dcinv2d dcinv2d.inp

Format of the control file dcinv2d.inp:

IREST  NITER
CHIFACT
OBS.DAT
FDMESH.DAT
TOPO.DAT
INI_MOD.CON
REF_MOD.CON
ALPHAS, ALPHAX, ALPHAZ
W.DAT

Control parameters and input files:

irest

restarting control parameter:
0 - begin inversion from scratch
1 - restart inversion from the previous iteration. This requires that the files dcinv2d.con, dcinv2d.out, and dcinv2d.log be present.

niter

number of iterations to be performed.

chifact

misfit parameter such that the final = N × chifact where N is the number of data. Usually chifact = 1.0 .
When a numerical value is entered, the program will find the model which has a misfit = N × chifact. If the misfit is detected to be plateauing at a higher value, the program will terminate and print advisory messages to both the screen and the log file.

When NULL or null is input, the program will start with a very small value for chifact and adjust it to a higher value when the plateauing of data misfit is detected. The final misfit will be 10% higher than the estimated asymptotic value of the data misfit function.

obs.dat

name of the file containing observed potential data and electrode configuration.
If the error standard deviation is provided, the program will use the user-provided error estimates in the inversion. If the standard deviation is missing, the program assigns error estimates to the data and then proceeds to the inversion.

fdmesh.dat

user-designed mesh file, or one of the following input values so that a mesh is generated by the program. If NULL or null is entered, a default mesh will be generated. If ncell, aspr is entered, a mesh will be generated with ncell cells between electrodes and with the cells near the surface having an aspect ratio of aspr. (Note: NULL generates a default mesh with (ncell, aspr)=(2, 2.0). )

topo.dat

name of the file containing surface topography. When NULL or null is entered, the surface is assumed flat at z = 0.

ini_mod.con

name of the file containing initial conductivity model. If the initial model is constant, the filename can be replaced by the constant value. If NULL or null is entered, the initial model will be equal to the reference model.

ref_mod.con

name of the file containing reference conductivity model. If the reference model is constant, the filename can be replaced by the constant value. If NULL or null is entered, the reference model will be the reciprocal of the average apparent resistivity.

alphas

coefficient for the smallest model component.

alphax

coefficient for the x-derivative term.

alphaz

coefficient for the z-derivative term.

There are two options for this input line:

1. The user supplies the values for the three coefficients and the program sets (_s , _x , _z) = (ALPHAS, ALPHAX, ALPHAZ).
2. When NULL or null is entered on this line, alphas is calculated based on array geometry and alphax and alphaz are set to 1.0 .

The program constructs a model using a standard model objective function and the three coefficients (_s , _x , _z) determine the relative importance of the three components in the model objective function. (See equations (8) and (9) )

w.dat

name of the file containing weighting matrix. If NULL or null is entered, default values of unity are used.

Both ini_mod.con and ref_mod.con are stored in model.con format.

Output files:

dcinv2d.con

conductivity model of the latest iteration. The model is stored in the model.con format. This file is overwritten at the end of each iteration.

dcinv2d.out

convergence information of past iterations. This file lists the values of data misfit, the model objective function, and the ridge regression parameter as functions of iteration.

dcinv2d.log

log file containing the detailed information about each iteration.

dcinv2d.pre

predicted potential data from the inverted model in the latest iteration. The predicted data is in the format of obs.dat with the field corresponding to data error omitted. This file is overwritten at the end of each iteration.

dcinv2d.msh

default mesh used in the inversion. The default mesh is in the format of fdmesh.dat. This file is output for use in the subsequent IP inversion.

Examples of DCINV2D Control File:

The following is an example control file. The inversion is restarted and five iterations are to be performed with chifact equal to one. The observed potential data are in obs.dat. The model discretization is defined by the finite difference mesh in fdmesh.dat and the surface topography in topo.dat. The initial and reference models are equal to 0.005 S/m. The default weighting function (unity) is used for the model objective function.

1  5             !!! restart, number of iterations
1.0              !!! chifact
obs.dat          !!! observed potential data file
fdmesh.dat       !!! finite difference mesh.
topo.dat         !!! topography (NULL for flat surface)
0.005            !!! initial model (file name or numerical value)
0.005            !!! reference model (file name or numerical value)
0.001 1. 1.      !!! alphas, alphax, alphaz (NULL for default)
NULL             !!! weighting matrix (NULL for default)

The following input uses all possible defaults and allows the user to perform a preliminary inversion of a data set:

0  20            !!! restart, number of iterations
null             !!! chifact
obs.dat          !!! observed potential data file
null             !!! finite difference mesh.
topo.dat         !!! topography (NULL for flat surface)
null             !!! initial model (file name or numerical value)
null             !!! reference model (file name or numerical value)
null             !!! alphas, alphax, alphaz (NULL for default)
null             !!! weighting matrix (NULL for default)

NOTE-1: To have the program display a sample input file, type: dcinv2d -inp

NOTE-2: If a special weighting (w.dat) is supplied by the user, care should be taken so that the final weighting matrix constructed by the program is positive definite. This property can be destroyed when the combination of the cell weighting coefficients and component coefficients (ALPHAS, ALPHAX, ALPHAZ, see the description of special weighting w.dat file) makes the diagonal elements too small. Under such circumstances, the program will usually print in the log file the indices of the rows which are not diagonally dominant. In the most severe situation, the program will output an error message indicating that the matrix may not be positive definite. When these messages appear, the user should stop the program execution and redesign the special weighting by increasing the value of _s or decreasing the dynamic range of W_S .

NOTE-3: DCINV2D will terminate before the specified maximum number of iterations is reached if the expected data misfit is achieved and if the model norm has plateaued. However, if the program exits when the maximum iteration is reached, the file dcinv2d.out should be checked to see if the desired (usually equal to the number of data) has been reached and if the model norm is no longer changing. If either of these conditions has not been met then the program should be restarted. If the desired misfit level is not achieved, but the model norm has plateaued and the model is not changing between successive iterations, then the user may want to adjust the target misfit to a higher value. Also an investigation as to which data are most poorly fit can be informative. It may be that the assigned standard deviations to specific data are unrealistically small. The program restarts using the information in dcinv2d.out and dcinv2d.con. Users are referred to Technical Note TN001 for more insight on these matters.

NOTE-4: The default mesh in DCINV2D is generated assuming that the data are collected by commonly used arrays and that the topographic relief is moderate. Thus the default mesh may not be optimal when the data are collected with an unusual electrode geometry or when data are collected over severe surface topography. In such cases, the user should redesign the mesh so that it is better suited for the particular needs of the data set.

NOTE-5: We stress that the default errors are only an initial guess and they facilitate a preliminary inversion of the data. The user will want to alter these error estimates for the final inversion used for interpretation. The data with the default errors are written in the file dcinv2d.log using the format of obs.dat. They can be copied to a file for finer adjustment of the error estimates or the user can supply his or her own errors directly.