Forward Modeling of the Induction Log Response of a Fractured Geologic Formation

Induction logging is a well-developed geophysical method with multiple applications. It has been used extensively in academic research as well as in industry. Induction logging is a controlled-source electromagnetic (CSEM) exploration method. It characterizes geologic formations through the measurements of induced magnetics fields. The purpose of this research project is to better understand induction logs and the effects fractured geologic formations have on them.

Computer modeling is used to generate synthetic logs for analysis in this research project. The original program required certain modifications to fit this research project?s goals. The computer program, Seatem is based on the finite element method. It is able to use a layered Earth model that is the basis for the synthetic log analysis. The geologic layers in this model are assigned various conductivities and also have the option of being assigned a geologic roughness value. The geologic roughness parameter is used to simulate fractured rocks in the subsurface.

The synthetic logs generated by the modified Seatem program produce some encouraging results. In a thinning bed analysis, it is shown that as a conductive bed is thinned in a step-size procedure, the resulting induction log underestimates the actual conductivity of the layer. It also shows that the boundary layers around the thinned layer are better characterized in the log. The next synthetic log was calculated for a fractured resistive layer. This log shows that as the layer becomes more fractured, there is an increase in the underestimation of the actual conductivity. This layer is then thinned down and another synthetic log is calculated. The resulting log shows similar traits to the thinning bed analysis and shows an underestimation of the apparent conductivity. The same procedure is performed for a fractured conductive layer. The analysis produce similar results; however, that are much more drastic changes in the induction logs. As the unit becomes more fractured, the apparent conductivity is lower then the actual conductivity, as in the resistive case. However, smaller increases in the roughness parameter produced more severe underestimations than larger increases in the roughness parameter did for the resistive layer.