This work reports on a method to measure the instrumental noise of a sensor in the presence of large background signals, based on measuring the coherence and output of two identical sensors exposed to the same input stimulus. The technique can be applied to any sensor characterization problem where there is interest in distinguishing random noise from common-mode noise. The method is demonstrated using geophones, a single-axis seismometer that has been an important sensor for geophysical measurements for decades. Previous experimental work has not published measurements of the geophone’s instrumental noise at frequencies near the geophone’s resonance because of the presence of large background seismic signals compared to the predicted instrumental noise, which is a minimum at the geophone’s resonant frequency. With this coherence technique, measurements performed in the presence of 1 μg/Hz seismic signals showed that the minimum instrumental noise was within a factor of 2 of the predicted minimum noise of 0.6 ng/Hz.

Abstract. Longstanding problems unique to marine seismology (in contrast to land-based studies) include high levels of environmental noise, unpredictable instrument placement, short measurement duration, and limited numbers of instruments. Traditional instruments are deployed and recovered from a surface ship with no capability to verify data acquisition until the end of the experiment. They are often poorly sited and usually poorly coupled to the seafloor. Experiments have typically been short in duration with only a minimum number of instruments. The result of these logistical problems is high background noise, sparse data recovery (compared to continental stations), and low fidelity data. In an effort to solve many of these problems, we have developed and deployed "corehole seismometers"(1-90 Hz) into small boreholes drilled by an underwater drilling system, using a tethered Remotely Operated Vehicle (ROV). The results are seismic data with a significantly lower background noise level, a lower threshold for recognition of small events, well-oriented (and useable) horizontal seismograms, and a much longer deployment capability. We present new data from these instruments and compare them with data from two types of traditional instruments deployed from a surface ship. These contemporaneous deployments were conducted during 1996 and 1997 in Monterey Bay.

This paper reports on the improved performance of a geo-phone obtained by measuring the displacement of its proof mass capacitively rather than inductively and operating the sensor as a closed loop system. A measurement of the displacement of the proof mass rather than the velocity measurement made with a conventional geophone provides better low frequency performance. As a result, a geophone can be considered for use in a broader array of applications. A capacitive geophone can operate in either an open loop or closed loop mode. This paper describes a closed loop geophone system with a frequency response similar to the state of the art Streckeisen seismometers.

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