Ground-probing radar as a tool for heterogeneity estimation in gravel deposits: advances in data-processing and facies analysis

Abstract

Pleistocene gravely braided-river deposits in river valleys constitute a large fraction of the natural ground-water reservoirs in Switzerland. The knowledge of the distribution and variability of hydraulic conductivity within these deposits are key factors for the estimation of water residence times and of description of large-scale mixing procesess in aquifers such as macrodispersion. It has been shown elsewhere that the spatial variability of hydraulic conductivity is related to the composition and the characteristic dimensions of sedimentary structures, which are themselves related to the dynamics of ancient braided-river systems. In many contamination problems, sedimentological information is sparse and drill-core descriptions and pumping-tests only give a limited picture of the geometry of inhomogeneities. The ground-probing radar (GPR) method is a promising tool for resolving changes of physical properties in gravel deposits at the scale of natural inhomogeneities arising from changing sedimentary composition. However, the main limitation of GPR is the rapid attenuation of electromagnetic waves in subsurface sediments such as gravels, which leads to a limited penetration of the order of 10 to 15 m for a 250 MHz antenna. The objectives of our present work are: 1. (1) To show how digital processing methods similar to reflection seismics may be applied for velocity and profile processing. These methods can improve both the resolution of radar profiles, in particular at greater depths, and the determination of velocity distributions from CDP experiments. 2. (2) To examine whether and to what extent the characteristic lithofacies of Pleistocene gravel deposits can be recognized as mappable reflection patterns on ground-probing radar (GPR) reflection profiles in order to gain information about the geometry of inhomogeneities. Using modern digital data processing methods, such as band pass, high- or low-cut filtering, deconvolution and velocity analysis, much more significant information can be obtained from the recorded GRP field data-sets. Our results demonstrate that on GPR reflection images the basic fluvial forms such as (1) pool deposits generated at the junction of two channels, and (2) channel deposits may be distiguished. Their shape and characteristic spatial dimensions may be recognized from a series of profiles in different directions. Because the method can detect changes in the water content, the reflection image may be related even to small changes in the degree of saturation of the sediments. Thus reflectors can indicate the changing composition of sediments.