Marine Geophysical Prospecting

Marine Geophysical Prospecting

 

prospecting done during the search for and study of mineral deposits on the continental shelf, continental slope, and floor of the world ocean. The first marine geophysical prospecting work was carried out in the 1930’s in the USSR, the United States, and France using electrical prospecting and gravimetry. In 1941 marine seismic prospecting was first carried out in the USSR on the Caspian Sea. Marine geophysical prospecting is usually accompanied by bathymetric measurements that provide a picture of the morphology of the ocean floor.

The tasks of marine geophysical prospecting are studying the deep structure of the earth’s crust beneath the waters of the seas and oceans, finding areas that are promising in terms of oil and gas and preparing them for exploratory drilling, and mapping subaqueous placer deposits. Marine geophysical prospecting uses the methods of magnetometry, gravimetry, nuclear geophysics, electrical prospecting, and seismic (also seismoacoustic) prospecting. The seismic exploration method is very important in finding structures that are promising for oil and gas.

The full geomagnetic-field intensity (T) or increase in this intensity (ΔT) is measured by ferroprobe, proton, or quantum magnetometers that are towed behind ships in submersible nacelles or are placed aboard aircraft. Measurements of the force of free fall at sea are made from surface vessels while they are in motion, using gravimeters on gyroplatforms; the precision of such measurements is ± 1−5 milligals. More exact observations (down to ± 0.03−0.05 milligals) are made by gravimeters set on the sea floor (at depths up to 1 km) and remote controlled by cable from a surface ship. Electrical prospecting is carried out in the form of profiling or vertical electrical logging by direct current at shallow sea depths to study the resistance of the bedrock under marine deposits. In seismic exploration (infrasonic frequencies) or seismoacoustic exploration (sonic and ultrasonic frequencies) the travel time of elastic waves created by a pulse or coherent source of emission (explosion, pneumatic clapper, electrical discharge, vibrator, and the like) and reflected or refracted primarily by the layers of sedimentary rock is recorded. Nuclear geophysical marine research is carried out either by studying the natural radiation emitted by the rock of the sea floor or by the method of induced radioactivity.

In marine geophysical prospecting it is very important to determine the coordinates of the points from which geophysical observations are made. In the open sea this is done by the radio-geodetic method, by determining the ship’s position in the radio wave field of shore stations, and by means of artificial earth satellites.

Marine geophysical prospecting is carried out on expeditionary ships, primarily small and medium ones with displacements between 300 and 1,500 tons. They have geophysical equipment, sonic depth finders, radio-navigational equipment, and onboard computers for quick processing of information received. Marine geophysical prospecting is ordinarily done while the ship is in motion; thus, even when the ship is traveling at a slow speed, productivity is high and the cost is lower than if the work were done on land.

REFERENCES

Shapirovskii, N. I., and R. M. Gadzhiev. Morskaia geofizicheskaia razvedka. Baku, 1962.
Fedynskii, V. V. Razvedochnaia geofizika, 2nd ed. Moscow, 1967.

V. V. FEDYNSKII

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