Gradient Reference Model

Use Constraints > Add Constraint > Gradient Reference Model or Gradient Reference Model > Modify to define the model that the inversion will tend towards, if there is no influence from the data.

Modify Gradient Reference Model Constraint dialog box options

Constraint type

By Default the gradient reference model is set to zero for all voxel elements. If the inversion model type is MVI then all three components of the vector voxel model elements are set to zero. For all model types other than MVI, you may provide a Constant other than zero for the entire model. You may have a prior knowledge of the characteristics of the study area, where as the geophysical response of the area is generally high and setting this value to a value >0 may speed up the process. Alternatively if you have derived through earlier processes a plausible Voxel model for the study area, you may choose to provide it instead. For the MVI model types, the Constant option is not available, because conceptually it is difficult to derive. If you however have created through earlier processes a plausible Vector Voxel model for the study area you may select it.

Filename

If theVector Voxeloption for MVI or the Voxel option for the other model type has been chosen, you are prompted to provide the name fo the file that defines the gradient model. This file is re-gridded to the VOXI document mesh interval and extrapolated to the extents of the padded mesh.

The Gradient Reference Model is often chosen to be the same as the Parameter Reference Model.

Application Notes

The Gradient Reference Model is used together with the EW | NS | Vertical Gradient Weighting constraint(s). It is the model to which the inversion will tend in the absence of influence from the data. The effect of this model is usually most pronounced at depth or when the data are sparsely sampled.

There are two end member situations: the user knows nothing about the physical property distribution in the inversion domain; or the user has a good model of the rate of change of the property distribution in the inversion domain, perhaps from activities such as extensive drilling. In the former case, the Gradient Reference Model can be left at the default setting of zero, whereas in the latter case, that known physical property model should be used as the Gradient Reference Model.