# Glossary

## 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.

## Apparent resistivity

A weighted average of the resistivity 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.

## Archie’s law

In petrophysics, Archie's law relates the in-situ electrical conductivity of a sedimentary rock to its porosity and brine saturation. (Source: Wikipedia)

## Depth penetration

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.

## Electrode arrays

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.

## Forward modeling

Calculate voltage or apparent resistivity 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.

## Infinity electrode

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.

## Inverse modeling

Calculate resistivity distribution from the field data, that can then be related to the subsurface geology.

## IP

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.

## Karst topography

A limestone or dolomitic area signified by caves, sinkholes and dissolution channels. Named after the Karst area in Slovenia, where this type of terrain can be seen as large caves, huge sinkholes and underground rivers.

## Mho

Unit of electrical conductance. The reciprocal relationship between conductance and resistance is indicated by spelling ohm backwards.

## Mise a-la-masse

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.

## Multi-channel system

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 channel instrument.

## Multi-electrode system

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 human error.

## Non-polarisable electrode

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.

## Ohm

Unit for resistance. Named in honor of the German physicist Georg Simon Ohm (1789-1854).

## Ohmmeter

Unit for resistivity. Resistivity is a property of matter.

## Ohms law

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 "infinity". 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 survey area.

## Profiling

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 changes.

## 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.

## Resistivity imaging

A method that can be seen as a data acquisition 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 methods.

## Roll along

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 fashion indefinitely.

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.

## Siemens

Unit for conductance. Named in honor of the German inventor Werner von Siemens (1816-1892).

## SP

Self potential, also known as, spontaneous potential or streaming potential. The natural voltage existing in the ground without any artificial current injection.

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.

## VES

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 electrodes.

## Wenner, Frank (1873-1954)

American physicist at the US Bureau of Standards who devised the Wenner configuration of electrodes. Wenner also gave a clear statement of the theorem of reciprocity in 1912.