Geophones are typically used for acquiring seismic data on land. Geophones are relatively simple devices, consisting a mass with wire coil wrapped around it hanging on a spring, surrounded by a magnet. When the ground moves, the mass tends to remain motionless, while the magnet moves with the ground. This creates an electrical voltage in the in the coil which is proportional to velocity at which the ground the ground is moving. In some geophones the components are reversed, that is a magnetic core is attached to the spring and a coil surrounds this core.
Figure 15 Geophone schematic, left, and geophone cases, right
The geophone is housed in a case, which usually has a long spike on the bottom. The case is anchored firmly to the ground by stepping on the top of the case to drive the spike all the way into the ground. Other case designs exist for use in different conditions that are encountered. The geophone is usually connected to a cable along that transmits the signal to a recording truck where it is digitized usually, at a sample rate of 0.5 to 8 ms, and recorded on magnetic tape. Some systems use radio to transmit the received signal to the recording truck instead of the cable. Several geophones may be connected together as a single receiver at a location, arranged in a pattern that reduces the surface wave, which is strong relative to the reflections, or other surface noise. This collection of geophones is referred to as a group.
Hydrophones , such as the one shown to the right, are most commonly used in marine surveys to acquire the seismic signal These are devices that convert changes in pressure to electrical signals. A piezoelectric transducer is usually used to perform the conversion.
Figure 16 Hydrophone
The hydrophones are connected at regular intervals on a cable to form a streamer, which is towed behind a boat. The signals are sent up the cable to the boat. Digitization of the signal may occur at the hydrophone itself or on the recording vessel where the recording is made. The position and depth of the hydrophones in the streamer are determined using acoustic ranging, laser ranging, or GPS and adjustments are made using control surfaces in the water. The picture below is a birdseye view of a recording vessel acquiring a 3D survey towing several streamers.
Figure 17 Birdseye view of a recording vessel with streamer