DELINEATION OF AIR WATER CAPILLARY TRANSITION ZONE FROM GPR DATA/

Spilled petroleum products spread laterally in the capillary transitio n zone above the water table. Ground penetrating radar (GPR) is a tech nique that can potentially delineate the air/water capillary transitio n 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 gra in size distribution and on the reservoir scale, mainly, by the permea bility distribution. We present the integral equations for the reflect ed signal from a inhomogeneous slab with a depth-dependent dielectric constant and electrical conductivity. The numerical solution of the eq uations allows us to compute the signal reflected from different sand profiles. The results show that the reflected signal is strongly influ enced by both the grain size distribution and the variation in permeab ility. A strong frequency dependence is also observed. We present a tw o-dimensional GPR profile measured in the field that shows a clear eff ect of the CTZ on the reflection of the radar signal. All these result s are considered of importance for the interpretation of GPR data invo lving the capillary transition zone.