The Potential of GPR AVO Analysis for Rock Fracture Characetrization

Successful characterisation of fractures requires quantification of the reflectivity (reflection coefficient (R)) of individual fractures. The AVO method involves measuring the reflection amplitude variation with offset (incidence angle)l. Measuring the full AVO response using CMP (Common Mid Point) surveys enables estimation of these fracture properties which has proved difficult to achieve with normal-incidence data such as that typically collected in Common Offset (CO) profiling. We carried out AVO analysis of CMP data acquired over a bedding plane fracture in Carboniferous Limestone in Yorkshire, United Kingdom. Raw amplitudes were corrected for spreading and antenna radiation/receiver patterns; offsets (0.29m-2.9m) were converted to incidence angles (8°-55°) to obtain a diagnostic AVO curve. The antenna patterns were measured through numerical modelling using the GprMax 3D modelling code. Being a low loss formation, conductive attenuation was considered insignificant over the ray-path length. Because of the difficulty in characterizing the source pulse strength, both theoretical and measured amplitudes were normalized to the minimum offset amplitude. Fracture properties are then obtained by comparing the normalised field derived AVO curve with normalized theoretical reflection coefficient curves for thin fractures with various apertures and fills.