Ultrasonic and Acoustic Emission Results from the Stripa Heater Experiments. Part I. Cross-Hole Investigation of a Rock Mass Subjected to Heating. Part II. Acoustic Emission Monitoring During Cool-down of the Stripa Heater Experim

摘要

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 were made prior to turning on the heater, during 398 days of heating, and after the heater was turned off. These measurements 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 before heating. Laboratory compressional and shear-wave velocity measurements were also 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, these measurements implied increases in horizontal normal stresses to between 30 and 40 MPa. Increases in 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. (ERA citation 07:052976)