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.
Reflection seismic data can provide both direct detection of drilling targets and a reduction in the number of holes required to define resources.
Reflection seismic data has been the mainstay of successful energy exploration for many decades. Here, we outline the strong case for incorporating the method in your mineral exploration workflow.
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 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 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.
When should seismic data be acquired?
A useful measure of the value of seismic data is to consider its cost in terms of drill holes. If a 20-hole diamond drillhole program is expected to cost $10M dollars, can a low-cost seismic cube save two or more holes? If so, the rich information gained from a seismic program can quickly pay for itself.
At the very beginning of an exploration program, even before tenement acquisition has begun, existing government seismic data may be utilised to assess the exploration potential of large swaths of land when used in conjunction with other pre-competitive datasets like magnetics and gravity data. Seismic used in this manner provides the depth dimension for structures and lithologies identified in the potential field data.
During exploration of existing tenements, one rule of thumb that may be applied for assessing the value of reflection seismic data might be the question “Is drilling going to be expensive?”
If the target is likely to be under considerable overburden or is thought to extend to depths where many deep diamond holes will be required to further define the mineralisation, seismic data can provide confidence in targeting the drill holes, reducing the chances of wasting holes.
In this situation, 2D seismic data might be applied early in the life of a project. If the target is thought to be large but has considerable cover, seismic data will refine the understanding of the target geometry.
If the concept of “fail fast” is applied to an exploration program, seismic data can help an exploration team rapidly assess the prospectivity of a project. Viable targets can be developed and tested within a compressed timeframe, encouraging either further expenditure or divestment, thus allowing the exploration team to focus on more productive work sooner.
3D Seismic data can be acquired soon after the decision to proceed to resource definition has been made to help define the geometry of the system using fewer drillholes. It may help raise a resource category from (say) inferred to indicated because the resource geologist can imply greater confidence in the geometry of mineralisation by using the seismic data.
At the other end of a project’s life, 3D seismic data might be acquired when near-mine exploration is beginning to exhaust options for new discoveries. Blind drilling for nearby potential targets is a gamble: A 3D seismic cube dramatically increases the probability of identifying structural settings for further mineralisation.
Do passive seismic methods work?
There is a mistaken perception growing in the minerals industry that passive seismic (PS) methods are both cheaper and can deliver similar results to those seen in reflection seismic data. Passive seismic is indeed cheaper, but the results delivered by the two methods are not comparable in any way.
The passive seismic methods differ vastly from the reflection seismic method. For mineral exploration, passive has very little in common with reflection seismic other than sharing the word “seismic” in their descriptions. Like magnetic and electromagnetic methods, they are not substitutes for each other even though they share the words “magnetic” and “seismic” in their respective descriptions.
Passive and reflection seismic techniques both rely on the transmission of seismic energy (somewhat akin to sound) through rock, but that is where the similarities end.
Passive seismic methods (HVSR and ANT) rely on ambient or “found” seismic energy existing in the earth to image either the highest contrast interface (e.g. depth of weathering) or seismic velocity.
Both of these passive seismic methods are best suited to defining vertical differentiation (e.g. layers) because they do not resolve lateral variation in velocity very well.
Horizontal to Vertical Spectral Ratio (HVSR) passive seismic makes use of “found” seismic energy to interrogate the lithologies directly below the single geophone used. It is a one-dimensional method well suited to (say) finding the depth of weathering at a given location. (“Tromino data” is a commercial example of HVSR.)
Ambient Noise Tomography (ANT) is a method of assessing seismic velocity information within a volume of rock utilising “found” energy. Typically, the frequency of the ambient noise utilised is less than 8 Hz, often as low as 0.1 Hz. With such low frequencies and inherently high seismic velocities of mineralised systems, the wavelengths of the “imaging signal” are usually many times greater than the dimensions of the features we seek when exploring for mineral deposits.
The resolving power of ANT is dependent on several factors, including:
- The spacing and extent of the acquisition geophone array,
- The frequency of the available energy,
- The depth of investigation,
- The completeness of azimuthal coverage of the energy in all seismic frequencies used, and
- The dimensions and orientations of the geology under investigation.
- The seismic velocity contrast of the geology in the volume under investigation.
If the required noise frequencies do not exist or their source direction coverage is incomplete, the method’s value is severely reduced.
Most mineral systems are invisible to ANT surveys because their dimensions are very much smaller than the wavelengths used in the technique and because there is insufficient velocity contrast for the technique to discriminate the mineral system or structures associated with it. In most cases, seismic velocities alone do not represent any particular rock type, structure or alteration mode within a project and can’t be used to model a system.
If the mineral system is large enough to find with ANT, it’s easily large enough to discover with potential field data (magnetics and gravity.) If drill targets are being developed, ANT surveys do not approach the resolution required to inform the targeting process.
In contrast, a reflection seismic survey is specifically designed to answer questions posed about the rock volume of interest. Nothing is left to chance: The geometry and density of the array, the frequencies used at the seismic energy source, the types of receivers utilised and even the processing methods used on the data are all customised (rather than “found”) to deliver verifiable results.
Where ANT surveys do offer value is as an adjunct to potential field inversion modelling, where the knowledge of depth of cover is a valuable constraint in the inversion modelling procedure.
In short, passive seismic methods work when the scale of investigation is appropriate for the methods to resolve. That is rarely the case in mineral exploration.
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.