Borehole radar applied to the characterization of hydraulically conductive fracture zones in crystalline rock

Journal Article · Sat Feb 01 04:00:00 EST 1992 · Geophysical Prospecting; (United States)

DOI:https://doi.org/10.1111/j.1365-2478.1992.tb00367.x· OSTI ID:5122168

Olsson, O; Falk, L; Forslund, O; Lundmark, L; Sandberg, E ^[1]

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1. ABEM AB, Skolgatan 11, S-930 70, Mala (SE)

This paper discusses the borehole radar system, RAMAC, developed within the framework of the International Stripa Project, which can be used in three different measuring modes; single-hole reflection, cross-hole reflection and cross-hole tomography. The reflection modes basically provide geometrical data on features located at some distance from the borehole. In addition the strength of the reflections indicate the contrast in electrical properties. Single-hole reflection data are cylindrically symmetrical with respect to the borehole, which means that a unique fracture orientation cannot be obtained. A method has been devised where absolute orientation of fracture zones is obtained by combining single-hole reflection data from adjacent holes. Similar methods for the analysis of cross-hole reflection data have also been developed and found to be efficient. The radar operates in the frequency range 20-60 MHz which gives a resolution of 1-3 m in crystalline rock. The investigation range obtained in the Stripa granite is approximately 100 m in the single-hole mode and 200-300 m in the cross-hole model. Variations in the arrival time and amplitude of the direct wave between transmitter and receiver have been used for cross-hole tomographic imaging to yield maps of radar velocity and attenuation. The cross-hole measurement configuration coupled with tomographic inversion has less resolution than the reflection methods but provides better quantitative estimates of the values of measured properties. The analysis of the radar data has provided a consistent description of the fracture zones at the Stripa Cross-hole site in agreement with both geological and geophysical observations.

• OSTI ID: 5122168
• Journal Information: Geophysical Prospecting; (United States), Journal Name: Geophysical Prospecting; (United States) Vol. 40:2; ISSN 0016-8025; ISSN GPPRA
• Country of Publication: United States
• Language: English

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