Geophysical Instruments

Geophysical Instruments are used by scientists and those seeking hard minerals and hydrocarbon deposits to study those parts of the earth hidden from direct view. Instruments used underwater include magnetometers (which are described in this glossary under their own listing), gravimeters, and seismometers.
Seismic devices are another class of underwater geophysical equipment used to generate shock waves so reflected signals can be used to detect structures under the sea floor. Dynamite was originally used to generate shock waves. Today mechanical devices such as air guns, bubble pulsers, boomers and sparkers are utilized. The reflected signals are detected by hydrophones that can either be placed on the seafloor or towed behind a ship. Streamer cables are arrays of hydrophones that are wired together and contained in oil-filled plastic hose.
In recent years three dimensional (3D) and three dimensional times series (4D) seismic survey techniques have been developed. The newest technique in this field is Continuous Source Electro-magnetic Measurements (CSEM). CSEM involves towing a source of low frequency electromagnetic signals to an array of seafloor receivers. The received data is used to determine the resistivity structure of the subsurface. This is of interest, because there is a significant contrast between resistive hydrocarbons reservoirs and conductive saline water saturated layers.