Interface

Simulating geophysical data from a physical property model requires three things: a computational mesh, a discretization of the model within that mesh, and a means to simulate the data. Plate simulation includes a module for generating a simple two-layer model with embedded plate anomalies within octree meshes. This section discusses all three of these components, their interface exposed by the ui.json file, and the storage of results.

Fig. 63 Merged images of both tabs of the ui.json rendered interface.

Geological Model

Plate simulation includes a module for generating plates embedded in a two-layer Earth model within octree meshes. Many permutations of this simple geological scenario result in a complex interface. To simplify this, the discussion is organized into two sub-sections: background (basement and overburden) and plates. All model values within plate-simulation must be provided in SI units that varies depending on the chosen forward simulation (g/cc, SI or Ohm.m)

Background

The basement resistivity is actually closer to a halfspace in the sense that it fills the model anywhere outside of the overburden and plate. Therefore, the basement resistivity should be chosen as an effective resistivity for the whole geological section. This approach is quite reasonable for most applications where the differences in resistivity between layers are much smaller than the difference between overburden and any anomalous bodies (plates).

Fig. 64 Basement resistivity option.

The overburden is discretized by the resistivity and thickness of the layer. The thickness is referenced to the earth-air interface and extends into the earth by the amount specified in the thickness parameter.

Fig. 65 Overburden resistivity and thickness options.

Fig. 66 Model section highlighting the overburden and basement boundary.

Plates

This section discusses the various plate options available through the ui.json and their impact on the resulting discretized model.

Fig. 67 Plate options available in the ui.json.

The first set of options allows the user to specify the number of plates and their spacing.

Fig. 68 Number of plates and spacing options.

For all choices of n>1, the plates are evenly spaced at the requested spacing and share the same resistivity, size, and orientation.

Fig. 69 Model created by choosing three plates spaced at 200m.

The plate resistivity must be entered in SI units (g/cc, SI or Ohm.m).

Fig. 70 Plate resistivity option.

The size of the plate is defined by three parameters: thickness, strike length, and dip length.

Fig. 71 Plate size options.

The image below shows a dipping plate with annotations indicating the size parameters for that particular plate.

Fig. 72 A dipping plate striking northeast with annotations for its thickness, strike length and dip length.

The plate orientation is defined in terms of dip and dip direction. The dip is the angle between the horizontal projection of the plate normal and the plate tangent sharing the same origin. The dip direction is measured between the horizontal projection of the plate normal and the North arrow. The image below provides a visual representation of these angles.

Fig. 73 Plate orientation options. Plate orientation is given as a dip and dip direction. The dip (b) is defined as the angle between the horizontal the projection of the plate normal (n') and the plate tangent sharing the same origin (t). The dip direction (a) is the angle measured between the horizontal projection of the plate normal (n') and due north (N).

The plate location can be specified in both relative and absolute terms. The position parameters are given as easting, northing, and elevation. If the relative locations checkbox is selected, the easting and northing are relative to the center of the survey and the elevation is relative to one of the available references. The elevation may be referenced to either the earth-air interface or the overburden via the Depth reference dropdown. Either choice can be relative to the minimum, maximum, or mean of the points making up the reference surface as given by the Reference type dropdown. In all these cases, the distance provided acts as a depth below the reference to the top of plate in the z negative down convention. If the relative locations checkbox is not selected, the easting, northing, and elevation specify the center location of the plate.

Fig. 74 Plate location options in relative mode. Notice the Elevation is given as negative to ensure the top of the plate is below the selected min of the overburden.

Fig. 75 Example of a relative elevation referenced 100m below the minimum of the overburden layer.

Data Simulation

The simulation parameters control the forward modeling of the plate model discretized within the octree mesh. Rather than exposing parameters within the plate simulation interface, the application allows the user to select an existing forward modelling SimPEG group. The user must ensure that the SimPEG group has been previously edited with appropriate options, includes at least a topography and survey object, and has selected one or more components to simulate. The user may also provide a name for the new SimPEG group to store the results.

Fig. 76 Selecting the initialized forward modelling SimPEG group and naming the group that will store the plate simulation results.

Create the required SimPEG group within Geoscience ANALYST through the Geophysics menu under SimPEG Python Interface entry.

Fig. 77 Creating a SimPEG group to be selected within the plate simulation interface.

Edit the options by right-clicking the group and selecting ‘Edit Options’.

Since plate-simulation creates its own mesh and model, the mesh and conductivity selections can be ignored. Selecting a value does not conflict with the plate-simulation objects and is simply ignored. In contrast, the survey, topography, and at least one component must be selected to run the simulation.

Fig. 78 Simulation options with annotations for required and not required components.

Octree Mesh

To accurately simulate the earth model, the mesh must be refined in key areas while remaining coarse enough elsewhere to efficiently simulate data. Plate simulation includes refinements at the earth-air interface, the transmitter and receiver sites, and on the surface of plates.

Fig. 79 Octree mesh refinement for earth-air interface, receiver sites, and within the mesh.

The meshing is controlled by options exposed in the ui.json. These options are significantly reduced compared with octree creation from grid-app, as many parameters have been tailored to suit the needs of plate simulation.

Fig. 80 Octree mesh parameters exposed in the ui.json.

Results

The results of the simulation are stored in the SimPEG group named in the simpeg group option section.

Fig. 81 Results group containing a survey object with all the simulated data channels stored in property groups, and an octree mesh containing the model parameterized in the interface.

To iterate on the design of experiment, copy the options, edit them, and run again.

Fig. 82 Copying the options to run a new simulation.