Geoelectric
Electrical prospecting uses a large variety of techniques, each based on some different electrical property or characteristic of materials in the earth. There for this techniques are divided into three major categories; for resistivities, being a method with a large applicability and complex, there are subcategories retaliated.
The resistivity method is designed to yield information on formations or bodies having anomalous electric conductivity and it has been used for a long time to map boundaries between layers having different conductivities. It is employed in engineering geophysics to map bedrock, for determining the homogeneity of the terrain and possible sinkholes, in groundwater studies to determine salinity and the depth to the water table. Most recently it has been applied in the search for geothermal power because subterranean steam affects the resistivity of formations in a way that can often be diagnostic.
It is convenient and customary for most resistivity techniques to define a response function called apparent resistivity, Roa , which can be evaluated or estimated from surface measurements. These apparent resistivities are usually functions of a variable that is related to the depth of penetration. Consequently, the apparent resistivities are of important intuitive value and in practical quality control and interpretation procedures.
Depending on the type and complexity of the array used for determining apparent rezistivity, this method is classified in three major categories, also adding a special section with the theory for the different arrays.
This method is by far the most used method for geo-electric surveying, because it is one of the cheapest geophysical method and it gives very good results in many area of interest.
The field measurements technique is adjustable for the different topographic conditions and the interpretation of the data can be done with specialized software, with a primary interpretation immediately after the measurements. The results of the measurements can be interpreted qualitatively as well as quantitatively. The quantitative interpretation, in the case of mineral exploration, can give the chance to evaluate the reserves in a certain area of interest.
This method is most useful in surveying at shallow depths (the first 300 m), but there were cases when the investigation depth was of thousands of meters. In cases like these powerful electric energy sources must be provided.
The principle of this method is to insert a electric current, of known intensity, through the ground with the help of two electrodes (power electrodes – AB) and measuring the electric potential difference with another two electrodes (measuring electrodes – MN). The investigation depth is proportional with the distance between the power electrodes.
Vertical electric sounding method, as all geophysical methods, needs for a good interpretation the presence of a contrast in resistvity. As the contrast is greater, the results are better.
The technology is applicable on rivers and lakes also, making it possible to find out the geology of under water terrain. This method has proven efficient in designing vertical guided boreholes and other engineer projects.
The electric resistivity profiling method uses the same type of arrays as in the vertical electric sounding (VES), just that in this case the interest is concentrated on several levels of depth. There for, there are arrays conceived with fixed distances between the electrodes so the measuring technology becomes simple and fast, the number of operators is smaller, the costs are smaller and the results can be visible immediately. This method is ideal when we need to survey for shallow depth ground properties but on long distances. It is successfully used in determining soil contamination (for pipe protection, in agriculture, etc.), determining depth to bedrock, discontinuities in mechanic properties of rocks (useful in designing pipe and cable diggings), and tracking of pollutants. The method has new applicability in engineering geophysics.
It can be considered the lead technique in geo-electric measurements. This method combines modern techniques of data acquisition with performing interpretation solutions.
The data acquisition is made uniform along the profile with a density established by the distance between the electrodes and the type of array used. With one length of the multielectrode cable we can obtain hundreds of resistivity measurements thus creating a 2D image of the underground resembling a tomography. This data sets are afterwards filtered and processed with a specialized software that does a 2D inversion, unlike the other methods, thus driving to the best results possible.
The high density of measuring points that are achieved with just one layout of the array make this method very useful in engineering projects. The resolution of the geoelectric tomography is very good thus making possible pointing out diverse objects in the ground.
Ussing special configurations of the measuring arrays, 3D images of the ground resistivity can be obtained thus making it very easy to visualize underground features (archeological, construction or natural features).
When a current is passed through earth materials not containing metallic minerals, the amount of current is related to the driving potential only by the ohmic resistance of the formations involved. When the formations contain metallic minerals, the currents give rise to an exchange of ions at the surface of contact between the minerals and the electrolytes dissolved in the fluid filling the intergranular pore spaces. This electrochemical exchange creates a voltage which opposes the current flow though the material, and an added voltage is necessary to overcome the barrier thus created. The extra voltage necessary to drive current though the barrier is sometimes referred to as the over voltage. When the externally applied current is turned off, the electrochemical voltages at the metallic grain surfaces are dissipated, but not instantaneously. The decaying voltages can be measured for a time after the current is switched off. The voltage is observed to vary with time. The ratio between the amplitude of the over voltage just after the current stops to that just before gives a measure of the concentration of metallic minerals in the material through which the current flows.
This method involves using special measuring electrodes – impolarizable electrodes. The technique requires careful handling on behalf of the operator and it can be influenced very easy by free running electric currents. It is the best method of investigating the disseminated mineralization, but it is also used for characterizing the hydro-geological conditions ( the clay in the aquifer layer has a manifestation of the induced polarization).
The method is derived from the pole-dipole method and is characterized by a long current line and a short distance between the measuring electrodes. The main characteristic is that one power electrode is put directly in contact with the solid body we are interested in. It is better if the rock body has a lower resistivity. Thus around the body a current field will be created that will be detectable on the surface. Through grid profiling, the main directions of current movement can be established and thus the properties of the solid body can be determined.
The method was successfully used in dam and artificial lake leakage surveying and hydrogeology.
The simplest array is one in which one of the current electrodes and one of the potential electrodes are placed so far away that they can be considered at infinity. This configuration with its formula for apparent resistivity is shown below.
Roa=V/I*2PIa
This array can actually be achieved for surveys of small overall dimension when it is possible to put the distant electrodes some practical distance away. For a survey in an area of a few square meters infinity can be on the order of a hundred meters. Pole-pole sounding data is plotted as apparent resistivity vs. “a”.
If only one of the current electrodes is placed at infinity the configuration and the apparent resistivity are as shown:
Roa = 2*PI*[b(a+b)/a]*V/I
This array is used frequently in resistivity surveying and the spacing are usually described, and taken, in integer multiples of the voltage electrode spacing b = na. The standard nomenclature is to call the potential electrode spacing “a” so the configuration and apparent resistivity becomes:
Roa=2*PI*an(n+1)*V/I
Pole-dipole sounding data is plotted as apparent resistivity vs. “a”.
The Wenner array is now seen to be a simple variant of the pole-dipole in which the distant pole at infinity is brought in and all the electrodes are given the same spacing, “a”, as seen in the following configuration
Roa=2*PI*a*V/I
The Wenner array is normally used for sounding and the apparent resistivities are plotted vs. “a” such as is shown in figure.
One of the first arrays used in the 1920 and still popular today is the Schlumberger array shown below with its formula for apparent resistivity. It is another variant of the pole-dipole, again with the second current electrode placed symmetrically opposite the first. The voltage difference is consequently doubled and so the apparent resistivity is the same as that for the general pole-dipole with a factor of 1/2 in the geometric factor. In a Schlumberger sounding the voltage electrodes are usually kept small and fixed while only the “b” spacing is changed.
Roa=V/I*PI*b(b+a)/a
Further, it is conventional to consider the spacing “b” to be the distance from the center of the array to the outermost electrodes. In this case “b” in the above expressions becomes AB/2. If a << AB/2 the above formulas for Roa are changed:
Roa = V/I*PI* b^2/a if a<<b
Data from a Schlumberger sounding is plotted vs. spacing in the same manner as the Wenner data of figure.
The dipole-dipole array is logistically the most convenient in the field, especially for large spacing. All the other arrays require significant lengths of wire to connect the power supply and voltmeter to their respective electrodes and these wires must be moved for every change in spacing as the array is either expanded for a sounding or moved along a line. The convention for the dipole-dipole array shown below is that current and voltage spacing is the same, “a”, and the spacing between them is an integer multiple of “a”.
The apparent resistivity is given by:
Roa=V/I*PI*an(n+1)(n+2)
The Hummel array is actually a more practical variant of the Schlumberger array, easy to adapt to difficult working conditions. In practical terms is a half of the Schlumberger array, one of the power electrodes being placed at infinity perpendicular on direction of the array.
The computing of the apparent resistivity is done using the same formula as for the Schlumberger array, but to be accurate the resistivity is doubled.
Roa =2* V/I*PI* b^2/a
Using this method we can reduce the personal number necessary for the operation of moving the electrodes and also the measuring time. This technique is useful when the geological conditions allow its use – relative horizontal layers.
