b'Coping with ambiguityFeaturesymbol were moved upwards to the axis and to the right byHanneson, JE, 2003. On the use of magnetics and gravity to the width of the symbol, it would define the sphere requireddiscriminate between gabbro and iron-rich ore-forming for gabbro with 2% magnetite. We must then suspend oursystems, Exploration Geophysics, 34 (1&2), pp110-113.calculations because lesser concentrations of magnetite yieldHawking, SW, 1988. A Brief History of Time, Bantam Books, spheres whose radii exceed the depth to the centre, which isLondon, 198pp.geologically disallowed because no gabbro was seen at theMooney, HM., and R. Bleifuss, 1953. Magnetic Susceptibility surface. Measurements in Minnesota, Part II, Analysis of Field Results, Geophysics, 18, pp383-393.References Nettleton, LL, 1942. Gravity and Magnetic Calculations, Geophysics, 7 (3), pp293-310.Blakely, R.J., 1995. Potential theory in Gravity and MagneticRussell, B., 2000. History of Western Philosophy, Routledge, New Applications; Cambridge University Press, 441pp. Fetter Lane, London, 842pp. (first published 1946).Cunliffe, B., 2001. The Extraordinary Voyage of Pytheas the Greek;Talwani, M., 1961. Computation with the help of a digital The Man who Discovered Britain, Allen Lane, The Penguincomputer of the magnetic anomalies caused by bodies of Press, 182pp. arbitrary shape, Geophysics, 26, p203.Esdale, D.J., Pridmore, D.F., Coggon, J.H., Muir, P.M., Williams, P.K.,Talwani, M., 1960. Rapid computation of gravitational attraction and F.P. Fritz, 1987, Olympic Dam depositgeophysical caseof three-dimensional bodies of arbitrary shape, Geophysics, history, Exploration Geophysics, 18, pp47-49. 25, p203.Henderson Byte: Deeper drilling for geothermal energyGeothermal resources at depths of 8 km and more can provide an unlimited heat source which is free of CO 2emissions. This energy source is, therefore, a highly desirable replacement for fossil fuels in a world increasingly conscious of a climate crisis. However, depths of the order of 8 km have generally been impossible to reach with existing drilling equipment, as the rocks are extremely hard and their heat (of the order of 400C to 500C) destroys mechanical drill bits. In fact, the maximum depth drilled in the world so far is 12 km, on the Kola Peninsula in Murmansk. It took 15 years from 1970 to 1994 to drill the hole, and the final temperature reached was a relatively low 180C.As a consequence, existing geothermal plants are located in places where conditions allow for energy extraction at relatively shallow depths and the number of geothermal energy plants being built has plateaued world-wide. However, geothermal heat at greater depths, if it can be accessed, has an almost unlimited potential.There are several newly proposed ways of drilling to great depths, including high energy laser beams and high temperature electrical plasmas, however, the technique that currently has the most promise is millimetre wave drilling - an outgrowth of fusion research at MITs Plasma Science and Fusion Centre guided by Paul Woskov. The technique employs a continuous beam of energy waves with wavelengths of only a few millimetres. These millimetre waves can be produced by a commercially available gyrotron, and the waves are guided straight down by a gravity-controlled waveguide. Due to the intense heat of the energy waves, the sides or lining of the borehole are melted and then solidified into a vitrified wall, creating a completely sealed borehole.Another method of drilling to greater depths is electrical plasma drilling. In this technique, an electric arc, formed by an electrical breakdown of a gas, produces an ongoing plasma discharge resulting from a current flowing through normally nonconductive media such as air or gas. It has several advantages over other technologies, including the ability to produce boreholes with a wide range of diameters, and drilling in the presence of water. A team at the Research Centre for Deep Drilling in the Slovak Academy of Sciences has developed this drilling concept based on utilisation of electrical plasma. One company involved is GA Drilling, headquartered in Bratislava, Slovakia.John Bishop, an active geothermalist and a Member of the ASEG since its inception, has been pursuing the goal of abundant energy from geothermal sources in Tasmania (as KUTh Energy) and now in the Hunter Valley, NSW (as Spa*ark Energy), where he hopes to make use of the existing coal power generation infrastructure present in that region.Further readingDrilling Deep, New Scientist, 13 August 2022, p.25.Tapping into the million-year energy source below our feet, Zach Winn, MIT News Office, June 28, 2022 https://news.mit.edu/2022/quaise-energy-geothermal-0628Roger Henderson history@aseg.org.au49 PREVIEW DECEMBER 2022'