Fast 3D inversion of “total field” resistive limit TEM data

Abstract

Rapid interpretation of 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 unpractically slow. Therefore, a fast approximate 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 resistive limit inversion scheme is suitable for airborne, ground, and downhole TEM, both dB/dt and B-field. Its efficacy is illustrated here via application to a heli-borne sub-audio magnetic (HeliSAM) data set recorded over the Lalor Zn-Cu-Au VMS deposit in Manitoba, Canada. The response from the deposit is clear in the “total field” EM (TFEM) data even though the mineralisation is very deep, extending from depth 575m to over 1100m. A three-stage inversion of resistive limits derived from the TFEM rapidly defined a 3D conductor below the uppermost pyrite-sphalerite lenses, enclosing a volume containing mainly pyrrhotite-chalcopyrite stringer sulphides. Total inversion time was less than one minute on a notebook PC.