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Subsurface fracture characterisation using full polarimetric borehole radar data analysis with numerical simulation validation

We report on the utilisation of a full polarimetric subsurface borehole radar measuring system for efficient characterisation of subsurface fractures. This system can measure the full polarisation (HH, HV, VV and VH) of electromagnetic waves for one borehole, and thus enables us to obtain more information about subsurface fractures compared to that obtained from conventional borehole radar systems, which usually use only single polarisation. Polarimetric datasets have been acquired at several sites, particularly at Mirror Lake, USA, which is a well known site for testing subsurface fractures. Nine fracture sets were observed in one borehole, FSE-1, in the Mirror Lake site. These were divided into four category fracture sets depending on polarimetric analysis of alpha, entropy and anisotropy decomposition analysis of scattering behaviour from fractures at frequency 30 MHz. We found that the characterised four fractures sets have the highest hydraulic permeable zones at depths of 24.75 m, and 47.80 m. The lowest hydraulic permeable zones were found to be at 28.50 m, 36.15 m and 44.80 m. These results show a good consistency with the hydraulic fractures permeability tracer test that was done by USGS. To validate these conclusions we implemented numerical simulation for a synthesised fractures property using the Finite Difference Time Domain (FDTD) method. Here, we used a plane wave as an electromagnetic source with frequency ranging from 1 MHz to 200 MHz, and monitored the electromagnetic scattering for various fractures. We found that distributions of alpha, entropy and anisotropy polarimetric parameters differ with the fracture roughness property which validates the polarimetric analysis of the measured data. The polarimetric borehole radar has the ability to characterise subsurface fractures. Nine fracture sets were characterised into four category fractures depending on polarimetric analysis of alpha, entropy and anisotropy decomposition. Validation of these results was implemented by numerical simulation for a synthesised fractures property using the Finite Difference Time Domain method.