Significance and Use
5.1 Concepts—The resistivity technique is used to measure the resistivity of subsurface materials. Although the resistivity of materials can be a good indicator of the type of subsurface material present, it is not a unique indicator. While the resistivity method is used to measure the resistivity of earth materials, it is the interpreter who, based on knowledge of local geologic conditions and other data, must interpret resistivity data and arrive at a reasonable geologic and hydrologic interpretation.
5.2 Parameter Being Measured and Representative Values:
5.2.1 Table 1 shows some general trends for resistivity values. Fig. 2 shows ranges in resistivity values for subsurface materials.
5.6.2 Schlumberger Array—The Schlumberger array consists of unequally spaced in-line electrodes (Fig. 3), where AB > 5 MN. The formula for calculating apparent resistivity from a Schlumberger measurement is:
| where: | ||
| AB | = | distance between current electrodes, and |
| MN | = | distance between potential electrodes. |
5.6.3 Dipole-Dipole Array—The dipole-dipole array consists of a pair of closely spaced current electrodes and a pair of closely spaced potential electrodes (Fig. 3). The formula for calculating apparent resistivity from a dipole-dipole measurement is:
| where: | ||
| na | = | distance between innermost electrodes measured as a number (n) of a-spacings, and |
| a | = | distance between the current electrodes and also the potential electrodes. |
5.6.4 Comparison of the Arrays:
5.6.4.1 Schlumberger Arrays:
(1) Schlumberger arrays are less susceptible to contact problems and the influence of nearby geologic conditions that may affect readings. The method provides a means to recognize the effects of lateral variations and to partially correct for them.
(2) Schlumberger arrays are slightly faster in field operations since only the current electrodes must be moved between readings.
5.6.4.2 Wenner Arrays:
(1) The Wenner array provides a higher signal to noise ratio than other arrays because its potential electrodes are always farther apart and located between the current electrodes. As a result, the Wenner array measures a larger voltage for a given current than is measured with other arrays.
(2) This array is good in high-noise environments such as urban areas.
(3) This array requires less current for a given depth capability. This translates into less severe instrumentation requirements for a given depth capability.
5.6.4.3 Dipole-Dipole Arrays:
(1) Relatively short cable lengths are required to explore large depths.
(2) Short cable lengths reduce current leakage.
(3) More detailed information on the direction of dip of electrical horizons is obtainable.
5.6.5 Other Arrays—There are several other arrays: Lee-partitioning array (Zohdy et al (2)), square array (Lane et al (11)), gradient array (Ward (2)) and pole-dipole (Ward (5)) and automated data acquisition and imaging systems that are not discussed in this guideline.
5.7 Sounding (Depth) Measurements—Sounding measurements are one of the most widespread uses for the resistivity technique. Soundings provide a means of measuring changes of electrical resistivity with depth at a single location. Several measurements are made with increasing electrode spacings. As the spacing of the electrodes is increased, there is an increase in the depth and volume of material measured (Fig. 4). The center point of the array remains fixed as the electrical spacing is increased.
FIG. 4 Increased Electrode Spacing Samples Greater Depth and Volume of Earth (from Benson et al, (8))
5.7.1 Sounding measurements result in a series of apparent electrical resistivity values at various electrode spacings. These apparent resistivity values are plotted against electrode spacing using a log-log scale (Fig. 5) and are interpreted using inversion techniques to derive true resistivity and thickness of subsurface layers.
FIG. 5 Resistivity Sounding Curve (from Benson et al, (8))
5.7.2 Successive electrode spacings should be (approximately) equally spaced on a logarithmic scale. Normally, 3 to 6 data points per decade should be measured. A resistivity sounding curve obtained from measurements of a uniform layered medium should follow a smooth curve, (Fig. 5). By using six points per decade, noise is generally less significant and a smooth sounding curve may be obtained. Data should be plotted in the field to ensure that an adequate number of noise-free measurements are made.
5.7.3 The depth of penetration for an inhomogeneous stratified earth depends upon the electrode separation and the resistivities of the earth's layers. In general, the overall array length could be many times the exploration depth.
5.8 Profiling Measurements—A series of profile measurements along a line is used to assess lateral changes in subsurface conditions at a given depth (Fig. 6). Electrical resistivity profiling is accomplished by making measurements with fixed electrode spacing and array geometry at several stations along a profile line (Fig. 7). A single profile measurement results in an apparent electrical resistivity value at a station. Several profiles over an area can be used to produce a contour map of changes in subsurface conditions (Fig. 8). These apparent resistivity profiles or maps cannot be interpreted in terms of layer resistivity values without sounding data or other additional information.
FIG. 6 Profiling to Assess Lateral Changes (from Zohdy et al, (12))
FIG. 7 Stations Along a Profile (from Benson et al, (8))
FIG. 8 Apparent Resistivity Contour Map (from Zohdy et al, (12))
5.8.1 Vertical soundings are used to determine the appropriate electrode spacing for profiling. Small electrode spacings can be used to emphasize shallow variations in resistivity that may affect the interpretation of deeper data. Spacing between measurements controls the lateral resolution that can be obtained from a series of profile measurements.
Scope
1.1 Purpose and Application:
1.1.1 This guide summarizes the equipment, field procedures, and interpretation methods for the assessment of the electrical properties of subsurface materials and their pore fluids, using the direct current (DC) resistivity method. Measurements of the electrical properties of subsurface materials are made from the land surface and yield an apparent resistivity. These data can then be interpreted to yield an estimate of the depth, thickness, voids, and resistivity of subsurface layer(s).
1.1.2 Resistivity measurements as described in this guide are applied in geological, geotechnical, environmental, and hydrologic investigations. The resistivity method is used to map geologic features such as lithology, structure, fractures, and stratigraphy; hydrologic features such as depth to water table, depth to aquitard, and groundwater salinity; and to delineate groundwater contaminants. General references are, Keller and Frischknecht (1),2 Zohdy et al (2), Koefoed (3), EPA(4), Ward (5), Griffiths and King (6), and Telford et al (7).
1.1.3 This guide does not address the use tomographic interpretation methods, commonly referred to as electrical resistivity tomography (ERT) or electrical resistivity imaging (ERI). While many of the principles apply the data acquisition and interpretation differ from those set forth in this guide.
1.2 Limitations:
1.2.1 This guide provides an overview of the Direct Current Resistivity Method. It does not address in detail the theory, field procedures, or interpretation of the data. Numerous references are included for that purpose and are considered an essential part of this guide. It is recommended that the user of the resistivity method be familiar with the references cited in the text and with the Guide D420, Practice D5088, Practice D5608, Guide D5730, Test Method G57, D6429, and D6235.
1.2.2 This guide is limited to the commonly used approach for resistivity measurements using sounding and profiling techniques with the Schlumberger, Wenner, or dipole-dipole arrays and modifications to those arrays. It does not cover the use of a wide range of specialized arrays. It also does not include the use of spontaneous potential (SP) measurements, induced polarization (IP) measurements, or complex resistivity methods.
1.2.3 The resistivity method has been adapted for a number of special uses, on land, within a borehole, or on water. Discussions of these adaptations of resistivity measurements are not included in this guide.
1.2.4 The approaches suggested in this guide for the resistivity method are the most commonly used, widely accepted and proven; however, other approaches or modifications to the resistivity method that are technically sound may be substituted if technically justified and documented.
1.2.5 This guide offers an organized collection of information or a series of options and does not recommend a specific course of action. This document cannot replace education or experience and should be used in conjunction with professional judgements. Not all aspects of this guide may be applicable in all circumstances. This ASTM standard is not intended to represent or replace the standard of care by which the adequacy of a given professional service must be judged, nor should this document be applied without consideration of a project's many unique aspects. The word “Standard” in the title of this document means only that the document has been approved through the ASTM consensus process.
1.3 Units—The values stated in SI units are to be regarded as standard. No other units of measurement are included in this standard. Reporting of test results in units other than SI shall not be regarded as nonconformance with this test method.
1.4 Precautions:
1.4.1 It is the responsibility of the user of this guide to follow any precautions in the equipment manufacturer's recommendations and to consider the safety implications when high voltages and currents are used.
1.4.2 If this guide is used at sites with hazardous materials, operations, or equipment, it is the responsibility of the user of this guide to establish appropriate safety and health practices and to determine the applicability of regulations prior to use.
1.5 This standard does not purport to address all of the safety concerns, if any, associated with its use. It is the responsibility of the user of this standard to establish appropriate safety, health, and environmental practices and determine the applicability of regulatory limitations prior to use.
1.6 This international standard was developed in accordance with internationally recognized principles on standardization established in the Decision on Principles for the Development of International Standards, Guides and Recommendations issued by the World Trade Organization Technical Barriers to Trade (TBT) Committee.
