Overview

The Ground based towed TEM module is a powerful processing and inversion tool supporting LCI and SCI inversion for groundbased towed TEM data. It is integrated with the GIS interface and are designed for handling big datasets. Efficient processing filters for GPS and voltage data is applied in order to obtain optimal data quality. Fine tuning of the automatic data processing is done easily in a specially designed user interface. To obtain the best possible resistivity model the inversion also models transmitter waveform and loop shape, front gate filter, and low pass filters. All of these parameters are entered as hard parameters or soft constraints. Modelling and presentation to the customer is done with the well-known functions of the Aarhus Workbench so integrating with boreholes, creating reports etc. is easy and only a few clicks away. 

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Key features​​

• Efficient and easy to use data processing tools and filters

• Integrated GIS user interface which also open up for display of boreholes

• Visualization of models in thematic maps and on cross sections

• Data and inversion Quality Control (QC) visualization tool

• Laterally or Spatial constrained inversion with the AarhusInv inversion algorithm

• Parallel computing and easy off-load of inversion jobs to all computers in the office

 

Processing

The Aarhus Workbench data processing module is specially designed to handle and process big datasets. The data are stored in a comprehensive database structure together with comprehensive information on the airborne system. This is all managed by the program and the user does not need to be an EM expert to process and invert EM data in Aarhus Workbench. 

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The database serves as an archive for both raw and processed data and it records all processing steps and processing settings for documentation. 

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In general, the processing is performed with user adjustable automatic data filtering and stacking. The automatic processing can be inspected at different levels and user corrections added through the interface. In brief, the processing features are:
 

GPS

• Path tracking to close minor gaps in the GPS-records and for filtering outliers.

• Lag correction to move the transmitter-receiver coil configuration to the optimal focus point

• Support for differential GPS and digital elevation model from the GPS or from external grid

 

Voltage data

• Filtering of distorted data due to EM coupling to power lines and cables

• Adjustable trapezoid shaped stacking for optimizing and balancing the signal-to-noise-ratio and lateral resolution.

• Data uncertainties estimated from the stacked data

• Manuel inspection and corrections to the single data point level.

 

 

Inversion

Data can be imported into Aarhus Workbench together with key system parameters grouped in a separate data-file. After import, you have full access to the comprehensive inversion modules as well as the processing module.

To have full control of the data processing phase we advise importing the most achievable raw data and then perform the data processing in Aarhus Workbench. Alternative pre-processed data can be imported and inverted.
 

 

Constraints

Inversion of the TEM data in Aarhus Workbench is performed with the AarhusInv inversion code. Locally we use a 1D model. The models are laterally and spatially constrained forming pseudo 2D and 3D model spaces. The inversion code has been customized to handle data from very big TEM surveys and support multi CPU cores with a very high parallel efficiency. Besides providing the resistivity models, the inversion code also calculates a depth of investigation (DOI) and a data residual for each resistivity model.

 

• LCI-setup: The models are laterally constrained along the flight lines forming at 2D model space.

• SCI-setup: The models are laterally constrainted along the flight lines and across the flight lines, resulting in 3D constrained model space. A full survey can be inverted as one single SCI setup.

The laterally constraints can either work in depths or elevation.

 

Model types

The smoothness or sharpness in the model results is control by the strength of the constraints and the regularization scheme. Aarhus Workbench offers three main type of model discretization/regularization scheme:

 

• Smooth: The resistivity model is discretize using several layers (~10-20) with fixed 
  layer boundaries. The regularization penalizes vertical changes in resistivity, resulting in a vertical smooth resistivity model.

• Sharp: The resistivity model is discretize using several layers (~10-20) with fixed
  layer boundaries. The regularization penalizes the number of vertical resistivity
  transitions of a certain size, resulting in resistivity models with relative sharp
  vertical resistivity transitions. The sharp regularization scheme can also be
  applied for the laterally constraints.

• Layer: The resistivity model is characterized by a few number of layers (~4-5).
  Both layer thickness and resistivity are model parameters. No vertical
  regularization is applied which result in distinctly layered resistivity models with
  a fixed number of layers. A built-in routine estimates the numbers of layers needed to fit the dataset based on a smooth model inversion results.
   

Prior constraints

Aarhus Workbench also supports prion constraints on any model parameter. The prion constraints can be initialized from grids and direct from the GIS map or be specified at borehole locations with decreasing strength moving away from the borehole locations.

 

Accurate system modelling

To obtain high quality inversion results it is important to model the EM-system in great detail. We therefore include the following parameters in the modelling:

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• Shape of transmitter loop

• Transmitter waveform

• System low pass filters

• Front gate filter

• Width of the individual time gates

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