Shear Velocity Model Appraisal In Shallow Surface Wave Inversion

Abstract

Uncertainty of surface wave dispersion from numerical modeling and field repeatability tests<br>shows a strong dependence on the spread length resolution limitation with frequency. High frequency<br>uncertainty is about 1% and rises nonlinearly at low frequency to over 30%, where Gaussian distribution<br>is invalid. The major source of dispersion error is additive noise to seismograms, static shifts and<br>geophone placement, with geophone coupling and tilt effects being minimal. Monte Carlo ensemble<br>analysis shows the half-space shear velocity to become unresolvable in the presence of realistic<br>dispersion error. Shear velocity of a shallow LVL under a caprock is well resolved, but becomes poorly<br>resolved when at depth underlying a buried HVL, as are the thicknesses of a LVL or HVL. Shear<br>velocity ‘smearing’ between interfaces occurs at depth, where velocity gradients are more likely to be<br>estimated instead of sharp contrasts, as well as an underestimate of half-space shear velocity. Gaussian<br>error propagation is invalid, with shear velocity likelihood showing broad and/or bi-modal distribution.<br>A 5 m thick, stiff layer at 20 m depth is found to be physically non-resolvable with dispersion measured<br>from an array at the surface.