1D resistivity survey
A survey where resistivity changes are resolved
only in the vertical direction. In most cases, a horizontally layered earth is
assumed. Vertical electrical sounding is an example of a 1D survey.
2D resistivity survey
A survey where electrodes are installed along a
profile line and/or within a plane for cross borehole imaging, and resistivity
variations in the vertical section are resolved. 2D surveys are based on
the assumption of a two-dimensional earth resistivity model with no variation
in the direction perpendicular to the plane (usually vertical) through
the line of electrodes.
3D resistivity survey
A survey where data is collected over an area
and/or in bore holes to determine resistivity variations in a volume.
A weighted average of the resistivities under four
electrodes A, B, M, N. Weighted because a larger part of the current
takes a path close to the surface, and average because the current travels
through all the material below the four electrodes. The apparent resistivity
is the same as the true resistivity if the ground is homogeneous
below the four electrodes.
Depends upon the electrode spread length and type of electrode array.
Typically the depth penetration is in the range 15-30% of the total electrode
spread length (before roll-along).
Dipole-dipole electrode array
An array where all electrodes are placed in line. The two current electrodes
are placed close together (as a dipole) and the the two potential electrodes are placed
closed together (as a dipole). The distance between the two dipoles is typically equal to or larger than the dipole size.
For one measurement, at least four electrodes are
needed. By tradition, these electrodes are placed in different geometric
configurations, where each configuration has its own name. For example,
Schlumberger, Wenner, or dipole-dipole configuration.
Calculate voltage or apparent resisistivity data from a
synthetic geological model with a particular array configuration.
Fox, Robert W. (1789-1877)
A British geologist and inventor who performed electrical geophysical
measurements already in 1830 in underground copper mines in Cornwall, England.
A remote electrode placed at such a distance that its
electric field does not distort the electric field in the survey area. The
infinity electrode is located away from the survey area and typically at a
distance of 5-10 times the largest electrode spacing of the survey area.
Calculate resistivity distribution from the field data,
that can then be related to the subsurface geology.
Induced Polarization. Measuring the voltage decay curve after the
injected current is shut off. This method has been extremely successful to
locate disseminated copper ore bodies.
A limestone or dolomitic area signified by caves, sinkholes and dissollution channels.
Named after the Karst area in Slovenia, where this type of terrain can be seen as
large caves, huge sinkholes and underground rivers.
Unit of electrical conductance. The reciprocal relationship between
conductance and resistance is indicated by spelling ohm backwards.
French meaning “charged body.” In this method one
current electrode is attached to a conductor, an ore body for example,
and the other current electrode is placed at infinity. The extent of the
body is then mapped by moving the two potential electrodes along the
surface, over the body.
A resistivity system with more than one receiver
so that more than one reading is taken for each current injection. For
example, an 8-channel instrument may record up to 8 measurements for
each current injection. A multi-channel instrument is faster than a single
A resistivity system where more than four electrodes
are placed on a line or in a grid. For each measurement at least
four electrodes are used, two for current injection and two for potential
measurement. The advantage is that the electrodes do not need to be
moved during the survey, thus saving time and reducing the chance of
An electrode which is designed so that there
will be no potential caused by electrochemical reaction between the electrode
and the ground. Also known as porous pot.
Unit for resistance. Named in honor of the German physicist Georg
Simon Ohm (1789-1854).
Unit for resistivity. Resistivity is a property of matter.
States that the strength of an electrical current is directly proportional
to the electro-motive force (potential), and inversely proportional
to the resistance of the circuit.
Pole-dipole electrode array
A method where one current electrode is placed at "infinitiy". In reality
this is of course not possible, so it is placed at a distance which should
be at least 10 times the size of the survey area. The other current electrode
is placed in the survey area and the potential dipole is used to measure the
resulting voltage, as the dipole is stepped away from the current electrode.
Pole-pole electrode array
Similar to the pole-dipole array, but two "infinity" electrodes are used, one
for current and one for potential. The two infinity electrodes are placed at
opposite side of the survey area. The voltage is measured as the single
voltage electrode is stepped away from the current electrode within the
A method where the four electrodes A, B, M, N are moved forward,
along a line, for each measurement, without changing the geometry
of the electrodes. This method is used to detect lateral sub-surface
Reciprocity, theorem of
The potential between two electrodes M and N,
caused by a current being injected between two electrodes A and B, is
the same, if the potential is measured between A and B and the same
current injected between M and N.
A method that can be seen as a data aquisition
combination of sounding and profiling. To collect data cost-effectively a
multi-electrode system is typically used for this type of survey. The complete
data set is inverted from apparent resistivity to resistivity using a
resistivity inversion software based on the finite element or finite difference
A technique where some of the first electrodes are moved to
the end of the line when the initial survey is ready. This way the survey
line can be extended even though there is a limited number of electrodes
at a fixed spacing. The electrodes can be moved forward in a leap frog
Schlumberger, Conrad (1878-1936)
Performed a series of electrical
experiments in Normandy 1912-1914. Founded the Schlumberger Well
Surveying Corporation in 1934.
Schlumberger electrode array
An array where the two outer electrodes
are current electrodes and the two inner electrodes placed close together
in the middle are the potential electrodes.
Unit for conductance. Named in honor of the German inventor
Werner von Siemens (1816-1892).
Self potential, also known as, spontaneous potential or streaming
potential. The natural voltage existing in the ground without any artificial
StrongGradient™ electrode array is used when performing resistivity
imaging in order to measure the electrical field gradient between electrodes
A and B. The electrodes A and B are placed 10 electrode spacings apart,
starting at electrodes 1 and 11. The resulting voltage is measured between
electrodes 2-3, 3-4,.....8-9 and 9-10. This procedure is repeated as the array
is moved one electrode spacing down the line until the end of the line.
Time lapse measurement
Repeating a resistivity survey to determine
the resistivity changes with time, that may be caused by contaminant
migration, groundwater recharge, salt-water intrusion, or remediation
process. A repeating 3D resistivity survey is also called a 4D resistivity
survey with the 4th dimension being time.
Vertical Electrical Sounding. A method where the electrodes are
expanded around the mid-point for each measurement. By expanding the
electrodes the current takes a somewhat deeper path so that a 1D interpretation
of the sub-surface can be achieved. This method is sometimes
also referred to as “electrical drilling.”
Wenner electrode array
The four electrodes are placed in a line with an
equal spacing, the two outer electrodes are the current electrodes and
the two inner ones are potential elelctrodes.
Wenner, Frank (1873-1954)
American physicist at the US Bureau of Standards who
deviced the Wenner configuration of electrodes. Wenner also gave a
clear statement of the theorem of reciprocity in 1912.