Options in Inversion tab

Inversion implies fitting model parameters to observations to reach the minimum misfit between measured and computed EM fields. The best fit parameters make up a resistivity section (Fig. 1). Depending on geological setting and availability of well logs, the model parameters are either locked or allowed to vary and fitted automatically. Inversion with reference to a priori geological data is more labor-consuming but of much a higher quality.


Example resistivity section obtained by inversion in TDEM Geomodel. Fig. 1. Example resistivity section obtained by inversion in TDEM Geomodel.


Inversion with reference to a priori geological data


The use of a priori geological data allows improving the quality of inversion by reducing the solution equivalence.

The procedure begins with soundings recorded near wells, which are used for reference. The first step is to obtain a model with locked thicknesses of layers distinguished using well logs (Fig. 2). At the next step, the reference responses are inverted to estimate layer resistivities.


Inversion EM data. Estimating layer resistivity at locked logging-derived layer thicknesses. Fig. 2. Estimating layer resistivity at locked logging-derived layer thicknesses.

Inversion of reference curves yields layer resistivity at the well site. Thus estimated resistivity of some layers can be locked as well (Fig. 3) to reduce the equivalence of inverse solutions. To use the edited resistivity model as a starting one for inversion of nearby curves of the same type, press (take starting model).


EM data inversion. Edit starting model. Fig. 3. Locking resistivity derived from a reference response and saving the starting model to be used in further inversion.

As the following step, select a group of nearby soundings of the same type and run automatic inversion (Figs. 4 and 5).

EM data inversion. Automatic inversion along profile.
Fig. 4. Selecting a group of nearby soundings of the same type for automatic inversion along profile.

EM data inversion. Automatic inversion along profile. Fig. 5. Result of automatic inversion of several similar soundings.

Inversion is run simultaneously for a group of stations selected previously in Profile tab. The resistivity section obtained by automatic inversion can be edited with reference to geological data to find the best fitting number of layers or vary equivalent parameters (e.g., resistivities and thicknesses of thin layers). Otherwise, inversion with reference to a priori evidence may use locked geological boundaries to estimate layer resistivities. This kind of inversion commonly applies to data with geological boundaries constrained by seismic petroleum exploration while resistivity estimates are expected to confirm (or not) the presence of oil or gas in a prospect zone.

Note that inversion with locked formation parameters allows reducing the equivalence and improving the quality of inversion. The geological boundaries and resistivities can be accurate to 5 %. Therefore, it is important to refer to these data, when available, and to use the inverted TEM data for geological interpretation.