On the use of dispersive APVO GPR curves for thin-bed properties estimation: Theory and application to fracture characterization

摘要

The presence of a thin layer embedded in any formation cre_ates complex reflection patterns caused by interferences within the thin bed. The generated reflectivity amplitude variations with offset have been increasingly used in seismic interpretation and more recently tested on ground-penetrating radar (GPR) data to characterize nonaqueous-phase liquid contaminants. Phase and frequency sensitivities of the reflected signals are generally not used, although they contain useful information. The present study aims to evaluate the potential of these combined properties to characterize a thin bed using GPR data acquired along a com_mon-midpoint (СМР) survey, carried out to assess velocity vari_ations in the ground. It has been restricted to the simple case of a thin bed embedded within a homogeneous formation, a situation often encountered in fractured media. Dispersive properties of the dielectric permittivity of investigated materials (homoge_neous formation, thin bed) are described using a Jonscher para-meterization, which permitted study of the dependency of amp_litude and phase variation with offset (APVO) curves on frequen_cy and thin-bed properties (filling nature, aperture). In the second part, we discuss and illustrate the validity of the thin-bed approx-imation as well as simplify assumptions and make necessary careful corrections to convert raw СМР data into dispersive APVO curves. Two different strategies are discussed to correct the data from propagation effects: a classical normal-moveout approach and an inverse method. Finally, the proposed method_ology is applied to a СМР GPR data set acquired along a vertical cliff. It allowed us to extract the characteristics of a subvertical fracture with satisfying resolution and confidence. The study mo_tivates interest to use dispersion dependency of the reflection coefficient variations for thin-bed characterization.