Fibre Optic Based Vibration Sensing: Nature of the Measurement

Within the industry there appears to be some confusion about what fibre optic based seismic systems are actually measuring. Using synthetic and real data we show that such systems measure instantaneous strain, which is equivalent to the sum of the perturbations of each section of the gauge length from its mean position divided by its length. The time derivative of strain, the strain rate, is proportional to the average velocity measured at all points along the gauge length. DVS measurements have the polarity of a hydrophone measurement but the directionality is highly complex and depends on wavenumber as well as the angle of incidence.