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Inversion of dispersive APVO GPR curves: a thin-layer approach for fracture characterization on a vertical cliff

Reliability of stability assessment of prone to fall rock masses suffers from the lack of information about the geometry and the properties of the fracture networks. GPR profiles associated to Common Mid-Point (CMP) data, all recorded on the cliff wall, recently proved their efficiency to image correctly the extension of fractures with a satisfying resolution. However, besides velocity, CMP data also contain information generally not used, i.e. Amplitude and Phase variations of the reflectivity for a given reflector as function of Offset (APVO) and frequency. In this study, we analyzed the potential of these curves in the context of thin layers, where multiple reflections generate interferences and complex patterns. We notably present an inversion of real APVO curves derived from a CMP profile acquired directly on a limestone cliff wall. The reflected wave was primary deconvoluated in order to correct wave propagation effects and to evaluate radiation patterns. In a second step, APVO curves were inverted considering a neighborhood algorithm. This procedure permitted to obtain the Jonscher parameters describing the complex permittivity of the thin-layer and of the limestone formation, as well as the aperture and depth of the studied fracture.