DC Resistivity
and Induced
Polarization
Interpex offers three software
tools that interpret resistivity and induced polarization soundings in terms
of 1 dimensional models. These tools belong to the RESIX family of software.
Use any one of these software packages to produce forward and inverse models
of resistivity data, or induced polarization data, or both.
MATHEMATICAL BASIS
For forward modeling, all RESIX
family programs use an Anderson-style adaptive linear digital filter for
most common arrays to provide you with resistivity contrasts of up to 10,000
to 1. For stable results for inversion, all programs use the Inman-style
ridge regression approach of nonlinear least squares curve fitting.
SPECIFICATION SUMMARY
Schlumberger, Wenner,
polar-dipole, equatorial dipole,
pole-dipole, dipole-dipole,
perpendicular dipole arrays
Maximum of 10 earth layers for
the forward model
Maximum of 8 earth layers in
the inversion
Maximum of 60 data points
Maximum of 200 data sets per
data base
RESIX is an interactive,
graphically oriented, forward and inverse modeling program for interpreting
resistivity sounding data in terms of a layered earth (1-D) model. Sounding
curves can be entered as a function of a for Wenner or pole-pole soundings,
AB/2 for Schlumberger soundings, R for dipole soundings or N for
dipole-dipole or pole-dipole soundings.
Sounding curves values are entered
as apparent resistivity as a function of spacing. Forward modeling enables
you to calculate a synthetic resistivity sounding curve for a model with up
to ten plane layers. Resistivity sounding curves are calculated using linear
filters.
Shifting of data with overlapping
curve segments is handled automatically by shifting the synthetic curve to
match the offsets between segments in the data. This is done by assuming the
data for the longest spacings is correct, and shifting segments at shorter
spacings to meet the data at longer spacings. Alternatively, you can shift
the data curve itself to meet either the longest or shortest spacing segment.
This can be done either automatically, or manually in the interactive
graphics mode using the Shift Segments option.
Masking of data is achieved in the
interactive graphics interpretation module by pointing and clicking on data
points. These points can be unmasked or deleted according to the users needs.
Masked points do not influence the error or inversion calculations.
Interactive model input via the
mouse allows the user to see changes in the calculated curve as he enters
the model graphically. Graphic entities of the plate output may also be
resized and positioned using the mouse.
Screen from RESIX showing
interactive model input using the mouse.
Inverse modeling allows you to
obtain a model that best fits the data in a least squares sense, using ridge
regression to interactively adjust the starting model parameters. You can
constrain some of the starting model parameters so the inversion will not
adjust them. Starting models for inversion can have up to 8 layers. Forward
models can have as many as 10 layers. Constants can be applied by fixing (or
freezing) a parameter, or by imposing limits on a parameter. Parameters can
be resistivity / thickness of layers, or resistivity / depth to bottom of
layers.
Results from forward or inverse
modeling can be directed to a printer or plotter for report-ready, hard-copy
output. Results can also be saved in a binary random access disk file for
later retrieval. Inverse modeling can be carried out in an unattended batch
mode.
RESIX provides several options to
present the observed data, the theoretical (forward) data, and the
geo-electrical model section used to calculate the forward response. This
information can be graphically presented individually on a dot matrix
printer, a laser printer, or a pen plotter. You can also print the data and
model section in a tabulated, paginated form directly to a text printer.
Data storage is handled by a new
data base file format which holds up to 200 data sets (or sounding curves).
The size of the file depends on the number of sounding curves it contains.
RESIX PLUS has all the features
of RESIX with
the following additions.
Sounding curves can be entered as
apparent resistivity versus spacing; or, the voltage, current, and electrode
spacings for both current and potential electrodes can be entered on the
worksheet. The worksheet calculates apparent resistivities as you enter the
data, using formulas that account for the finite electrode spacings.
Data Edit Worksheet from RESIX
Plus.
Direct inversion allows you to
estimate the layered model directly from the data curve, without having to
manually construct the number of layers and layer resistivity and thickness.
Extension of the input data curve for short and long electrode spacings, and
curve resampling and transformation, are done automatically and are
transparent.
Smooth modeling enables you to
automatically interpret resistivity sounding data in terms of a smooth model
with up to 19 layers. The model depths are logarithmically spaced and are
determined from a minimum and maximum depth. The depth range can be
user-specified or automatically generated. Model resistivities are normally
initialized to the average apparent resistivity. Inversion can be carried
out in either interactive or batch mode, using ridge regression or William
of Occam's smooth model concept. Results can be plotted, printed, listed on
the screen or written to an ASCII file for use by user-supplied or
third-party software.
Equivalence analysis allows you to
generate a set of equivalent models, (that is, alternative models that fit
the data nearly as well as the best-fit model, but differ from this model).
Equivalence analysis also indicates the allowable range of model parameters.
Plot of equivalent models from
RESIX Plus.
The MODEL SUITE command enables
you to construct a collection of forward models by specifying one or more
different values for a parameter. You can also generate new electrode
spacings. This enables you to see the results of a specific change to the
original model.
RESIX PLUS offers the user the
capability to enter Offset Wenner Sounding resistances in to the Offset
Wenner worksheet or read them from Campus Geophysical Instruments ASCII
files thereby replacing the OFFIX and BOSSIX programs.
RESIX IP is an interactive,
graphically oriented, forward and inverse modeling program for interpreting
induced polarization (IP) and resistivity sounding data in terms of a
layered earth (1-D) model.
RESIX IP has all the features of RESIX PLUS
with the following exceptions where the program was adapted to incorporate
Induced Polarization (IP) data.
Sounding curves can be entered as
a function of a for Wenner or pole-pole soundings, AB/2 for Schlumberger
soundings, R for dipole soundings or N for dipole-dipole or pole-dipole
soundings. Apparent resistivity data can be interpreted with or without
apparent polarization data.
Interactive modeling in RESIX
IP.
Forward modeling enables you to
calculate synthetic IP/resistivity sounding curves for a model with up to 10
plane layers. IP readings can be entered in units of mSec-V/V or in percent
frequency effect (PFE).
Inverse modeling enables you to
obtain a model that best fits the data in a least squares sense. Starting
models for inversion can have up to 7 layers if both resistivity and
polarization parameters are included, or up to 10 layers if only resistivity
parameters are included. Forward models can have as many as 10 layers.
Constraints can be applied by fixing (or freezing) a parameter, or by
imposing limits on a parameter. Parameters can be resistivity + thickness of
layers, or resistivity + depth to bottom of layers. If IP data are present,
each layer has an additional parameter, either chargeability or PFE,
according to the choice of data type.
Equivalence analysis functions
similarly to equivalence analysis in RESIX PLUS, but RESIX IP generates
equivalent models for both resistivity and IP models.
The MODEL SUITE command functions
similarly to the MODEL SUITE command in RESIX PLUS, but RESIX IP generates
suites of models for both resistivity and IP data.
Screen from RESIX IP showing
model suites and calculated results for a range of resistivity values
visible.
Smooth modeling functions
similarly to smooth modeling in RESIX PLUS, but RESIX IP generates smooth
models for both resistivity and IP data.
Screen from RESIX IP showing
smooth model plot on same plot axis as resistivity and IP data.
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