A borehole probe for use in monitoring microseismic activity associated with a variety of geomechanics field projects has recently been developed. It was designed for mounting in a BX borehole and may be used in underground or surface applications. The probe consists of a brass body containing an expandible central section inside of which is mounted a microseismic transducer (accelerometer or velocity gage). In use the probe is inserted into a suitable borehole, positioned to the desired location, and pressurized. The application of this pressure, supplied by a nitrogen gas supply, expands the centre section of the probe effectively locking the transducer to the surface of the borehole. The main application of the probe to date has been in vertical boreholes, drilled from surface, over the structure to be monitored. It has been used with great success in boreholes up to 300 feet deep (often containing water), and over extended periods of time. The design and development of the probe will be described in this paper along with a brief discussion of its application in a number of field projects associated with coal mine and underground gas storage reservoir stability.
The phenomenon of microseismic activity, or acoustic emission, as it is sometimes referred to, is associated with the fact that when a structure composed of materials such as rock is loaded, microlevel acoustic signals are generated within the structure, often at stresses well below the failure point. These microseismic signals are indicative of the stability of the structure, and with suitable instrumentation they may be detected at considerable distances from their source. In the general area of geomechanics basic and applied research associated with microseismic activity has been underway for some thirty five years, although the major developments have been made during the last ten years. For further details on the general concept of the phenomenon and its range of application in the geomechanics field, the reader is referred to the recent publication by Hardy and Leighton (1977). Since 1970 the Rock Mechanics Laboratory at The Pennsylvania State University has been involved in microseismic studies associated with the evaluation of stability in geologic structures. To date the major field effort has been concentrated on two projects, namely: the stability evaluation of underground gas storage reservoirs (Hardy et al., 1976), and the investigation of the mechanical stability of underground coal mines (Hardy and Mowrey, 1976). Both projects are directly related to the economics and safety interests of the gas and coal industry, and as such are particularly appropriate in light of the current energy shortage. Although there has been an increasing interest in the use of microseismic techniques in recent years, the above projects have been unique in that measurements have been made using near-surface rather than underground transducers. This paper deals with one of the transducer installation techniques, the borehole probe, which may be installed from surface over the structure to be investigated.
MICROSEISMIC MONITORING SYSTEM
Basically the system for detecting and monitoring microseismic activity involves a transducer, an amplifying and filtering system, and a recorder.