ABSTRACT: This paper presents an analysis of fracture depth and intensity due to subsidence over the longwall panel. Sonic reflection techniques were utilized to determine fracture depth, and a variation of p-wave velocity was used as an indication of the fracture intensity. Instrumentation has been tested for consistency and accuracy first in the laboratory using small scale models, then applied in the field in two mine sites with different mining geometry and geologic conditions. A new sonic viewer was utilized for sonic velocity measurements. Regular hammering method was an acoustic source. The results were concurrent with monitored horizontal strain profiles and measured open fractures over the longwall panels.
Techniques involving the propagation of acoustic or seismic waves are becoming of increasing importance in the characterization of rock masses in mineral exploration, mining operations, site investigations and other engineering application. McCann et (1975) described the use of cross-hole acoustic measurements to delineate interfaces between homogeneous media, to detect localized, irregular features, and to estimate the degree of fracture in rock asses. Palmer et al., (1981) discussed the fracture detection in crystalline rocks using ultrasonic reflection techniques. Meister (1974) used ultrasonic pulse attenuation to determine the depth of fracturing behind excavation tunnel walls. In the area above longwall mining, discontinuous ground disturbances (i.e., open cracks, steps, cave-in pits), will occur along the surface of the subsidence trough, when mining thick seams or groups of seams which are under soft rock strata. The source of these disturbances are either static or dynamic loading of the overburden strata due to undermining. Static fractures are produced when ground movements cease and ground stresses reach their final equilibrium state, and are usually developed at the edge or border lines of excavations (see Fig. 1). Dynamic fractures develop during excavation, parallel to the longwall face at some distance ahead of the face in tension zone (see Fig. 2) and often close as the face passes by the fracture zone and leave fracture zones in the gob area, compression zone. The length, width, depth, and intensity of these disturbances, fractures, are dependent on mining geometry, depth of overburden, geology, topography of the area, rate of mining and direction of mining with respect to the topographic slope (Khair et al., 1988). Using the principal of sonic wave propagation and utilizing wave travel time and velocity attenuation, disturbance in terms of fracture depth and intensity can be quantified. The travel time of energy between two points in a medium is governed by Format's "minimum time" principle which suggests that the wave which reaches the target point first has followed the path of the minimum travel time. That wave path may not necessarily correspond to the minimum distance between the two points. In a perfectly elastic medium, energy would be fully transmitted between two points. However, since no such perfect medium exists, part of the transmitted energy is absorbed and the wave amplitude is attenuated. Furthermore, higher frequency components of pulse will attenuate more rapidly than the lower frequency components, leading to a decrease in the sharpness of the pulse with increased distance of propagation, thus resulting in pulse broadening.