Modeling

Geophysical Modeling and Inversion Software:
Resistivity and IP

2DIP

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.

TS2DIP

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.

Utilities

CCINV

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 spectral curves.

S2DPLOT

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.

MODSECT

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 "p".

MAPDAT

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.

Reference Papers

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.