New nationwide airborne geophysical survey in Sierra Leone The government of Sierra Leone has commissioned a high-resolution airborne geophysical survey of the entire country (71 740 km2). Funded by the World Bank, and executed by the Ministry of Mines and Mineral Resources, the survey will collect more than 543 000 line km of magnetic and radiometric data at a flight line spacing of 150 m and a nominal terrain clearance (subject to safety constraints) of 50 m. Five survey aircraft have been employed in data acquisition over a field season that will last approximately six months. The survey has been progressing well so far, with more than 50% (or nearly 273 000 line km) of field data being collected and independently approved within the first 90 days of survey operation. The survey was officially launched on 29 January 2019 by HE Julius Maada Bio, the President of Sierra Leone, who stated that the lack of geological data has constrained effective policies required to boost the mineral resources sector, and added that a high quality national dataset will reposition the need for natural resources development. The main objective of the nationwide survey is to obtain integrated geophysical data that will form the basis for a review of the country’s geology and mineral resources and their distribution. The ultimate goal is to gain improved knowledge of the quantity and distribution of the country’s mineral resources so as to ensure their sustainable exploitation in future. Data acquisition is being carried out by South African/Spanish contractor Xcalibur Airborne Geophysics, which has operational experience in most African countries, the Middle East, Asia and Canada. Survey supervision, including data QC, is being provided by UK-based Reid Geophysics. As explained by Sierra Leone’s Director of Geological Survey, Prince Cuffey, the survey benefits from several interesting features. These include the use of aircraft specifically designed to fly low and slow for improved geological imaging, magnetic sensors mounted on each wingtip to improve geophysical data quality, and an externally-mounted head- up display for better and safer survey navigation. Each aircraft is additionally equipped with 40 l of radiometric crystal pack, mounted in a belly pod to provide maximum detection capability on this once-in-a-lifetime nationwide exploration initiative. Tim Archer Reid Geophysics Ltd tim@reid-geophys.co.uk Henderson byte: Georg Ohm You know that ohm is the SI unit for electrical resistance but, do you know who Ohm was? The person whose name is used for the unit of electrical resistance was Georg Simon Ohm (1789 - 1854), born into a family of seven children in Erlangen, Germany. Georg and his brother Martin, who also became a well-known mathematician, were initially taught by their father, who brought them to a high standard in mathematics, physics, chemistry and philosophy. In September 1806 Ohm left home to accept a position as a mathematics teacher in a school near Bern, Switzerland. This was followed by other postings until, in April 1811, he became a lecturer in mathematics at the University of Erlangen where his previous private studies prepared him for his doctorate. He ended his somewhat turbulent academic career by becoming a professor of experimental physics at the University of Munich in 1852, just two years before his death. Using equipment of his own creation, Ohm found that there is a direct proportionality between the potential difference (voltage) applied across a conductor and the resultant electric current. The constant of proportionality is the resistance of the circuit. This relationship is known as Ohm’s law. Ohm’s most important publication was, in 1827, Die galvanische Kette, mathematisch bearbeitet (The galvanic circuit investigated mathematically) in which he outlined his complete theory of electricity. Ohm’s law first appeared in this publication. His work was finally recognized by the Royal Society in 1841 with the award of the Copley Medal. Ohm also gave his name to Ohmmeter, an instrument for measuring electrical resistance and ohmic, for electrical devices that obey Ohm’s Law. Roger Henderson rogah@tpg.com.au News Industry 16 PREVIEW JUNE 2019