A Broadband Ocean-Bottom Seismometer System Based on a 1-Hz Natural Period Geophone

Abstract

Recent work with broadband sensors has demonstrated that useful data can be obtained from seismic sensors deployed on the deep seafloor for periods as long as 300 sec, but ocean-bottom seismometer (OBS) systems using conventional broadband sensors have proven fragile and expensive to build and operate. The intrinsic thermal (Brownian motion) noise limit for large 1-Hz geophone sensors such as the Mark Products L-4 is far below typical noise levels at seafloor sites. In response to the need for large numbers of OBS systems for experiments studying the oceanic upper mantle and crust, we have developed a very low-noise amplifier that can be used with short-period sensors. The new amplifiers push noise levels at the 100 sec period some 40 dB below standard short-period OBS noise levels. Useful data can then be obtained from these short-period sensors in the broad-frequency range from 0.005 to 50 Hz. Noise levels rise from 5 × 10 –16 ( m /sec 2 ) 2 /Hz at a 20-sec period to 5 × 10 –14 ( m /sec 2 ) 2 /Hz at a 200-sec period. Power requirements are less than 35 mW for three channels. These amplifiers have been installed in the first 24 instruments of the new NSF-sponsored LDEO OBS Instrument Center. We believe this system is an excellent compromise between cost, reliability, and noise level. The new amplifiers may also prove useful for work on land, extending the useful range of short-period sensors to a much longer period.