| Geophones are utilized in many different areas, for example in mining and petroleum engineering, seismology, seismic prospecting, and structure evaluation. They are also often used as sensors in most acoustic emission (AE) field applications due to the rapid attenuation of the high frequency components of the associated AE signals. Past research indicates that calibration data supplied by manufacturer with new geophones often lacks sufficient detail if more than routine applications are anticipated. Furthermore, geophones retrieved from long-term field installation need to be re-calibrated in order to determine their current characteristics and performance levels, if they are to be reused in later studies. Since some low-frequency geophones used in recent studies cost {dollar}3000 to {dollar}5000 apiece it is critical that facilities be available for accurate recalibration.; At the Penn State Rock Mechanics Laboratory (PSRML), a computer-based calibration facility, incorporating a dual-channel signal analyzer, a vibration exciter, and an associated computer program, has been developed to determine the "true" frequency response of geophones. As a means of evaluating the developed facility, several types of commercially available geophones, types commonly used in Penn State field studies, have been evaluated utilizing both the "step-sine (single-frequency sweep)" and "random-noise" calibration methods. In addition, one type of geophone has been investigated to evaluate the effect of installation angle on geophone sensitivity and overall behavior.; The results have indicated that the frequency response curves obtained are similar to the theoretical curves within a certain frequency range and installation angle, however, outside this range the actual behavior often deviates considerably from the theoretical. The calibration data have also illustrated the limits of the useful operating frequency range, and information on the spurious frequencies caused by lateral movement of the geophone sensing element when installed improperly. In general the calibration facility has proven to be highly successful and cost effective in obtaining accurate response data for geophones. |
