Geologically constrained inversion of airborne TEM data

Two computer programs have been written to perform rapid TEM inversion in a 3D framework. Both programs operate on a 3D geological model, which facilitates integrated interpretation. In greenfields areas the starting model can degenerate into a discretised homogeneous half-space, to permit “unconstrained” inversion. VPem1D performs 1D inversion at each TEM data location. It is therefore a hybrid 1D/3D approach, ideal for quasi-layered environments. VPem3D performs 3D inversion on time-integrated (resistive limit) data, and is well suited for interpretation of compact conductive targets. Because the programs can invert a geological model, they permit a variety of inversion styles. For example, if one or more geological units are considered uniform in conductivity, the optimal conductivities can be determined for the entire survey area via homogeneous unit inversion. If some geological units are variable in conductivity, heterogeneous unit inversion can be applied to those units alone. Alternatively, because geological interfaces are captured in the model, geometry inversion can be used to adjust the shape of contacts or structures. For example, interpretation of cover thickness is a natural application for VPem1D. Both programs are applicable to data from variety of systems including (but not limited to) Geotem, Tempest, VTEM, Spectrem, SkyTEM, MegaTEM and Hoistem. This paper will illustrate some of the different inversion options on Spectrem data sets from Australia and Brazil.