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Geophysical Exploration

From surface, different geophysical methods are used to explore subsurface formations, based on the physical properties of rock, and metal bearing minerals such as magnetism, gravity, electrical conductivity, radioactivity, and sound velocity. Two or more geophysical methods are often combined in one survey, to acquire more reliable data. Results from the surveys are compiled, and matched with geological information from surface, and records from any core drilling, to decide if it is worth proceeding with further exploration.

Surveys

Magnetic surveys measure variations in the Earth's magnetic field caused by magnetic properties of subsurface rock formations. In prospecting for metallic minerals, these techniques are particularly useful for locating magnetite, pyrrhotite and ilmenite.

Electromagnetic surveys are based on variations of electric conductivity in the rock mass. An electric conductor is used to create a primary alternating electromagnetic field. Induced currents produce a secondary field in the rock mass. The resultant field can be traced and measured, thus revealing the conductivity of the underground masses. Electromagnetic surveys are mainly used to map geological structures, and to discover mineral deposits, such as sulphides containing copper or lead, magnetite, pyrite, graphite, and certain man­ganese minerals.

Electric surveys measure either the natural flow of electricity in the ground, or "galvanic" currents led into the ground and accurately controlled. Electrical surveys are used to locate mineral deposits at shallow depth, and map geological structures to determine the depth of overburden to bedrock, or to locate the ground-water table.

Gravimetric surveys measure small variations in the gravitational field caused by the pull of underlying rock masses. The variation in gravity may be caused by faults, anticlines, and salt domes that are often associated with oil-bearing formations. Gravimetric surveys are also used to detect high-density minerals, like iron ore, pyrites and lead/zinc mineralizations.

In regions where rock formations contain radioactive minerals, the intensity of radiation will be considerably higher than the normal background level. Measuring radiation levels helps locate deposits containing uranium, thorium and other minerals associated with radioactive substances.

The seismic survey is based on variations of sound velocity experienced in different geological strata. The time is measured for sound to travel from a source on surface, through the underlying layers, and up again to one or more detectors placed at some distance on surface. The source of sound might be the blow of a sledgehammer, a heavy falling weight, a mechanical vibrator, or an explosive charge. Seismic surveys determine the quality of bedrock, and can locate the contact surface of geological layers, or of a compact mineral deposit deep in the ground. Seismic surveys are also used to locate oil-bearing strata.



Geochemical surveying is another exploration technology featuring several specialities, the main one being to detect the presence of metals in the topsoil cover. By taking a large number of samples over an extended area, and analyzing the contents of each metal, regions of interest are identified. The area is then selected for more detailed studies.


Date: 2015-12-24; view: 687


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