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 soils, geologic conditions, and other data, must interpret resistivity data and arrive at a reasonable soils, 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.

5.8.1 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.

5.9 Multielectrode Arrays—In the late 1980’s multielectrode data acquisition were developed which allow for the rapid acquisition of sounding and profiling data. General references include Griffiths and Turnbull (15), Griffiths (16), and Loke and Barker (17 and 18). Arrays with tens to hundreds of electrodes are now in common use. Current injection and potential measurements are supplied by a programmable controller. The controller is programed to inject current at two electrodes and measure the resulting potential at one or more nearby electrode pairs. Hundreds of apparent resistivity measurements can be completed within several minutes. Wenner, Schlumberger or Dipole-Dipole electrode arrangements are commonly used, but most systems are capable of programming data acquisition using other arrays or a combination of electrode arrangements to optimize results (Stummer et al (19)).

5.9.1 Apparent resistivity data are inverted using finite element algorithms that compare modeled apparent resistivity values to measured values. The technique is commonly referred to as Electrical Resistivity Tomography (ERT) or Electrical Resistivity Imaging (ERI).

5.9.2 Two dimensional models (cross-sections) are produced if the data are collected along a linear array. Three dimensional models are produced if data are collected from an electrode grid or by contouring results from adjacent 2D models.

1.1 Purpose and Application: 

1.1.1 This guide is one in a series of documents that describe geophysical site investigation methods.

1.1.2 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 or in water and yield an apparent resistivity. These data can then be interpreted to yield an estimate of the depth, thickness, voids, and resistivity of subsurface materials.

1.1.3 Bulk electrical resistivity in the shallow subsurface is primarily controlled by electrolytic conduction in aqueous fluids that are either distributed across grain boundaries or contained in pores, fractures, and faults. Resistivity measurements as described in this guide are applied in geological, geotechnical, environmental, and hydrologic investigations. The resistivity method is used to map soils and/or 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), Telford et al (7), and Daily et al (8).

1.1.4 This guide includes the use of tomographic data acquisition and interpretation methods, commonly referred to as electrical resistivity tomography (ERT) or electrical resistivity imaging (ERI).

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 approaches for resistivity measurements using 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 in 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.

Note 1: Even in countries that use inch-pound units conductivity and resistivity are reported in SI units, albeit in S/cm and Ωcm (instead of S/m and Ωm) respectively. It is recommended, however, that S/m and Ωm are used.

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.