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Electrical and Electromagnetic Method
Electrical methods employ a variety of measurements of the effects of electrical current flow within the earth. The trends that can be measured include current flow, electrical potential (voltages) and electromagnetic fields. Properties that affect the resistivity (or conductivity) of a soil or rock include porosity, water content, composition (clay mineral and metal content), salinity of the pore water, and grain size distribution. In mineral exploration, electrical method is mainly applied to directly constrain conductive bodies (eg. iron ore, sulphide mineralisation), or to locate faults along which conductive minerals have concentrated (eg. graphite, sulphides). The two main techniques used in the mineral exploration industry are induced polarisation (IP) and electromagnetic (EM) methods.
Induced polarisation (IP)
This method measures the decaying potential difference as a function of time (time domain). Another technique is to study the effect of alternating currents on the measured value of resistivity (frequency domain). Certain layers in the ground can become electrically polarised, forming a battery when energised with an electric current. IP is commonly used in exploration for orebodies, principally of disseminated sulphides. Most equipment employed can take both IP and resistivity surveys at the same time.
Depending on the geological environments and the types of targets sought, a number of configurations and techniques can be applied. The main methods currently used in mineral exploration are offset pole-dipole, dipole-dipole and gradient-array methods.
This includes frequency domain EM (FDEM) methods and time domain EM (TDEM or TEM) methods. The primary fields used are normally generated by passing alternating or pulsed current through long wires or coils. This induces eddy currents that flow in any good electrical conductor and generate secondary fields. EM is used for locating conductive base-metal deposits, locating buried pipes and cables, detecting unexploded ordnance, and near-surface geophysical mapping.
A large variety of methods are available for EM techniques due to the various current configurations, frequency and conductivity. EM survey can be carried out on land or in the air (VTEM – EM37).