larger signals from local sources. It is possible though to recover the small signals caused by large but distant events by correlating signals from several geophones deployed in an array. Signals which are registered only at one or few geophones can be attributed to unwanted, local events and thus discarded. It can be assumed that small signals that register uniformly at all geophones in an array can be attributed to a distant and therefore significant event. The sensitivity of passive geophones is typically 30 volts per (meter per second), so they are in general not a replacement for broadband seismometers. Conversely, some applications of geophones are interested only in very local events. A notable example is in the application of remote ground sensors (RGS) incorporated in unattended ground sensor (UGS) systems. In such an application there is an area of interest which when penetrated a system operator is to be informed, perhaps by an alert which could be accompanied by supporting photographic data. A network of geophones has been used on the lunar surface. See also Accelerometer Hydrophone Michelson interferometer Seismometer References External links PSR-1 Seismic Intrusion Detector (Vietnam era military device) Seismology instruments