The electrical resistivity imaging (ERI) technique maps differences in the resistivity of geologic materials. These differences can result from variations in lithology, water content, pore-water chemistry, or contaminant plumes. The method involves transmitting an electric current into the ground between two electrodes and measuring the voltage between two other electrodes. The direct measurement is the apparent resistivity of the area beneath the electrodes. The measurements include deeper layers as the electrode spacing is increased. Recent advances in technology permit rapid collection of multiple soundings, using up to 224 electrodes for each spread. The data are modeled to create a 2-D geo-electric cross-section that is useful for mapping both vertical and horizontal variations of the subsurface strata.
The IP method studies the decaying potential difference as a function of time. In this method the geophysicist looks for portions of the earth where current flow is maintained for a short time after the applied current is terminated.
When a metal electrode is immersed in a solution of ions of a certain concentration and valence, a potential difference is established between the metal and the solution sides of the interface. This difference in potential is an explicit function of the ion concentration, valence, etc. When an external voltage is applied across the interface, a current is caused to flow and the potential drop across the interface changes from its initial value. The change in interface voltage is called the "over voltage" or "polarization" potential of the electrode. Over voltages are due to an accumulation of ions on the electrolyte side of the interface. The time constant of buildup and decay is typically several tenths of a second.
Various potentials are produced in native ground or within the subsurface altered by our actions. Natural potentials occur about dissimilar materials, near varying concentrations of electrolytic solutions, and due to the flow of fluids. Sulfide ore bodies have been sought by the self-potential generated by ore bodies acting as batteries. Other occurrences produce spontaneous potentials, which may be mapped to determine the information about the subsurface. Spontaneous potential can be produced by mineralization differences, electro-chemical action, geothermal activity, and bioelectric generation of vegetation.
Four different electrical potentials are recognized. Electrokinetic, or streaming, potential is due to the flow of a fluid with certain electrical properties passing through a pipe or porous medium with different electrical properties. Liquid-junction, or diffusion, potential is caused by the displacement of ionic solutions of dissimilar concentrations. Mineralization, or electrolytic contact, potential is produced at the surface of a conductor with another medium. Nernst, or shale, potential occurs when similar conductors have a solution of differing concentrations about them. Generally, the SP method is qualitative and does not attempt to quantify the anomalous volume size, owing to the unknown volumetric shapes, concentration/density of various masses, and electrical properties of the sought causative media.
Recognition of different spontaneous-potential sources is important to eliminate noise, the low background voltages. Some engineering and environmental occurrences may be mapped by contouring surficial voltages between base/reference electrode(s) and the mobile electrodes. Flow of gasses and fluids in pipes, leakage of a reservoir within the foundation or abutment of a dam, movement of ionic fluids to or within the groundwater, flow of geothermal fluids, and movement of water into or through a karst system can be the origin of streaming potentials. These potentials may exceed the background voltage variation of a site.
The Mise-a-la-Messe technique has been used in the mining industry since the 1920s for delineating electrically conductive subsurface ore bodies (or fractures). In that application, a current electrode is placed in the conductive body (either at a surface exposure or in a drill hole), and a second current electrode (to complete the circuit) is placed in the ground at electrical infinity (actually just a distance greater than 5 times the largest expected dimension of the subsurface body). This causes the entire body to radiate electric current. The equipotential lines from this current flow pattern can be mapped with a voltmeter and two mobile probes. The shapes of these equipotentials typically mimic to some degree the footprint of the conductive body.
Geomembrane Defect Detection (GDD) surveys to accurately delineate landfill liner defects and mechanical damage caused during construction. A GDD survey consists of applying a DC voltage between the earth material outside the liner and the cover material inside. The resulting potential field is small but generally uniformly distributed over the entire liner. If the liner is ruptured, current will flow through the defect and the measured potential gradient will peak around the location of the rupture.
The IP method studies the decaying potential difference as a function of time. In this method the geophysicist looks for portions of the earth where current flow is maintained for a short time after the applied current is terminated.
When a metal electrode is immersed in a solution of ions of a certain concentration and valence, a potential difference is established between the metal and the solution sides of the interface. This difference in potential is an explicit function of the ion concentration, valence, etc. When an external voltage is applied across the interface, a current is caused to flow and the potential drop across the interface changes from its initial value. The change in interface voltage is called the "over voltage" or "polarization" potential of the electrode. Over voltages are due to an accumulation of ions on the electrolyte side of the interface. The time constant of buildup and decay is typically several tenths of a second.
