As you explore for your next big mine project, spare a thought for where the process water will come from, water issues are more than ever the fatal flaw or the thorn-in-the-side of mining projects.
We live on the driest inhabited continent on earth; our groundwater resources are finite; and the amount of water we need to produce a unit of metal is increasing exponentially as we mine at ever lower cut-off grades. But, in our favour, Australia is the home of airborne geophysics which is dramatically improving our targeting success and cutting the costs of water exploration. As water becomes increasingly scarce, the successful projects of the future will be those which embrace the new exploration technologies.
Australia is a mature mining environment; our readily accessible high grade greenfields deposits were mined out at the turn of the last century and we currently rely on obtaining scale efficiencies so that we can mine down the grade curve. Just 20 years ago most metalliferous mines would have been processing less than 0.3 mtpa of run of mine (ROM) ore, but today mine start-ups look to process between five and 50 mtpa of ROM ore at grades that were considered well below the economic cut-off of 20 years ago. As a result, the amount of water consumed to produce one tonne of metal has been increasing exponentially year on year because water consumption in the mill is measured on ROM throughput and the ROM tonnes per tonne of recovered metal increases as we mine down the grade curve. In effect, the cost of water as a proportion of the mine production cash costs is increasing.
Remembering that we are on the driest inhabited continent on earth, it should come as no surprise that more projects are failing each year as groundwater resources reach their full allocation and project water demands become increasingly unrealistic. Twenty years ago a typical metaliferous mine start-up would have had a modest water demand of around 1,000 L/day, but to achieve scale efficiencies needed for today’s start-ups, projects are being designed around water expectations of between 15,000,000 and 40,000,000 L/day in totally inappropriate hydrogeological environments. The challenge for hydrogeologists is to accurately and responsibly convey hydrogeological prospects to the mine planner, whilst working smarter at finding and defining larger water supplies amongst dwindling regional groundwater resources.
Fortunately over the past three decades Australian companies such as Aerodata, World Geoscience and GPX Airborne have pioneered the development and refinement of airborne geophysical exploration technologies such as magnetics, spectral radiometrics, micro-gravity, frequency and time domain electromagnetics. These technologies have revolutionised water exploration by giving hydrogeologists a tool to rapidly explore vast regional areas with a great deal of confidence at little cost.
With geophysics, It’s a matter of ‘horses for courses’ ;and the geophysicist picks the tools and designs the survey to best match the objectives to the water supply. For instance, high resolution aeromagnetics makes mapping faults, dykes and geological contacts in t he bedrock a relatively easy task; whilst micro gravity is brilliant in sedimentary and karstic environments because of the huge density contrast between sediment and underlying bedrock or between voids and limestone. Radiometrics is also a useful tool for mapping the near surface and regolith from the amount of uranium, thorium and potassium radiation emitted. However, by far the most exciting technological development from a hydrogeologists perspective has been evolution of time domain electromagnetic (TDEM) technologies. These technologies directly measure the electrical conductance of the earth to depths of 150m and deeper with high vertical and spatial resolution. Since the ground conductance is mostly dictated by the amount of salt and clay, we can directly map palaeochannels, changes in groundwater quality or map out the regolith. The computing power now available allows large volumes of TDEM data to be processed in the field to slice and dice the earth and produce cross sections, conductivity slices at various depths or three dimensional conductivity surfaces.
The use of TDEM in water exploration was successfully used in the 1990’s, when ground TDEM surveys were used to map the deepest parts of Western Australia’s palaeochannels for borefield development. Laying transmitter cables out on the ground, however, was time consuming. At best a ground crew could complete a vertical electrical sounding every 30 minutes, which meant a typical palaeochannel survey of about 1500 stations could take over a year to complete. World Geoscience overcame this problem by stringing the transmitter around a fixed wing aircraft, which allowed over 50,000 electrical soundings to be collected in a day. In 2000 Anaconda Nickel successfully used fixed wing TDEM to cover 60,000 km2 of the Officer Basin and within months locate a massive 250,000,000 L/day brackish water resource. While the fixed wing approach dramatically reduced the time add cost of water exploration, having the receiver “bird” trailing behind the transmitter causes an annoying artificial asymmetry in the data, smearing the response in the direction of flight. The development of HoistEMheli TDEM system (now replaced by XTEM by GPX/Newmont overcame this problem by keeping the transmitter and receiver in symmetry. The heli XTEM gives much higher spatial resolution than fixed wing TDEM surveys, allowing detailed mapping of shallow to medium depth groundwater resources.
In 2003, the United Arab Emirates commissioned the most comprehensive demonstration of the value of airborne geophysics in water exploration by flying a blind XTEM survey over an area of desert previously explored by drilling hundreds of bores at enormous cost. In just five days of flying and five days of processing, the survey produced over four million airborne electrical soundings, with each sounding having almost as much information as a borehole. The acquired data was processed into multiple maps, cross sections and 3D images that mapped out the fresh water resources in very high resolution never seen before. The whole survey cost less than just one of the hundreds of bores drilled using the traditional scattergun drilling approach. Since this demonstration the XTEM system has become part of mainstream groundwater exploration throughout Australia.
As project water needs continue to rise to meet increasing scale efficiencies and the remaining groundwater resources become harder to locate, the ability to stay ahead of advances in airborne geophysical technologies will be the critical success factor for future water development.
By Don Scott (Pennington Scott) and Greg Reudavey (GPX Airborne)