BOREHOLE RADAR APPLIED TO THE CHARACTERIZATION OF HYDRAULICALLY CONDUCTIVE FRACTURE ZONES IN CRYSTALLINE ROCK¹

Abstract

OLSSON, O., FALK, L., FORSLUND, O., LUNDMARK, L. and SANDBERG, E. 1992. Borehole radar applied to the characterization of hydraulically conductive fracture zones in crystalline rock. Geophysical Prospecting 40, 109-142.The borehole radar system, RAMAC, developed within the framework of the International Stripa Project, can be used in three different measuring modes; single-hole reflection, crosshole 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 2W300 m in the cross-hole mode.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. 109 110 OLLE OLSSON ET A L .Comparison of the radar results with seismic cross-hole data showed excellent agreement with respect to shape and location of the fracture zones in space. Comparison with hydraulic data shows that the features identified by radar are of hydrogeological significance.

Accounting for imperfect forward modeling in geophysical inverse problems — Exemplified for crosshole tomography

“…In this study, we consider an example from GPR cross-borehole tomography. This method has become popular during the past few decades and has various applications, such as mapping of tunnels and voids (Moran and Greenfield, 1993), mapping of bedrock fractures and fracture zones (Olsson et al, 1992;Lane et al, 1998), estimation of hydrological parameters and delineation of flow paths in the unsaturated zone (Hubbard et al, 1997;Looms et al, 2008aLooms et al, , 2008b, and delineation of geologic structures and lithologies (Fullagar et al, 2000;Bellefleur and Chouteau, 2001;Tronicke et al, 2004).…”

Estimation of position and radius of cylindrical targets using MUSIC algorithm in cross‐borehole radar

“…There are configurations, such as buried pipes under common trenches, that cannot be physically measured from the ground surface. In such situations, it is considered effective to use measurement based on boreholes with a diameter of 10 cm [4][5][6]. The wavelength used in borehole radar in the soil is about 70 to 40 cm due to limitations on the antenna shape in the borehole and the low-pass characteristic of the soil (a relative permittivity of 36 at 70 to 125 MHz).…”

Surface and borehole ground-penetrating-radar developments

“…Slob et al Olsson et al, 1992;Yi et al, 2005͒ or signals of a pair of orthogonalloop antennas ͑Nickel et al, 1983͒. These antenna pairs can steer the direction angle of the maximum amplitude in the radiation pattern ͑equation 7͒ and estimate the direction of the arriving signal.…”