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Using ground noise or other seismic signals, an unknown sensor can be
calibrated against a known one by operating the two sensors side by side
[Pavlis & Vernon 1994].
As a method of relative (frequency-response) calibration, this
is limited to a frequency band where suitable seismic signals are
well above the instrumental noise level, and are spatially coherent
the instruments. However, when the frequency response of the sensor has
been measured electrically, then its absolute gain may be determined
quite accurately with this method. The two responses should be
digitally equalized before the amplitudes are compared.
Calibration against a reference sensor
In a similar way, the orientation of a three-component borehole sensor may
be determined by comparison with a reference instrument at the surface.