Maurice Jones reports on the growth of non-contact geophysical techniques
Although the traditional prospector and drilling activities are the first images that may first come to mind when the subject of mineral exploration comes up, the greatest growth in recent years has been in non-contact (or at least non-drilling) geophysical techniques. Some of these may be possible from airborne and seaborne carriers, and even from space, offering several important advantages over more traditional methods.
Within this category there is also a wide range of measurement techniques that can be employed including electromagnetic, gravity, magnetic, electrical, radiometric and even ground-penetrating radar. While these may not provide the detail necessary for accurate resource/reserve mapping, the carriers have the advantage of being able to cover wide areas, highlighting geological structures and anomalies that may indicate the presence of mineralisation. In the case of airborne and space carriers they are unlikely to be deterred by potentially hostile terrain, whether physical, climatic or even political.
Area selection within a region is important to afford the best chance of finding mineral deposits economically. To this end reference to existing knowledge from experience of ore genesis procedures, overall geological structures, previous surveys and mine workings will help in identifying good targets.
Demonstrating the capabilities, recently junior miner Xmet reported results of its third airborne electromagnetic survey. This was run over 437 line kilometres (273 miles) covering 20,000 acres (8,090 hectares) of its Blackfire Project property near Hearst, Ontario, Canada. The results discovered two large conductive anomalies and further delineated the survey’s prime target. The aerial survey was conducted by Geotech (www.geotechuk.com) using Versatile Time Domain Electromagnetic (VTEM) surveying, also assisting in interpreting results.
Airborne surveys can be carried out from fixed-wing aeroplanes or helicopters depending, to some extent, of the instrumentation response speed. Satellites will have reliable, dedicated instrumentation to collect images of the Earth beneath its transit route using various filtered portions of the electromagnetic spectrum.
Most marine geophysical surveys are associated with the oil and gas industries or other offshore structures but the relatively new activity of submarine mining is encouraging their use for mineral investigations. In addition to a large fleet of dedicated airborne carriers, the Fugro (www.fugro.com) fleet of survey vessels and AUVs (Autonomous Underwater Vehicles), stationed at various ports throughout the world, can carry out seabed sampling and testing as well as non-contact surveys. Their crews include specialists such as geophysicists, environmental scientists and specialist engineers.
Nearer the earth electromagnetic (EM) survey instruments can be used in ‘walkover’ surveys, preferably avoiding direct contact, so that they can be used with less effect from surface features such as on ‘brownfield’ sites. These surveys are claimed more rapid and cost-effective than conventional resistivity surveys. Using the appropriate frequencies and length of instrument EM surveys can be used to detect bedrock discontinuities and mineral veins and, also useful in mining, locating old shafts and other voids such as from old workings.
References: Dentith M & Mudge ST. Geophysics for the mineral exploration scientist, to be published May 2014, Cambridge University Press; Kearey P, Brooks M, & Hill I. An introduction to geophysical exploration 3rd edition, Black Science 2002, Oxford
Video 1: Video of the Beep Mat miniaturised electromagnetic survey instrument to detect conductive and magnetics materials under up to three metres of overburden, by GDD Instrumentation of Canada
Video 2: Animation of gravity surveying, by Australia’s Queensland Resources Council