Geophysical Modeling and Inversion Software:
Resistivity and IP
Two-dimensional Resistivity and IP Modeling
2DIP is a finite element program which computes the resistivity
and IP responses of two-dimensional models. Models may include both subsurface
structure and surface topography on a cross sections up to ninety dipole
lengths long and nine dipole lengths deep. Default electrode configurations
for dipole-dipole, pole-dipole, or gradient arrays are available, or electrode
positions may be specified for arbitrary array geometries. Model responses are
calculated as apparent resistivity and IP phase or chargeability for up to 728
data points. Model data are stored in a tabular file format which allows plotting
by generic contouring programs.
Smooth-Model Resistivity and IP Inversion with Topography
Smooth-model inversion is a robust method for converting resistivity and IP
measurements to smoothly varying model cross-sections. Dipole-dipole or pole-dipole
field data may be input from Zonge, Geosoft or spreadsheet format files. Either
time-domain or frequency-domain data may be used with lines up to 200 dipoles long
and n-spacings between 0.25 and 100. Observed apparent resistivities are averaged to
initialize a background resistivity model while background-model IP values are set to
one. Interactive tools allow background model editing to include known geology.
Resistivity and IP values in the two-dimensional model section are then iteratively
modified until calculated data values match observed data as closely as possible,
subject to constraints on model smoothness and the difference between background and
inverted model values.
Constraints control the character of TS2DIP's inversion models. Separate constraint
parameters are included for vertical smoothness, horizontal smoothness and for difference
from an arbitrary background model. Constraint weighting can be varied to suit geologic
conditions. Increasing the weighting of vertical smoothness constraints is appropriate in
areas with steeply dipping geology, while increasing horizontal smoothness-constraint
weighting is more suitable for flat lying geology. Constraining model parameter values to
stay close to a background model is useful for incorporating independent geologic information
in the inversion.
The finite-element forward-modeling algorithm used in TS2DIP v4.20 calculates apparent
resistivity and phase values generated by two-dimensional models to an accuracy of about 5
percent. When topographic profile information is included during model setup, TS2DIP's
finite-element mesh is draped over the terrain.
Inversion results are output into tabular ASCII files, which can be contoured and displayed
with general purpose plotting packages. Two plotting program drivers are included. One for
creating GeoSoft plots of inversion results and one for creating plots using Golden Software's
Surfer for Windows.
(provided with modeling programs)
CR to Cole-Cole inversion program
CCINV inverts spectral complex resistivity data to Cole-Cole models with
one to three additive Cole-Cole dispersions. Array type options are either Dipole-Dipole or
Pole-Dipole. Dipole lengths are shown both in terms of station numbers and length units of
m or ft. Station numbers represent distance along line, but may not be scaled to directly
indicate length units. CCINV allows inversion to either Cole-Cole or Zonge dispersion models.
One to three dispersions may be used in the inversion model, although one is usually
sufficient, particularly when using the Zonge model. Two Cole-Cole dispersions may be
required to fit double peak spectra, but the Zonge model can match double peak spectra with
a single dispersion. On rare occasions it may take three Cole-Cole dispersions to fit some
Color-Filled-Contour Plots of 2D Resistivity/IP Inversion Results
S2DPLOT reads Zonge TS2DIP inversion-program files (*.s2d, *.ipm and *.ipd)
and creates multi-panel plots of inversion-model sections and data pseudosections. Screen
plots may be exported to the Windows Print Manager, to Windows metafiles (wmf), to portable
network graphics (png) raster image files, to Surfer script and data files or to Oasis montaj
control and data files. Output files are given the same filename stem as the source inversion
model file, plus a suffix characterizing the plot number, 1 or 2. By default resistivity
inversion results are shown on plot 1 and IP results on plot 2, but resistivity and IP plot
panels can be combined within a single plot.
Color-Filled-Contour Plots of Inversion-Model Sections
MODSECT reads Zonge inversion-program model files and creates color-filled
contour plots of inversion-model-section resistivity or IP (one panel per plot). Modsect can
read scsinv m1d (CSAMT), steminv m1d (TEM), ts2dip IPM (resistivity/IP) or scs2d .mtm and .mtd
(far-field CSAMT/NSAMT) files. Plots may be viewed on screen or exported for hardcopy.
Modsect can generate script and data files for use with Surfer v6 or v7. It can also export
GeoSoft Oasis montaj control and data files which MODSECTGX.GX will turn into finished plots.
Modsect also exports plots directly to the Windows Printer Manger, windows metafiles (wmf)
or portable network graphics (png) raster image files. Output files are given the same
filename stem as the source inversion model file, plus a one-letter suffix. Resistivity
section plot-file names end with a "r" while IP model-section plot-file names end with a
Interpolation to Plan-Map Data file
MAPDAT reads SCSINV and STEMINV *.M1D files, TS2DIP *.IPM or SCS2D *.MTM
files, interpolates to a constant depth or elevation and then writes interpolated values
to a tabular-format *.MAP file. *.MAP files have a simple spreadsheet format which can be
used by Geosoft or Surfer.
Making plan maps requires starting with a consistent grid coordinate
system in *.STN files for each line, so that inversion results from multiple lines can
be combined. Concatenate *.M1D, *.IPM or *.MTM files for a project area into a single
large file. MAPDAT v3.01 can handle up to 16384 stations, ie 128 stations on 128 lines.
Type "MAPDAT AUBELL.M1D" to extract an elevation or depth slice from AUBELL.M1D. MAPDAT
will place the interpolated values into AUBELL.MAP. When you are generating multiple depth
slices, you can rename the *.MAP file to something like ABZ500.DAT to avoid overwriting.
Note that S2DIP and SCS2D model sections extend past data coverage at
each end of the survey line, but the model-section extensions are poorly resolved and may
include spurious features. MAPDAT does not include automatic clipping on data coverage,
so it is worthwhile to trim *.IPM and *.MTM model-sections back to the extent of data
coverage before concatenating multiple lines into one large file.
You may select either constant elevation or constant depth slices in
MAPDAT. You may also control the number of decimal places used in stations numbers.
MAPDAT will read keywords from *.MDE files, parse keywords from the command line or will
prompt you to enter values.
2-dimensional Inversion of Resistivity and IP data with Topography
Two-dimensional, smooth-model inversion of resistivity and induced polarization
data produces image-like, electrical property sections which improve the data's interpretability.
Recent software improvements enable routine smooth-model inversion of resistivity and induced
polarization (IP) data. Nearly uniform starting models are generated by running broad moving-average
filters over lines of dipole-dipole or pole-dipole data. Model resistivity and IP properties are then
adjusted iteratively until calculated data values match observed values as closely as possible, given
constraints which keep the model section smooth. Calculated values are generated with a finite element
algorithm which can be adapted for accurate two-dimensional modeling of data collected in rough
terrain. Smooth-model inversion of sample data show the method's utility as an interpretation aid
and the importance of modeling topography in areas with significant relief.