Delineation of Air/Water Capillary Transition Zone From GPR Data

Abstract

Summary Spilled petroleum products spread laterally in the capillary transition zone above the water table. Ground penetrating radar (GPR) is a technique that can potentially delineate the air/water capillary transition zone (CTZ), because a varying water content strongly influences the reflected signal through its effect on the electrical properties. The vertical water distribution is determined on the pore scale by the grain size distribution and on the reservoir scale, mainly, by the permeability distribution. We present the integral equations for the reflected signal from a inhomogeneous slab with a depth-dependent dielectric constant and electrical conductivity. The numerical solution of the equations allows us to compute the signal reflected from different sand profiles. The results show that the reflected signal is strongly influenced by both the grain size distribution and the variation in permeability. A strong frequency dependence is also observed. We present a two-dimensional GPR profile measured in the field that shows a clear effect of the CTZ on the reflection of the radar signal. All these results are considered of importance for the interpretation of GPR data involving the capillary transition zone.