Technique for measurement of the noise of a sensor in the presence of large background signals

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 mu g/root Hz seismic signals showed that the minimum instrumental noise was within a factor of 2 of the predicted minimum noise of 0.6 ng/root Hz. (C) 1998 American Institute of Physics. [S0034-6748(98)02305-3].