USING GPR DATA TO ASSESS HETEROGENEITIES IN SEDIMENTARY STRATA

From forward modeling of the waveform and amplitude of GPR events it is possible to extract constraints on the gross characteristics of variations in permittivity and conductivity between and across reflecting horizons. Finite difference time domain (FDTD) modeling of radar response to sharp and gradational contacts is used to examine how the form of a contact affects radar reflections. This insight is then applied to find and constrain a set of models compatible with a given radar profile. For example, in zones of increasing ground conductivity the amplitude and shape of the radar return are strongly dependent on the nature of conductivity increase at the outermost edge of the conductive zone.

Examples are shown from surveys on deltaic gravel deposits and sandy barrier island strata. Tighter constraints on layer properties can be placed (a) when target horizons are surveyed with several antenna frequencies, (b) in settings where layering is cyclic or repetitive in nature, and (c) on dipping units that can be imaged from the surface both via reflections and direct wave travel across the units. In general, upper bounds can be placed on contrasts in permittivity across sharp contacts and on bulk conductivity. At the sites studied here, strata radar properties (permittivity and/or conductivity) differ significantly in places from the pattern expected from visual inspection of outcrop or core.