Introduction
The reflection seismic method is like an ultrasound image of your project. It is a vision of the future development of your exploration project, with data that is both rapidly acquired and proven to be effective.
Reflection seismic data adds value to the more readily available data sets (mapping, magnetics, gravity, shallow drilling, etc.) As shallow exploration targets are exhausted, the reflection seismic method is playing an increasingly important role in improving the effectiveness of our exploration drilling budgets.
What is the reflection seismic method?
During a reflection seismic survey, we introduce controlled energy into the ground and listen for its echoes with microphones laid out on the surface, and through a series of processing steps, we produce visual representations of those echoes.
The very term “reflection seismic” implies the energy is reflected from something. That something is the location where there is an abrupt and significant change in acoustic impedance, which itself is the product of seismic velocity (the “speed of sound”, Vp) and the specific gravity of the rock mass (SG). When either the Vp or the SG (or both) change, we may expect that location to be a reflective interface.
Unless otherwise stated, hereinafter, the term “seismic data” refers to reflection seismic data.
What can we image with reflection seismic data?
When used for mineral exploration, the reflection seismic method can image not only lithological contacts, but structures (brittle faults and shear zones), massive sulphide bodies, pegmatite dykes, and alteration zones. We may even see underground development in many instances.
Resolving power is broadly maintained with depth and is dependent on:
- The style of seismic acquisition (2D or 3D)
- The frequency of the source energy,
- The spacing of the source and receiver array
- The seismic velocity of the rock, and
- The thickness, acoustic impedance contrast and geometry of the imaged reflector.
Under the best circumstances, reflection seismic data will resolve reflectors to within a few metres at depths beyond 2000 metres.
Why use reflection seismic in mineral exploration?
Seismic data adds value to all other data sets in a mineral exploration project.
The strongest selling point of reflection seismic method is the retention of its high resolving power at depth. No other geophysical method has the ability to retain its resolution as the depth of investigation increases. All other methods - including passive seismic - lose their imaging capability rapidly as depth increases.
Unlike other geophysical methods, the reflection seismic method readily images repetitions of targets beneath known deposits. Deep targets are not masked by shallow ones, often a problem with magnetics, gravity and electrical methods.
At the regional scale, 2D seismic data helps support a “mineral systems approach” to exploration, enabling the informed selection of tenements and projects to acquire. Deep-seated structures identified with seismic data are often the most prospective and most reflective due to alteration associated with fluid transfer.
At the project exploration scale, 2D seismic data is useful in identifying potentially fertile structures. The method is very efficient for extending known mineralisation to depth and identifying structures and lithologies with similar characteristics.
At this scale, 2D seismic data allows a more robust interpretation of potential field data (magnetics and gravity) and electrical data (IP and EM), and in return, those data sets also help with the interpretation of the seismic data too.
3D seismic data acquired at the tenement scale allows for direct imaging of targets and is especially useful for massive sulphide exploration, since these deposits are very good reflectors of seismic energy. It can also identify repetitions of known mineralisation structural settings.
At the mine scale, 3D seismic data can successfully image drill targets within striking distance of existing infrastructure, extending the productive life of the mine.
In each case, seismic stretches the drilling budget further, with fewer holes required for discovery and resource definition.
Which commodities can benefit from reflection seismic data?
The method has been successfully applied in exploration for gold, nickel (massive sulphides), VMS base metal systems, lithium-bearing pegmatites, stratigraphic and porphyry copper, manganese, paleochannel uranium and igneous precious and base metals.
Any mineral system in which structures are an important factor for ore genesis may benefit from the application of the reflection seismic method, since it so clearly delineates where structures may be found.
In mineral systems where the ore itself is highly reflective (e.g. massive sulphides, pegmatite), the reflection seismic method can be a geophysical “direct detection” method, effective at depths well beyond those sampled by the more conventional geophysical methods. Indeed, there are no geophysical methods that come close to seismic’s ability to define pegmatites, which are notoriously hard to identify in other geophysical datasets.
How much does a reflection seismic program cost?
In years past, the cost of reflection seismic in the minerals industry had been considered as very high; a luxury item that only the largest explorers could afford.
Those days are gone, with much of the “hard-rock seismic” knowledge and skill that had been isolated within one company now being distributed to a broader selection of vendors. This has had the effect of lowering the price to the point where it’s now a viable proposition for even junior companies to consider applying reflection seismic techniques to their exploration projects.
As an example, in Australia, all of the major land seismic acquisition companies are now offering packages at prices that are typically half the prices seen just three years ago. Some of these acquisition companies offer very small seismic source vehicles, requiring little or no line clearing (saving even more time and money.)
Several processing companies are also able to deliver well processed seismic data that’s bespoke to mineral exploration (which is a significantly different product to that seen in the energy sector.)
All of these developments combine to offer prices for processed 2D seismic data plummet
A well planned and executed 3D seismic survey can cost around the same as ten to fifteen deep diamond holes but it delivers information over a much greater volume. If the timing of the survey is optimised, many drillholes can be trimmed from the resource definition stages of drilling.
Conclusion
Reflection seismic data retains much more of its resolving power with depth than any other geophysical method.
Reflection seismic data adds value to a mineral exploration project by completing the missing dimension in most exploration datasets: Depth. In doing so, it greatly expands the utility of the more readily available data sets (mapping, magnetics, gravity, shallow drilling, etc.), and in the case of 3D reflection seismic data, it can provide both direct detection of drilling targets and a reduction in the number of holes required to define resources.
As shallow exploration targets become rarer, the reflection seismic method is playing an increasingly important role in improving the effectiveness of our exploration drilling budgets.