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Fully automated operation was another major design goal. A specific requirement was to determine seismic event locations, occurzence times, and magnitudes accurately and rapidly and to superimpose the data on a mine map without human interaction (e.g., manual picking of seismic P-wave arrival times). This led to the decision to use seismic arrays with a modest number of single-axis (e.g., vertical component) sensors instead of arrays with a fewer number of triaxial sensors. While shear-(S) wave arrivals help constrain seismic event location solutions, and triaxial seismic data allow Swave arrivals to be picked, it is more difficult [o achieve a high degree of accuracy in the S-wave arrival pick in software than i[ is the firs[-arziving P-wave. Thus, for high- volume, fully automatic seismic data processing, no attempt was made [o pick and use Swave arzival times in the event location process. It was also important to minimize the difficulty and expense of sensor installation; therefore, most sensors were attached directly to roof and rib surfaces without [he use of boreholes. Although sensor placement in boreholes reduces the influence of mine openings and fractured rock surzounding these openings, which is a requirement for more sophisticated, full-waveform inversions and related analyses (e.g„ Mendecki, 1990), these influences typically are not significant sources of error in elementary event location solutions (Swanson et al., 1992). The seismic data acquisition and processing software made available through the International Association of Seismology and Physics of the Earth's Interior (IASPEI) (Lee, 1994) was selected to serve as the initial basic building block for data acquisition in order to take advantage of as much freely available software as possible. A common thread [o much of [he IASPEI software is [he use of the Seismic Unified Data System (SUDS) data format based on C language data structures (Ward, 1989; Banfill, 1996). While providing a starting point for collecting mine-wide seismic data, much of [he IASPEI processing software is geared toward larger-scale earthquake networks and is not directly applicable to smaller-scale mining. SYSTEM HARDWARE Sensors Both high-sensitivity accelerometers (40 V/g) and inexpensive 4.5-Hz, moving-coil geophones with variable gain amplifiers are used in the sensor arrays. Sensors are anchored directly to competent roof or rib surfaces or epoxied to the end of tensioned roof bolts. On the surface above amine, most geophone sensors are pressed into the soil bottom of a shallow hole. Data Acquisition Methods Several different types of digitizing systems are used. With the first kind, analog seismic signals are transmitted via cable to a commercial off-the-shelf A/D converter attached [o a local data acquisition PC. In the second type, seismic signals are digitized near the sensors, and the data are transmitted digitally [o a data acquisition PC on the network. In [he third approach, low-power PC104 format computers with A/D converters collect signals and transmit the data over a wireless local area network (LAN). In each approach, data acquisition PC's are dedicated solely to collecting waveform files either continuously or in a triggered event-capture mode. (/) Digitizing signals of a centralized network computer. The PC-based A/D conversion boards used in this par[ of the development effort are all commen;ially available (Table t). Sampling rates are typically between 500 and 2,500 samples per second per channel for systems with 4 to 64 channels.