Geophysical Exploration

THEORETICAL BACKGROUND OF THE GEOPHYSICAL METHODS OF INVESTIGATION
3.1 Preamble
Geophysical survey methods have wide scope of utilizations, there is always one physical property for which a particular method is exceptionally sensitive. And few, if any, geophysical methods provide a unique solution to a particular geophysical situation. It is possible to obtain a very large number of geophysical solutions to some problems. However, a number of geophysical methods may be applied simultaneously (integrated geophysical exploration) in solving certain geophysical problems, and such approach, greatly reduces the problem of ambiguity, which is the inherent drawback in the interpretations of results from one method, by complementing the information

In the resistivity method, artificially generated electric currents are introduced into the ground and the resulting potential differences are measured at the surface. Deviations from the pattern of potential differences expected from homogeneous ground provide information on the form and electrical properties of the subsurface inhomogeneity (Kearey et al., 2002).
The purpose of electrical resistivity surveys is to determine the subsurface resistivity distribution by making measurements on the ground surface. From these measurements, the true resistivity of the subsurface can be estimated. The ground resistivity is related to various geological parameters such as the mineral and fluid content, porosity and degree of water saturation in the rock. Electrical resistivity surveys have been used for many decades in hydrogeological, mining and geotechnical investigation (Loke, 1999).
The method has been used to locate fault zones, zones of deep weathering and cavities. It can also be used in the exploration of alluvial deposits where permeable gravel and sand beds can

c) Porous or fractured rock bearing free water has resistivity, which depends on the resistivity of the water and on the porosity of the rock.
d) Impermeable clay layer, which is wet, has low resistivity but may not contain enough yields for successful groundwater exploitation.

In resistivity measurements, highest resistivity are associated with igneous rocks. Sedimentary rocks tend to be most conductive due to their high fluid content. Metamorphic rocks have intermediate resistivity. Granites and quartzite have high resistivity ranges; sandstone and shale have intermediate resistivity ranges (Bernard, 2003). The resistivity therefore in a particular geological environment has an influence on the aquifer resistivity Numerical values for various types of water are outlined.
In resistivity measurements current is injected into the ground via electrodes and the resulting potential is measured also by electrodes in the ground. The outer electrodes show the current electrodes for injecting current into the ground and the inner electrodes are the potential electrodes connected to the voltmeter. 27

Table 3.1 Resistivity’s of different rocks types taken from (Loke,