A cross-hole high-frequency acoustic investigation of a granitic rock mass subjected to sustained heating is reported. Compressional and shear-wave velocity measurements along four different paths between four vertical boreholes made prior to turning on the heater, during 398 days of heating and after the heater was turned off correlated well with the presence of fracture zones, in which the fractures were closed by thermal expansion of the rock upon heating. When the rock mass cooled, the velocity measurements indicated a greater intensity of fracturing than had existed prior to heating. Laboratory compressional and shear-wave velocity measurements have been made on intact rock specimens obtained from the site and subjected to axial stress. When used to interpret the increases in velocities measured in the field upon heating the rock mass, increases in horizontal normal stresses to between 30 and 40 MPa were inferred. Increases of these magnitudes agree with stress measurements made by the other techniques. The ratio of measured compressional to shear-wave velocity appears to provide a sensitive measure of the fraction of crack porosity containing water or gas.

INTRODUCTION

One of the more promising methods developed in the past few years for geotechnical site investigation and the characterization of rock masses is the higher-frequency acoustic wave technique. The high frequencies employed permit detection of discontinuities and the outlining of zones having different physical properties between boreholes or behind surface boundaries in much more detail than the conventional low-frequency seismic methods.

Price, Malone and Knill (1970), McCann, Grainger and McCann (1975) and Auld (1977) describe the use of acoustic measurements between boreholes for geotechnical purposes. Price and colleagues employed the results of their study to determine the optimum rock-bolt pattern to stabilize a rock mass. McCann and colleagues used the between-hole technique to delineate interfaces between homogeneous media, to detect localized, irregular features and to estimate the degree of fracturing in the rock mass. Auld used between-hole acoustic measurements to determine the elastic properties of the rock mass.

Acoustic techniques employed within a borehole have been described by Geyer and Myung (1971), Myung and Baltosser (1972) and by King and colleagues (1975, 1978). The application of acoustic borehole logs in detecting fractures, for rock classification and in determining the in situ elastic properties of rock have been discussed by these workers and by Carroll (1966, 1969) and Coon and Merritt (1970).

In this paper are described the results of a research project involving cross-hole acoustic measurements in a fractured granite rock mass subjected to thermal stresses. The acoustic research project is itself part of a comprehensive rock mechanics and geophysics research programme associated with large-scale heater tests in an abandoned iron-ore mine in centralSweden, as described by Witherspoon and colleagues (1979).

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