Estimation of hydraulic conductivity of an unconfined aquifer using cokriging of GPR and hydrostratigraphic data

Densely sampled geophysical data can supplement hydrogeological data for es timating the spatial distribution of porosity and hydraulic conductivity ov er an aquifer. A 3D Ground Penetrating Radar (GPR) survey was performed ove r a shallow unconfined aquifer consisting of a coarse to medium sand sequen ce overlying an impermeable clay layer. The site is instrumented with piezo meters and water levels are frequently monitored. Vertical determination of moisture and granulometry at a resolution of 10 cm were made at a few loca tions. The GPR reflection times were correlated with piezometric and strati graphic information cokriging of both data yields the spatial distribution of the radar velocities within the layers. Porosity and hydraulic conductiv ities are estimated using the Complex Refractive Index Method (CRIM) and Ko zeny-Carman formulations, respectively. A pumping test and a tracer test, b oth done using a well in the center of the survey zone, provide a measure o f the average hydraulic conductivity and its anisotropy. The results from c okriging in the saturated zone show that the estimated parameters agree ver y well with the measured hydrogeological data. The geometric mean of the po rosity is close to the laboratory measurements. The geometric mean of the G PR-derived hydraulic conductivities fits the values obtained from the pumpi ng and tracer tests. The range of estimated hydraulic conductivities is qui te large and indicates that flow could be faster or slower than the one pre dicted from the pumping test in some places. Radar attenuation is also foun d to be a good indicator of porosity distribution. From the observed (high) GPR attenuations and electrical conductivities of water sampled in the pie zometers, porosity is determined using Archie's formula. In the vadose zone , moisture content estimated from the GPR velocities using either CRIM or T opp formulations agree well with the ones from the laboratory measurements. Cokriging of the radar reflection times and of the hydrogeological/stratig raphic data leads to an accurate estimate of the radar velocities with a pr ecision and a spatial resolution much higher than the CDP technique. Within the limits of the interpretative models, porosity, saturation and hydrauli c conductivities can accurately be estimated with a high spatial resolution over the survey zone. (C) 2001 Published by Elsevier Science B.V.