Investigations of Fracture Zones in Crystalline Rock by Borehole Radar

Abstract

AbstractA borehole pulse radar system has been developed as part of the International Stripa Project with the objective to identify and characterize fracture zones at a considerable distance from boreholes. The radar uses very short pulses, which are transmitted and received by dipole antennas inserted into the boreholes. The pulses are extremely broadband with center frequencies of 25–60 MHz corresponding to wavelengths of a few meters in the rock. At 25 MHz the attenuation in the Stripa granite is 28 dB/100 m and the pulse velocity is approximately 128 000 km/s. Reflection measurements have been used to identify fracture zones and determine their position and orientation. The zones often cause strong and well-defined reflections. Improvements in the pulse form and numerical filtering of the data have consequently made the radar a very efficient instrument for locating fracture zones. During measurements in Stripa reflections from fracture zones have been observed more than 100 m from the borehole.