Classification of subsurface fracture roughness by polarimetric borehole radar

Abstract

Polarimetric borehole radar systems, which can measure full polarimetry in drilled boreholes, were developed and applied to subsurface fracture detection and evaluation. Radar polarimetry can be used to obtain precise information from the radar targets beyond the limitation of wavelength. This is especially important in borehole radar, where radar resolution is poor due to the high attenuation of subsurface material. The authors use 100 MHz in order to obtain a deeper penetration depth, where the aperture of the subsurface fractures is typically less than 1 mm. Although direct observation or inverse approach cannot be used for subsurface fracture measurement, classification of subsurface fractures based on the radar polarimetry is possible. Field data was acquired in Mirror Lake test site, where water permeable fractures are known. Full polarimetric radar profiles show the difference of reflectivity depending on the polarization status. They found that the polarization information is related to fracture roughness and provides information on water permeability.