ASTM G57-06(2012)

Standard Test Method for Field Measurement of Soil Resistivity Using the Wenner Four-Electrode Method

SIGNIFICANCE AND USE
Measurement of soil resistivity is used for the control of corrosion of buried structures. Soil resistivity is used both for the estimation of expected corrosion rates and for the design of cathodic protection systems. As an essential design parameter for cathodic protection systems, it is important to take as many measurements as necessary so as to get a sufficiently representative characterization of the soil environment that the entire buried structure will experience.
SCOPE
1.1 This test method covers the equipment and procedures for the field measurement of soil resistivity, both in situ and for samples removed from the ground, for use in the control of corrosion of buried structures.
1.2 To convert cm (metric unit) to metre (SI unit), divide by 100.
1.3 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 and health practices and determine the applicability of regulatory limitations prior to use.

General Information

• Status: Historical
• Publication Date: 30-Apr-2012
• ICS: 93.020 - Earthworks. Excavations. Foundation construction. Underground works
• Technical Committee: G01 - Corrosion of Metals
• Drafting Committee: G01.10 - Corrosion in Soils

Standards Content (Sample)

NOTICE: This standard has either been superseded and replaced by a new version or withdrawn.
Contact ASTM International (www.astm.org) for the latest information
Designation: G57 − 06 (Reapproved 2012)
Standard Test Method for
Field Measurement of Soil Resistivity Using the Wenner
Four-Electrode Method
This standard is issued under the fixed designation G57; the number immediately following the designation indicates the year of original
adoption or, in the case of revision, the year of last revision.Anumber in parentheses indicates the year of last reapproval.Asuperscript
epsilon (´) indicates an editorial change since the last revision or reapproval.
1. Scope sents the average resistivity of a hemisphere of soil of a radius
equal to the electrode separation.
1.1 This test method covers the equipment and procedures
for the field measurement of soil resistivity, both in situ and for
3.2 A voltage is impressed between the outer electrodes,
samples removed from the ground, for use in the control of
causing current to flow, and the voltage drop between the inner
corrosion of buried structures.
electrodes is measured using a sensitive voltmeter.
1.2 To convert cm (metric unit) to metre (SI unit), divide by Alternatively, the resistance can be measured directly. The
100. resistivity, ρ, is then:
1.3 This standard does not purport to address all of the
ρ, Ω·cm 5 2π aR ~a in cm! (1)
safety concerns, if any, associated with its use. It is the
5191.5 aR a in ft
responsibility of the user of this standard to establish appro- ~ !
priate safety and health practices and determine the applica-
where:
bility of regulatory limitations prior to use.
a = electrode separation, and
R = resistance, Ω.
2. Terminology
Using dimensional analysis, the correct unit for resistivity is
2.1 Definitions:
ohm-centimetre.
2.1.1 resistivity—the electrical resistance between opposite
faces of a unit cube of material; the reciprocal of conductivity.
3.3 If the current-carrying (outside) electrodes are not
Resistivity is used in preference to conductivity as an expres-
spaced at the same interval as the potential-measuring (inside)
sion of the electrical character of soils (and waters) since it is
electrodes, the resistivity, ρ, is:
expressed in whole numbers.
b
2.1.1.1 Discussion—Resistivity measurements indicate the
ρ, Ω·cm 5 95.76bR/ 1 2 (2)
S D
b1a
relative ability of a medium to carry electrical currents. When
a metallic structure is immersed in a conductive medium, the
where:
ability of the medium to carry current will influence the
b = outer electrode spacing, ft,
magnitude of galvanic currents and cathodic protection cur-
a = inner electrode spacing, ft, and
rents. The degree of electrode polarization will also affect the
R = resistance, Ω.
size of such currents.
or:
3. Summary of Test Method
b
ρ, Ω·cm 5πbR/ 1 2 (3)
S D
3.1 The Wenner four-electrode method requires that four b1a
metal electrodes be placed with equal separation in a straight
where:
line in the surface of the soil to a depth not exceeding 5 % of
b = outer electrode spacing, cm,
the minimum separation of the electrodes. The electrode
a = inner electrode spacing, cm, and
separation should be selected with consideration of the soil
R = resistance, Ω.
strata of interest. The resulting resistivity measurement repre-
3.4 For soil contained in a soil box similar to the one shown
in Fig. 1, the resistivity, ρ, is:
This test method is under the jurisdiction of ASTM Committee G01 on
Corrosion of Metals and is the direct responsibility of Subcommittee G01.10 on ρ, Ω·cm 5RA/a (4)
Corrosion in Soils.
Current edition approved May 1, 2012. Published June 2012. Originally where:
approved in 1978. Last previous edition approved in 2006 as G57–06. DOI:
R = resistance, Ω,
10.1520/G0057-06R12.
Copyright © ASTM International, 100 Barr Harbor Drive, PO Box C700, West Conshohocken, PA 19428-2959. United States
G57 − 06 (2012)
FIG. 1 Typical Connections for Use of Soil Box with Various Types of Instruments
5.1.3 Voltmeter—The voltmeter shall not draw appreciable
A = cross sectional area of the container perpendicular to the
current from the circuit to avoid polarization effects. A galva-
current flow, cm , and
nometer type of movement is preferred but an electronic type
a = inner electrode spacing, cm.
instrument will yield satisfactory results if the meter input
NOTE 1—The spacing between the inner electrodes should be measured
impedance is at least 10 megaohm.
from the inner edges of the electrode pins, and not from the center of the
5.1.4 Electrodes fabricated from mild steel or martensitic
electrodes.
3 1
stainless steel 0.475 to 0.635 cm ( ⁄16 to ⁄4 in.) in diameter and
4. Significance and Use
30 to 60 cm (1 to 2 ft) in length are satisfactory for most field
measurements. Both materials may require heat treatment so
4.1 Measurement of soil resistivity is used for the control of
that they are sufficiently rigid to be inserted in dry or gravel
corrosion of buried structures. Soil resistivity is used both for
soils. The electrodes should be formed with a handle and a
the estimation of expected corrosion rates and for the design of
terminal for wire attachment.
cathodic protection systems. As an essential design parameter
5.1.5 Wiring, 18 to 22-gage insulated stranded copper wire.
for cathodic protection systems, it is important to take as many
Terminals should be of good quality to ensure that low-
measurements as necessary so as to get a sufficiently represen-
resistance contact is made at the electrodes and at the meter.
tative characterization of the soil environment that the entire
Where regular surveys are to be made at fixed electrode
buried structure will experience.
spacing,ashieldedmulticonductorcablecanbefabricatedwith
terminals permanently located at the required intervals.
5. Apparatus
5.1 At-Grade Measurements in situ: 5.2 Soil Sample Measurement:
5.1.1 The equipment required for field resistivity measure-
5.2.1 The equipment required for the measurement of the
ments to be taken at grade consists of a current source, a
resistivity of soil samples, either in the field or in the
suitable voltmeter, ammeter, or galvanometer, four metal
laboratory, is identical to that needed for at-grade measure-
electrodes, and the necessary wiring to make the connections
ments except that the electrodes are replaced with an inert
shown in Fig. 2.
container containing four permanently mounted electrodes (see
5.1.2 Current Source—An ac source, usually 97 Hz, is
Fig. 1).
preferred since the use of dc will cause polarization of most
5.2.2 If the current-carrying (outside) electrodes are not
metalelectrodes,resultinginerror.Thecurrentcanbeprovided
spaced at the same interval as the potential-measuring (inside)
by either a cranked ac generator or a vibrator-equipped dc
electrodes, the resistivity, ρ, is:
source.An unaltered dc source can be used if the electrodes are
b
abraded to bright metal before immersion, polarity is regularly
ρ, Ω·cm 5 95.76bR/ 1 2 (5)
S D
b1a
reversed during measurement, and measurements are averaged
for each polarity. where:
G57 − 06 (2012)
FIG. 2 Wiring Diagram for Typical dc Vibrator-Current Source
boulders, concrete foundations, and so forth, which are not
b = outer electrode spacing, ft,
representative of the soil of interest, in the electrode span.
a = inner electrode spacing, ft, and
Conductive structures such as pipes and cables should not be
R = resistance, Ω.
within ⁄2 a of the electrode span unless they are at right angles
or:
to the span.
b
7.1.2 Select electrode spacings with regard to the structure
ρ, Ω·cm 5πbR/ 1 2 (6)
S D
b1a
of interest. Since most pipelines are installed at depths of from
where: 1.5 to 4.5 m (5 to 15 ft), electrode spacings of 1.5, 3.0, and 4.5
m (5, 10, and 15 ft) are commonly used. The a spacing should
b = outer electrode spacing, cm
equal the maximum depth of interest. To facilitate field
a = inner electrode spacing, cm, and
calculation of resistivities, spacings of 1.58, 3.16, and 4.75 m
R = resistance, Ω.
(5.2, 10.4, and 15.6 ft), which result in multiplication factors of
5.2.3 The dimensions of the box can be established so that
1000, 2000, and 3000, can be used when a d-c vibrator-
resistivity is read directly from the voltmeter without further
galvanometer instrument is used.
calculation. The box should be readily cleanable to avoid
7.1.3 Impress a voltage across the outer electrodes. Measure
contamination by previous samples.
the voltage drop across the inner electrodes and record both the
current and voltage drop if a separate ammeter and voltmeter
6. Standardization
are used. Where a resistivity meter is used, read the resistance
6.1 Periodically check the accuracy of resistance meters
directly and record.
using a commercial resistance decade box. Meter error should
7.1.4 Make a record of electrode spacing, resistance or
not exceed 5 % over the range of the instrument. If error
amperes and volts, date, time, air temperature, topography,
exceeds this limit, prepare a calibration curve and correct all
drainage, and indications of contamination to facilitate subse-
measurements accordingly. A soil box can be calibrated using
quent interpretation.
solutions of known resistivity. Solutions of sodium chloride
and distilled water with resistivities of 1000, 5000, and 10 000
7.2 Soil Sample Measurement:
Ω·cm are recommended for this purpose. These solutions
7.2.1 Soil samples should be representative of the area of
should be prepared under laboratory conditions using a com-
interest where the stratum of interest contains a variety of soil
mercial conductivity meter, itself calibrated to standard solu-
types. It is desirable to sample each type separately. It will also
tions at 20°C (68°F).
be necessary to prepare a mixed sample. The sample should be
reasonably large and thoroughly mixed so that it will be
7. Field Procedures
representative. The soil should be well-compacted in layers in
7.1 At-Grade Measurements:
the so
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