Investigation Methods and Procedures - Geotechnical Engineering Investigation - page 57

Theoretical Basis
Various subsurface materials have characteristic conductances for direct currents of elec-
tricity. Electrolytic action, made possible by the presence of moisture and dissolved salts
within the soil and rock formations, permits the passage of current between electrodes
placed in the surface soils. In general, conductance is good in materials such as moist clays
and silts, and poor in materials such as dry loose sands, gravels, and sound rocks.
Resistivity refers to the resistance to current flow developed in geologic materials and is
expressed as Ω cm 2 /cm, or simply as Ω cm or Ω ft. Some typical values of resistivity for

various geologic materials are given in Table 2.11.

Apparatus
The electrical resistivity apparatus consists of a battery as energy source, a milliammeter,
a potentiometer, and electrodes ( Figure 2.31) . There are three basic electrode configura-
tions as shown in Figure 2.32.
Wenner array: Commonly used in the United States, it employs four equally spaced elec-
trodes. All four electrodes are moved between successive operations.
Schlumberger array: Commonly used in Europe, it is similar to the Wenner, except that
the spacing between the two center electrodes is made smaller than that between the other
two. In operation, the inner electrodes remain fixed and the outer electrodes are moved.
The test is repeated changing the spacing between the inner electrodes.
Dipole-dipole array: The source dipole is separated from the receiving dipole and the dis-
tances between the two dipoles are varied.
Pole-dipole array (not shown in Figure 2.32): One of the current electrodes is placed at a
distance from the survey area approximately 5 to 10 times the desired survey depth. The
receiving dipoles are stepped away from the current electrode, providing resistivity meas-
urements that are reasonably representative of media encountered at a specific depth and
distance from the current electrode.

Operational Procedures
With a battery as a direct-current source, a current flow is established between the two
outer electrodes. The current drop is detected by the two inner electrodes and recorded on

TABLE 2.11
Typical Resistivity Values for Geologic Materials a

Resistivity

Materials Ω ft Ω m

Clayey soils: wet to moist 5-10 1.5-3.0

Silty clay and silty soils: wet to moist 10-50 3-15

Silty and sandy soils: moist to dry 50-500 15-150

Bedrock: well fractured to slightly fractured with 500-1000 150-300

moist soil-filled cracks
Sand and gravel with silt About 1000 About 300

Sand and gravel with silt layers 1000-8000 300-2400

Bedrock: slightly fractured with dry soil-filled cracks 100-8000 300-2400

Sand and gravel deposits: coarse and dry 8000 2400

Bedrock: massive and hard 8000 2400

Freshwater 67-200 20-60

Seawater 0.6-0.8 0.18-0.24

a From Soilest, Inc.
Note:

(1) In soils, resistivity is controlled more by water content than by soil minerals. (2) The resistivity of the
pore or cleft water is related to the number and type of dissolved ions and the water temperature.