ASTM G57-95A
(Test Method)Standard Test Method for Field Measurement of Soil Resistivity Using the Wenner Four-Electrode Method
Standard Test Method for Field Measurement of Soil Resistivity Using the Wenner Four-Electrode Method
SCOPE
1.1 This method covers the equipment and procedures for the field measurement of soil resistivity, both in situand 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.
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Designation: G 57 – 95a
Standard Test Method for
Field Measurement of Soil Resistivity Using the Wenner
Four-Electrode Method
This standard is issued under the fixed designation G 57; the number immediately following the designation indicates the year of original
adoption or, in the case of revision, the year of last revision. A number in parentheses indicates the year of last reapproval. A superscript
epsilon (e) indicates an editorial change since the last revision or reapproval.
1. Scope electrodes is measured using a sensitive voltmeter. Alterna-
tively, the resistance can be measured directly. The resistivity,
1.1 This method covers the equipment and procedures for
r, is then:
the field measurement of soil resistivity, both in situ and for
samples removed from the ground, for use in the control of r,V·cm 5 2p aR ~a in cm!
corrosion of buried structures.
5 191.5 aR~a in ft!
1.2 This standard does not purport to address all of the
safety concerns, if any, associated with its use. It is the
where:
responsibility of the user of this standard to establish appro-
a 5 electrode separation, and
priate safety and health practices and determine the applica-
R 5 resistance, V.
bility of regulatory limitations prior to use.
Using dimensional analysis, the correct unit for resistivity is
1.3 To convert cm (metric unit) to metre (SI unit), divide by
ohm-centimetre.
100.
3.3 If the current-carrying (outside) electrodes are not
spaced at the same interval as the potential-measuring (inside)
2. Terminology
electrodes, the resistivity, r is:
2.1 Definition:
b
2.1.1 resistivity—the electrical resistance between opposite
r, V·cm 5 95.76 bR/ 1 2
S D
b 1 a
faces of a unit cube of material; the reciprocal of conductivity.
Resistivity is used in preference to conductivity as an expres-
where:
sion of the electrical character of soils (and waters) since it is
b 5 outer electrode spacing, ft,
expressed in whole numbers.
a 5 inner electrode spacing, ft, and
2.1.2 Resistivity measurements indicate the relative ability
R 5 resistance, V.
of a medium to carry electrical currents. When a metallic or:
structure is immersed in a conductive medium, the ability of
b
r, V·cm5p bR/ 1 2
the medium to carry current will influence the magnitude of S D
b 1 a
galvanic currents and cathodic protection currents. The degree
where:
of electrode polarization will also affect the size of such
b 5 outer electrode spacing, cm,
currents.
a 5 inner electrode spacing, cm, and
3. Summary of Test Method
R 5 resistance, V.
3.4 For soil contained in a soil box similar to the one shown
3.1 The Wenner four-electrode method requires that four
in Fig. 1, the resistivity, r, is:
metal electrodes be placed with equal separation in a straight
line in the surface of the soil to a depth not exceeding 5 % of r, V·cm 5 RA/a
the minimum separation of the electrodes. The electrode
where:
separation should be selected with consideration of the soil
R 5 resistance, V,
strata of interest. The resulting resistivity measurement repre-
A 5 cross sectional area of the container perpendicular to
sents the average resistivity of a hemisphere of soil of a radius
the current flow, cm , and
equal to the electrode separation.
a 5 inner electrode spacing, cm.
3.2 A voltage is impressed between the outer electrodes,
causing current to flow, and the voltage drop between the inner NOTE 1—The spacing between the inner electrodes should be measured
from the inner edges of the electrode pins, and not from the center of the
electrodes.
This method is under the jurisdiction of ASTM Committee G-1 on Corrosion of
Metals, and is the direct responsibility of Subcommittee G01.10 on Corrosion in
4. Apparatus
Soils.
Current edition approved April 15, 1995. Published June 1995. Originally 4.1 At-Grade Measurements in situ:
published as G 57 – 78. Last previous edition G 57 – 95.
Copyright © ASTM, 100 Barr Harbor Drive, West Conshohocken, PA 19428-2959, United States.
G57
FIG. 1 Typical Connections for Use of Soil Box with Various Types of Instruments
4.1.1 The equipment required for field resistivity measure- 4.1.3 Voltmeter—The voltmeter shall not draw appreciable
ments to be taken at grade consists of a current source, a current from the circuit to avoid polarization effects. A galva-
suitable voltmeter, ammeter, or galvanometer, four metal nometer type of movement is preferred but an electronic type
electrodes, and the necessary wiring to make the connections instrument will yield satisfactory results if the meter input
shown in Fig. 2. impedance is at least 10 megaohm.
4.1.2 Current Source—An ac source, usually 97 Hz, is 4.1.4 Electrodes fabricated from mild steel or martensitic
3 1
preferred since the use of dc will cause polarization of most stainless steel 0.475 to 0.635 cm ( ⁄16 to ⁄4 in.) in diameter and
metal electrodes, resulting in error. The current can be provided 30 to 60 cm (1 to 2 ft) in length are satisfactory for most field
by either a cranked ac generator or a vibrator-equipped dc measurements. Both materials may require heat treatment so
source. An unaltered dc source can be used if the electrodes are that they are sufficiently rigid to be inserted in dry or gravel
abraded to bright metal before immersion, polarity is regularly soils. The electrodes should be formed with a handle and a
reversed during measurement, and measurements are averaged terminal for wire attachment.
for each polarity. 4.1.5 Wiring, 18 to 22-gage insulated stranded copper wire.
FIG. 2 Wiring Diagram for Typical dc Vibrator-Current Source
G57
Terminals should be of good quality to ensure that low- 6.1.2 Select electrode spacings with regard to the structure
resistance contact is made at the electrodes and at the meter. of interest. Since most pipelines are installed at depths of from
Where regular surveys are to be made at fixed electrode 1.5 to 4.5 m (5 to 15 ft), electrode spacings of 1.5, 3.0, and 4.5
spacing, a shielded multiconductor cable can be fabricated with m (5, 10, and 15 ft) are commonly used. The a spacing should
terminals permanently located at the required intervals. equal the maximum depth of interest. To facilitate field
4.2 Soil Sample Measurement: calculation of resistivities, spacings of 1.58, 3.16, and 4.75 m
4.2.1 The equipment required for the measurement of the (5.2, 10.4, and 15.6 ft), which result in multiplication factors of
resistivity of soil samples, either in the field or in the 1000, 2000, and 3000, can be used when a d-c vibrator-
laboratory, is identical to that needed for at-grade measure- galvanometer instrument is used.
ments except that the electrodes are replaced with an inert
6.1.3 Impress a voltage across the outer electrodes. Measure
container containing four permanently mounted electrodes (see
the voltage drop across the inner electrodes and record both the
Fig. 1).
current and voltage drop if a separate ammeter and voltmeter
4.2.2 If the current-carrying (outside) electrodes are not
are used. Where a resistivity meter is used, read the resistance
spaced at the same interval as the potential-measuring (inside)
directly and record.
electrodes, the resistivity, r, is:
6.1.4 Make a record of electrode spacing, resistance or
amperes and volts, date, time, air temperature, topography,
b
r,V·cm 5 95.76 bR / 1 2
S D
b 1 a drainage, and indications of contamination to facilitate subse-
quent interpretation.
where:
6.2 Soil Sample Measurement:
b 5 outer electrode spacing, ft,
6.2.1 Soil samples should be representative of the area of
a 5 inner electrode spacing, ft, and
interest where the stratum of interest contains a variety of soil
R 5 resistance, V.
types. It is desirable to sample each type separately. It will also
or:
be necessary to prepare a mixed sample. The sample should be
b
reasonably large and thoroughly mixed so that it will be
r,V·cm5pbR / 1 2
S D
b 1 a
representative. The soil should be well-compacted in layers in
the soil box, with air spaces eliminated as far as practicable.
where:
b 5 outer electrode spacing, cm Fill the box flush to the top and take measurements as
a 5 inner electrode spacing, cm, and previously detailed (6.1.3). The meter used may limit the upper
R 5 resistance, V.
range of resistivity, which can be measured. In such cases, the
4.2.3 The dimensions of the box can be established so that
resistivity should be recorded as <10 000 V·cm, etc.
resistivity is read directly from the voltmeter without further
6.2.2 The measured resistivity will be dependent on the
calculation. The box should be readily cleanable to avoid
degree of compaction, moisture content, constituent solubility,
contamination by previous samples.
and temperature. The effect of variations in compaction and
moisture content can be reduced by fully saturating the sample
5. Standardization
before placing it in the box. This can be done by preparing a
5.1 Periodically check the accuracy of resistance meters
stiff slurry of the sample, adding only sufficient water to
using a commercial resistance decade box. Meter error should
produce a slight amount of surface water, which should be
not exceed 5 % over the range of the instrument. If error
allowed to evaporate before the slurry is remixed and placed in
exceeds this limit, prepare a calibration curve and correct all
the box. Where available, use ground water from the sample
measurements accordingly. A soil box can be calibrated using
ex
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