High range resolution directional borehole radar for 3-D fracture delineation

Abstract

Directional borehole radar was developed for detection of three-dimensional (3-D) target localization in single-hole radar measurement. Phase differences among four dipole elements of receiving circular array uniquely determine an azimuth direction of a reflected wave. Receiving voltages of dipole elements are measured by optical electric field sensors whose high electrical isolation feature enables data acquisition of highly correlated signal between the channels. Besides, a switching unit to control resonant frequency of dipole elements was newly developed to reduce mutual coupling between the dipole elements. Laboratory experiments have demonstrated that approximately 30% frequency bandwidth enlargement is achieved by the switching operation without been affected by the mutual coupling in air. The directional borehole radar system was tested in a field test site in Kamaishi-Mine in Japan. All the boreholes available in this test site is filled with water and past borehole radar surveys conducted in this test site revealed presence of complex fracture system. Cross-hole and single-hole borehole radar measurement were conducted to clarify the performance of the resonant switching control and also to detect 3-D geometrical structure of fractures in this test site. 3-D analysis of data acquired by the directional borehole radar in a single-hole measurement clarified azimuth orientation fractures up to a range distance of 15m with a range resolution less than 1m. High reliability of the result was inferred from the fact that individual fracture pattern in a reflection profile showed a consistent color along the depth and also high repeatability of the result was obtained by repeating measurements.