Because ZTEM data are collected using an airborne system, the method is ideally suited to cover large survey areas. However, tipper data are only sensitive to 3D structures so the method would not be suited for homogeneous or layered subsurfaces.
The reference (base) station is used to account for the unknown source fields and polarization and measures the horizontal fields. It has been said that the reference station should ideally be placed in a region that is relatively uniform or horizontally layered. However, this may not be necessary because the horizontal fields at the reference station and the observation locations are not expected to vary largely [HO10b]. Additionally, the reference station location is included when inverting the ZTEM data and thus no assumptions about the resistivity structures have been applied [HO10b].
Geotech’s helicopter-borne ZTEM system collects data at six frequencies: 30, 45, 90, 180, 360, and 720 Hz, thus providing information at different depths. If we consider that the skin depth and a reasonable background resistivity of 100 \(\Omega\) m, then the ZTEM system provides information for depths ranging from 190 m to 920 m.