Mark Dransfield

Airborne gravity gradiometry: Terrain corrections and elevation error

Terrain corrections for airborne gravity gradiometry data are calculated from a digital elevation model (DEM) grid. The relative proximity of the terrain to the gravity gradiometer and the relative magnitude of the density contrast often result in a terrain correction that is larger than the geologic signal of interest in resource exploration. Residual errors in the terrain correction can lead to errors in data interpretation. Such errors may emerge from a DEM that is too coarsely sampled, errors in the density assumed in the calculations, elevation errors in the DEM, or navigation errors in the aircraft position. Simple mathematical terrains lead to the heuristic proposition that terrain-correction errors from elevation errors in the DEM are linear in the elevation error but follow an inverse power law in the ground clearance of the aircraft. Simulations of the effect of elevation error on terrain-correction error over four measured DEMs support this proposition. This power-law relation may be used in selecting an optimum survey flying height over a known terrain, given a desired terrain-correction error.

High resolution gravity surveys from a fixed wing aircraft

BHP has in operation two FALCON airborne gravity gradiometer (AGG) systems providing gravity surveys with sufficient sensitivity and resolution for mapping the gravity anomalies associated with mineral deposits. Their development was motivated by the need for greater selectivity in regional reconnaissance surveying. This is achieved through the addition of density information to magnetic susceptibility and electrical conductivity which are available from other airborne survey technologies. The capability of the FALCON AGG systems is demonstrated with three examples. The King George anomaly was identified as an isolated magnetic anomaly in government release regional magnetic survey data. A FALCON survey of 2500 line km over the anomaly was completed in 5 days and demonstrates a clearly defined gravity anomaly of 7 mGal coincident with the magnetic anomaly. This gravity information is sufficient to upgrade the significance of the magnetic anomaly as a potential iron-oxide-copper-gold (IOCG) target. The Middleback Ranges, South Australia are an iron ore province supplying the OneSteel steelworks at Whyalla. The survey area includes relief of 300 m and some sharp escarpments, requiring accurate topographic correction of the gravity data. The FALCON survey has been compared with the ground gravity data in both gravity and gravity gradient form.

Noise and Repeatability of Airborne Gravity Gradiometry

When evaluating the capability of any Airborne Gravity Gradiometer (AGG) system some of the most useful inputs are data collected over areas with good-quality ground truth. A gravity test range has been established at Kauring in Western Australia for such comparisons. CGG (then Fugro Airborne Surveys) flew the fixed-wing FALCON AGG system over the Kauring AGG Test Site over three periods in July 2011, November 2011 and February 2012. Comparison between the FALCON AGG survey data and the high resolution ground gravity data over the Kauring AGG Test site indicates that the FALCON vertical gravity, gD, has an error of /- 0.18 mGal, and that the FALCON vertical gravity gradient GDD has an error of /- 5.6 eotvos at 300m full wavelength low-pass filtering. Analysis of repeat surveying over the Kauring AGG Test site suggest slightly lower errors of the order of /- 0.10 mGal for the FALCON vertical gravity, gD; and that the FALCON vertical gravity gradient GDD has an error of /- 4.7 eotvos after 300m full wavelength low-pass filtering. We strongly recommend the collection and publication of comparative analyses over Kauring and areas with similar quality ground gravity data to establish the capability of AGG systems.