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.
General Information
Relations
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NOTICE: This standard has either been superseded and replaced by a new version or withdrawn.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:G57–95a (Reapproved 2001)
Standard Test Method for
Field Measurement of Soil Resistivity Using the Wenner
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:
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.
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
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
G57–95a (2001)
abraded to bright metal before immersion, polarity is regularly V·cm are recommended for this purpose. These solutions
reversed during measurement, and measurements are averaged should be prepared under laboratory conditions using a com-
for each polarity. mercial conductivity meter, itself calibrated to standard solu-
4.1.3 Voltmeter—The voltmeter shall not draw appreciable tions at 20°C (68°F).
current from the circuit to avoid polarization effects. A galva-
6. Field Procedures
nometer type of movement is preferred but an electronic type
instrument will yield satisfactory results if the meter input
6.1 At-Grade Measurements:
impedance is at least 10 megaohm. 6.1.1 Select the alignment of the measurement to include
4.1.4 Electrodes fabricated from mild steel or martensitic uniform topography over the limits of the electrode span. Do
3 1
stainless steel 0.475 to 0.635 cm ( ⁄16 to ⁄4 in.) in diameter and not include large nonconductive bodies such as frozen soil,
30 to 60 cm (1 to 2 ft) in length are satisfactory for most field
boulders, concrete foundations, etc., which are not representa-
measurements. Both materials may require heat treatment so tive of the soil of interest, in the electrode span. Conductive
that they are sufficiently rigid to be inserted in dry or gravel
structures such as pipes and cables should not be within ⁄2 a of
soils. The electrodes should be formed with a handle and a the electrode span unless they are at right angles to the span.
terminal for wire attachment.
6.1.2 Select electrode spacings with regard to the structure
4.1.5 Wiring, 18 to 22-gage insulated stranded copper wire. of interest. Since most pipelines are installed at depths of from
Terminals should be of good quality to ensure that low- 1.5 to 4.5 m (5 to 15 ft), electrode spacings of 1.5, 3.0, and 4.5
resistance contact is made at the electrodes and at the meter. m (5, 10, and 15 ft) are commonly used. The a spacing should
Where regular surveys are to be made at fixed electrode equal the maximum depth of interest. To facilitate field
spacing,ashieldedmulticonductorcablecanbefabricatedwith calculation of resistivities, spacings of 1.58, 3.16, and 4.75 m
terminals permanently located at the required intervals. (5.2, 10.4, and 15.6 ft), which result in multiplication factors of
4.2 Soil Sample Measurement: 1000, 2000, and 3000, can be used when a d-c vibrator-
galvanometer instrument is used.
4.2.1 The equipment required for the measurement of the
resistivity of soil samples, either in the field or in the 6.1.3 Impress a voltage across the outer electrodes. Measure
the voltage drop across the inner electrodes and record both the
laboratory, is identical to that needed for at-grade measure-
ments except that the electrodes are replaced with an inert current and voltage drop if a separate ammeter and voltmeter
are used. Where a resistivity meter is used, read the resistance
container containing four permanently mounted electrodes (see
Fig. 1). directly and record.
6.1.4 Make a record of electrode spacing, resistance or
4.2.2 If the current-carrying (outside) electrodes are not
spaced at the same interval as the potential-measuring (inside) amperes and volts, date, time, air temperature, topography,
drainage, and indications of contamination to facilitate subse-
electrodes, the resistivity, r, is:
quent interpretation.
b
r,V·cm 5 95.76bR / 1 2
S D 6.2 Soil Sample Measurement:
b 1 a
6.2.1 Soil samples should be representative of the area of
where:
interest where the stratum of interest contains a variety of soil
b = outer electrode spacing, ft,
types. It is desirable to sample each type separately. It will also
a = inner electrode spacing, ft, and
be necessary to prepare a mixed sample. The sample should be
R = resistance, V.
reasonably large and thoroughly mixed so that it will be
or:
representative. The soil should be well-compacted in layers in
the soil box, with air spaces eliminated as far as practicable.
b
r,V·cm5pbR / 1 2
S D
b 1 a
Fill the box flush to the top and take measurements as
previously detailed (6.1.3).The meter used may limit the upper
where:
range of resistivity, which can be measured. In such cases, the
b = outer electrode spacing, cm
resistivity should be recorded as <10 000 V·cm, etc.
a = inner electrode spacing, cm, and
6.2.2 The measured resistivity will be dependent on the
R = resistance, V.
degree of compaction, moisture content, constituent solubility,
4.2.3 The dimensions of the box can be established so that
and temperature. The effect of variations in compaction and
resistivity is read directly from the voltmeter without further
moisture content can be reduced by fully saturating
...
Questions, Comments and Discussion
Ask us and Technical Secretary will try to provide an answer. You can facilitate discussion about the standard in here.