ASTM G57-95a(2001)
(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:G57–95a (Reapproved 2001)
Standard Test Method for
Field Measurement of Soil Resistivity Using the Wenner
1
Four-Electrode Method
ThisstandardisissuedunderthefixeddesignationG 57;thenumberimmediatelyfollowingthedesignationindicatestheyearoforiginal
adoption or, in the case of revision, the year of last revision.Anumber in parentheses indicates the year of last reapproval.Asuperscript
epsilon (e) indicates an editorial change since the last revision or reapproval.
1. Scope strata of interest. The resulting resistivity measurement repre-
sents the average resistivity of a hemisphere of soil of a radius
1.1 This method covers the equipment and procedures for
equal to the electrode separation.
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. Alterna-
1.2 To convert cm (metric unit) to metre (SI unit), divide by
tively, the resistance can be measured directly. The resistivity,
100.
r, is then:
1.3 This standard does not purport to address all of the
safety concerns, if any, associated with its use. It is the r,V·cm 5 2p aR ~a in cm!
responsibility of the user of this standard to establish appro-
5 191.5 aR a in ft!
~
priate safety and health practices and determine the applica-
bility of regulatory limitations prior to use.
where:
a = electrode separation, and
2. Terminology
R = resistance, V.
2.1 Definition:
Using dimensional analysis, the correct unit for resistivity is
2.1.1 resistivity—the electrical resistance between opposite
ohm-centimetre.
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, r is:
expressed in whole numbers.
b
2.1.2 Resistivity measurements indicate the relative ability
r, V·cm 5 95.76bR/ 1 2
S D
b 1 a
of a medium to carry electrical currents. When a metallic
structure is immersed in a conductive medium, the ability of
where:
the medium to carry current will influence the magnitude of
b = outer electrode spacing, ft,
galvanic currents and cathodic protection currents. The degree
a = inner electrode spacing, ft, and
of electrode polarization will also affect the size of such
R = resistance, V.
currents.
or:
b
3. Summary of Test Method
r, V·cm5pbR/ 1 2
S D
b 1 a
3.1 The Wenner four-electrode method requires that four
metal electrodes be placed with equal separation in a straight where:
b = outer electrode spacing, cm,
line in the surface of the soil to a depth not exceeding 5 % of
a = inner electrode spacing, cm, and
the minimum separation of the electrodes. The electrode
R = resistance, V.
separation should be selected with consideration of the soil
3.4 For soil contained in a soil box similar to the one shown
in Fig. 1, the resistivity, r, is:
1
This method is under the jurisdiction of ASTM Committee G01 on Corrosion
r, V·cm 5RA/a
of Metals, and is the direct responsibility of Subcommittee G01.10 on Corrosion in
Soils.
Current edition approved April 15, 1995. Published June 1995. Originally
published as G 57 – 78. Last previous edition G 57 – 95.
Copyright © ASTM International, 100 Barr Harbor Drive, PO Box C700, West Conshohocken, PA 19428-2959, United States.
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G57–95a (2001)
FIG. 1 Typical Connections for Use of Soil Box with Various Types of Instruments
4.1.1 The equipment required for field resistivity measure-
where:
ments to be taken at grade consists of a current source, a
R = resistance, V,
A = cross sectional area of the container perpendicular to
suitable voltmeter, ammeter, or galvanometer, four metal
2
the current flow, cm , and
electrodes, and the necessary wiring to make the connections
a = inner electrode spacing, cm.
shown in Fig. 2.
NOTE 1—The spacing between the inner electrodes should be measured 4.1.2 Current Source—An ac source, usually 97 Hz, is
from the inner edges of the electrode pins, and not from the center of the
preferred since the use of dc will cause polarization of most
electrodes.
metalelectrodes,resultinginerror.Thecurrentcanbeprovided
by either a cranked ac generator or a vibrator-equipped dc
4. Apparatus
source.An unaltered dc source can be used if the electrodes are
4.1 At-Grade Measurements in situ:
FIG. 2 Wiring Diagram for Typical dc Vibrator-Current Source
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G57–95a (2001)
abraded to bright metal before im
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