Inversion Methodology of Dispersive Amplitude and Phase versus Offset of GPR Curves (DAPVO) for Thin Beds
Abstract
The presence of a thin layer embedded in any formation creates complex reflection patterns because of interferences within the thin bed. Amplitude variation with offset (AVO) is used increasingly in seismic interpretation and has been tested more recently on ground-penetrating radar (GPR) data to characterize nonaqueous-phase liquid contaminants. In those analyses, phase and dispersion properties of the reflected signals generally are omitted, although they contain useful information. An inversion methodology to examine thin-bed properties -- dispersive amplitude and phase versus offset (DAPVO) -- combines all reflectivity properties (amplitude, phase, and dispersion) of the reflected GPR signal generated by a thin bed embedded within a homogeneous material. A brief description of electromagnetic (EM) phenomena is presented. The dispersive properties of the dielectric permittivity of investigated materials can be described using a Jonscher parameterization, which allows the study of the dependency of amplitude and phase versus offset (APVO) curves on the frequency of thin-bed properties (filling nature, aperture). Simplifying assumptions and using careful corrections are necessary to convert raw common-midpoint (CMP) reflected data into DAPVO curves and to study the propagation and radiation-pattern corrections. The inversion methodology is explained and validated to a synthetic set of CMP GPR data and can be illustrated with a real CMP data set acquired along a vertical cliff. This allows for extraction of the characteristics of a subvertical fracture while simultaneously satisfying resolution and confidence. Such a study motivates interest in combining the dispersion dependency of the reflection-coefficient variations with classical AVO analysis for thin-bed characterization.