Geophysical Investigations

GEOPHYSICAL INVESTIGATIONS

A. Electrical Methods

Principle.

Ø    All electrical method are based on the fundamental fact that different materials of earth' s crust possess widely different electrical properties.

Ø    Resistivity, electrochemical activity and dielectrical constant are some of these properties that are generally studied through these methods

Ø    potential-drop methods: the natural potential may be due to electrochemical reactions between the solutions and the surrounding - subsurface rocks.

These reactions are not always of the same order throughout the dimensions of the rock masses thereby creating a potential difference and conditions for flow of current from one end to the other end.

Ø    Elongated ore bodies of magnetite and pyrite etc. are easily delineated by this method.

Ø     Natural electrical potential is measured with the help of nonpolarising electrodes along definite directions and results are plotted in terms of potential gradient along horizontal distances which are then interpreted.

Potential Drop Methods.

Ø     These include a variety of methods in which electrical current is artificially introduced from an external source at certain points and then its flow through subsurface materials recorded at different distances.

Ø    In the Equipotential Method two primary electrodes are inserted into the ground, 6-7 meters apart from each other, across which current is introduced.

Ø     The position of these primary electrodes remains fixed in the subsequent investigations.

Potential between these primary electrodes is determined with the help of two search electrodes and points of equal potential found out along the entire region under investigations, which are jointed to get equipotential lines.

Ø     Under normal conditions, that is, when the material below is of uniform nature, electrically c the ec lines would be regular in character.

Ø     But in cases when the material w is not of uniform character (that is, it contains patches of high or low conductivity), equipotential lines would show clear distortions or irregularities which would include probable location of rock masses of different characteritics.

Ø     The Resistivity Method is similar to equipotential method but in this case it is the resistivity of the material of the subsurface which is determined and from which important interpretations are made

Here also, a known current is introduced through two electrodes- current electrodes, which are inserted at some distances apart from each other.,

Investigation.

Ø     The depth of penetration of electrical current in these investigations is broadly equal to although there are many conditions attached to this generalization.

Ø     The resistivity method envisages interpretation of the qualitative as well as quantitative characters of the subsurface materials which are governed by two basic principles

(i)                If material below is of uniform nature, the resistivity values would be of regular character.

(ii)             If the material is non-uniform, that is, it consists of layers or masses of different character, then these would be indicated by irregularities or anomalies in the resistivity values.

(iii)           The depths at which these anomalies occur can be calculated and also the nature of the subsurface material broadly understood.

Applications:

(a) In Prospecting: The electrical methods have been successfully employed in delineation of ore bodies occurring at shallower depths. For such surveys at great depths, these are not of much help.

In table 1, some typical value-ranges of resistivity are given. As may be seen, rocks exhibit a great variation ranging from as high resistivity as > io ohms-meters in igneous rocks to as low as less than I ohm-rn for clayey mans.

In Civil Engineering: Resistivity methods have been widely used in engineering investigation for determination of

Depth to the bed rock -as for instance, in important projects like dams, buildings and bridge foundations, where it would be desirable that the structure should rest on sound hard rocks rather than on overburden or soil

Location of geological structures -like folds, buried valleys, crushed and fractured zones due to shearing and faulting.

Location of Aquifers -and other water bearing zones which could be easily interpreted on the basis of known resistivity values of moisture rich rocks and dry rocks.