Let’s get geophysical

Materials World magazine
,
3 Jul 2017

Geophysical surveys represent a highly cost-effective and accurate method of minerals exploration. Michael Schwartz explains why.

Geophysical surveys offer flexibility as they can be undertaken from a variety of platforms, such as fixed-wing aircraft, helicopters, satellites and drones, as well as at ground level. The surveys can also take many forms including magnetics, gamma spectrometry, radiometrics, gravity, ground penetrating radar, seismics and electromagnetics.

The minerals that can be surveyed are formed of, among others, base and precious metals such as kimberlites, hydrocarbons, uranium and rare earths. In turn, geophysical surveys can help with mine planning and construction that requires accuracy to more than 20cm in elevation – calculating volumes shifted can also be accomplished.

Materials World interviewed three geophysical operators on their views of the challenges and opportunities within geophysical surveys. Terraquest, a privately owned Canadian airborne survey company, founded in 1984, has conducted thousands of fixed wing and helicopter surveys in 25 countries around the world. At present, it operates five systems variously equipped with highly sensitive magnetometers and gamma ray spectrometers to map subsurface geology before ground exploration and drilling. Clients have included DeBeers, Shell, Chevron, Teck-Cominco, Goldfields, Rio Tinto and ALCOA.

Modern Mag, Australia, conducts geophysical surveys for the coal, mineral exploration, mining, geotechnical and archaeological industries. The company employs the GEM Systems GSMP-35 potassium magnetometer, which is carried in lightweight custom backpacks and is used for archaeology and geotechnical surveys. Modern Mag has more than 17 years of ground magnetic survey experience in Australia, Africa, Canada and Russia. If ground geophysical surveys are not suitable, drones using the same potassium magnetometer are used.

Sander Geophysics Limited (SGL), Canada, provides worldwide airborne geophysical surveys for petroleum and mineral exploration, and geological and environmental mapping. Services offered include high-resolution airborne gravity, magnetic, electromagnetic and radiometric surveys using fixed-wing aircraft and helicopters. SGL conducted its first airborne survey in 1957, and has since operated in exceptionally diverse conditions, methane sensing being just one service. SGL currently owns and operates 17 aircraft including eight turbines – Cessna Grand Caravans – all equipped for geophysical surveys.

Materials World asked the three companies whether they develop their own specialist technologies. Modern Mag’s principal geophysicist Justin Ward replied, ‘By far our most exciting development is our non-magnetic backpack that can hold any magnetometer from any manufacturer and control it with a handheld computer and software. We received a Victorian Government grant for this research and are selling the units now. These backpacks are lighter, safer, more reliable and easier to use in the field.’

Malcom Argyle, SGL’s Marketing Manager, said, ‘We do develop our own specialist technologies when we feel that there is a unique or better solution than what is available on the market. An example would be our AIRGrav airborne gravity system – a gravity system capable of measuring both the vertical and horizontal components of the gravity field. We have also developed numerous proprietary data processing techniques, and all of our acquisition and processing software has been developed in-house.’ 

Howard Barrie, President and Owner of Terraquest, said, ‘Terraquest was the first commercial operation in Canada to develop and fly a fixed wing horizontal magnetic gradiometer, and was the first company to provide digital very low frequency electromagnetics (VLF-EM) data with inversions to show correlations that match ground IP data.’

Working in remote areas

As may be expected, geophysical surveys are often carried out in isolated locations. Barrie produced a list of challenges emanating from these areas, including ‘permitting, availability of fuel, security, lack of aviation support facilities and navigational aids.’

Ward points out the difficulties of the 1980s and 1990s, but, ‘now with GPS, satellite phones, satellite internet, Google Earth and better vehicles and recovery gear, the outback has opened up. It is more difficult working in other countries in remote areas because of the language and cultural barrier. Under those circumstances, we need more client involvement or use subcontractors with local knowledge of the area. Working in Sri Lanka was difficult, but we got the surveys done using modern equipment.’

There are advantages to working in remote conditions, as Argyle points out, ‘In general, bureaucracy and permitting are no more difficult in remote areas than in other areas, and often they are much easier since fewer people, buildings and aircraft are located in these areas. Operationally, there are challenges, which can range from minor to extreme, and the specific type of challenge depends on where the remote area is.’ Argyle summarised the greatly differing conditions from several areas. In the Arctic, for example, finding a suitable base reasonably near the survey area can be challenging. In addition, ‘sourcing sufficient fuel can be difficult and expensive. Installing and maintaining outdoor equipment, such as GPS antennas and mag reference stations, can be slow and painful. Working on aircraft is difficult when hangars are not available, and tend to be expensive. Equipment can also not operate in extreme cold. Reliable electrical power may be difficult to find, meaning portable generators are required. Extreme cold may mean we can not fly due to safety concerns.’

These safety concerns apply to the other extreme of the desert, when there is extreme turbulence. The problems continue in the jungle, as ‘electronic equipment does not like humidity, so reliability can be an issue, especially for longer surveys. As with the desert, heat can also be a problem, as can reliable power,’ said Argyle.

Helicopter, fixed wing or drone?

Among airborne methods, the debate as to whether helicopter or fixed-wing is more effective has been joined by the arrival of the drone. For Argyle, the main advantage of helicopters is that, ‘they fly slower than fixed-wing aircraft, which provides better resolution data. Helicopters can also climb and descend more steeply than fixed-wing aircraft, so they can follow the terrain more closely in mountainous terrains. The main advantages of fixed-wing aircraft are that they can fly longer and faster, which improves productivity – they are generally slightly cheaper to operate, which keeps costs down, and they usually can carry a larger payload than a helicopter, which allows more instruments to be carried at the same time.’

Barrie replied that helicopters are used for high-resolution acquisition and mapping in mountainous terrain, while fixed-wing surveys are very good at providing cost-effective regional mapping including offshore magnetic, electromagnetic and gravity surveys.

But Modern Mag finds that, ‘Fixed-wing aircraft are cheaper than helicopters, so I commission those surveys using agricultural fixed-wing aircraft that can fly 20m off the ground. In some cases, helicopters are required due to steep terrain for example. If we can’t do the survey on the ground, then we use drones. The development of our drone system has taken years, but we now have a reliable system that acquires excellent data quality.’ 

Regarding drones, Barrie is more cautious, ‘Drones are in their infancy and the current regulations concerning operations restrict their usefulness. For example, if you have a property that you'd like to be mapped in a remote area, the cost to get a couple of people into the site would probably be the same as if the survey were to be performed by a light aircraft such as the Cessna 206.’

Argyle also identifies certain issues even though SGL has been involved in unmanned aerial vehicles (UAVs) for several years. Argyle notes that, ‘We have not had an opportunity where using a UAV would have made sense, but if such an opportunity came along we would pursue it.’ His list of issues comprises regulations, payload limitations, reliability, operational limitations, data quality and economics. In addition, these issues mean that UAVs may be competing against ground-based rather than piloted surveys.

Ward is somewhat keener on the use of drones, ‘everybody wants one – we have three. It is an emerging technology. I still think it has two-to-five years to mature into a reliable technology that will not crash. For now, drone magnetic surveys are not much cheaper than conventional magnetic surveys.’

Future developments

For all the disadvantages discussed, the geophysical sector continues to develop relevant technology – computer modelling of data that permits the explorationist to ‘extract’ information from multiple data sets when they overlie each other, high-resolution airborne gravity and gravity gradiometry, multi-parameter surveys where four or more sensors are flown simultaneously, extremely sensitive gas-sensing, helicopter time-domain electromagnetic systems and the miniaturisation of some sensors for use on UAVs, cloud computing for inversion and modelling of geophysical data – popular in Australia – and, finally, software to create 3D workspaces that incorporate a geological model and the geophysical data.

Considerations of space mean that certain very difficult projects cannot be discussed here. Geophysicists must, however, be prepared to tackle misleading statements. As Ward states, ‘For example, they may say that it’s a ‘small hill’ and a ‘few trees’, but when you get to the site, it is an impenetrable mountain forest. At this point our field manager would turn to the offsider and say, release the drone.’