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

The same tool is used to add data to VOXI the first time and to subsequently modify it.

Use Data > Add Data on the VOXI Manager tree to set the type of inversion and provide the data channels for the geophysical modelling.

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

Measurements dialog options

Type of system

You can invert both Conductivity/Resistivity and IP and Spectral IP data. Select the appropriate system based on the database supplied in the previous dialog.

Type of model

The model types are a function of the IP system specified above and in the first dialog. The model type selection list is contextual and is populated dynamically.

The supported model types for the Conductivity/Resistivity and IP system are:

  • Conductivity or Resistivity (subject to the selection in the first dialog)
  • Chargeability
You must first invert the data to obtain a conductivity or resistivity model in order to be able to proceed and invert for a chargeability model.

The supported model types for the Spectral IP system are:

  • Complex conductivity or resistivity (subject to the selection in the first dialog)
  • Conductivity or Resistivity amplitude
  • Conductivity or Resistivity phase
The Complex conductivity/resistivity option inverts both the amplitude and phase models. These can also be modelled separately.

Field data

This is a contextual parameter subject to the type of system and model. You will be prompted for the data necessary to invert the selected model(s).

You will not be able to proceed until you have provided all the necessary information.

If you are modelling:

  • Conductivity of a Conductivity/Resistivity and IP system - you will be prompted for Voltage.

  • Chargeability of a Conductivity/Resistivity and IP system - you will be prompted for IP.

    The conductivity/resistivity model is an input to the chargeability inversion.

If you select:

  • The Spectral IP system - you will be prompted for both the Amplitude and Phase data, regardless of the desired output model.

IP type

This parameter is contextual and appears if you have selected Chargeability as the model type. Different types of data can be used to model chargeability, all of which are mathematically interrelated. Specify the nature of the field data you have supplied for the inversion process.

  • When inverting time domain data, the common IP type selection is Apparent Chargeability.
  • The Vs/Vp option triggers a true non-linear inversion of the selected Field data channel.
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    Mask channel

    		 Prior to modelling, the data must be QAQC’d. During the QAQC stage, some data values are flagged for exclusion. Good points are flagged as "1" and bad points as dummies. This information is stored in a channel. Select the Mask channel to exclude the bad data from being used in the inversion process.

    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 expense 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 (Error value parameter) 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 as 5% relative error (Fraction parameter), and the minimum threshold  is set to a value which is equivalent to 5% of the signal input data for voltage (Minimum error parameter).

    Data error channel

    		 You may also define the fit error for each data point by providing an error channel (Error channel parameter). 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 it will be defined as a fraction of standard deviation of the data. The default fraction of standard deviation is 5% of the standard deviation (Fraction of standard deviation parameter).

    Application Notes

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

    • The VOXI inversion process - subject to the constraints provided - will attempt to completely explain the measurements through the physical parameters of the model.

    To learn more about effective workflows and key concepts, access the Seequent Learning Centre Catalogue, select VOXI from the Category list, and explore your learning options.

    See Also: