ZTEM is a passive helicopter-borne system that measures the magnetic field response in the frequency range 30-720 Hz of naturally occurring currents in the subsurface. The resolution of this system is analyzed by forward modeling and inverting synthetic ZTEM data using a 2D algorithm for a range of conductivity scenarios. Figure 1: Flight path of VTEM and ZTEM surveys overlain on the geology of the Mt Milligan porphyry system. ZTEM data acquired across the Mt Milligan Cu-Au porphyry system are compared with overlapping VTEM data. Conductivity-depth sections derived from both data sets show broad agreement, but indicate better spatial resolution for the VTEM data. Neither data set appears to show a significant response from the Cu-Au mineralization. Products derived from the ZTEM data, including apparent conductivity, phase and Karous-Hjelt filtered grids appear to map geologic structure.
The ZTEM system has been available for commercial surveys for only a couple of years and reported case studies are therefore limited (Legault et al., 2009a, 2009b). It measures the magnetic-field response of naturally occurring subsurface currents, induced by far-away lightning discharges. By comparison, the VTEM system measures the magnetic-field response due to currents induced in the subsurface by the transmitter the system is carrying and many case studies have been reported in the literature (e.g. Witherly and Irvine, 2007).
Synthetic ZTEM profiles were forward modeled and inverted using a 2D MT algorithm, developed by Constable and Wannamaker (deGroot-Hedlin and Constable, 1990; Wannamaker at al., 1987; deLugao and Wannamaker, 1996). The inversion fitted the data to a RMS error of 1.3, which has been determined as a representative target RMS for the modeling of survey data. As part of Geoscience BC’s QUEST project, both the Geotech VTEM and ZTEM systems have been flown over Mount Milligan, an alkalic Cu-Au porphyry system located in British Columbia, 155 km northwest of Prince George. It should be noted that this type of mineralization is expected to be a difficult target for an electromagnetic system due to generally being associated with relatively low conductivities. However, by being sensitive to conductivity contrasts, the ZTEM system might have an advantage in resistive terrain. The overlap of data from both surveys allow for a direct comparison of the spatial resolution and depth penetration of the two systems.
Synthetic ZTEM profiles were forward modeled and inverted using a 2D MT algorithm, developed by Constable and Wannamaker (deGroot-Hedlin and Constable, 1990; Wannamaker at al., 1987; deLugao and Wannamaker, 1996). The inversion fitted the data to a RMS error of 1.3, which has been determined as a representative target RMS for the modeling of survey data. Synthetic modeling results are shown in Figures 2-4. A flying height of 80 m was modeled. The insensitivity of the ZTEM system to one-dimensional or layered conductivity structures is illustrated in Figure 2. The ZTEM system shows a strong Tzx response at the contact of the two quarter-spaces and the 2D inversion resolves the conductivity contrast well.