Applying GPR-amplitude wave maps and Am-scans as a semi-quantitative approach to the internal structure of sediments

Abstract

Abstract In this work we analyse the applicability of amplitude grid maps to the routine of geological surveys by means of GPR (ground penetrating radar). Although amplitude grid maps have been commonly used in archaeological surveys, their use in geological prospecting (including the detection of voids and determination of the internal geometry of sedimentary bodies) is not widespread. The direct analysis of GPR-profiles permits the analysis of geometrical features and other qualitative aspects that can be related to changes in EM properties. Aspects such as changes in the density of the banded disposition in radargrams, loss of reflector definition or higher scattering in particular zones of the profiles can give useful, though non-quantitative, information. The GPR wave-amplitude is a qualitative measurement of magnetic properties that can be processed as a semi-quantitative layout. The main differences observed in changing wave amplitude are related to the surveyed materials and their geometry. These changes produce variations in the relative wave amplitude or vertical wave-phase changes related to differences in the propagation velocity, the attenuation factor, the reflectivity and the geometry of the materials. Maps based on the lateral correlation between profiles (C-scans) or the lateral correlation of wave amplitudes along the same profile (as a tomography or Am-scans) permit the analysis of these changes. Variations in amplitude grid maps or Am-scans are related to (i) geometrical changes of surveyed materials, (ii) changes in the dielectric constant, and (iii) changes in the potential penetration depth (higher attenuation in particular zones of a profile) than can be correlated to the type, state or clay content of subsoil materials. Direct analysis of exposures helps to constrain interpretation using the geometrical features in radargrams resulting from the geological structure. In the same way, analysis of geometrical features in radargrams, together with time-slices of C-scans, can be used to determine the areal distribution of changes in the subsoil and approach the changes in EM properties. An example with parallel profiles and different devices, constrained by means a broadband multifrequency EM survey is shown.