Geophysical Prospecting Methods

the study of the structure of the earth’s crust by physical methods for the location and surveying of minerals; geophysical prospecting is an integral part of geophysics.

Geophysical prospecting methods are based on the study of physical fields (gravitational, magnetic, electrical, thermal elastic vibration, radiation, and nuclear radiation). Measurements of the parameters of these fields are made on the surface of the earth (land and water), in the air, and underground (in wells and shafts). The information obtained is used to determine the location of geological structures, ore bodies, and so forth and their fundamental characteristics. This allows the selection of the most proper guidelines for expensive drilling and mining operations, thereby increasing their efficiency.

Geophysical prospecting methods utilize both natural and artificially created physical fields. The resolution, that is, the ability to distinguish specifically the sought-for features of the environment, is significantly higher, as a rule, for artificial field methods. The facilities for research by natural field methods are relatively inexpensive and transportable and yield uniform, readily comparable results for vast territories. In view of this, geophysical prospecting methods utilize natural fields (for example, in magnetic prospecting) primarily in the reconnaissance stage and artificial physical fields mainly for more detailed work, such as seismic surveying. A series of geophysical prospecting methods is employed in most cases, because each physical field provides specific characteristics for only one aspect of the geological objective. (For example, magnetic prospecting yields data only on the magnetic properties of rock.) The various geophysical prospecting methods are distinguished according to the nature of the physical fields utilized: gravitational, which is based on the study of the earth’s field of gravity; magnetic, which studies the earth’s natural magnetic field; electrical, which utilizes induced direct or alternating (current) electromagnetic fields and sometimes measurement of natural earth (telluric) fields; seismic, which studies elastic vibration fields created by the detonation of explosive charges (trotyl or powder) or mechanical shocks and emitted within the earth’s core; and geothermal, which is based on temperature measurements in wells, and which utilizes the thermal conductivity differences in rock that account for changes near the earth’s surface in the amount of heat coming from the interior of the earth. Nuclear geophysics, a new specialization in geophysical prospecting methods, investigates natural radioactive emanation, mostly gamma radiation, of rocks and ores and their interaction with elementary particles (neutrons, protons, electrons) and radiations whose sources are radioactive isotopes or special accelerators (neutron generators).

All geophysical prospecting methods are based on the use of physicomathematical principles for the development of theory; high-precision instruments with electronic, radio-engineering, precision-mechanical, and optical components for field measurements; and elements of computer technology, including modern electronic computers, for processing the results.

Research in boreholes is conducted by means of all of the geophysical methods. Geophysical measurements in bore-holes are performed by instruments whose readings are transmitted by cable to the earth’s surface. Electrical, acoustical, and nuclear-geophysical well-logging are the most important techniques. The drilling of deep wells is done with the necessary well-logging, which makes it possible to clearly define the limits of a rock (core) sample and to increase the sinking rate. Geophysical measurements in wells and mine shafts are also used to search for ore bodies in the spaces between them (so-called borehole geophysics). Finally, geophysical methods are used for studying the technical conditions of wells (identification of recesses and projections, control of pipe-space cementing quality, and so forth).

Geophysical prospecting methods are undergoing rapid development and successfully solving the problems of mineral prospecting, exploration, and surveying, particularly in regions covered with layers of soft sediments, at great depths, and under the floors of seas and oceans.