A typical geophone measures the displacement of an inertial mass relative to a rigid case, which is excited by the seismic signal. They are basically a mass-spring system, in which the fiber measures the strain in the spring. The main difference between the hydrophone and the geophone is that the hydrophone measures changes in a surrounding pressure field and, at low frequencies, is omnidirectional, while a geophone is measuring a component of acceleration and is inherently directional at all frequencies. Geophones are normally used in a three-axis configuration, with three orthogonally mounted devices. Typically, geophones will be used at lower frequencies than hydrophones, normally below 100 Hz.
Laser light enters the sensor and is split into two arms, one sensing and one reference.
The sensing arm consists of fiber wound on a hollow air-backed cylinder. The fiber on the reference arm is wound around a solid mandrel that is insensitive to pressure variations. Any variation in the relative path lengths between the sensor and the reference legs will show up as an interference pattern or phase change in the recombined light signal emerging from the sensor. As sound waves or pressure changes occur around the hydrophone, the hollow cylinder changes its diameter, just slightly. Pressure changes are measured by the change in diameter of a hollow mandrel on the sensing leg of a Mach-Zehnder interferometer.