ERTLab employs the following commands. These may be entered
directly in the configuration settings displayed in both the inversion and
forward workspaces, through an input file, or by using the menu system.
Mesh settings
Type of mesh
Only mesh type 0 is available.
Interior Grid Element X Size (m)
Nominal X element size. If the mesh
is adapted to electrode positions then this will vary modestly.
Interior Grid Element Y Size (m)
Nominal Y element size. If the mesh
is adapted to electrode positions then this will vary modestly.
Interior Grid Element Z Size (m)
Nominal Z element size. If the mesh
is adapted to electrode positions then this will vary modestly.
Mesh
Minimum X Distance (m)
Nominal Z element size. If the mesh
is adapted to electrode positions then this will vary modestly.
Mesh Maximum X Distance (m)
Nominal largest extent of the mesh in the X direction (value in
meters).
Mesh
Minimum Y Distance (m)
Nominal smallest extent of the mesh in the Y direction (value in
meters).
Mesh
Maximum Y Distance (m)
Nominal largest extent of the mesh in the Y direction (value in
meters).
Minimum
Z Value (m)
Nominal smallest extent of the mesh in the Z direction (value in
meters).
Maximum
Z Value (m)
Nominal largest extent of the mesh in the Z direction (value in
meters).
Grid Right (+X) Pads (interior X grid units)
Pad elements that define the
background region of the mesh in the
positive X direction. For each number given, a single pad element is appended
starting at the specified edge of the foreground
region. Each subsequent number represents the ultimate size of the
background region in this direction in terms of the equivalent number of
interior mesh elements for that direction. The size of any single pad element
is determined by subtracting the given value from the previous value and
multiplying by the interior grid element size for
the given direction. Default values are: 1 2 4 8 16 24 32 40 48 56 64;
which specifies 11 pad elements whose sizes in meters are 1, 1, 2, 4, 8, 8, 8,
8, 8, 8 and 8 times the interior grid element size.
Grid Left (-X) Pads (interior X grid units)
Pad elements that define the
background region of the mesh in the
negative X direction. For each number given, a single pad element is appended
starting at the specified edge of the foreground
region. Each subsequent number represents the ultimate size of the
background region in this direction in terms of the equivalent number of
interior mesh elements for that direction. The size of any single pad element
is determined by subtracting the given value from the previous value and
multiplying by the interior grid
element size for the given direction. Default values are: 1 2 4 8
16 24 32 40 48 56 64; which specifies 11 pad elements whose sizes in meters are
1, 1, 2, 4, 8, 8, 8, 8, 8, 8 and 8 times the interior grid element size.
Grid Back (-Y) Pads (interior Y grid units)
Pad elements that define the
background region of the mesh in the
positive Y direction. For each number given, a single pad element is appended
starting at the specified edge of the foreground
region. Each subsequent number represents the ultimate size of the
background region in this direction in terms of the equivalent number of
interior mesh elements for that direction. The size of any single pad element
is determined by subtracting the given value from the previous value and
multiplying by the interior grid
element size for the given direction. Default values are: 1 2 4 8
16 24 32 40 48 56 64; which specifies 11 pad elements whose sizes in meters are
1, 1, 2, 4, 8, 8, 8, 8, 8, 8 and 8 times the interior grid element size.
Grid Front (+Y) Pads (interior Y grid units)
Pad elements that define the
background region of the mesh in the
negative Y direction. For each number given, a single pad element is appended
starting at the specified edge of the foreground
region. Each subsequent number represents the ultimate size of the
background region in this direction in terms of the equivalent number of
interior mesh elements for that direction. The size of any single pad element
is determined by subtracting the given value from the previous value and
multiplying by the interior grid
element size for the given direction. Default values are: 1 2 4 8
16 24 32 40 48 56 64; which specifies 11 pad elements whose sizes in meters are
1, 1, 2, 4, 8, 8, 8, 8, 8, 8 and 8 times the interior grid element size.
Bottom (-Z) Pads (interior Z grid units)
Pad elements that define the
background region of the mesh in the
negative Z direction. For each number given, a single pad element is appended
starting at the specified edge of the foreground
region. Each subsequent number represents the ultimate size of the
background region in this direction in terms of the equivalent number of
interior mesh elements for that direction. The size of any single pad element
is determined by subtracting the given value from the previous value and
multiplying by the interior grid
element size for the given direction. Default values are: 1 2 4 8
16 24 32 40 48 56 64; which specifies 11 pad elements whose sizes in meters are
1, 1, 2, 4, 8, 8, 8, 8, 8, 8 and 8 times the interior grid element size. Note
that pad elements are not required in the positive Z direction because the
boundary is always Neuman, i.e. no current flow.
Flat grid -1; Grid with topography -2
Use 1 for a mesh without topographic distortion, 2 to
automatically adapt the surface (+Z) to the topographic relief specified either
in the electrode
ZTerrain or using a separate topography file.
Add electrodes Z terrain to
topography: Yes-1; No-2
Use 1 to add the ZTerrain information from the
electrode definition to the list of topographic points.
Update elevation for
Surface electrodes: Yes-1; No-2
Use 1 to move Z coordinate of the surface electrodes to the
topographic surface that was specified by importing a topography file. (Note
that X and Y coordinates of the electrodes must be in the same coordinate
system as the topography file).
Adapt grid to electrodes: Yes-1; No-2
Use 1 to snap (move) the nearest mesh node
to the electrode location. Electrodes
must fall on mesh nodes. If 2 is used then any electrodes that do not fall on
nodes will be moved to the nearest node. Adapting the mesh is generally more
accurate than moving the electrodes.
Starting
model
Type of resistivity model
Only model type 0 is available.
Background Resistivity (ohm m)
The value entered here will be assigned to each element of the
starting mesh, except when
anomalies are supplied then those values will overwrite the background
value supplied here.
Number of Resistivity Anomalies
The number of resistivity anomalies that will be employed in the
starting model. Anomalies are used for forward modeling, and occaisionally to
provide a more accurate starting guess for inversion.
Res. Anomalies: -X,+X -Y,+Y
-Z,+Z Resistivity
The location of a rectilinear resistivity anomaly. Seven
values are expected, the negative and positive X extents (in meters), the
negative and positive Y extents (in meters), the negative and positive Z
extents (in meters) and the resistivity of the block (in ohm-m). More than one
anomaly may be specified. Each subsequently specified anomaly overwrites the
previous one if overlap occurs. Shapes other than rectilinear must be built
using a series of these rectilinear anomalies.
Type of IP model
Only model type 0 is available.
Background Ip Value
The value entered here will be assigned to each element of the
starting mesh, except when
anomalies are supplied then those values will overwrite the background
value supplied here.
Number of IP Anomalies
The number of IP anomalies that
will be employed in the starting model. Anomalies are used for forward
modeling, and occaisionally to provide a more accurate starting guess for
inversion.
IP Anomalies: -X,+X -Y,+Y
-Z,+Z IP
The location of a rectilinear IP anomaly. Seven values are
expected, the negative and positive X extents (in meters), the negative and
positive Y extents (in meters), the negative and positive Z extents (in meters)
and the IP of the block (in chargeability).
More than one anomaly may be specified. Each subsequently specified anomaly
overwrites the previous one if overlap occurs. Shapes other than rectilinear
must be built using a series of these rectilinear anomalies.
IP Scale Factor (1000 for Chargebility)
A scale factor may be applied to IP data to convert from alternative units.
Add anomalies to flat grid: Yes-1; No-2
A value of 1 will add any anomalies (both resistivity and
IP) to a flat, pre-topographic distortion
mesh. Adaptations to a topographic surface (if used) are distributed
throughout the mesh to minimize element distortion.
Since any anomalies applied must completely fill the elements they affect, the
rectilinearly defined anomalies will necessarily be distorted in the final
mesh. This distortion is minimized if the anomalies are applied to the flat
grid before the mesh is adapted to topography.
Forward
solver parameters
[-/+X;-/+Y;-Z] Boundaries (0-Dir, 1-Mixed,
2-Neu)
Used
to set the type of boundary conditions for each face of the model (edge of the mesh) except plus Z.
The plus Z direction is the
earth’s surface and is always 2 (Neuman or no-flow). –X and
–Y indicate the first elements of the mesh in the X and Y directions,
+X and +Y indicate the last mesh elements in the X and Y directions.
Defaults are mixed (1) everywhere. Dirichlet or zero voltage
boundaries (0) may be used to represent a
grounded electrode; and Neuman or non-flow
boundaries (2) may be used to represent an electrically isolated area
such as a water tank or similar laboratory studies.
Omega:
Used for SSOR precond
This parameter is used to adjust the
convergence rate of the forward model. It
controls the error reduction factor. Allowable values are 1.0 to 2.0.
Forward Solver Tolerance
This parameter indicates the stop criteria
for the forward model. The smaller the
value the more accurate the forward model, but longer processing time is
required.
Maximum Number Forward Iterations
Indicates
the maximum number of iterations for the
forward model that will be
performed. The larger the number the slower the solution.
Skip TX electrodes with no RX-1; Calc. All -2
A value of 1 will skip calculations for electrodes used only
as transmitters. This may be useful to
speed calcuation times when remote electrodes,
which are typically only used as transmitters, are employed.
Model IP Data: No-1; Yes-2
A value of 2 will cause IP data to be modeled (either
forward or inverse). This is
performed after resistivity data are
modeled.
Inversion
parameters
Simple Inversion: Yes-1; No-2
ERTLab inversion strategy employs trials to determine the
optimal roughness parameter that
should be used for each outer iteration.
Since this strategy results in significantly longer calculation times for each
outer iteration for which it is employed, the Simple Inversion switch allows
the user to turn the trials off for all but the first outer iteration. If
simple inversion is used then the roughness parameter used for subsequent outer
iterations is the previous roughness parameter divided by the user controlled
multiplier.
Maximum Number Inversion Iterations-Rho
This is the maximum number of outer iterations the user will
allow the program to perform during inversion of
resistivity data. Fifteen is usually a good starting point. The code
will usually converge in fewer
iterations.
Maximum Number Inversion Iterations-IP
This is the maximum number of outer iterations the user will
allow the program to perform during inversion of IP
data. Resistivity inversion happens
before IP inversion. They do not happen concurrently. Ten is usually a good
starting point. The code will usually converge
in fewer iterations.
Maximum Internal Inverse PCG
Iterations-Rho
This is the maximum number of inner iterations the user will allow the program to perform during
inversion of resistivity data. Twelve
is usually a good starting point.
Maximum Internal Inverse PCG Iterations-IP
This is the maximum number of inner iterations the user will allow the program to perform during
inversion of IP data. Twenty is usually a good
starting point.
Tolerance for Inverse PCG iterations-Rho
This parameter indicates the stop criteria for the resistivity
inversion. Bigger numbers
result in earlier stopping. Allowable values are .1 to .001.
Tolerance for Inverse PCG iterations-IP
This parameter indicates the stop criteria for the IP
inversion. Bigger numbers result in earlier
stopping. Allowable values are .1 to .001.
Factor <1 for choosing optimal
Roughness Factor-Rho
This factor controls the identification of the optimal
roughness parameter.
Factor <1 for choosing optimal
Roughness Factor-IP
This factor controls the identification of the optimal
roughness parameter.
Multiplier for Changing Roughness
Factor-Rho
This factor controls the degree of change between roughness
parameters when stepping to the next outer
iteration during simple inversion.
Multiplier for Changing Roughness
Factor-IP
This factor controls the degree of change between roughness
parameters when stepping to the next outer
iteration during simple
inversion.
Initial Roughness Factor-Rho
This is the initial roughness parameter for the
first outer iteration trial.
Initial Roughness Factor-IP
This is the initial roughness parameter for the
first outer iteration trial.
Constant Value for Param X Weights
Used to control layering, factors smaller than 1 will increase layering in the X direction.
Factors greater than 1 will increase smoothness.
Constant Value for Param Y Weights
Used to control layering, factors smaller than 1 will increase layering in the Y direction.
Factors greater than 1 will increase smoothness.
Constant Value for Param Z Weights
Used to control layering, factors smaller than 1 will increase layering in the Z direction.
Factors greater than 1 will increase smoothness. The default is to impose
layering in the Z direction.
Noise
Rho Data Percent Errors (%)
This parameter controls the acceptable percent difference between the data and the model (the
inversion will converge to this
error level). The value entered here should reflect the estimated degree of
data repeatability (i.e. the actual noise in the field data), which is easily
obtained if reciprocal data were
collected. This term works in conjunction with the constant
error term to determine convergence.
Rho Data Constant Error Term
This parameter indicates the acceptable absolute difference between the data and the model that is
acceptable (the inversion will
converge to this error level). The value entered here should reflect
the estimated lower noise threshold of the instrument used to collect the data.
This term works in conjunction with the percent error
term to determine convergence.
IP Data Percent Errors (%)
This parameter controls the acceptable percent difference between the data and the model (the
inversion will converge to this
error level). The value entered here should reflect the estimated degree of
data repeatability (i.e. the actual noise in the field data), which is easily
obtained if reciprocal data were
collected. This term works in conjunction with the constant
error term to determine convergence.
IP Data Constant Error Term
This parameter indicates the acceptable absolute difference between the data and the model that is
acceptable (the inversion will
converge to this error level). The value entered here should reflect
the estimated lower noise threshold of the instrument used to collect the data.
This term works in conjunction with the percent error
term to determine convergence.
Use robust inversion (data errors
reweight)-1; No -2
A value of 1 sets use of robust inversion. This strategy
allows the code to ignore data outliers (those data with the worst
model fit) so they are no longer important to the solution. Except in
the most rigorously controlled field situations this strategy should be used.
When inverting synthetic or forward data
this option should be turned off. Even when robust is used it is
possible that the inversion will not converge.
This may happen when all of the data have approximately the same deviations and
no outliers can be identified.
Number of reweight iterations-Rho
This parameter controls the number of trials that may be attempted when searching for an
optimal roughness parameter.
Number of reweight iterations-IP
This parameter controls the number of trials that may be attempted when searching for an
optimal roughness parameter.
Saving
and Plotting parameters
Plot bounds: -X +X -Y +Y -Z +Z
These parameters control the extents of the output model.
Typically only the foreground region is
output. However for thin models (e.g. 2D models),
the extents output should be widened. The units are in meters.
Save inverted model for each
iteration: Yes-1; No-2
A value of 1 causes a model to be output (written to disk) after
each outer iteration.
|