134 l Finding Gold – Time for a Seismic Shift in Exploration Thinking Greg Turner and HiSeis staff
Finding Gold – Time for a Seismic Shift in
Exploration Thinking
Greg Turner and HiSeis staff
Introduction
At shallow depths, gold explorers have a broad arsenal
of targeting tools and drill testing is relatively cheap. At
greater depths, where each drillhole represents a substantial
expense, the effectiveness of most current targeting tools is
greatly reduced. Reflection seismic is one geophysical tool
which retains its resolution, and therefore effectiveness, as a
function of depth.
Reflection seismic operates by transmitting vibrational waves
into the ground (see Figure 1). These waves propagate until
there is a change in the elastic properties (e.g. lithological
boundaries, alteration zones, faults and shears). At each
boundary a small portion of the energy is reflected whilst
the majority of the energy is transmitted down to the next
boundary where the process is repeated - and so forth. The
reflected signals are measured by an array of sensors on Figure 1. Schematic showing configuration of a seismic survey. A source
the surface and can be used to provide a detailed image of transmits vibrational waves into the ground. These waves are reflected
the subsurface. The sensor array can be along a single line off subsurface interfaces and the reflections are recorded by sensors on the
(2D survey) or a grid (3D survey). Surveys are built up surface. In a 2D survey the sources and sensors will be on a single line.
by transmitting the vibrational waves into the ground at In a 3D survey the sources and receivers are on a grid.
multiple locations.
The key parameter that determines the strength of
reflections is the acoustic impedance. This is equal to the
product of density and seismic velocity. Seismic velocity is
Case History 1 – Epithermal Gold
broadly correlated with rock strength. Geological features In July and August 2013 2D seismic surveys were completed
are imaged both directly by reflections off specific boundaries at Cracow and Pajingo to assist with exploration for
and by interruptions to the reflections and changes in texture epithermal gold deposits. The mineralisation at both Cracow
of the reflections. and Pajingo occurs as thin (1-15m) structurally controlled,
Reflection seismic is a critical component of almost all steeply dipping quartz veins. Both surveys identified
oil and gas exploration projects and its use has massively multiple sub-vertical structures interrupting reflections
improved drill success rates (Aylor, 1999). With some from andesitic lavas, tuffs and fragmentals. The survey also
exceptions (e.g. Stevenson and Durrheim, 1997), reflection provided much greater detail on the structural architecture
seismic has rarely been used in mineral exploration. In of the regions. Figure 2 shows a structural interpretation
the past this was for both technical and financial reasons. from the 2D seismic data and other available information
Significant developments on both fronts mean that seismic from drilling, mining and potential field data.
surveys can now offer substantial benefits to gold explorers. Drilling showed these structures to be coincident with veins
Our presentation will show the results of recent seismic some of which contained gold. Based on the success of these
surveys to assist exploration for high-grade gold in terranes surveys, Evolution initiated 3D surveys at each of the 2 sites
with known gold endowment. in May and June 2014. Figure 3 shows an initial depth slice
1. HiSeis Pty Ltd., Suite 4, Enterprise Unit 3, 9 De Laeter Way, Bentley, WA 6102 Australia Corresponding author: Greg Turner Email: g.turner@hiseis.com
Gold14@Kalgoorlie – Western Australia
135 l Finding Gold – Time for a Seismic Shift in Exploration Thinking Greg Turner and HiSeis staff
Figure 2. Structural interpretation of 14km cross-section at Pajingo based on 2D seismic data and other
available information from drilling, mining and potential field data. The blue lines indicate interpreted
post-mineral normal faults and syn-mineral inversion. The blue and green shading indicate stratigraphic
markers in the volcanic sequence (reproduced from Klein 2014).
through the 3D data cube with the interpreted location of
faults some of which correspond to the location of known
faults and epithermal orebodies.
Case History 2 – Orogenic Gold
A significant benefit of seismic reflection imaging is not
only the ability to detect structures which may be difficult to
detect via sub-vertical drilling but to provide a continuous
map of these structures. In many gold deposits high grade
zones occur in dip or strike flexures of host structures (see
for example Figure 4). Seismic surveys can potentially map
these structures whilst they can be difficult to detect by
drilling particularly at larger depths. Two 2D surveys at Figure 3. Depth slice from Cracow 3D seismic data at 484m below
Tropicana in December 2012 and July 2013 demonstrated surface. Grey lines represent faults, some of which correspond to the
the ability of seismic surveys to image the structures location of known faults and epithermal orebodies (reproduced from
controlling mineralisation. Figure 5 shows the seismic Klein, 2014).
sections from these two surveys. As a result of the success
of the 2D seismic surveys, AngloGold Ashanti and the
Independence Group decided to proceed with a 3D seismic
survey to image the 3D geometry of
these structures down dip from the
existing ore reserves. The 3D survey
covered an area of approximately 10
square kms (see Figure 6) and was
completed in July 2014.
Conclusion
Surface based reflection seismic can
provide continuous 3 dimensional
images of prospective geological
terranes. Its strengths are:
• its ability to provide images over
depth ranges from less than 100m
to many kilometres;
• its high resolution; and
• that the high resolution is Figure 4. Image of some of the shears in the Havana pit at Tropicana
essentially independent of depth. Mine illustrating flexures which influence the high grade zones within
the mineralisation (reproduced from Gibbs, 2014).
Gold14@Kalgoorlie – Western Australia
136 l Finding Gold – Time for a Seismic Shift in Exploration Thinking Greg Turner and HiSeis staff
Figure 5. Two 2D seismic sections from Tropicana and their relation to Figure 6. Plan of the Tropicana mine showing the outline of the 3D
mineralisation (shown in yellow) (reproduced from Doyle, 2014). seismic survey recently acquired to image 3D geometry of these structures
down dip from the existing ore reserves (reproduced from Gibbs, 2014).
Thus reflection seismic provides gold explorers with a way
to develop a full 3 dimensional understanding of their
References
mineral prospects and therefore rapidly focus in on the Aylor, W.K., 1999, Measuring the impact of 3D seismic on business
most prospective areas. Recent results indicate that this performance, Journal of petroleum technology.
can provide a new highly cost effective approach for gold Doyle, M., 2014, Tropicana - An integrated approach to understanding
granulite hosted gold, WA Department of Mines and Petroleum Albany
exploration.
-Fraser Orogen seismic and MT Workshop, 9 April.
Acknowledgements Klein, J., 2014 Evolution Mining June 2014 Quarterly Results Presentation.
Gibbs, D., 2014, AngloGold Ashanti/Independence Group July 2014
The case histories reported in this paper are from Tropicana site visit presentation.
commercial surveys completed by Evolution Mining and Stevenson, F., and Durrheim, R.J., 1997, Reflection Seismics for gold,
AngloGold. The authors would like to acknowledge their platinum and base metal exploration and mining in southern Africa,
Proceedings of Exploration 07, Toronto, Canada, September 1997.
support and the considerable inputs from Roric Smith,
Jeremy Cook, Armelle Kloppenburg, Keith Martin,
Greg Cant, Jeni Savage, Mark Doyle and other geoscientists
and field staff which have led to the results presented.
Biography
Greg turner’s previous roles have included being Geoscience Manager for WMC’s Technology Group and a co-
founder of the Geoforce geophysical service company .
He is currently Director New Business Development at HiSeis which is a company that uses seismic methods to fast-
track exploration and improve mine planning.
Gold14@Kalgoorlie – Western Australia
