Add/Modify Data EM (2/3) - Measurements

This same tool is used to provide further information about the database that contains the observed data and to define the channels to use for the inversion when using for the first time, and to subsequently modify it.

Use Data > Add Data on the VOXI Manager tree to provide information about the database containing the observed data, for the modelling.

Subsequently, to modify the data of an existing VOXI document use Data > Data Source > Modify.

Measurements dialog options

Model type

 The content of the dropdown list is dynamic and depends on the selected data source. If the data source is FD-Electromagnetics, Conductivity will be the only available Model type.

Type of data

The supported data types for the Conductivity model in frequency domain are:

• DIGHEM

• RESOLVE

Each data type requires its own specific entries. The system configuration of the selected data type determines the supported frequencies and dynamically updates the dialog to prompt you for them.

Field data

Prior to modelling, the data must be corrected to minimize the noise, lag and drift. One tab per supported frequency is dynamically added to the dialog. The in-phase and quadrature channels for each frequency are provided in the appropriate tab. For each frequency, if the corresponding in-phase and quadrature channels are auto-populated if they are present in the database. You can override the selection. If you would like to exclude specific channels from the inversion process, set the entry to <Not used>. Channels should be provided in pairs. The check mark on the tab label indicates which frequencies will be inverted.

Fit error

VOXI will attempt to fit your data until the difference between the model prediction (the fit) and the measured data is on average less than the Fit error. Increasing the Fit error focuses the inversion on features of a larger magnitude at the expensed of overlooking the small variations, while decreasing the Fit Error focuses on the subtler features and enhances the structure. Four fit error methods are available. After you select the type, the dialog will adjust to prompt you for the related parameters as described below.

Absolute error

A single constant error value is used for all data points. By default, the Absolute error value is set to 5% of the standard deviation of the data. You can typically accept this default, which gives acceptable results in most cases. If your data is well controlled with little noise, i.e., the model should explain all the data, then you may choose to reduce the Absolute error. In this case, you may start by halving this value. On the other hand, if you find your data is very noisy and the modelling fits the noise, increase the Absolute error.

Relative error

The error will be a fraction of the measurement at each observation above a minimum error threshold. The default errors are populated by as 10% relative error, and the minimum threshold is set to 5 ppm.

Data error channel

 You may also define the fit error for each data point by providing an error channel. This might be useful where you have some other reason for fitting parts of the measured data relatively less or more tightly than other parts.

Fraction of standard deviation

 In this case, the error will be a fraction of the overall measurements, and will be defined as a Fraction of standard deviation of the data. The default fraction of standard deviation is 5% of the standard deviation.

EM Frequency Domain system

The standard system configuration parameters are always loaded. The coil index belonging to the frequency that is at the front (in focus) are displayed. To cycle through the coin parameters, simply cycle through the tabs.

You can customize the system configuration and Save it to a file under a different name for future use. The latter configuration will become part of your system and will be available as a Type of data in your future VOXI-EM modelling sessions.

  • Although defaults are calculated for all the entries of the Measurements tab, it is recommended that you review them prior to inverting the data. The intent is to make sure you accept the default settings.

  • The mesh is generated according to the selected "Type of data", as a result if you subsequently opt to modify this VOXI document, you are encouraged to maintain the mesh resolution within a reasonable range.

    • Application Notes

    The VOXI inversion process - subject to the constraints provided - will attempt to completely explain the residual measurements through the physical parameters of the model. Response from sources outside of the VOXI model (including padding) would have a broad regional effect and is of no interest to the project, thus it should be removed from the data. In order to eliminate these effects, the data subject to inversion should be the residual data.

    Access Seequent Online Learning and select the VOXI guided paths to learn more about effective workflows and key concepts.

    See Also: