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Geophysical Theme

A geophysical theme is a derivation of geophysical properties from the models of a database query.

A geophysical theme appears as a subordinate to a Model Selection node in the Workspace Manager and if put on the map directly, simply show the positions of the models from which it was possible to extract the specified geophysical property. The extracted geophysical properties may then subsequently either be shown as a point theme node (PointTheme) or be gridded (GridTheme) and then given color to be shown as an image node (NewImage). These nodes appears as subordinate to the theme in the Workspace Manager.

To create geophysical themes, the user must first select a Model Selection in the Workspace Manager and then select Theme from the Visualization ribbon. The user is then asked to select the Theme Type. There is a number of different Theme Types, with a few exceptions they all share Blinding Settings that can be used to exclude part of the model from the theme.

No Blinding allows the themes to be made from the full model with the last layer going down to infinite depth.

Blind at DOI Conservative or Blind at DOI Standard exclude the part of the model that is below that selected DOI value.

Blind at Factor, exclude the part of the model that is below a depth of the chosen dropdown factor, times the depth of the last layer boundary.

Usually it is a good idea to blind at DOI Standard since the data had very little sensitivity to the information below that value.

Also common for all themes is the Edit Display options where the user can select the Symbol, Size and Color of the theme points.

Finally there is a Histogram (HistogramAndStatistics) option that can be used to see a histogram and some statistics about the models.

Mean Parameters

This theme is used to create several depth or elevation slices of the mean resistivity / conductivity and IP parameters Chargeability/Phi, TauRho/TauPhi and C. Each interval appears as an independent geophysical theme in the Workspace Manager.

In the dropdown menu Property to extract the user can choose between the above parameters. The vertical resistivity is simply the mean model resistivity in the depth interval, while the horizontal resistivity is the mean of the inverse model conductivity in the depth interval. A relatively conductive layer weighs heavier in the latter approach.

Note that the currents with the TEM method only flows horizontally in the ground, it can therefore be argued that one should be using Horizontal Resistivity rather than Vertical Resistivity for TEM data, not everyone agrees with this however.

The intervals are calculated for either Depth or Elevation.

Top of the set of layers, an Interval Length, and a Number of Intervals. The resulting set of layers is listed further down on the form. The value for Top is given as a Depth or as an Elevation depending on the type of intervals to be calculated. When Apply is pressed the layers from the list is created and added to the Workspace Manager under the DBQ. The names of the layers will automaticly reflect the intervals, but it can still be a good idea to add on D for depth or E for elevation to the names.

The user can continue to specify additional sets of layers and also add those to the Workspace Manager.

For elevation intervals it is practical to use the Histogram button to see the listed elevation of the topography for the models in the area to get an upper limit. When blinding at the DOI, it can similarly be practical to see the listed DOI vales for the models in the area to get a lower limit to how far down it at most makes sense to make intervals. If the layer would have been emtpy, it will simply not be made.

The final option is the Surface Slope Correction (by DEM). Using a DEM node that has been imported to the Workspace, it is possible to correct the position of the theme interval positions, based on the slope of the DEM at the surface of the model positions, so that the theme interval positions are placed normal to the surface rather than simply vertical from the surface.

Layer Parameters

This theme derives properties for specific layers.

In the dropdown menu Property to extract the user can choose between Resistivity, Conductivity, Thickness, Depth top of layer, Depth bottom of layer, Elevation top of layer, Elevation bottom of layer, Resistivity STD, Conductivity STD, Thickness STD and Depth bottom STD, and also the equivalent IP parameters for Chargeability/Phi, TauRho/TauPhi and C.

Layer is the layer or layers from which the properties are extracted. The layers can be specified with a line like 1-5,8,11-13 and will, when there is more than one, get _LayerX added to the node name.

From Top or From Bottom decide whether Layer is counted from the top or bottom.

This theme searches every model for a layer that match the user-defined criteria. When a matching layer is encountered, the search moves to next model.

In the first dropdown menu Property to extract the user can choose the property to extract from the layers that meet the criteria defined below. The user can choose from Resistivity, Conductivity, Thickness, Depth top of layer, Depth bottom of layer, Elevation top of layer, Elevation bottom of layer, Resistivity STD, Conductivity STD, Thickness STD, Depth bottom STD and also the equivalent IP parameters for Chargeability/Phi, TauRho/TauPhi and C.

  • Layer thickness activates a criterion on layer thickness.
     
  • Depth top of layer activates a criterion on the depth top of the layer.
     
  • Elev. top of layer activates a criterion on the top of the layer.

In the second dropdown Layer Property the user chooses which property to use as criterion.

  • Layer resistivity. If checked a criterion is activated on the layer resistivities. The criterion is defined as an interval. If a “larger than” or “smaller than” search is desired, extreme values are entered as one of the interval limits. For example layers with resistivities higher than 80 ohm-m are found by entering “80” in Min. and “99999” in Max.
     
  • Res. Layer above activates a criterion on the resistivity of the layer above.
     
  • Res. Layer below activates a criterion on the resistivity of the layer below.

 

The equivalent properties for IP parameters Chargeability/Phi, TauRho/TauPhi and C is also available.

Finally, the user must decide whether to initiate the Search from top or Search from bottom. This is important if more than one layer in a model match the criteria above.

Cumulative Layer Seach

This theme searches every model for a cumulative layer that match the user-defined criteria. When a matching cumulative layer is encountered, the search moves to next model. This is a variant of General Layer Seach.

In the dropdown menu Property to extract the user can choose the property to extract from the layers that meet the criteria defined below. The user can choose from Cumulative Thickness, Depth top of layer, Depth bottom of layer, Elevation top of layer and Elevation bottom of layer.

  • Layer thickness activates a criterion on layer thickness.
     
  • Depth top of layer activates a criterion on the depth top of the layer.
     
  • Elev. top of layer activates a criterion on the top of the layer.


In the second dropdown Layer Property the user chooses which property to use as criterion.
 

  • Layer resistivity. If checked a criterion is activated on the layer resistivities. The criterion is defined as an interval. If a “larger than” or “smaller than” search is desired, extreme values are entered as one of the interval limits. E.g. layers with resistivities higher than 80 ohm-m are found by entering “80” in Min. and “99999” in 
    Max.

The equivalent properties for IP parameters Chargeability/Phi, TauRho/TauPhi and C is also available.

Finally, the user must decide whether to initiate the Search from top or Search from bottom. This is important if more than one layer in a model match the criteria above.

For cumulative thickness, it is worth noting that it always will include the thickness of the layer as a whole, even if only part of the layer live up to the other criterion. For example one could be looking for the cumulative thickness of layers with layer resistivity below 10 within 100 m from the surface, but unless the layer exactly stopped at 100 m, one could end up adding the full layer thickness of the layer at 100 m depth.

Low Resistivity Layer

This theme searches every model for a low resistivity layer that match the user-defined criteria. When a matching low resistivity layer is encountered, the search moves to next model. This theme is a special case of a General Layer Seach that often is relevant in the Danish geological settings. In geological settings with double descending low-resistive layers it will not be applicable.

In the dropdown menu Property to extract the user can choose the property to extract from the layers that meet the criteria defined below. The user can choose from Resistivity, Conductivity, Depth top of layer, Depth bottom of layer, Elevation top of layer, Elevation bottom of layer, Resistivity STD, Conductivity STD, Depth bottom STD and Layer Number. If Depth/Elevation bottom of layer is extracted and the criteria is met by the deepest layer of the model, the model is NOT included in the extraction as the bottom of the deepest layer is undefined.

Min. Res in Layer Above activates a criterion on the min. resistivity of the layer above the low resistivity layer. For example, if this value is set 50 ohm-m, the low resistivity layer defined below must be overlain by a layer with a resistivity above 50 ohm-m. This is relevant if the user wants the properties of a low resistivity layer constituting the lower boundary of a sandy aquifer (50-80 ohm-m).

Max. Layer Resistivity activates a criterion on the max. resitivity of the low resistivity layer.

Min. Layer Thickness defines the minimum thickness of low resistivity layer. This criterion is only checked if the box also is checked.

Low Resistivity Layer always find the deepest layer that fulfills the defined criteria (equivalent to using the Search from bottom option with General Layer Search).

Clay Thickness

This theme has been developed in cooperation with the County of Aarhus, Denmark. It is meant as an estimate of the clay content in the upper part of the earth. The clay content is interesting as a high clay content serves as a membrane protecting an underlying aquifer. The settings are:

Depth limit is the lower limit of the interval in which the clay thickness is calculated. The upper limit is always the surface. The default value is 30 m, i.e. the clay thickness is calculated in the depth interval from 0 to 30 m.

Resistivity filter defines the translation from interval resistivity to interval clay thickness. The settings mean:

Cut off begin is the “beginning to the end” of the resistivity interval, where clay is supposed to be present. If a layer in the depth interval 0 to Lower depth limit has a resistivity below Cut off begin, the entire thickness of the layer is translated to clay thickness. For example if a model has a first layer with a resistivity of 15 ohm-m and a thickness of 50 m, the Clay thickness in this model is 30 m (if Lower depth limit is the default 30 m). In other words: If a layer is defined in the depth interval between 0 and Lower depth limit and has a resistivity below Cut off begin, it is assigned a weight of 1 in the calculation of clay thickness.

Cut off end is the “end of the end” of resistivity interval, where clay is supposed to be present. If a layer has a resistivity above Cut off end it is assigned the weight 0 in calculation of Clay thickness. If a layer has a resistivity between Cut off begin and Cut off end it is assigned a weight between 1 and 0 depending on the filter type described next.

The thickness weight assigned layers with resistivities between Cut off begin and Cut off end is calculated using a logarithmic function, where the end points are (Cut off begin,1) and (Cut off end,0). Example: Lower depth limit = 30 m, Cut off begin = 40 ohm-m, Cut off end = 60 ohm-m, resistivity first layer = 56 ohm-m, thickness first layer = 19 m, resistivity second layer = 91 ohm-m, thickness second layer = 65 m and resistivity third and final layer = 10 ohm-m. As the third layer is not defined in the depth interval 0 to 30 m, it does not contribute to the Clay thickness. The second layer lies within the depth interval, but the resistivity of the layer is more than Cut off end so this layer does not contribute to the clay thickness either. The resistivity of the first layer lies between Cut off begin and Cut off end. The thickness weight is 1 - (Log(Layer resistivity / Cut off begin) / (Log(Cut off end / Cut off begin)) = 1 - (Log(56 / 40) / (Log(60 / 40)) = 0.17. Hence, the Clay thickness is 0.17 * 19 m = 3.2 m.

This rather simple Clay thickness transform has proved its worth in the aquifer vulnerability work of the County of Aarhus, Denmark. The transform is not necessarily applicable anywhere else in the world, and the settings must be thoroughly considered prior to application.

DOI (Depth of Investigation)

This theme searches every model for the DOI of the model.

In the dropdown menu Property to extract the user can choose the property to extract from the layers that meet the criteria defined below. The user can choose from DOI Conservative, DOI Standard, DOI Conservative as elevation and DOI Standard as elevation.

In the second dropdown, the user can choose the parameter from which the DOI should be extracted.