Potential methods
Magnetic prospecting maps variations in the magnetic field of the earth that are attributable to changes of structure, magnetic susceptibility, or remanence in certain near-surface rocks. Sedimentary rocks generally have a very small susceptibility compared with igneous or metamorphic rocks, which tend to have a much higher magnetite content, and most magnetic surveys are designed to map structure on or inside the basement or to detect magnetic minerals directly. The magnetic method was initially used for petroleum exploration in areas where the structure in oil-bearing sedimentary layer appeared to be controlled by topographic features, such as ridges or faults, on the basement surface.
Since the development of aeromagnetic methods, most magnetic surveys undertake for oil exploration are carried out to ascertain the thickness of the sedimentary section in areas where such information is not otherwise available (usually frontier areas). Interpretation of such data is complicated by the fact that intrabasement susceptibility changes usually have a much more significant effect on the observed magnetic field than does structural relief on the basement surface itself.
In mining exploration, magnetic methods are employed for direct location of ores containing magnetic minerals such as magnetite. Intrusive bodies such as dike can often be distinguished on the basis of magnetic observations alone.
Interpretation of magnetic data is subject to the lack of uniqueness inherent in all potential methods. Here again, the more geological information is available, the less the uncertainty in the final interpretation.
The gravity method is useful wherever the formations of interest have densities that are appreciably different from those of surrounding formations. It is an effective means of mapping sedimentary basins where the basement rocks have a consistently higher density than the sediments. It is also suitable for locating and mapping salt bodies because of the generally low density of salt compared with that of surrounding formations. Occasionally it can be used for groundwater studies and for direct detection of heavy minerals such as chromites. Recently, extremely sensitive gravimeters have been used to detect underground tunnels and the locations of burial chambers in pyramids.
Data from gravity surveys are more subject to ambiguity in interpretation than with seismic surveys; because any gravity field can be accounted for equally well by widely different mass distributions. Additional geophysical or geological information over a gravity anomaly will reduce the ambiguity and increase the usefulness of the gravity data.
Gravity measurements are routinely made in conjunction with marine seismic work and are used as a minor supplement. Gravity surveys, unaccompanied by other methods, are no longer employed in oil and gas exploration except on rare occasions.
Self Potential (SP) surveying, one of the oldest geophysical methods, involves passively measuring natural earth voltages. The method is commonly used in hydrogeological and geotechnical applications.
The SP method involves measuring natural earth voltages using two electrodes at the surface. The natural earth voltages are typically caused by moving water and ions through pore spaces of soil and rock (electro kinetic potential). The potential drop (voltage) is measured between two non-polarizable electrodes at specific locations along the survey line. Typically, one electrode is placed in a fixed location, while the mobile electrode is placed at each reading location.
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The method has been successfully used in determining the infiltrations zones of water from compressed soil dams.
