Controlled Source Audio
Magnetotelluric (CSAMT) surveys were carried out by Glass Earth (New
Zealand) Limited (Glass Earth) in 2006 - 2007
to investigate epithermal gold targets in the central North Island of New
Zealand. The surveys were conducted using a Zonge GDP 32 8-channels receiver and
a Zonge XMT 32 transmitter powered by a 16 KW generator. The electric field
dipole was maintained at 20 m length (five dipoles per 100 m spread) and the
frequency ranged from 4 KHz to 4 Hz. Only scalar (TM mode) signals were
recorded, using one magnetic coil per spread. The transmitter bipoles were 1 - 4
km in length and located about 5 to 10 km from each survey area.

Sixteen individual
target areas were surveyed by June 2007. The CSAMT survey lines (100 - 400 m
spacing) were determined mainly on results from previous ultra-detailed airborne
magnetic and gravity surveys conducted by Glass Earth in 2005. The data from
each CSAMT line was carefully examined to remove any spurious signals and
processed using Zonge SCS2D software, which can produce smooth 2D resistivity
inversions of the field data. The results were presented as resistivity sections
and maps to help the identification of exploratory drilling targets, mainly from
the existence of subsurface high resistivity zones (resistors) that can be
interpreted as caused by silicification associated with epithermal
mineralization.

An
example of processing and interpretation of the CSAMT data is presented in this
paper on results from Glass Earth's Tahunaatara target, where CSAMT surveys have
identified zones of epithermal mineralization confirmed by exploratory
drilling.