A broadband ocean-bottom seismometer system based on a 1-Hz natural periodgeophone

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 conventi onal broadband sensors have proven fragile and expensive to build and opera te. The intrinsic thermal (Brownian motion) noise limit for large 1-Hz geop hone sensors such as the Mark Products L-4 is far below typical noise level s at seafloor sites. In response to the need for large numbers of OBS syste ms for experiments studying the oceanic tipper mantle and crust. we have de veloped a very low-noise amplifier that can be used with short-period senso rs. The new amplifiers push noise levels at the 100 sec period some 40 dB b elow standard short-period OBS noise levels. Useful data can then be obtain ed from these short-period sensors in the broad-frequency range from 0.005 to 50 Hz. Noise levels rise from 5 x 10(-16) (m/sec(2))(2)/Hz at a 20-sec p eriod to 5 x 10(-14) (m/sec(2))(2)/Hz at a 200-sec period. Power requiremen ts are less than 35 mW for three channels. These amplifiers have been insta lled in the first 24 instruments of the neu NSF-sponsored LDEO OBS Instrume nt 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.