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FACT SHEET
- ELECTRICAL RESISTIVITY PROFILING
RUDEN AS provides a wide range of
geophysical services, mainly related to
water, geological and environmental
prospecting as well as mapping of
natural resources. One of our main
geophysical services is Electrical
Resistivity Profiling (ERT).
Overview
Resistivity profiling method is a non-invasive tool
for subsurface geological, geophysical and
hydrological exploration. It is capable of depict
the underground resistivity distribution in 2D
2.5D or 3D without the need for digging or
drilling.
The measured apparent resistivity of the
subsurface formations is the combined value of
the rock and the pore-filling material.
Changes in the subsurface conditions can be
seen if there is a contrast between these
combined values (either because of the change
in the pore-filling material or the host rock).
During electrical resistivity profiling direct current
is injected into the ground and the induced
electric potential is measured by electrodes
planted into the ground. In modern equipment
that have switching capability the current and
potential electrodes are chosen automatically
from
numerous
preinstalled
electrodes,
accelerating the measuring procedure very
effectively.
Electrical resistivity imaging is a very effective
tool for shallow to medium depth underground
exploration in:
 Water prospecting
o Determine groundwater level
o Locate underground water aquifers
o Locate underground formations that are
suitable aquifers
o Locate underground aquitard (clay)
layers
o Differentiate between fresh and salt
water reservoirs
o Determine fresh to salt water surface
 Environmental studies
o Delineate contaminant plumes (e.g. oil
spill or landfill leachate)
o Assess quick clay hazard
o Environmental baseline studies and
monitoring of dams, landfills, mines
o Monitoring changes in groundwater level
and flow (induced by construction work or
any kind of intervention to natural
processes)
 Structural and lithological surveying
o Determine depth to bedrock
o Fault and dyke investigation
o Exploration of sedimentary layering –
differentiation by grain size
 Ore prospecting
 Cavity and cave exploration
Inverted resistivity distribution of a 2D profile
2D resistivity measurement layout and electrode
connection to the main cable
RUDEN AS
Moloveien 2.
N-1628 Engelsviken
NORWAY
Inverted
resistivity
distribution
of a
3D profile
Tel:
+47 69 36 41 41
Cell:
+47 97 69 85 12
Email: [email protected]
Methodology
An electrical resistivity profiling survey is
composed of sometimes several thousands of
single measurements (called quadripoles). In a
single quadripole 4 electrodes are used. 2
electrodes are used for injecting current into the
ground while the other two are used to measure
the induced voltage difference between them (in
multichannel equipment several potential
electrode pairs can be used simultaneously for
potential measurements).
electrodes. According to these variations the
apparent resistivity values can be drawn into a
pseudosection. Pseudosections give a quick
glance how resistive the ground is, but has to be
treated with suspicion as it is not the true
resistivity.
Design of a 3D resistivity survey (apparent
resistivity will be measured for every red point)
Quadripole
Depending on the actual spacing and
arrangement of the current and potential
electrode pairs different types of arrays exist:






Wenner
Schlumberger
Dipole-dipole
Pole-dipole
Pole-pole
and many more
The length of an electrical resistivity profile is not
limited by the maximum electrodes that a
certain equipment can handle at a same time. In
various modern equipment the rolling technique
is built in. At rolling the first set of electrodes
(connected to the same cable) are moved to the
end of the cable layout and the new
measurement starts. However, only the new
quadripoles – includes at least one electrode
from the moved cable – will be measured.
The rolling concept
Electrode configuration of some array types
The measured apparent resistivity is then the
product of an array dependent parameter (k,
geometric factor) and the measured resistance
between the potential electrodes. This value is
not the true underground resistivity, but the
resistivity of a homogeneous half space that
would give the same reading for the same array
as the actual measurement.
Measuring several quadripoles in a row gives
many apparent resistivity values that correspond
to different parts of the measured profile
depending on the spacing between the
RUDEN AS
Moloveien 2.
N-1628 Engelsviken
NORWAY
To obtain the true resistivity values the
pseudosection has to be inverted. The inversion
is an iterative process where a resistivity model
of the subsurface is built up and its response to
the measurement process is calculated. If the
actual measurement values and the calculated
model response are closer than a threshold
value the iteration stops and the last resistivity
model will be the result of the inversion.
2D inversion in the RES2DINV software
Tel:
+47 69 36 41 41
Cell:
+47 97 69 85 12
Email: [email protected]