There are several ways to improve the electrical contacts ranging from just wiping the sample with a suitable solvent to soldering directly on the sample or even pressing some pieces of soft metals onto the contact area. Cables are made of conductors, meaning that the material has very low resistivities. Built by scientists, for scientists. What makes sheet resistance interesting is that it is independent of the size of the square and the thickness of the sheet is not required to measure sheet resistance. It has to be noted and understood that in this case the current is flowing between the two inner electrodes (in the case of four-pole cell) and that the voltage is measured between the two outer electrodes. General electronic configuration of a two-pole resistivity cell . (4) and (5) are combined : I is the current (A) flowing through the specific sample, V is the voltage (V) applied across this specific sample, and R is the resistance (Ω) of this specific sample. The distances between the electrodes can define the cell constant based on the electric fields built up as shown in Figure 5: General electrode positioning for the four-pole resistivity cell . The most common sources of error arise from doing a two-point measurement on a material that has any of the contact problems discussed earlier. The basic principle behind electrical methods is the injection of current into the ground using a pair of electrodes. These correction factors are available for the most commonly faced sample geometries . This is usually done by pressing four collinear and equally spaced contacts into the film. Any changes in size and shape of the sample can cause changes in its total resistance, while those changes will not affect the resistivity of the sample since that is a property of the material alone. Use flow-through cell to avoid carbon dioxide from dissolving in the solution. 2018). This indicates that the current does not necessarily flow in the direction of the applied electric field. A theoretical assumption has been made when designing resistivity cells that the electric field lines are straight lines from one pole to the other and that they are not affected by surrounding objects. The main advantage of toroidal conductivity cell is that the coils do not come in contact with the solution. In those cases, high frequencies are used, and for high resistivities where the ionic content is low, lower frequencies are used. Electrode spacing is increased and the survey is moved along a profile in order to measure both vertical and horizontal resistivity. In this chapter, isotropic and homogeneous materials are assumed, so ρ is considered to be a scalar. Variations in the resistivity of Electrical resistivity prospecting is a very useful and attractive method for soil characterization. The basic principle behind electrical methods is the injection of current into the ground using a pair of electrodes. Electrical resistivity imaging (ERI) is a non-destructive method for subsurface investigations. The total resistance of a cable is proportional to its length. The resistance of the cable can induce an error on the readings of the cell, and therefore, it should be compensated for accurate measurements. Variations of the method include electrical resistivity, self potential, and induced polarisation. Submitted: August 31st 2016Reviewed: February 6th 2017Published: May 31st 2017, Home > Books > Electrical Resistivity and Conductivity. The Resistivity technique is a useful method for characterising the sub-surface materials in terms of their electrical properties. The electrical resistivity of concrete is an important value for designing electrochemical anti-corrosive methods or a durability indicator of reinforced concrete structures. One advantage of the four-electrode method is that measurements are not sensitive to contact resistance at the potential electrodes so long as it is low enough that a measurement can be made, because observations are made with the system adjusted so that there is no current in the potential electrodes. It employs an artificial source which is introduced into the ground though a pair of electrodes. This chapter is distributed under the terms of the Creative Commons Attribution 3.0 License, which permits unrestricted use, distribution, and reproduction in any medium, provided the original work is properly cited. Electrical resistivity of the soil can be considered as a proxy for the spatial and temporal variability of many other soil physical properties (i.e. In solutions, the polarisation effect is of high importance due to the accumulation of ions near the surface of the electrodes. Good soil models are the basis of all grounding designs and they are developed from accurate soil resistivity testing. The level of the induced current will be proportional to the resistivity of the medium inserted within the coils (Figure 2). Electric imaging is able to survey both vertical and horizontal changes in resistivity. Under those circumstances, geometric correction factors are used to compensate in order to accurately measure the sheet resistance. ●The resistivity method is used in the study of horizontal and vertical discontinuities in the electrical properties of the ground. Considering the bar-shaped sample in Figure 1: General two-point resistivity measuring technique . Like strength, electrical resistivity is a property of any given concrete. Flow through type resistivity cells are designed for those cases. That will decrease the error between the geometrical cell constant and the actual cell constant by providing a better estimate of the effective volume of the sample. The most common problem with flow through cells is that they need a closed liquid system protected from air. This method depends on differences in the electrical resistance of different soil (and rock) types. Soil resistivity testing is the single most critical factor in electrical grounding design. In order to improve the reliability and accuracy of the measurements, the source of errors must be identified. The conductivity of the solution can be calculated by : where κTrefis the conductivity at reference temperature, κT is the conductivity at the temperature of the measurement, Tref is the reference temperature, T is the sample temperature, and θ is the temperature coefficient (%/°C). Environmental studies (contaminant mapping, landfill sites); Mineral exploration (induced polarisation (IP) surveys). Eq. To avoid this problem, use as low current as possible while the voltage is still readable on the metre or use a temperature sensor on the resistivity cell itself. Furthermore, in the attempt to measure the sample only, the impedance caused by polarisation of the electrodes and the field effects, interfere with the measurement, and both impedances are measured. In cases where the sample is a solution, cell constant can change due to changes on the electrodes. As this voltage measurement takes place with a negligible current, these two electrodes are not polarised, and therefore, their resistance is effectively zero. lows. That ionic movement causes and accumulation of ions and therefore charge at the electrodes’ surface which can further cause the initiation of chemical reactions (Figure 8). Login to your personal dashboard for more detailed statistics on your publications. It is well known that the four-probe method (the Wenner method) When the measurement requires very high precision and accuracy, the measurements are taken in a temperature controlled environment to ensure temperature stability and for a more accurate determination of the cell constant at that temperature . The van der Pauw technique is explored as a step-by-step procedure to estimate the resistivity of a material with no arbitrary shape. Furthermore, toroidal cells are typically larger and the solution current induced by the toroid occupies a volume around the sensor. The gravity method also enables a prediction of the total anomalous mass (ore tonnage) responsible for an anomaly. In addition, the way to experimentally determine the cell constant of a cell is described and the necessity for calibration is clearly explained. To provide a more accurate version of resistivity that can be applied for every material, the more general form of Ohm’s law is used : In this equation, E is a vector that represents the electric field generated in the material (V/m), J is also a vector that represents the current density within the material (A/m−2), and ρ is a tensor which is basically the proportionality coefficient (Ωm). This value can provide a range of information regarding the material being tested. High cell constant (1 cm−1) or higher if possible. The implications and challenges faced during resistivity measurements are explored and explained with ways to compensate for errors due to temperature and capacitance changes. DC (direct current) resistivity methods involve injecting a steady state electrical current into the ground The van der Pauw technique is explored as a step-by-step procedure to estimate the resistivity of a material with no arbitrary shape. • The resulting models can provide accurate estimates of depth, thickness and electrical resistivity of subsurface layers. Brief introduction to this section that descibes Open Access especially from an IntechOpen perspective, Want to get in touch? There are several precautions that can be used to minimise or eliminate polarisation: Using an AC current will eliminate the polarisation effect since the polarity of the electrodes will constantly change, and the ions in the solution will not accumulate on one side since they will be sequentially repelled and attracted by the electrode. method, we overview the variation of electrical resistivity as a function of soil properties, we listed the main electrical device to performed one-, two- or three-dimensional surveys, and explain the basic principles of the data interpretation. Ideally, electrodes placed at specific distances with a known effective surface area. There are non-Ohmic materials that change their resistance depending on the applied voltage or current across them. Therefore, it is logical to do four-point measurements whenever possible. Locate water bearing zones. In those cases, the sheet resistance is usually used to compare between different thin films of materials. 2. One way to minimise the polarisation effect is to decrease the current density. It provides explanations for two-, three-, and four-pole as well as toroidal resistivity cells. That minimises the effect of having special field fluctuations and eliminates the influences of external factors, such as the size of the beaker and the distance between the beaker walls and the poles, on the resistivity measurements. Electrical resistance surveys (also called earth resistance or resistivity survey) are one of a number of methods used in archaeological geophysics, as well as in engineering geological investigations.In this type of survey electrical resistance meters are used to detect and map subsurface archaeological features and patterning. Polarisation issues in high conductivity samples. Overview of Electrical Conductivity. Other than the common experimental errors and some ways to prevent them, further measurement improvements, can be achieved when following these simple rules [2, 6, 7]: The cell constant is the most important component for accurate resistivity measurements. Electrical measurements on-site are inexpensive, swift, reliable, and non-destructive. The method allows a quantitative result to be obtained by using a controlled source of specific dimensions. Scraping the sample’s surface with a razor blade, exposing a fresh surface, and using alligator clips or silver-painting are also appropriate options. In general, the geometry of the cell is vital. Resistivity is not proportional to concentration at high levels. In most conductivity metres, the frequency is automatically increased with decreasing resistance of the sample, to avoid polarisation errors at low resistivities . Discuss in detail the use of electrical resistivity surveys in groundwater studies. Electrical resistivity method is one of the most useful techniques in groundwater geophysical exploration, because the resistivity of rocks is sensitive to its ionic content (Alile, et al., 2011). Hereafter, toroidal cells need more surrounding space and therefore are mounted in larger pipes . Although the calculated resistance includes the resistance of the electrodes as well, in cases where the sample is a solution, its resistance is much higher than the resistance of the electrodes, and therefore, it can be neglected. 1.7 Electrical resistivity has been used to measure the quality of concrete since the early 1910s, but in recent years state of the art devices have been developed improving this measurement process. The main downside of this cell type is that the cell is susceptible to coating and corrosion, which severely decreases the performance of the cell. Furthermore, when using the two-pole cells, the determination of the cell constant must be done at close resistivities to the resistivity of the sample since the cell constant is also resistivity dependent. (1) is Ohm’s law in a more general context where E and J are vectors, and ρ is a tensor. When measuring resistivity of solutions with medium to low resistivity, the linear temperature correction is used. DC Resistivity Survey (Schlumberger Array) At Coso Geothermal Area (1977) Coso Geothermal Area 1977 1977 Schlumberger soundings, audio-magnetotelluric soundings and telluric mapping in and around the Coso Range 1974 If the two resistivities ρx and ρy are not within 10% of each other, then either the contacts are bad or the sample is non-uniform. Polarisation can cause error on the measurements as it is a parasitic component to the solution resistance. Available from: Electric Machines for Smart Grids Applications, Holistic Electronics Research Lab, Department of Electrical & Computer Engineering, University of Cyprus, Nicosia, Cyprus. Thus, the method of measuring electrical resistivity or conductivity using four-electrode probe has been applied in geology and soil science for almost a century and the theory of the method is well developed. Therefore, another technique called van der Pauw technique  is used. In reality, although the majority of the field lines do form in straight lines, some of them form curves (Figure 9). Electrical resistance surveys (also called earth resistance or resistivity survey) are one of a number of methods used in archaeological geophysics, as well as in engineering geological investigations.In this type of survey electrical resistance meters are used to detect and map subsurface archaeological features and patterning. Sometimes, voltmeter leads age and their contacts with the voltmeter are damaged. Any remarkably low resistance value measured between the electrodes should indicate short circuit. An obvious but not trivial point is to ensure the circuit integrity. In particular, for pure water resistivity measurements, it is very important to use a flow cell since contact with air will dissolve carbon dioxide and for carbonate ions changing the resistivity of the sample . Soil Resistivity Testing Methods – The Wenner 4 Probe So soil resistivity testing can be carried out using different methods. The resistivity of soils is a complicated function of porosity, permeability, ionic content of the pore fluids, and clay mineralization. In reality, sheet resistance is exactly the same as the resistance of a square film of a material. the cable capacitance of the resistivity cell is >350 pF. Determine depth to bedrock and overburden thickness. It is ideal to determine the cell constant of the cell right before any measurement, but a frequent cell constant calibration is advisable. The resistivity cell should be calibrated before measuring any material samples. Resistivity of semiconductor by four probe method 1. Identify sinkholes. The two techniques are the inductive and the contact-based methods. Resistivity measurements are temperature dependent; if the temperature increases, resistivity decreases. These cells are customised for this kind of measurements but show several disadvantages. It is also part 3 in a series of short posts on Soil Resistivity Testing Methods. In solutions, for example, the distance between the cell and the wall can be a major source of error since the electric field is bounded and altered by the beaker walls. Licensee IntechOpen. However, the resistivity method based on the electrical properties and the seismic refraction methods based on the elastic properties of the deposits have been used widely in large civil engineering projects. Therefore, just like the bar sample in Figure 1, the resistivity can be calculated by : where ρ is the sample resistivity (Wm), V is the voltage measured by the voltmeter (V), w is the width of the sample (m), h is the thickness of the sample (m), I is the current the ammeter measures flowing through the sample (A), and l is the length of the film (m). Relatively small current magnitudes are needed to produce measurable potential differences. There are five conditions to be fulfilled in order to correctly use that technique: All four contacts must be located at the edges. There is still some field effect present for the four electrodes cell due to the fact that when field lines do not flow directly to the other electrode, the distance travelled by the current is different from the distance between the two electrodes. Soil Resistivity Testing Methods a popular post. Alter the contact points to measure R43,12 and R14,23. The input impedance is usually listed in the equipment specifications. This current causes a potential difference in the ground which is measured by a separate pair of electrodes. Qualitative explana- There are also cases that the samples are of arbitrary shape and the sample might be damaged in the attempt to make it into the desired shape. It permits measurements of resistivity in samples having a wide variety of shapes, including the resistivity of small volumes within bigger pieces of semiconductor. It further provides information on common experimental errors and a short guideline to improve the reliability and accuracy of the measurements. Determine depth to groundwater. Originally published in 2013 and now updated in 2020. The linear correction method is useful and correct only when the reference temperature and the temperature of the measurement are close. Contact area of any individual contact must be at least an order of magnitude smaller than the area of the entire sample. That can have a major effect on the cell constant. It is not necessary to measure the width or length of the sample. The most convenient and common way to do that is to cover the electrodes with platinum black. 1996;Alamryetal.2017). Unlike conventional resistivity techniques, no ground contact is required. A number of different electrode configurations can be employed for electrical surveys, including: Then, the temperature coefficient is calculated by using the following equation : T2 must be a typical sample temperature, usually room temperature, and should be approximately 10°C different from T1. It is also a common technique to measure the resistance of films of arbitrary size and shape. The voltage measured can then, using the parameters of the survey, be converted into an apparent resistivity value. By Wei Shan, Zhaoguang Hu, Ying Guo, Chengcheng Zhang and Yao Liu. They are usually surrounded by a polymeric material. A common technique is to use platinum black to cover the electrodes’ surfaces because it has a very high electrochemical surface area. Conductivity (σ) is in principle the same property of the material, but it is calculated as the inverse of resistivity, and it is measured in Siemens per metre (S/m). These field lines can affect the measurement especially when another object or another field interferes with them. The easiest way to measure sheet resistance is to make the material into a square film having equal length and width. The mineral particles of soil are poor conductors of current. Thus, electrical methods are employed for those applications in which a knowledge of resistivity or the resistivity distribution will solve or shed light on the problem at hand. Different types of soil compositions have different resistivities. Marios Sophocleous (May 31st 2017). Do not attempt to dilute the solutions to bring them down to the measuring range of the cell. Ensure that the instrumentation is within its operating ranges in terms of current, voltage, and frequency. The resistivity of soils is a complicated function of porosity, permeability, ionic content of the pore fluids, and clay The procedure involves measurement of potential difference between other two electrodes in the vicinity of current flow. Field effects must be in the centre of the vessel. Therefore, before making accurate measurements, the linearity between voltage and current across the sample should be investigated. The cell constant can be calculated using Poisson’s equation: Some specific approaches are shown in Figure 6 where d is the distance between the electrodes (m) and n is an integer to show that it is a multiple of d. Typical electrode configurations for four-pole resistivity . Electrical Resistivity Test of Soil. The most common errors in resistivity measurements are those produced by field effects. A Review of Measuring with the Wenner Method. Part 1 & Part 3. The general step-by-step procedure for doing a van der Pauw measurement is as follows: Define resistance Rab,cd = Vcd/Iab, where Vcd = Vc – Vd and is the voltage between points c and d, while Iab is the current flowing from contact a to contact b. Current density can be decreased by increasing the electrochemical surface area of the electrodes. Help us write another book on this subject and reach those readers. where ρx is the resistivity (Wm), d is the thickness of the sample (m), resistances RH and RL are measured in W, and ln 4 is approximately 1.3863. Soundings are sometimes referred to as VES; Vertical Electrical Sounding. There are cases where the current applied to the outer electrodes is kept constant, and the voltage is measured between the two inner poles. We are a community of more than 103,000 authors and editors from 3,291 institutions spanning 160 countries, including Nobel Prize winners and some of the world’s most-cited researchers. If platinum black is damaged or scratched, it will alter the cell constant and the properties of the cell. The method described here overcomes the difficulties mentioned above and also offers several other advantages. The applied voltage or current can cause self-heating of the material or even the cell itself, which can change the resistivity measurements. The basic idea for this correction method is to correct the measured conductivity from the measurement temperature to 25°C to give K25 . This method essentially combines the other two methods. Unsuitable for micro-samples depth of immersion 3–4 cm. It further provides information to overcome the standard problem of polarisation when the resistivity of solutions with high ionic content is investigated. The measurement range over which the cell stays linear gets broader as the number of poles increases. If it does not fall to zero and gives a random number, then that indicates open circuit. A short comparison between the classical two-pole resistivity cell and the more advanced four-pole resistivity cell is shown in Table 1: Good with viscous media or samples with suspension. Soil electrical resistivity can be monitored using a variety of electrode con-figurations including one-, two-, and four-electrode methods (Samson et al. Platinised cells are very powerful because their cell constant is linear over 2–3 decades of resistivity, while without platinum black it is only linear for approximately one decade. This chapter discusses and explains the basic operating principles of various measuring methods of resistivity for materials in both liquid and solid phase. The purpose for adding the third pole was to direct and constrain the electric field lines. They also measure the temperature of the resistivity measurement, and they use a function to translate the measured resistivity to reference resistivity. The curing temperature has a significant influence on the electrical resistivity for fly ash based geopolymers, while the influence of curing temperature for metakaolin based geopolymer is negligible with a prolonged curing period. When using a two-pole cell, deposits on the electrode’s surface can have a similar effect to polarisation since the electrodes’ resistance changes, while in the case of the four-pole cells, contamination has no effect . When the measured resistance is high, the cable capacitance is not negligible and must be taken into account. ELECTRICAL RESISTIVITY SURVEYS Electrical resistivity is a geophysical method in which an electrical current is injected into the ground through steel electrodes in an attempt to measure the electrical properties of the subsurface. ELECTROMAGNETIC METHODS Introduction: Electromagnetic induction (EM), as the name implies, uses the principle of induction to measure the electrical conductivity of the subsurface. Cells with up to K = 10 cm−1 are best for very low resistivity samples. The resistivity method can be adapted to an induced polarisation method with the addition of switches and changes to the instrumentation. It is advisable to use platinum black for measurements in non-viscous samples, without suspensions and frequent calibrations. In particular, in the case of platinised cells, calibration should be performed even more frequently due to the elevated risk of contamination and physical or chemical alteration of the platinum layer. structure, water content, or fluid composition). Also, its one of the three most popular Electrical Resistivity Method (ERM) is one of geophysical tool that offer very attractive technique for subsurface profile characterization in larger area. Applying a potential difference or an electrical current to the electrodes of the cell, depending on the polarity, ions will be attracted or repelled from the electrodes. 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