Ground Penetrating Radar Investigations on the Relationship between Horizontal Sub-wavelength ‘Thin-layer’ Bedrock Fractures and Reflection Amplitudes

Several theoretical equations that predict sub-wavelength ‘thin-layer’ reflection amplitudes are compared to the results of a series of controlled ground penetrating radar surveys using 1 GHz transducers over a physical model of a horizontal bedrock fracture. Two large plastic (UHMW-PE) blocks, separated by one or more stacked inserts (polyethylene; ~0.1 mm thick) for a total of 101 surveys, generate a modeled fracture with an aperture ranging from 0-300 mm. All existing theoretical reflection coefficient equations fail to predict observed reflection amplitude oscillations in the data when the fracture aperture is less than 1/48 of a wavelength. The only theoretical formulation to properly predict any significant aspect of the fracture EM reflectivity is the Widess equation; however, the best fit only occurs where aperture sizes are less than 1/16, not 1/8 of the wavelength as predicted. Thermal expansion and temperature fluctuations do not sufficiently account for the oscillations.

The influence of salinity on a water-filled sub-wavelength constant aperture (5 mm) fracture using 1 GHz antennas is also investigated. Results indicate that at this frequency, the reflection amplitude has a slight negative correlation with changes in salinity from 0-5700 mS/m.