a real case study involving a P-wave data set acquired in a geotechnical borehole.