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Glossary of Terminology used in ERTLab

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ERTLab employs the following termininology for aspects of ERT, XIP, computer systems and data:

3D
Three dimensional or three dimensions. This is a reference to maps of volume or space that use 3 orthogonal cartesian coordinates (XYZ) to identify a point location.

2D
Two dimensional or two dimensions. This is a reference to maps of volume or space that use 2 orthogonal cartesian coordinates (typically X and Z) to identify a point location. A single straight line of data collected at surface is usually considered to be a two dimensional dataset because the target area is directly below the survey line.

Absolute Roughness
See Roughness.

ABMN
Common four electrode (quadrapole) array designation. A and B are the transmitter electrodes, M and N are the receiver electrodes.

Apparent Resistivity
Resistivity of a halfspace the corresponds to an individual quadrapole configuration.

Blocks
Used interchangeably with elements.

Background Region
The portion of the mesh outside the main area of interest. The background region is used when simulating (modeling) a real earth to extend the mesh in +/- X, +/- Y and -Z directions to create a pseudo-infinite halfspace for mitigation of boundary effects.

Boundary Conditions
The first and last elements of a mesh may terminate in a variety of electrically reasonable ways. The most obvious is no-electrical current flow outside the boundary, but zero-voltage (grounded) is also reasonable, and mixed conditions are often implmented.

Cable
Any logical grouping of electrodes (e.g. one multiconductor cable with takeouts or one instrumented borehole may be identified as a single cable).

Chargeability
Induced polarization measured in the time domain, a physical property often thought of interms of capacitance or the ability of the earth to store a charge. Units are expressed as off-time voltage times a factor of 1000, divided by on-time voltage (mV/V). The measurement of chargeability is complex and no clear standard is available, especially when waveforms other than the .125 Hz rectangular wave (2 seconds on / 2 seconds off) are used. Usually the off-time voltage  is measured as a series of 'windows' values collected a different delays after turn-off but before the transmitter turns on again. These windows are most commonly averaged so that an equivalent value for the approximate middle of the turn-off period is presented.

Convergence
A measure of model fit. When the the model has fit the data we say it has converged. The term is used because at each iteration the model comes closer and closer to (converges) on a reasonable fit. We compare the total number of ERT data to the data error (misfit or residual) as a means of determining whether the model has converged. This is commonly called the ?2error.

Data Residual
The root of the sum of the squares of the difference between input data and modeled data.

Dipole
A pair of electrodes. The name is not typically applied unless the electrode pair is used as either a transmitter or receiver.

Electrode
Any electrical connection to the earth. This may be accomplished using stainless steel,  grounding rod, graphite (usually used in marine environments), lead, silver-silver chloride (usually used in lab experiments), titanium, etc.

Element
A 3D tetrahedral space that is assigned a unique resistivity or IP value.

EIT
Electrical Impedance Tomography. The practice of collecting resistivity data and inverting them to produce a tomographic or 3D image.

ERT
Electrical Resistivity Tomography. The practice of collecting resistivity data and inverting them to produce a tomographic or 3D image.

Foreground Region
The portion of the mesh that encompases the main area of interest.

Forward modeling
Forward modeling is the process of creating a data suite (schedule) from a 3D resistivity dispersion (in a 3D mesh).

Flag
Programmers terminology for a record (data field) that contains a number or string (a word) that provides the answer to a question that typically has a small set of answers, usually Yes / No, or True/False. For example we implement a skip flag of 0 to indicate the datapoint should be skipped and a 1 to indicate it should be used.

Flat Grid
Modeling mesh created without topography (i.e. the top or +Z direction is flat).

Halfspace
A semi-infinite region divided in two with half the volume assigned a constant finite resistivity and half the volume assigned a constant infinite resistivity.

Inversion
Inversion is the process of creating a resistivity dispersion (in a 3D mesh) that matches  the  data available.

IP
Induced polarization, a physical property often thought of interms of capacitance or the ability of the earth to store a charge. Units are chargeability, expressed as off-time voltage times a factor of 1000, divided by on-time voltage (mV/V).

Iteration
Cycle. During the inversion process a resistivity model is generated, the data errors are calculated and a new resistivity model is generated. Therefore at the end of each iteration a resistivity model is available.

Mesh
The mesh is a 3D array of elements.

Model
Combination of a mesh whose elements have been assigned a resistivity and / or IP value with a listing of electrodes and thier locations and a schedule of quadrapoles based on these electrodes with measured and / or synthetic data for each quadrapole.

Model Fit
The degree to which the resistivitydistribution in the model is capable of generating the observed data.

Node
A verticy of a mesh element.

Pad (Pads or Padding element)
A mesh element that is a member of the mesh background region.

Quadrapole
A four electrode array or distribution of electrodes. The first two electrodes identify the positive and negative transmitter (AB); and the second two electrodes identify the positive and negative receiver (MN). In ERTLab we express a quadrapole in terms of electrode ID following the Lawrence Livermore Cable Electrode format (both a cable ID and an electrode ID are required to identify an individual electrode).

Receiver
A dipole or pair of electrodes use to receive voltage.

Reciprocals
The theory of reciprocity states that for electrical data transmitters and receivers should be interchangable. That is, the same data should be collected whether dipole A is used as transmitter and dipole B as a receiver or dipole B as transmitter and dipole A as receiver. In practice this generally holds true unless one of the dipoles is much larger than the other (i.e. a remote electrode is used).

Remote Electrodes
Electrodes used only as reference (zero or ground) which are usually placed outside the foreground region. These may be employed, for example, by certain named quadrapole arrays such as pole-dipole, gradient, and pole-pole.

Resistivity
Electrical resistance X a unit of length. Expressed in  Ohm-m.

Resistance
Electrical resistance from Ohm's law (R=V/I) where V is voltage and I is current. Expressed in Ohm.

Reweight Ratio
A measure of the amount of data that has effectively been ignored by the model. This parameter only applies if Robust inversion is being used. Numbers larger than 5 indicate a significant portion of the data has been ignored.

Roughness
A measure of the heterogeneity in model. Larger numbers represent more heterogeneity. Also called Absolute Roughness.

Roughness Parameter
This controls the ability of the inversion to make large changes to the model heterogeneity. The larger the number, the greater the changes allowed. Also called the roughness factor.

Schedule
A listing of quadrapoles. Data may or may not follow each quadrapole listing depending on the intended use or stage.

Transmitter
A dipole or pair of electrodes to which a voltage is applied.

XIP
Cross-hole IP. The practice of collecting IP data in boreholes and inverting them to produce a tomographic or 3D image.

ZTerrain
Data used in the electrode definition to specify the elevation of surface above an electrode.


 



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