2018
EXPLORATION
MEGATEM development team
en Witherly (BHP) and John Gingrich
(Noranda) were users of the equipment
that recognised the potential of the
system and drove its development from the
demand perspective. This demand was not just
in their organisations, but also from
governments and public organisations.
Mike Carson led the interaction with BHP,
while George Nader and David Fountain
interacted with Noranda. All were champions
within the Geoterrex corporate structure and
collectively convinced senior management and
employees to get behind the development
project.
Richard Smith and Peter Annan were involved
with the geophysical justifications for the
system and design, and development of the
system and the prior successful developments
that were incorporated into the MEGATEM
development. They provided strong technical
support and gave confidence that the system
would be a successful exploration project.
Tom Payne, Bruce Magnes and Al Proulx
designed and tested the electronic and software
systems for the MEGATEM developments and
upgrades. Proulx operated the system in the
early period when the bugs were being worked
out of the system.
Brenda Sharp processed the data from the
MEGATEM and identified an anomaly that led to
the discovery of the $4 billion Perseverance
deposit near Matagami, Quebec. This discovery
was critical at an early stage, when the system
was just starting and needed success to gather
momentum.
The strengths of the system were explained
to potential users by Smith and Fountain, who
were critical in the technical marketing of the
system to the exploration community.
MEGATEM was an extremely successful
system, being able to fly at very high latitudes
in Chile and Peru. The system identified an
anomaly of the Spence deposit prior to its
discovery, but it was not drilled at that time for
geological reasons.
In Canada, the system was used by the
Ontario Geological Survey and Discover Abitibi
to refly areas in Ontario and virtually all the
greenstone belts in the Abitibi subprovince of
Quebec were reflown by Noranda with support
from the Quebec ministry. During the uranium
boom in the first decade of the new millennium,
it was extensively flown in the Athabasca basin
for its ability to detect the prospective graphite
horizons at more than 700 m depth. The use of
the MEGATEM technology became a critical part
K
HoF 2 International Mining | JUNE 2019 Supplement
of junior companies’ exploration strategy and
was mentioned in press releases in order to
raise funds on the stock exchanges.
In the mid-1980s, Geoterrex (later Fugro
Airborne Surveys) introduced the GEOTEM
system, a fully digital airborne electromagnetic
receiver, using the same transmitter that had
previously been used, very successfully, on the
INPUT system. The GEOTEM system was
mounted on a twin engine CASA 212 aircraft,
and the transmitter loop was wound around the
nose, wing tips and the tail of the aircraft. A
transmitter excites eddy currents in the
subsurface with periodic pulses of the ‘primary’
magnetic field. The decay of these currents is
measured with a receiver that is towed behind
the aircraft in a ‘bird’. When the eddy currents
decay slowly, this is generally indicative of
material in the subsurface that is conductive.
Airborne electromagnetic systems typically
also measure the intensity of the Earth’s
geomagnetic field. For the GEOTEM system, the
magnetometer is in a second bird that is also
towed behind and below the aircraft. Having
magnetic data is useful for mineral exploration,
as it can help to distinguish between conductive
suphides (which are often associated with
magnetic sulphides – for example pyrrhotite)
and other conductive features like clay,
graphites and shear zones (which are not
magnetic). The magnetometer towed-bird
location puts the magnetometer sensor close to
the ground and provides high-resolution data.
In the mid 1990s, the GEOTEM system on a
CASA had been deployed by BHP for exploration
at relatively high altitudes (less than 2,400 m)
in the Altiplano regions of the Andes Mountains
of South America. By 1996, BHP was interested
in exploring in areas above 2,400 m altitude.
The CASA fixed-wing aircraft carrying a
GEOTEM system is limited to flying in areas
A schematic diagram of the GEOTEM airborne
electromagnetic system. The receiver sensor is
towed in a ‘bird’ 130 m below and 50 m behind
the transmitter. There is also a magnetometer
bird for measuring the intensity of the Earth’s
geomagnetic field
The MEGATEM system refuelling while on a
government survey
when the altitude is less than 2,400 m. In order
for a twin engine aircraft to fly surveys safely,
the aircraft must be able to maintain altitude (or
climb) after it has lost the use of one engine.
When the aircraft is surveying at high altitudes
(and temperatures) the ability of the aircraft’s
wing to provide lift is reduced.
Effectively this means that the aircraft cannot
fly safely at high altitudes, particularly when the
temperature is high either at the airport or in
the survey area. The additional weight and drag
of the transmitter loop mounted on the aircraft
means that the performance of the CASA with a
GEOTEM system is not equivalent to a standard
CASA.
BHP wanted to extend the capability of the
existing GEOTEM system without compromising
safety. In order to satisfy BHP’s requirement for
airborne electromagnetic surveys at the higher
altitudes, it was felt a new type of aircraft was
needed.
As a consequence of this need, BHP was
supportive of installing the GEOTEM system on
another aircraft. The aircraft selected was the
Dash-7, primarily because this aircraft had four
engines, and losing a single engine would not
be as critical – there would still be three engines
remaining in operation to maintain the altitude
of the aircraft.
The Dash-7 aircraft is capable of supplying
more electrical power and carrying heavier EM
equipment than the CASA aircraft. With
Noranda’s encouragement and support, the
MEGATEM system underwent a further upgrade
in early 2001. This upgrade took advantage of
the power available from the AC generators on
each engine. The total power available is about
40 kW, this power was converted to DC and
used to drive an upgraded transmitter pulser.
The four-fold increase in power resulted in a
two-fold increase in peak transmitter dipole
moment to 2.2 million Am 2 . This upgrade to the
MEGATEM system was designated MEGATEM II.
In 2005, Fugro introduced the HeliGEOTEM.
This system brings together the proven
GEOTEM/MEGATEM technology with the greater
operational flexibility and improved lateral
resolution of helicopter-mounted AEM systems.
This HeliGEOTEM system has undergone
significant improvements since its introduction
and is now marketed as HELITEM by CGG (to
whom Fugro Airborne Surveys was sold).