Simplified interpretation method by combining interval and direct methods for seismic downhole tests

Abstract

Conventional downhole survey interpretation utilizes a straight raypath assumption from source to receiver, neglecting subsurface refractions along the travel path, which simplifies the estimation of shear wave velocity (\({V}_{S}\)) profiles and average velocity (\({{V}^{z}_{S}}\)). Out of the most commonly used direct method (DM) and interval method (IM), IM is susceptible to sudden changes in subsurface stiffness due to high impedance contrast and anomalies, prompting the use of more detailed refracted raypath method (RRM) in such cases. However, RRM requires velocity of all the layers above the depth concerned. To avoid such dependency on the previous layers, a new interpretation approach combined direct interval method (CDIM) is proposed. The interfaces at impedance contrasts were observed to cause errors in \({V}_{S}\) estimation while using IM. A parametric model study was conducted with varying depths and magnitude of impedance contrasts. Model study and downhole data acquired from five test sites showed that the errors associated with \({V}_{S}\) estimation using CDIM are substantially lower than IM. The interval length for all the interpretation methods converges to the testing interval as the depth increases, which signifies that for greater depths and in the absence of high impedance contrasts in shallow depths, proposed CDIM offers a streamlined and fast interpretation, applicable for any geology and subsurface layering condition and can provide quick resolution of \({V}_{S}\) profiles.