Electrical Properties Associated with Rocks, Physics tutorial

Electric Current Methods:

Several geophysical surveys depend on measurements of voltages or magnetic fields related with electric currents flowing in ground. Some of the currents exist independently, being sustained by natural oxidation-reduction reactions or variations in ionospheric or atmospheric magnetic fields, but most are produced artificially. Current can be made to flow by direct injection, by capacitative coupling or by electromagnetic induction.

Surveys containing direct injection via electrodes at ground surface are usually referred to as direct current or DC surveys although in practice direction of current is reversed at regular intervals to cancel few forms of natural background noise. Currents which are driven by electric fields applying either through electrodes or capacitatively (rather than inductively, by differing magnetic fields) are at times termed galvanic. Surveys in which currents are made to flow inductively are defined as electromagnetic or EM surveys.

Resistivity and Conductivity:

Metals and most metallic sulphides conduct electricity proficiently by flow of electrons, and electrical methods are thus significant in environmental investigations, where metallic objects are frequently targets, and in search for sulphide ores. Graphite is also good electronic conductor and, as it is not itself useful mineral, is source of noise in mineral exploration. Most rock-forming minerals are extremely poor conductors, and ground currents are thus carried mostly by ions in pore waters. Pure water is ionized to only very small degree and electrical conductivity of pore waters depends on presence of dissolved salts, mostly sodium chloride. Clay minerals are ionically active and clays conduct well if even slightly moist. This is Ohm's law. Constant of proportionality, R, is called as resistance and is estimated in ohms when current (I) is in amps and voltage (V) is in volts. Reciprocal, conductance is estimated in siemens, also called as mhos. Resistance of unit cube to current flowing between opposite faces is known as its resistivity (ρ) and is estimated in ohm-metres (Ωm). Reciprocal, conductivity is stated in siemens per metre (Sm-1) or mhos per metre. Resistance of rectangular block estimated between opposite faces is proportional to resistivity and to distance x between faces, and inversely proportional to their cross-sectional area, A, i.e.

R = Ρ(x/A)

Isotropic materials have same resistivity in all directions. The majority of rocks are reasonably isotropic but strongly laminated slates and shales are more resistive across laminations than parallel to them.

Electrical resistivities of rocks and minerals:

Resistivity of several rocks is roughly equal to resistivity of pore fluids divided by fractional porosity. Archie's law, that defines that resistivity is inversely proportional to fractional porosity raised to power that differs between about 1.2 and 1.8 according to shape of matrix grains, gives a closer approximation in most cases. Departures from linearity are not large for common values of porosity.

Apparent resistivity:

The resistivity value of completely homogeneous ground (homogeneous half-space) which would generate same result when examined in exactly the same way. This quantity is called as clear resistivity. Variations in apparent resistivity or reciprocal, apparent conductivity give raw material for interpretation in most electrical surveys. Where electromagnetic methods are being utilized to detect very good conductors like sulphide ores or steel drums, target location is more significant than determination of precise electrical parameters. As it is hard to separate effects of target size from target conductivity for small targets, results are at times presented in terms of conductivity -thickness product.

Overburden effects:

Build-ups of salts in soil generate high conductivity in near-surface layers in several arid tropical areas. Such effectively short-circuit current produced at surface, allowing very little to penetrate to deeper levels. Conductive overburden therefore presents problems for all electrical methods, with continuous wave electromagnetic surveys being the most strictly affected. Highly resistive surface layers are obstacles only in DC surveys. They may really be beneficial when EM methods are being utilized, as attenuation is reduced and depth of investigation is increased.

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