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Instrumentation, Electrical Resistivity
- Electrical survey. Mapping subsurface resistivity by injecting an electrical current into the ground.
- Resistivity meter. An instrument used to carry out resistivity surveys that usually has a current transmitter and voltage-measuring circuitry.
- Electrode. A conductor planted into the ground through which current is passed, or which is used to measure the voltage caused by the current.
- Apparent resistivity. The apparent resistivity is the resistivity of an equivalent homogeneous earth model that will give the same potential value as the true earth model for the same current and electrodes arrangement.
- Multi-core cable. A cable with a number of independent wires.
The resistivity survey method is more than 100 years old and is one of the most commonly used geophysical exploration methods (Reynolds, 1997). It has been used to image targets from the millimeter scale to structures with dimensions of kilometers (Linderholm et al., 2008; Storz et al., 2000). It is widely used in environmental and engineering (Dahlin, 2001; Chambers et al., 2006) and mineral exploration (White et al., 2001; Legault et al., 2008) surveys. There have been many recent advances in instrumentation and data interpretation resulting in more efficient surveys and accurate earth models. In its most basic form, the resistivity meter injects a current into the ground through two metal stakes (electrodes), and measures the resulting voltage difference on the ground surface between two other points (Figure 1). The current (I) and voltage (V) values are normally combined into a single quantity, the apparent resistivity, which is given by the following relationship:
ra 1/4 kV=I
The apparent resistivity is the resistivity of an equivalent homogeneous earth model that will give the same potential value as the true earth model for the same current and electrodes arrangement. The geometric factor k depends on the arrangement of the electrodes (see Electrical Resistivity Surveys and Data Interpretation). The flow of the current through the ground depends on the resistivity of the material (rocks) through which it passes and is reflected in the measured voltage values. Subsurface resistivity distribution can be mapped by making measurements with the current and voltage electrodes at different positions.
Basic resistivity instruments
The basic parts of a resistivity measurement system include a source of electrical current, a voltage measuring system, and the cables to connect these components to the electrodes. These components have undergone major modifications over the years to improve the efficiency of the survey procedure, the quality of the data, and to increase the depth of investigation. The power source for the current is typically a battery for shallow surveys. Some small portable resistivity meter systems (Figure 2a) for very shallow surveys of up to a few meters depth weigh less than a kilogram (Pozdnyakov et al., 2009). A typical system (with associated cables and electrodes) for environmental and engineering surveys that uses an internal battery is shown in Figure 2b. Such systems usually weigh between 10 and 50 kg. The current source and voltage measuring circuitry are integrated into a single unit (the resistivity meter). Such battery-based systems can provide currents of up to about 2 A and have been used for survey depths of up to about 200 m. For deeper surveys where currents of up to 10 A are used, a petrol/diesel engine– powered electric generator is usually used. Such systems can weigh several hundred kilograms, and are commonly used for induced polarization (IP) surveys in mineral exploration (White et al., 2001). The current transmitter is separate from the receivers that measure the potential signal (Figure 2c).
In the interpretation of the survey data, it is assumed that a direct current (DC) is used. However, in practice, a low-frequency square-wave alternating current (AC) is normally used. The frequency is sufficiently low (typically less than 100 Hz) such that EM effects are negligible. Figure 3 shows an example of the shape of the input current used and the resulting voltage signal. The amplitude of the voltage signal is typically in the millivolt range, but smaller voltages can be resolved if the signal-to-noise ratio is sufficiently high or signal-stacking techniques are employed. A 24-bit ADC (analog-to-digital converter) is widely used in modern systems to digitize the voltage signal. In the absence of an input current, the ground has a natural self-potential (SP) voltage between different points on the surface. This can be caused by variations in the chemical properties of the soil or currents induced by changes of the geomagnetic field with time. The SP voltage can vary significantly with time. The voltage measuring system in the resistivity meter has a SP buck-out mechanism to remove the SP voltage signal. It usually also has notch filters (e.g., 50,60, 16.7 Hz) to remove noise due to electric power lines and other technical infrastructure such as electrified railway systems.
IP surveys are widely used in mineral exploration, and recently there has been increasing interest in the environmental and engineering sector (Aristodemou and Thomas-Betts, 2000; Dahlin et al., 2002). While both time and frequency domain (Telford et al., 1990; Reynolds, 1997) measurement techniques have been used in mineral exploration, the time domain method appears to be the norm for the less powerful equipment used in environmental and engineering surveys.
Metal stakes (Lu and Macnae, 1998) are commonly used for the current electrodes as well as the potential electrodes in resistivity surveys. For some ground conditions where it is difficult to insert a stake electrode, flat-base (or plate) electrodes have been used (Athanasiou et al., 2007; Tsokas et al., 2008); galvanic contact with the ground is achieved using an electrically conductive gel or mud at the base of the electrode. In IP surveys, nonpolarizable electrodes (Reynolds, 1997) are widely used as the potential electrodes to reduce SP noise. This makes it difficult to carry out IP surveys using multielectrode systems for 2-D (two-dimensional) imaging surveys. There has, however, been significant progress in using normal steel stakes as the potential electrodes (Dahlin et al., 2002; LaBrecque and Daily, 2008).
Multielectrode and multichannel systems
The development of multielectrode systems over the past 20 years has sparked a revolution in resistivity surveying. The advent of 2-D and 3-D (three-dimensional) resistivity tomography has opened up whole new application areas to electrical methods. Before early 1990s, the electrical resistivity method was mainly used in resistivity sounding, profiling, and mapping surveys (Telford et al., 1990). Quantitative interpretation was mainly confined to 1-D (one-dimensional) structure of the subsurface consisting of horizontal layers (Koefoed, 1979). The multielectrode systems made it practical to carry out 2-D imaging surveys that give a more accurate picture of the subsurface (Dahlin, 2001) in a routine manner.
Unlike the conventional 4 electrodes system, a multielectrode system has about 25 or more electrodes connected to the resistivity meter via a multi-core cable (Figure 4). A switching circuitry controlled by an internal microcomputer within the resistivity meter automatically selects the appropriate 4 electrodes for each measurement. The system is usually programmable and almost any electrode array configuration can be used. Several variants of the multielectrode cable system exist, where the distinguishing factor tends to be the physical location of the switching circuitry. Some resistivity meters contain all the switching capability within the main unit, so that two or more long multi-core cables can be attached directly to the meter. The cables then merely serve as a physical extension between the switch and the electrodes, which have no other function than to act as a galvanic contact between the meter and the ground (also known as “dumb” electrodes). These systems have a practical limit on the number of electrode takeouts (commonly 32) in each cable. In the second type of system, an external switch box is connected to the main meter via a system bus. The switching capacity can then be extended by daisy-chaining switch boxes, each connecting to cable segments with multiple “dumb” electrodes. This design can vastly increase the possible number of electrodes available for simultaneous measurements, but it can also be less portable than the first arrangement, particularly in forested and rugged terrain. The third type of system uses “intelligent” electrodes, where the switching circuitry is decentralized and placed at individual electrodes. Even the actual voltage measurement and analog-to-digital conversion may be carried out locally in such systems, potentially reducing interference problems (Stummer and Maurer, 2001). A more recent development is multichannel capability for multielectrode systems that can greatly reduce the survey time. Only two electrodes can be used as the current electrodes at a single time, but the voltage can be measured between different pairs of potential electrodes. Commercial systems with 4–10 channels are available and some research systems have more than 100 channels (Stummer and Maurer, 2001).
There has been considerable research on systems that record the full waveform of the potential signal in an effort to extract more information from the data and to improve the signal-to-noise ratio using signal processing techniques (Friedel and Jacobs, 1998; Storz et al., 2000; Matthews and Zonge, 2003; Rowston et al., 2003; Zhe et al., 2007). This is often the only practical approach for resistivity surveys in which large depths of investigation are required (such as, regional surveys or crustal studies) or where the nature of the target requires large dipole offsets (for example, investigations on volcanoes).
Towed systems for dynamic measurements
The multichannel multielectrode systems have been adapted for continuous profiling water-borne surveys using floating electrodes attached to a cable pulled by a boat (Loke and Lane, 2005; Mansoor and Slater, 2007; Goto et al., 2007). A PC coordinates the resistivity meter system data acquisition together with a GPS and water depth sounder. Continuous measuring systems for land surveys have also been developed. Some systems use cylindrical steel electrodes based on an in-line array geometry (Sørensen, 1996), while others use spiked wheels to achieve continuous galvanic contact with the soil (Panissod et al., 1998; Dabas, 2009).
Capacitively coupled systems
It is difficult or impossible to use conventional resistivity meter systems that inject a galvanic current into the ground in areas with very resistive surface materials or paved surfaces. Capacitively coupled systems can be used in such areas. These instruments use an oscillating, non-grounded electric dipole to generate current flow in the ground and a second similar dipole to measure the resulting potential distribution at the ground surface (Kuras et al., 2006). Two major configurations have been used in commercial and research instruments of this type. The first configuration (line antenna type) uses cylindrical transmitters and receivers, which are towed behind an operator (Figure 5a). It gives measurements that are comparable to the galvanically coupled in-line dipole–dipole array (Møller, 2001) and have maximum depths of investigation of 1–20 m, depending on the lengths of transmitter and receiver and the distance between them. The second type (electrostatic quadrupole) uses flat metallic conductors (Figure 5b) in an equatorial dipole–dipole configuration (Panissod et al., 1998; Kuras et al., 2007). This type has been used for survey depths of up to a few meters. Both configurations are dynamic measuring systems where the array configuration is fixed but the entire setup is moved during the survey for lateral coverage. 2-D and 3-D surveys with dense lateral coverage can be rapidly conducted with these systems (Kuras et al., 2007).
Automated resistivity monitoring systems
Automatic PC controlled monitoring systems have been developed for detecting transient phenomena such as water seepage from dams, landslides, and solute transport (Oldenborger et al., 2007; Sjödahl et al., 2008; Supper et al., 2008; Kuras et al., 2009). Monitoring systems are becoming increasingly sophisticated, and are now being deployed remotely with permanently installed electrode arrays, telemetric control, and data transfer, supported by automated data management systems to handle the large data volumes generated through time-lapse data acquisition (Ogilvy et al., 2009).
There have been major advancements over the past few decades in the instrumentation for the electrical resistivity method. In addition to traditional 1-D surveys, the new multielectrode and multichannel systems have made it possible to efficiently carry out 2-D and 3-D imaging surveys that provide more accurate models of the subsurface geology. Computerized dynamic towed systems can map large areas on land and water rapidly. Capacitively coupled systems make it possible to survey areas with resistive surfaces where conventional galvanic systems cannot be used. Sophisticated PC controlled monitoring systems are now available to automatically detect and record transient phenomena.
We wish to thank Geometrics Inc, Landviser LLC, Abem Instrument AB and the British Geological Survey (NERC) for permission to use their illustrations in this paper.