Three-Dimensional Modeling of Transient Electromagnetic Data from Queensland, Australia
An airborne transient electromagnetic (EM) survey for mineral deposits in Queensland, Australia, showed an anomaly, which later was resurveyed with moving in-loop and large fixed-loop transient EM systems. The moving in-loop profiles show a broad positive anomaly at early times and a corresponding negative anomaly at late times, possibly caused by a shallow polarizable source. The fixed-loop profiles show a strong crossover anomaly with the possibility of a deep conductor. To determine whether the anomaly was caused by a paleochannel or a deep thick vertical conductor with potential mineralization, 3-D numerical modeling was done with the program SYSEM.
A comparison of modeling results and field data led to the conclusion that the source of the EM anomaly was likely to be a polarizable paleochannel. This was proved by subsequent drilling. The strong anomaly observed in the fixed-loop data and the shift of its position for two transmitter positions were caused mainly by channeling currents concentrating near the far edge of the paleochannel away from the transmitter. Numerical results show that a long strike length of a paleochannel is required to cause strong channeling currents and the associated fixed-loop anomaly. The low on the profiles due to the IP effect is, however, not much affected by the strike length. The moving in-loop profiles for the paleochannel model show a positive anomaly at early times and a negative anomaly at late times consistent with the field data. The moving in-loop profiles for the thick vertical conductor show a sharp peak at early times, caused by horizontal current flow induced at the top of the conductor, and an M-shaped anomaly at late times caused by induced currents circulating in the vertical plane of the conductor at late times. These patterns do not appear in the field data.