Feasability Study of Intrinsically Digital Broadband Seismometer

Low-noise, high quality seismometers are needed to support DOE responsibilities with respect to the monitoring of nuclear explosions. The ideal seismometer would inherently generate a digital signal as the inertial mass moves relative to the seismometer frame or ground reference. Because of the difficulty of building a seismometer suspension that is immune to temperature variations, it is desirable to incorporate this digital signal into a force feedback loop. In this project, several digitization methods will be investigated: laser interferometry, sigma-delta, and charge couple device (CCD). Two ways to close the feedback loop also will be investigated: (1) a digital/analog (D/A) converter to feed a current into the coil; and (2) a bipolar-high-frequency-pulse-width-modulated signal fed directly into the coil, thereby avoiding the digital-to-analog converter. Noise levels and dynamic range will be addressed as well. Commercial Applications and other Benefits as described by the awardee: An intrinsically digital borehole seismometer should be of great interest to the nuclear nonproliferation community because of its simpler implementation, installation, lower power, easier interface to digital communication links, and ease of control. Other government users include the USGS, which eventually should be interested in replacing existing analog instruments as they become obsolete and/or adding new measurement sites.