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A coil is suspended by springs in a magnetic field (the magnet may be integral with the case of the instrument). A seismic wave moves the case and the magnet, but the coil remains relatively stationary because of its inertia. The relative movement of a magnetic field with respect to the coil generates a voltage across the coil, the voltage being proportional to the relative velocity of the coil with respect to the magnet (when above the natural frequency of the geophone). Below the natural frequency, the output (for input of constant velocity of magnet motion) is proportional to frequency and hence to the acceleration involved in the seismic wave passage.[1]

Compare hydrophone and streamer.

  1. Evenden, B. S; Stone, D. R.; Anstey, N. A (1971). Seismic prospecting instruments. 1. Gebru¨der-Borntraeger. ISBN 978-3443130039.