Geological Survey of South Australia: An update on South Australia’s gravity calibration ranges An absolute gravity meter measures the acceleration due to gravity at one place. Typically, this value is approximately 9.8 ms−2 (equivalent to 980 000 mGals, or 9 800 000 μms−2). A relative gravity meter measures a number in (say) mGals, but it is not an absolute value. For example, a Scintrex CG5 gravity meter measures values between 1000 and 9000 mGals, which need to be tied to an absolute measurement to recover the absolute values. However, the values also need scaling. A range of (say) 100 mGals as measured by an absolute gravity meter might be measured as a range of (say) 101 mGals by a relative gravity meter. This can be seen by taking two measurements with a gravity meter, one at the bottom of a hill and one at the top of the hill, where the absolute gravity is known at each point. The difference in values should be the same but will typically be out by a very small amount. Even though it is a small amount, the effect of not applying a scale factor to gravity measurements is noticeable and can ultimately affect geological interpretation (Heath 2018). It is therefore important to calculate this scale factor prior to gravity surveys. Two gravity calibration ranges exist in South Australia to calculate scale factors for relative gravity meters. One is in Adelaide, and the other a short distance southeast of Port Augusta. The Adelaide Gravity Calibration range has recently suffered a minor setback. The Norton Summit Cemetery Australian Fundamental Gravity Network (AFGN) site (code 2012999208) has been destroyed. The site was near a sheltered park bench and served as the upper half of the Adelaide Gravity Calibration range. The blue plug has gone, and the bench and small shelter are in a new location a short distance away. The ground underneath them is not suitable for a new AFGN site. However, the AFGN site at the Norton Summit Hotel (1960910208) is still available for use. It is very close to a couple of roads and so is not ideal for gravity readings, but will suffice until a new, quieter site is established. If any readers have suggestions for new AFGN sites in the Adelaide Hills – preferably a short distance from the established sites at Kensington Park – they would be very welcome. The established sites at Kensington Oval (2001910108) and the Burnside Rugby Club (2015909408) are still in good condition, although you may have to weave between some parked motorcycles and/or scooters at the Kensington Oval site. Transit from the Burnside Rugby Club to Norton Summit is slightly quicker than from the Kensington Oval site. Absolute gravity values at these three sites are: Norton Summit Hotel absolute gravity: 9796300.22 μms−2 (AAGD07) Kensington Oval absolute gravity: 9796985.05 μms−2 (AAGD07) Burnside Rugby Club absolute gravity: 9796927.08 μms−2 (AAGD07) Reference Heath, P. 2018. Quantifying the differences between gravity reduction techniques. Exploration Geophysics 49: 735–43. doi:10.1071/EG17094. Norton Summit Hotel AFGN site Philip Heath Geological Survey of South Australia Philip.Heath@sa.gov.au News Geophysics in the Surveys 23 PREVIEW JUNE 2019