The electrical resistivity method uses four electrodes to measure the resistivity of soil and rock formations. Resistivity depends on factors like material type, water content, and salt concentration. Rocks have higher resistivity than saturated clays. This method involves expanding the electrode spacing to measure resistivity at increasing depths. It can detect subsurface variations and locate deposits like sand and gravel within fine-grained soils. Interpretation can be challenging as resistivity changes gradually at interfaces rather than abruptly.

1. Electrical Resistivity Method
Submitted by Abhineet Godayal
M.Tech Geotechnical Engineering

2.  The resistivity of a material depends upon the
type of material, its water content,
concentration of dissolved ions and many
other factors
 Rocks and dry soils have a greater resistivity
than saturated clays
 Therefore resistivity of soil decreases as both
water content and concentration of salt
increases.

3. Type of
Rock Soil
Sound
Rock
Weathered
Rock
Gravel Sand Clayey
Sand
Saturated
Clay and Silt
Resistivity
(ohm-m)
>5000 1500 to
2500
1500 to
4500
500 to
1500
200 to
500
2 to 100

4.  In this method four electrodes are used at
constant spacing “a”
 To conduct this test ,four electrodes in the form
of metal spikes are driven in to the ground.
 Two outer electrodes are called current
electrodes and inner two electrodes are potential
electrodes.
 The mean resistivity of strata is determined by
applying D.C. to outer electrodes and measuring
the potential drop between inner electrodes.

5. --This gives
mean resistivity
up to depth a.
--This test is
repeated after
changing the
spacing and
again
determining
the mean
resistivity for
new spacing.
Fig 1.Wenner arrangement

6.  Electrical Profiling Method is used for locating
sand and gravel deposits with in fine-grained
soil deposit.
Electrical Resistivity Measurements

7.  In this method, the electrode system
consisting of four electrodes is expanded
about a fixed point.
 The spacing in first sitting is a1, which is
increased to a2 in the second sitting and to a3
in the third sitting.
 The spacing is thus increased to depth of
exploration

8. Fig 2.Electrode arrangement for electrical
sounding method
--This method is
useful in studying the
changes in the strata
with increasing depth
at a point.
--This method can
indicate the subsurface
variation when a hard
layer overlies soft layer
or vice versa.

9.  The method capable of detecting only the
strata having different electrical resistivity.
 The results considerably influenced by
surface irregularities, wetness of strata and
electrolytic concentration of ground water
 As the resistivity of different strata at
interface changes gradually and not abruptly
as assumed, the interpretation of results
becomes difficult.