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ERIGRAPH – USER GUIDE
2007-10-18
Introduction
Erigraph is primarily designed to facilitate graphical presentation of 2D resistivity
imaging (CVES) data and associated inverted model sections. It also supports time
domain IP data. Measured apparent resistivities and chargeabilities can be plotted as
pseudosections in colour or grey scale. Inverted sections can be plotted as continuous
(smooth) plots or as rectangular blocks. 2D inversion must be carried out using a
separate inversion program, such as for example Res2dinv.
Pseudosection Plotting
Pseudosection plotting is useful for getting a first rough idea of the resistivity of an
investigated site, and for data quality assessment purposes. An example of a
pseudosection plot of a Wenner array data set is shown in Figure 1. A number of
buttons along the top panel of the software window can be used to control the plots,
and the palette can be changed via “Diagram/Palette”.
Figure 1. Example of pseudosection plot of a Wenner array data set.
In all cases the pseudodepth used is equal to the median depth of current penetration,
i.e. the level at which half the current passes above and below. For simple array types
like Wenner and pole-pole array one pseudosection plot of the data is enough to give
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a complete picture. When using arrays like Schlumberger, pole-dipole, dipole-dipole
and multiple gradient array several different a-spacings and n-factors can be
combined in a data file. In such cases all data may not be visible in the same plot as
data points fall on top of each other, and the mixing of different electrode geometries
may give a noisy appearance of good quality data.
An example of a pseudosection plot of a multiple gradient array data set is shown in
Figure 2. In such cases it is useful to make separate plots for either each a-spacing or
n-factor, as shown in Figure 3. For this data set eight different pseudosections can be
plotted, one for each of the n-factors ranging from -4 to 4. Alternatively one
pseudosection can be plotted for each of the electrode spacings, but all data points
will then not be visible since data measured with the same potential electrode location
but mirrored current electrode layouts fall on top of each other in the pseudosections.
Figure 2. Example of pseudosection plot of a multiple gradient array data set.
Figure 3. Examples of pseudosection plots for a couple of different n-factors from the same data
set; a) n = 3, b) n = -3.
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To change the diagram settings, open the settings menu by right clicking on the
diagram or by clicking . The a-spacings and n-factors to plot for a pseudosection
are controlled at the bottom of the settings menu. Depending on the data set this may
make sense or not. For example, if only one a-spacing was used in the survey in a
dipole-dipole data set, it is not possible to make a pseudosection for any single
n-factor.
Figure 4. Plot settings menu.
A variety of different properties of the plot can be modified from the different tabs of
the settings menu, e.g. geometrical limits, resistivity limits etc. Please play around and
try out the different options.
The apparent resistivity data can be stored either in a conventional ASCII format with
one set of x, a, rho-a on each line or in the format used by Res2dinv (in both cases
DAT filename extension is assumed). For the latter format please refer to the manual
of Res2dinv.
Field data files from the ABEM Lund Imaging System can be converted into DATfiles
using ERIGRAPH or the Terrameter SAS1000 / SAS4000 Utility Software. The
topography can be automatically incorporated in the DAT file, if it is saved in an
ASCII file in the same directory as the OHM file to be converted. The topography file
is automatically recognised if saved with the same filename as the data but with ALT
extension. The topography file format is as follows:
x(1) alt(1)
x(2) alt(2)
...
...
x(n) alt(n)
In addition to the .DAT file the program searches for a file with .INF extension, which
is created automatically when data is converted with Erigraph or when a data file is
first opened (see appendix for details).
IP Data
Induced polarisation (IP) data can be plotted as pseudosections either as chargeability
or normalised chageability, via selection on the “Settings” form.
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Merging Data Files
Erigraph has a function for merging two or more DAT files into one via the meny
choice “File/Merge/Data Files”, to be used for example for combining two data files
measured with different electrode arrays on the same layout. In the present version
there is no option for automatic exclusion of duplicate data points.
Merging Time-lapse Data Files
Two DAT files can be merged into a time-lapse DAT file via the menu choice
“File/Merge/Time-lapse”. The two DAT files must of course be measured over the
same line using the same measurement protocol(s). The data points need not be saved
in the same order, and if data points are missing in either of the data files they will be
omitted automatically. If IP data is present in the input data files it will not included in
the time-lapse DAT file.
Difference Plotting
For time-lapse data files there are options for plotting either of the time step data sets
as a pseudosection, or to plot the difference between the files calculated as:
2/)( 10
10
rhorho
rhorho
Diff
+
−
=
Inverted Model Plotting
Erigraph can plot inverted models saved in RHO-format (Figure 5), and it has built-in
conversion tools for converting from INV files created by Res2dinv. If IP data is
available the inverted chargeability model can be plotted as well. An inverted model
can be associated with a pseudosection data file if the DAT file is first opened and
then “2D model section plot” is selected. If the inverted model is saved with the same
filename as the DAT file it will automatically be associated with it, otherwise the
model file in question has to be selected.
By default the models are plotted in smooth style (Figure 5a), but an option is to plot
the inverted model in rectangular block style (Figure 5b). The latter shows the actual
model cells as defined in the inversion software.
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Figure 5. Example plots of inverted data set plotted in; a) smooth style, b) rectangular block
style.
The RHO-1 format is used when converting directly from the INV file, and it works
well in cases without topography of with smoothly varying topography. Res2dinv has
an option for exporting inverted models in Lund-format, however in some versions of
Res2dinv there is a bug so it will export non-sense models. Furthermore, topography
will not be included in the converted model.
If a data file is opened and an associated inverted model is plotted selecting "2D
section", the models saved in RHO-1 format will be plotted with topography
provided the DAT-file contains topography (please consult the Res2dinv manual for
details). Alternatively, the topography may be saved in a file with ALT extension.
However, in any case it is necessary to open a data file and then plot the associated
2D section since the RHO-1 model file does not contain any topography information.
The RHO-2 format includes topography in the model file.
The RHO-2 format is better in cases with strong gradients in the topography (see
example in Figure 6). The depth is in this case the bottom of each layer. There is a
built-in RHO-2 format conversion tool in Erigraph for conversion from XYZ files
saved by Res2dinv (do not use negative depths). This conversion tool is only designed
for models inverted with topography.
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Figure 6. Example plots of inverted data set from line with steep topography; a) inverted model
converted directly from INV file, b) inverted model converted from XYZ file saved
with topography.
User Defined Text
The text strings on the axes of the diagrams can be customised via the menu
“Settings/Diagram Labels”.
Distance Markers
Distance markers (Figure 7) can be added to the plots by clicking , for example to
mark out where a perpendicular line crosses. The vertical line type (Figure 8) will be
plotted on pseudosections as well as inverted sections, whereas the other types only
apply to inverted sections.
Figure 7. Distance markers with labels can be added.
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Figure 8. Distance markers edit menu.
Level Markers
Level markes can be included in the model plots by saving the level marker data in an
ASCII file with MRK extension in the data/model directory (see appendix for file
format).
Reference Data
Drilling reference data can be included in the model plots by clicking . The
available fill patterns are for plotting the layers are shown in Figure 11, with
corresponding material codes.
Figure 9. Inverted model section with drilling reference data.
Figure 10. Menus used for adding and specifying reference data from e.g. drilling.
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The text strings for the layer materials are by default written in black with white label
background. Select transparent labels under “Settings/Plot Options/Transparent layer
labels” if preferred.
Figure 11. Availble rasters for drilling reference data.
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Appendix: File Formats
Pseudosecion Data
The apparent resistivity data can be stored either in a conventional ASCII format with
one set of x, a, rho-a on each line or in the format used by Res2dinv (in both cases
DAT filename extension is assumed). For the latter format please refer to the manual
of Res2dinv. Field data files from the ABEM Lund Imaging System can be converted
into DAT-files using ERIGRAPH or the Terrameter SAS1000 / SAS4000 Utility
Software.
In addition to the .DAT file the program searches for a file with .INF extension, which
holds information in the following format:
comment string
date of measurement
array type
blank (line contents not used)
direction label left side (optional)
direction label right side (optional)
nl no of marker labels (optional)
x[1] flag[1] marker text string[1] distance, type flag, text string (optional)
x[2] flag[2] marker text string[2]
...
x[nl] flag[nl] marker text string[nl]
The topography can be automatically incorporated in the DAT file, if it is saved in an
ASCII file in the same directory as the OHM file to be converted. The topography file
should have the same filename as the data but with ALT extension. The file format is
as follows:
x(1) alt(1)
x(2) alt(2)
...
...
x(n) alt(n)
Inverted Models
Erigraph can plot inverted models in RHO-files (ASCII files) with the following
format:
RHO-1 [IP]
nd Depth(1) Depth(1) ... Depth(nd)
x(1) rho(1,1) rho(1,2) ... rho(1,nd) [ip(1,1) ip(1,2) ... ip(1,nd)]
x(2) rho(2,1) rho(2,2) ... rho(2,nd) [ip(2,1) ip(2,2) ... ip(2,nd)]
...
...
x(nx) rho(nx,1) rho(nx,2) ... rho(nx,nd) [ip(nx,1) ip(nx,2) ... ip(nx,nd)]
/Model residuals:
[optional]
/Inversion type header:
[optional]
/IP parameter/unit
[optional]
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The specified depth is in this case the midpoint of each layer. Blank model cells
should be entered as asterisks (*). If IP data is available it can be included as
indicated, the square parantheses [] indicate that it is optional. The IP parameter/unit
line would similar to the corresponding line in the DAT files.
Alternatively the RHO-2 format can be used:
RHO-2 [IP]
x(1) alt(1)
nd(1)
rho(1,1) Depth(1,1) [ip(1,1)]
rho(1,2) Depth(1,2) [ip(1,2)]
...
rho(1,nd-1) Depth(1,nd-1) [ip(1,nd-1)]
rho(nd) [ip(nd)]
x(2) alt(2)
nd(2)
rho(2,1) Depth(2,1) [ip(2,1)]
rho(2,2) Depth(2,2) [ip(2,2)]
...
rho(2,nd-1) Depth(2,nd-1) [ip(2,nd-1)]
rho(2,nd) [ip(2,nd)]
...
...
...
...
x(nx) alt(nx)
nd(nx)
rho(nx,1) Depth(nx,1) [ip(nx,1)]
rho(nx,2) Depth(nx,2) [ip(nx,1)]
...
rho(nx,nd-1) Depth(nx,nd-1) [ip(nx,nd-1)]
rho(nx,nd). [ip(nx,nd)]
/Model residuals:
[optional]
/Inversion type header:
[optional]
/IP parameter/unit
[optional]
The RHO-2 format is better in cases with strong gradients in the topography (see
example in Figure 6). The depth is in this case the bottom of each layer. There is a
built-in RHO-2 format conversion tool in Erigraph for conversion from XYZ files
saved by Res2dinv (do not use negative depths). This conversion tool is only designed
for models inverted with topography.
For both model formats an unlimited number of comment lines may be added at the
end of the file, where each comment line should start with a slash (/). This can be
useful for saving information on e.g. inversion parameters together with the model.
User Defined Text
For the model sections the file must be named MODSEC.TXT and have the following
structure:
Header
Distance label
Depth scale label
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Topography scale label
Resisitivity legend label
Example of a MODSEC.TXT file:
2D INVERTED SECTION
Distance/[m]
Depth/[m]
Level/[m]
Resistivity/[ohm-m]
Level Markers
Level markes can be included in the model plots by saving the level marker data in an
ASCII file with MRK extension in the data/model directory. Use the following
format:
"comment" "comment" / first marker line
x(1,1) depth(1,1)
x(1,2) depth(1,2)
...
...
x(1,n) depth(1,n)
"comment" "comment" / second marker line
x(2,1) depth(2,1)
x(2,2) depth(2,2)
...
...
x(2,n) depth(2,n)
Reference Data
Drilling reference data file format:
x(1) / distance
id(1) / reference point id text string (optional)
gwl(1)/ groundwater level (optional)
nl(1) / no of layers
x(1,1) d(1,1) code(1,1) col(1,1) txt(1,1) / dist, depth, materialcode, textcolour, text lable
x(1,2) d(1,2) code(1,2) col(1,2) txt(1,2)...
...
x(1,nl) d(1,nl) code(1,nl) col(1,nl) txt(1,nl)
x(2) / distance
id(2) / reference point id text string (optional)
gwl(2)/ groundwater level (optional)
nl(2) / no of layers
x(2,1) d(2,1) code(2,1) col(2,1) txt(2,1) / dist, depth, materialcode, textcolour, text lable
x(2,2) d(2,2) code(2,2) col(2,2) txt(2,2)...
...
x(2,nl) d(2,nl) code(2,nl) col(2,nl) txt(2,nl)
The text strings for the layer materials are written in black if the text colour is set to 0,
and white is set to 1 (in the latter case select transparent labels under “Settings/Plot
Options/Transparent layer labels” in order to see the text).