Scattering versus intrinsic attenuation in the vadose zone: A VSP experiment

Abstract

We studied scattering versus intrinsic attenuation estimates in the vadose zone from a shallow VSP experiment conducted in the Lawrence Livermore National Laboratory (LLNL) facility. Using permanent downhole geophones and a vertical impact source, we estimated effective attenuation of the downgoing transmitted P-wave. We compared theoretical scattering attenuation estimates and finite-difference synthetics to the measured field [Formula: see text] values ([Formula: see text] being a measure of attenuation). Using a selected range of impedance profiles of variance typical for a sedimentary basin, our estimates of [Formula: see text] are in the order of 20–85. Given the short propagation pathlengths involved ([Formula: see text]), we show that attenuation due to lateral heterogeneity is not significant. We analyzed additional distorting factors, including near-field presence, local impedance effects, and interference from reflections originating beneath receivers, and found that they may significantly impact attenuation measurement in near-surface studies, and result in biased [Formula: see text] values. From a comparison of the analyzed ranges of [Formula: see text] to the measured [Formula: see text] values, we deduced [Formula: see text], which is consistently low, but whose inferred frequency dependence depends strongly on the scattering model assumed. The ranges for the [Formula: see text] factor resulting from scattering and distorting factors, and the intrinsic [Formula: see text] value were estimated as, respectively, 4–7, and 7–4. We identified two potential mechanisms which could lead to low [Formula: see text] values in the vadose zone: patchy saturation and squirt flow. We found through viscoelastic 3D synthetic modeling using a standard linear solid (SLS), that the field [Formula: see text] frequency dependence can be reproduced, although nonuniquely, for the studied range of impedance variance.