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Physics of shear‐wave intrinsic dispersion and estimation of in situ soil properties: a synthetic VSP appraisal
- Source: Near Surface Geophysics, Volume 10, Issue 6, Dec 2012, p. 613 - 629
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- 01 Aug 2011
- 01 May 2012
- 01 May 2012
Abstract
Estimates of in situ porosity and permeability in saturated soils are important in various disciplines. In this research, a recently proposed concept for quantitative integration of dispersive seismic velocity and attenuation in soft soils, based on the underlying physics, is tested on a realistic synthetic shear‐wave (S‐wave) vertical seismic profiling (VSP) dataset. The effects of error in the determination of layer‐specific dispersive velocity and attenuation from VSP data without and with noise, as well as those of error in the used poroelasticity model, on the estimated values of porosity and permeability are investigated. A methodology involving extraction of layer‐specific intrinsic dispersion from VSP data and enhancement of robustness and reliability through use of multiple receivers within the thickness of a given layer, in combination with source stacking, is presented. For the assumed frequency band (50–140 Hz) for S‐wave data, stable values of both porosity and permeability can be obtained for all but a very low‐permeability clayey layer. The results show that if the model error is not large, then both porosity and permeability can be estimated quite accurately even when the data are rather noisy. However, when the model used is grossly inaccurate and there is large noise in the data and hence a large error in the estimated dispersion, then though the absolute error in porosity is still within 2–3%, the permeability can be off by an order of magnitude. In general, in this approach, if the poroelasticity model is so chosen that it explains reasonably well the observed dispersion, then the estimates of in situ porosity and permeability should both be quite accurate.