The "electric" quadrupole method consists of introducing a continuous electrical current on the surface of the ground through two electrodes of "current". Voltage is measured by another pair of electrodes. From the value of the injected current and the measured voltage, is obtained the "apparent resistivity" of subsoil.
Each type of material has a "real" resistivity range more or less characteristic. The empty cavities (air filled) show apparent resistivity which tends to infinity: the saturated soils are highly conductive and therefore have low resistivity, etc.. The electrical method is one of the oldest geophysical techniques and has been evolving based on the capabilities of the data aquisition and processing equipments.
Depending on the position of the electrodes, the current penetrates more or less on the ground. In practice, a line is spread with several electrodes which are selected in groups of four by the field team (resistivity). This way, many points of apparent resistivities are obtained.
The specific objective of this technique is to determine the value of the real resistivity and its distribution in the subsurface from surface measurements at many points of a profile, interpolating the data later to make a cut of ground resistivity.
Subsequently the data is processed with algorithms using computer tools which, after a process of iterations, approximates the measured section to a real theoretical model.
The ending result is a distance-depth section with the distribution of real resistivity of the subsoil, easily understood in hydrogeological, geological and / or geotechnical terms.
In its broadest sense, tomography is a geophysical technique for subsurface study whose aim is to determine the distribution of a physical parameter characteristic of the area within a limited space, from a very large number of measurements taken from the ground surface or from probing.
The specific aim of electrical Tomography is to determine the actual distribution of the resistivity of subsoil in the area between two drillings or in a certain range of depth along a profile measurement, from the apparent resistivity values obtained by measurements made by conventional methods of current. This presentation will be limited to profile studies from the ground surface.
A key factor of this technique is the number and distribution of field measurement, since from it are depending the resolution and research depth. As a general rule, a study using electrical tomography requires obtaining a large number of data, with a small spacing between measures to obtain the necessary lateral resolution, as well as the carried out measures should progressively involve various depth ranges.
Electrical tomography requires the use of specific instrumentation capable of performing a large number of measurements in a fast and reliable way. Apparent resistivity measurements of the land for a study through electrical tomography techniques are made by the current with the possibility of employing a wide variety of devices as far as the electrodes distribution is concerned. Among the most commonly used devices are included pole-dipole, dipole-dipole, Schlumberger, etc...
Properly combining the lateral resolution and the research depth, electrical tomography is undoubtedly one of the more effective non-destructive tools for the study and characterization of potential subsurface discontinuities in the range from a few meters to hundreds of meters deep.
It also exists a variant, the 3D tomography. In it, the arrangement of the electrodes in the field is done closed form, allowing the realization of a large number of measures between each one of the different electrodes. This provides a much larger amount of data, allowing a subsequent three-dimensional reconstruction of the ground which is the one comprised inside the wiring device.
For their operational capacity to investigate depths which can reach hundreds of meters, electrical tomography is applicable to any study of the subsoil where is interesting to identify any accidents or discontinuities that represent a sufficient contrast in the resistivity distribution of the rocky medium.
Among the most common objectives to be solved by this technique are included the following:
- Fault detection and its characterization by determining its area of influence, direction, dip slope and depth extent.
- Contact detection between lithological units of different nature, determining the morphology and the precise location of such discontinuities.
- Detection and characterization of cavities and voids, such as karst accidents, chutes, tanks, clay fillers, etc.
- Determination of aquifer units, groundwater levels, seawater intrusion, etc...
Electrical tomography, due to its vertical penetration capability, is useful for applications in archeology, geotechnical, environmental, etc.
Like any other geophysical method, electrical tomography can be very effective or it may be useless depending on several factors to be present. The most important are the following:
- Proper planning of its application based on a good definition of the problem to be solved.
- Use of appropriate instrumentation to obtain and process economically the large volume of data which are required in tomographic measures.
- Implementation of these studies by personnel skilled in both field measurements and further processing and interpretation. The risk of semiautomatic processes in the application of geophysical methods is that they can lead to aberrant results from the geological point of view even though they were mathematically correct.
The participation of qualified technicians from Water Technologies in the planning and execution of such geophysical studies is unavoidable, in order to ensure the obtention of results capable of solving the problems with the accuracy that technical community demands and the electrical tomography method can provide.