aglo

DCIP2D:
FILE: obs.dat
 

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This file contains the observed measurements and the associated electrode locations. Both potential data and apparent chargeability data are stored in the same format. This will be the format of all the data files that are output from DCIPF2D, and are input to DCINV2D and IPINV2D.

It is assumed that the survey is carried out along a line in the x-direction which is perpendicular to geologic strike and that all current and potential electrodes lie on this line. It is also assumed that all electrodes are on the surface so the coordinates of any electrode are completely specified by its location along the x-axis. The z-coordinate of the electrode is assumed to coincide with the topography at location x. Programs in the DCIP2D package will automatically place the electrodes on the surface according to the topography. If the file topo.dat is not supplied, all electrodes will be at z=0. When the topography is specified, the electrode will be placed on the topographic surface that is discretized onto the finite difference mesh according to file topo.dat.

All array configurations can be accommodated for inversion, but only pole-pole, pole-dipole, dipole-dipole, Wenner or RealSection can be in the data input file (NOTE the special form of specifying RealSection or Wenner data in the first two lines). For Schlumberger, gradient, or irregular survey arrays, you must enter all electrode positions explicitely and specify dipole-dipole on the second line of obs.dat. Note also that the Graphical User interface can estimate default parameters and display pseudosections for pole-pole, pole-dipole, dipole-dipole Wenner or RealSection arrays only. See the GUI documentation for details. Only one type of configuration can be included in a single file, but arrays with different spacings of the same configuration can be mixed in the same file. For example, two pole-dipole arrays with a 50 m dipole and a 100 m dipole respectively can be in the same file.

The obs.dat file can have two different formats; the "standard" format and the "common-current" format. The type of format chosen to store the data does not make any difference to the DCIP2D programs and so the choice is determined by the user's preference. At the beginning of execution, the programs check to see which format is used. The output files will be stored in the same format. 

(1) Standard Format:

In this format, each observation record is stored on one line. Each record consists of six columns specifying the X locations of the current electrodes (XA,XB) and potential electrodes (XM,XN), data and standard deviation (optional). The data records can be arranged in any order. The file structure for the standard format obs.dat is as follows: 


TITLE
TYPE
XA1 XB1 XM1 XN1 VAL1 ERR1
XA2 XB2 XM2 XN2 VAL2 ERR2
XA3 XB3 XM3 XN3 VAL3 ERR3
:
TITLE
this line is not used in the programs. It is simply a file header describing the data. It must be on the first line of the file and cannot be preceded by blanklines. (New, Dec 2005 ('experimental') - to view Wenner or Realsection data, enter RealSection for the TITLE and dipole-dipole for TYPE.)
TYPE
type of electrode configuration. This should be one of the following four values:
  1. pole-pole,
  2. pole-dipole,
  3. dipole-pole,
  4. dipole-dipole,
  5. RealSection or Wenner: **New, Dec 2005 ('experimental') - to view Wenner or Realsection data, enter RealSection for the TITLE and dipole-dipole for TYPE.

where the first word denotes the configuration of the corrent electrodes and the second word denote the potential electrodes. For example, pole-dipole means current is input by a pole and potential is measured by a dipole. For Wenner, Schlumberger, gradient or irregular arrays, specify dipole-dipole and ensure that all current and potential electrode positions are specified explicitly. See the GUI documentation for more details.

XAi, XBi
X positions (in metres) of the current electrodes used to measure the datum VALi. When current is input by a dipole, XB will be taken as the position of the negative current electrode. When current is input by a pole, XB is not used, but the input field must remain.

XMi, XNi
X positions (in metres) of the potential electrodes used to measure the datum VALi. When potential is measured by a dipole, XN will be taken as the position of the negative potential electrode. When potential is measured by a pole, XN is not used, but the input field must remain.

VALi
value of observed datum. The potential measurements must be measured value in Volts for potential, or a dimensionless real number (not percentage) for apparent chargeability. (The potential value is always normalized to unit current amplitude.) There are four types of IP data generally in use; two gathered in the time domain and two gathered in the frequency domain. For small chargeabilities, as is nearly always the case for earth materials, all data types can be used as input for inversion, and resulting models will have chargeabilities in the same units.

ERRi
standard deviation of the datum VALi. This is an absolute value and should not be specified as a percentage. This value should always be positive. If the first datum does not have an error field (or the error field is commented out by placing a "!" before it), the default errors for the entire data set will be calculated from the data.
If the default standard deviation is requested, it will be calculated from the data using the following rules:
  • DC: ERRi = 0.05 × (| i | + nmax ) , where nmax is the potential at the maximum electrode separation averaged over 5 adjacent stations.
  • IP: ERRi = 0.05 × | ai | + amin , where amin is the estimated standard deviation of the background apparent chargeabilities in the data set.

(2) Common-current Format:

In this format, data are grouped by the current electrode locations. For each current (XA,XB), there should be N lines containing the potential electrode locations, data and standard deviation (optional). The file structure for the "common-current" format obs.dat is as follows: 

TITLE
NCUR IDP IDC
XA1 XB1 N1
XM1 XN1 VAL1 ERR1
XM2 XN2 VAL2 ERR2
:
XA2 XB2 N2
:
TITLE
this line is not used in the programs. It is simply a file header describing the data and must be on the first line of the file and cannot be preceded by blank lines.
NCUR
number of current locations.
IDP
equals 0 if the potential is measured by a dipole with one electrode at infinity (ie. a "pole" recording), IDP=1 if the potential difference is measured with a dipole.
IDC
equals 0 if current is input by a dipole with one electrode at infinite (i.e. a "pole" source), IDC=1 if the current is input with a dipole.
XAn, XBn
X positions of the nth current electrodes in meters. When IDC=1, XB will be taken as the position of the negative current electrode. When IDC=0, XB is not used but the input field must remain.
Nn
number of measurements of potential corresponding to source location XAn and XBn.
XMi, XNi
X positions of the ith potential electrodes in metres for a given source location. There must be Nn potential locations for the nth source location. When IDP=1, XN will be taken as the position of the negative potential electrode. When IDP=0, XN is not used but the input field must remain.
VALi
measured value in Volts for potential, or a dimensionless real number (not percentage) for apparent chargeability at the ith potential electrodes. (The potential value is always normalized to unit current amplitude.) There are four types of IP data generally in use; two gathered in the time domain and two gathered in the frequency domain. For small chargeabilities, as is nearly always the case for earth materials, all data types can be used as input for inversion, and resulting models will have chargeabilities in the same units.
ERRi
standard deviation of the datum VALi. This is an absolute value and should not be specified as a percentage. This value should always be positive. If the first datum does not have an error field (or the error field is commented out by placing a "!" before it), the default errors for the entire data set will be calculated from the data.


Examples of the obs.dat file:

The following examples correspond to pole-dipole acquisition and having the potential electrodes on the left. Note that the electrode configuration is the same as that in the potgrid.dat example. Example-1 has user-supplied errors, while Example-2 and Example-3 cause the inversion program to calculate the default errors. 

Example-1: Data errors are supplied by the user in column-6. ("Standard" format) 

TOTAL POTENTIALS
pole-dipole
   -100.00   -100.00    -80.00    -70.00   8.47942E+00   4.31066E-01
   -100.00   -100.00    -70.00    -60.00   2.71912E+00   1.28015E-01
   -100.00   -100.00    -60.00    -50.00   1.14801E+00   5.61510E-02
   -100.00   -100.00    -50.00    -40.00   6.27136E-01   2.99415E-02
   -100.00   -100.00    -40.00    -30.00   2.88175E-01   1.47640E-02
    -90.00    -90.00    -80.00    -70.00   4.98173E+01   2.66027E+00
    -90.00    -90.00    -70.00    -60.00   8.77796E+00   4.74392E-01
    -90.00    -90.00    -60.00    -50.00   2.80881E+00   1.36039E-01
    -90.00    -90.00    -50.00    -40.00   1.10303E+00   5.59810E-02
    -90.00    -90.00    -40.00    -30.00   4.58344E-01   2.46371E-02
    -90.00    -90.00    -30.00    -20.00   1.60491E-01   8.69555E-03
     :
     :

Example-2: Default errors are used. Column-6 is blank. ("Standard" format) 

TOTAL POTENTIALS
pole-dipole
   -100.00   -100.00    -80.00    -70.00   8.47942E+00
   -100.00   -100.00    -70.00    -60.00   2.71912E+00
   -100.00   -100.00    -60.00    -50.00   1.14801E+00
   -100.00   -100.00    -50.00    -40.00   6.27136E-01
   -100.00   -100.00    -40.00    -30.00   2.88175E-01
    -90.00    -90.00    -80.00    -70.00   4.98173E+01
    -90.00    -90.00    -70.00    -60.00   8.77796E+00
    -90.00    -90.00    -60.00    -50.00   2.80881E+00
    -90.00    -90.00    -50.00    -40.00   1.10303E+00
    -90.00    -90.00    -40.00    -30.00   4.58344E-01
    -90.00    -90.00    -30.00    -20.00   1.60491E-01
     :
     :

Example-3: default errors are used. The column-4 of the first datum is commented out by an "!". This allows the user to override any previously assigned errors. ("Common-current" format) 

 TOTAL POTENTIALS                               
  17   1   0

   -100.00   -100.00    5
    -80.00    -70.00   0.847942E+01 !  0.431066E+00
    -70.00    -60.00   0.271912E+01   0.128015E+00
    -60.00    -50.00   0.114801E+01   0.561510E-01
    -50.00    -40.00   0.627136E+00   0.299415E-01
    -40.00    -30.00   0.288175E+00   0.147640E-01

    -90.00    -90.00    6
    -80.00    -70.00   0.498173E+02   0.266027E+01
    -70.00    -60.00   0.877796E+01   0.474392E+00
    -60.00    -50.00   0.280881E+01   0.136039E+00
    -50.00    -40.00   0.110303E+01   0.559810E-01
    -40.00    -30.00   0.458344E+00   0.246371E-01
    -30.00    -20.00   0.160491E+00   0.869555E-02
     :
     :

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