3.3.3. Observations File¶
This file is input when inverting field-collected data. This file contains the survey information, field observations and data uncertainties.
Important
As of May 2018, the user must invert all 4 components of the impedance tensor for MT data OR both components of the transfer function for ZTEM data. Also, flags cannot be used to omit data points.
3.3.3.1. Format¶
Note
Bolded entries are fixed flags recognized by the Fortran codes and blue hyperlinked entries are values/regular expressions specified by the user
Each unique data type, frequency and set of observation locations corresponds to a unique “transmitter”; e.g. 2 transmitters must be specified if the same data are collected at the same locations at 2 different frequencies.
Each block contains DATATYPE, FREQUENCY, N_RECV and the data array
The lines of a data file with one or more transmitters are formatted as follows:
Fig. 3.8 Example data file for MTZ data.¶
3.3.3.2. Parameter Descriptions¶
(A) Number of transmitters: In line 1, the number of transmitters/groups of natural source EM data is specified. Example: N_TRX 3
(B) Flag to ignore data entries: A regular expression is entered, signifying data in the data structure which is ignored during the inversion. Example: !IGNORE -0
(C) Data type:. For the data corresponding to each transmitter, this line sets the type of data. Example: DATATYPE MTZ. There are 4 options for DATATYPE:
“MTZ” - MT data (Both real and imaginary impedance tensor data)
“MTT” - ZTEM data (Hx and Hy constant at first receiver location and first receiver station defines base station)
“MTE” - ZTEM data where Hx, Hy are calculated at the base station from the initial model
“MTH” - ZTEM data (reference is at the data points - no base station)
Important
When modeling MT and ZTEM data simultaneously, you must choose either type MTZ and MTT or MT and MTE or MTZ and MTH; e.g. you cannot have MTT, MTE and MTH in the same observations file.
(D) Frequency: Frequency at which the corresponding set of field observations are made. Example: FREQUENCY 1.0000E+002.
(E) Number of receivers: Number of receivers collecting data at the aforementioned frequency for the aforementioned data type. Example: N_RECV 900.
3.3.3.3. Data Arrays by Type¶
MT data (DATATYPE = MTZ):
Each row in the array contains the elements of the impedance tensor at a particular location separated into real and imaginary components, along with the corresponding uncertainties. The units for MT data are (V/A). Download MT data example . The columns for this data format are as follows:
such that each \(Z_{ij} \; data\) is comprised of 4 columns:
where
\(Z^\prime_{ij}\) is the real component of entry i,j of the impedance tensor
\(Z^{\prime\prime}_{ij}\) is the imaginary component of entry i,j of the impedance tensor
\(U^\prime_{ij}\) is the uncertainty on \(Z^\prime_{ij}\)
\(U^{\prime\prime}_{ij}\) is the uncertainty on \(Z^{\prime\prime}_{ij}\)
Important
For standard MT data, X = Northing, Y = Easting and Z = Down; which this code uses! Thus \(Z_{xy}\) is essentially the ratio of the electric field along the Northing and the magnetic field along the Easting. For more, see the theory section.
ZTEM data (DATATYPE = MTT, MTE or MTH):
Each row in the array contains the elements of the transfer function at a particular location separated into real and imaginary components, along with the corresponding uncertainties. Data values and uncertainties are unitless with no normalization factor. Download ZTEM data example . The columns for this data format are as follows:
such that each \(T_{zx} \; data\) is comprised of 4 columns:
where
\(T^\prime_{zx}\) is the real component of \(T_{zx}\)
\(T^{\prime\prime}_{zx}\) is the imaginary component of \(T_{zx}\)
\(U^\prime_{zx}\) is the uncertainty on \(T^\prime_{zx}\)
\(U^{\prime\prime}_{zx}\) is the uncertainty on \(T^{\prime\prime}_{zx}\)
and similarly for \(y\).
Important
For standard natural source data, X = Northing, Y = Easting and Z = Down; which this code uses! Thus \(T_{zx}\) is the transfer function related to an incident plane wave whose electric field is polarized along the Northing direction; which produces magnetic fields with components in the Easting direction. For more, see the theory section.
Important
If MT and/or ZTEM data are being modeled, the frequencies do not need to match nor do the locations for each frequency.
For MTT and MTE data (ZTEM), the first line in the array refers to the base/reference station location. Only the Easting, Northing and elevation is required. However, each remaining field must be given a flag value of “i”. Example for first row: \(350 \;\; 200 \;\; 0 \;\; i \;\; i \;\; i \;\; i \;\; i \;\; i \;\; i \;\; i\)
For MTH data (ZTEM), measurements Hx, Hy and Hz are taken at different locations. Data and uncertainty values are required for all rows.
For MTT and MTE data (ZTEM), the first line in the array refers to the base/reference station location. Thus if there are \(N\) receiver locations specified for a given array with data type “MTT”, the inversion will output \(N-1\) rows of predicted data in the predicted data files.
For MTH data (ZTEM), measurements Hx, Hy and Hz are taken at the same location. Thus if there are \(N\) receiver locations specified for a given array with data type “MTH”, the inversion model will output \(N\) rows of predicted data in the predicted data files.