Contact us for an initial consultation and cost estimate.

Electrical measurements of various types are made at the ground surface to investigate subsurface conditions in an area. Electrical surveying methods are categorized into active and passive. The active techniques, such as direct current (DC) electrical resistivity and induced polarization (IP), require introducing artificially generated electrical currents into the ground with pairs of electrodes. The passive method, spontaneous or self-potential (SP), uses naturally occurring electric fields within the Earth. These are standard electrical methods that utilize DC or low-frequency alternating currents to investigate the electrical properties of the subsurface and are used for a wide range of applications. The basic principle behind active electrical methods is the injection of current into the ground using a pair of electrodes. This current causes a potential difference in the ground which is measured by a separate pair of electrodes. The measured voltage is converted into an apparent resistivity value using the survey’s parameters. The apparent resistivity value can provide a range of information regarding the material being tested.

The direct current (DC) electrical resistivity method measures subsurface electrical resistivity (inverse of conductivity). In electrical resistivity surveying, artificially generated electric currents are introduced into the ground with an array of electrodes pushed into the ground or in boreholes, and the resulting potential differences are measured at the surface. Anomalous conditions or inhomogeneities within the subsurface, such as electrically better or poorer conducting layers, are inferred because they deflect the current and distort the normal potentials. Lithology, mineralogy, pore fluid chemistry, clay content, organic matter content, and water content affect the electrical resistivity. The electrical readings are recorded using an electrical resistivity instrument, the so-called resistivitymeter.

The electrical resistivity method was first designed to determine the vertical structure of layered Earth as vertical electrical sounding (VES); however, more sophisticated electrical resistivity imaging (ERI) methods in 2D, 3D, and 4D are now widely used to map lateral and vertical electrical variations in the subsurface. Different types of soil, sediment, and rock compositions have different resistivities. ERI surveying allows determining the spatial distribution of subsurface earth materials’ resistive or conductive characteristics in 1D, 2D, 3D, and 4D. Cordillera Geo-Services uses ERI with arrays of multiple electrodes to produce 1D insights and 2D, 3D, and 4D images of the subsurface.

Induced polarization (IP) surveying deals with the capacitance of the subsurface materials. It is a complementary technique to electrical resistivity surveying. The subsurface materials (e.g., soils, fluids, sediments, and rocks) can both dissipate (resistance) and store (capacitance) the energy associated with an injected electric current flowing through it. Hence, the resistive and capacitive properties of the subsurface materials are measured with a resistivity meter in an IP survey. IP and resistivity methods are deployed similarly; however, non-polarizing potential electrodes should be, ideally, used in an IP survey. IP measurements can be made in time-domain or frequency-domain mode. Cordillera Geo-Services mainly uses the time-domain IP mode. The time-domain IP imaging method measures how much injected energy is stored (capacitance) in the subsurface materials.

The capacitive action of the subsurface is evaluated by determining its chargeability in milliVolts/Volt (mV/V). When an injected electrical current passes through the subsurface materials, a small charge is stored, and the subsurface becomes charged. When the current is turned off, this injected charge gradually decays over a discrete-time. This decay is seen in the recorded potentials at two potential electrodes. By measuring the rate of this decay, it is possible to calculate the chargeability distribution of the subsurface materials. The gradual decrease in measured voltage is a complex function of the electrical charge polarization at the fluid-grain interface and the conduction within the pore fluid and along the grain boundaries. Grain polarization and membrane polarization are two main mechanisms of rock polarization. Two materials that possess the same resistivity might have contrasting chargeabilities. As such, IP imaging can provide additional discrimination of subsurface materials.

The spontaneous polarization or self-potential (SP) method is based on the surface measurement of natural potential differences (voltages) between points on the surface resulting from electrochemical reactions in the subsurface. Typical SP anomalies may have an amplitude of several hundred millivolts (mV) with respect to barren ground. A resolution of 1 mV is sufficient for SP field measurements. Field equipment consists of a pair of non-polarizing electrodes connected by an insulated cable to a high-impedance millivoltmeter. Electrode spacing is typically less than 30 meters. Traverses may be performed by leapfrogging successive electrodes or, more commonly, fixing one electrode in a barren ground and moving the other over the survey area. Four source mechanisms can originate self-potentials: electrofiltration potentials, thermoelectric potentials, electrochemical potentials, and mineralization potentials.