Recent Development in High
Resolution Geophysics for Mineral Exploration : Airborne, Ground and
Cross-hole Techniques
Gianfranco
Morelli1 and Quentin Yarie2
1Primo
Asia M&D – Philippines
2Geotech – Canada
Abstract
Efficient
mining exploration requires the correct application of the most
appropriate geophysical methods to resolve the exploration problem
at hand. Exploring through cover is not related only to seeking
anomalous responses for drilling targets, but it also a tool to
map layers, structures, alteration and associated
mineralization.
The
current routine can include low-level detailed aeromagnetic data and
semi-detailed ground gravity, Induced Polarization and radiometrics.
A more advanced mapping can be obtained with Electromagnetic (EM)
techniques, where differences in the conductivity structure of the
subsurface are detected and this information is used for direct
targeting of mineral deposits and alteration systems or mapping of
rock units and structures . In general, a wire coil transmitter is
used to induce a time-varying primary EM field by moving the
transmitter over a prospect area or using a fixed transmitter loop
sitting over a mineralized zone or borehole.
The
primary EM field will induce a secondary EM field around a
conductive ore body or geological horizon sitting in the subsurface.
A wire coil or sensitive magnetic receiver is used to record the
secondary EM field as a change in signal amplitude and phase shift
in the case of frequency domain electromagnetics (FDEM), or as a
slow rate of secondary decay in the case of time-domain
electromagnetic (TEM). Most EM systems do not have to come into
electrical contact with the ground, and therefore may be employed on
moving platforms, such as fixed wing aircraft and helicopters.
In recent
times the development of airborne TEM systems, such as the Tempest,
HoisTEM, SkyTEM and VTEM, with the ability to acquire data of
similar quality from the air as on the ground (for a about 15% of
the cost) has led to the routine use (in countries like Canada and
Australia, now rapidly expanding worldwide) of these systems for
mapping of cover thickness and investigation of underlying geology.
In the
last 30 years, nothing has fundamentally changed in regard to the
physical theory and basic methodology behind electromagnetic
prospecting. However in the last 10 years, there have been
technological advances that have propelled EM prospecting to the
forefront of many mineral exploration programs. During this time,
there have been a number of mineral deposit discoveries made with
the aid of EM prospecting, mostly for nickel sulphide deposits.
The technological advances include:
-
Digital data acquisition and signal processing technology using
sample frequencies greater than 1,000 readings per second (>1
kHz).
-
Computing power for rapid data acquisition, data storage, and
intensive filtering/processing routines.
-
Larger and faster switching transmitter systems, providing
cleaner signal and greater power into the ground.
-
#000000 More channels being recorded and out to longer decay
times or frequencies, providing greater depth penetration and
resolution.
-
Longer stacking times to increase the signal-to-noise response.
-
Development of computer code to rapidly image conductivity with
depth information, as conductivity depth images and layered
earth inversions
-
Spatial imaging of conductivity results and anomalies.
With
greater power and resolution, explorers are now able to detect
conductive EM targets at great resolution in the upper 100m, and as
far down as 400 m depth; even below 100 m of moderately conductive
overburden.
EM
results, used in combination with other forms of geophysical,
geological and geochemical exploration information can greatly
reduce exploration risk in covered areas where conductivity contrast
and conductivity structure can be detected.
In
combination with airborne EM surveys, Induced polarization can find
useful fields of application during all the ages of a mining
prospect, since grass-root to the development stage. Large-dipole
widely spaced IP lines can be profitably effected since the early
phases of porphyry-copper exploration. Closely-spaced lines, with
smaller dipolar lengths, will help in reconstructing the 3D geometry
of mineralized structures, and locating the position of high-grade
ore-shoots. In this presentation, we introduce new developments in
Resistivity and IP surveying:
Surface
High Resolution IP (HIRIP) , that uses a high number of
non-polarizing receivers ( up to 100 ) and spacings of 10-20 meters
( instead of the classic 50-100 m ), to obtain depths of exploration
of 300-400 meters with lateral resolution of 5-10 meters ;
Cross-Hole tomography, employed in new or existing holes,
used to optimize drilling programs and to supply cost-effective
information about the dip and plunge of intersected or near-miss
mineralized bodies ; recent, sophisticated 2D and 3D inversion
algorithms can produce final models with a high degree of
confidence, and spatial resolutions that are hard to achieve from
surface ;
Spectral IP (either in the time and frequency domain), can
supply crucial information for discriminating mineral responses and
mitigate or cancel out the effects of e.m. coupling, or, by using
the Cole-Cole parameter Tau, increasing the effective depth of
exploration ; high resolution, wide-band Complex Resistivity
laboratory measurements can help in designing the acquisition
rationale of IP surveys, both conventional and spectral, and can
individuate, in some cases, subtle physical contrasts useful for
discriminating diagnostic units.
Examples
of geophysical exploration using airborne TEM and different kinds of
Resistivity/IP methods will be presented, from sites in SE Asia,
Canada, Africa , Europe. |