Fast 3D inversion of transient electromagnetic (TEM) resistive limit data

Rapid interpretation transient electromagnetic (TEM) data sets is highly desirable for timely decision-making in exploration. However, full solution 3D inversion of TEM data sets is often still not feasible on current day PCs. Therefore, a fast 3D TEM inversion scheme has been developed for time-integrated (resistive limit) data. The resistive limits are amenable to linear 3D magnetic inversion, which is up to 100 times faster than “rigorous” 3D TEM inversion. The evolution of the decay lost during time integration can be recovered in large part by constructing a starting model based on conductivity-depth images (CDIs) or 1D inversion, by applying depth weights, and by imposing geological constraints if available. Incorporation of geological constraints reduces the non-uniqueness of any 3D TEM inversion. Integrated interpretation is facilitated here by performing inversion on a geological model, i.e. one attributed with lithology as well as conductivity. Geological models also offer a number of practical advantages over pure property models during inversion. In particular, they permit adjustment of geological boundaries. In addition, optimal conductivities can be determined for homogeneous units. The resistive limit inversion scheme has been successfully tested on both synthetic and real airborne, ground, and downhole TEM. It is illustrated here via application to a Spectrem data set from Brazil.