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For airborne gravity gradiometry in rugged terrain, helicopters offer a significant advantage over fixed‐wing aircraft: their ability to maintain much lower ground clearances. Crucially, this provides both better signal‐to‐noise and better spatial resolution than is possible with a fixed‐wing survey in the same terrain. Comparing surveys over gentle terrain at Margaret Lake, Canada, and over rugged terrain at Mount Aso, Japan, demonstrates that there is some loss of spatial resolution in the more rugged terrain. The slightly higher altitudes forced by rugged terrain make the requirements for terrain correction easier than for gentle terrain. Transforming the curvature gradients measured by the Falcon gravity gradiometer into gravity and the complete set of tensor components is done by a Fourier method over gentle terrain and an equivalent source method for rugged terrain. The Fourier method is perfectly stable and uses iterative padding to improve the accuracy of the longer wavelengths. The equivalent source method relies on a smooth model inversion, and the source distribution must be designed to suit the survey design.

Three major developments provide improvements in the spatial resolution, long wavelength response and noise reduction in airborne gravity gradiometer data. The world’s first commercial strap down gravimeter, sGrav, has achieved the same levels of data quality at long wavelengths as stabilized platform systems. With a repeatability of 0.71 mGal RMS at 300 s filtering, it is ideally suited to augment the long wavelength airborne gravity gradiometer data. The sGrav data is incorporated into the airborne gravity gradiometer processing stream by a modified conforming technique that first converts the gravity data to gravity gradients and then merges the result with the gradients from the gravity gradiometer. This improved process has advantages for conforming gravity gradiometer data to any regional gravity data. Processing gravity gradient data routinely involves low-pass filtering which limits the spatial resolution. A new processing method increases this resolution by splitting the acquisition noise from the geologic noise and then removing the acquisition noise. This method has reduced noise amplitude densities, in some cases by 50%.

Airborne gravity gradiometry (AGG) is becoming a widely accepted tool in exploration. It provides rapid acquisition of accurate gravity data at high spatial resolution with complete coverage over large areas. The data are of significant value to explorers for a wide range of commodities throughout the world. With the advent of airborne gravity gradiometer technology, it is now possible to collect high-resolution gravity with all the attendant advantages.

The application of equipotential surface curvatures to airborne gravity gradient data is presented. The differential curvature, as measured by the FALCON? airborne gravity gradiometer system, the mean curvature, and the direction of maximum curvature of the equipotential surface should improve the understanding and geological interpretation of gravity gradient data. It has been shown that the horizontal gradients of the vertical component of the gravity vector form the curvature of the gravity field line. As this line is orthogonal to the equipotential surface, its curvature is related to how successive equipotential surfaces change and it may not be needed to interpret. The theoretical basis for the method is discussed. Practical applications of utilising the curvature method are presented on a synthetic model and FALCON? airborne gravity gradiometer data from the western zone of the Halls Creek Orogen, Western Australia.

SUMMARY Computing the cross-gradient product of airborne vertical gravity and magnetic intensity data provides a means to combine information about two different physical properties into the same image, thereby aiding interpretation. The method utilises the angle and cross-product values between the horizontal gradients of each dataset to produce two images in which structural similarities are enhanced. The theoretical basis for the method and results of its application to the survey data are discussed. The cross-gradient method was tested on a synthetic model and applied to FALCON™ airborne gravity gradiometer data from the Halls Creek Orogen, Western Australia. The vertical component of the gravity vector and the reduced to magnetic pole total magnetic intensity, were used as inputs.

Gravity derived only from airborne gravity gradient measurements with a normal error distribution will have an error that increases with wavelength. It is straightforward in principle to use sparsely sampled regional gravimeter data to provide the long wavelength information, thereby conforming the derived gravity to the regional gravity. Regional surface or airborne gravimeter data are not always available and can be difficult and expensive to collect in many of the areas where an airborne gravity gradiometer survey is flown. However the recent release by the Danish National Space Centre of the DNSC08 global gravity anomaly data has provided regional gravity data for the entire earth of adequate quality for this purpose. Studies over three areas, including comparisons with ground, marine and airborne gravimetry, demonstrate the validity of this approach. Future improvements in global gravity anomaly data are expected, particularly as the product from the recently launched Gravity field and steady-state Ocean Circulation Explorer (GOCE) satellite becomes available and these will lead directly to an improvement in the very wide bandwidth gravity available after conforming gravity derived from gravity gradiometry with the global gravity.

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.

For the past 15 years airborne gravity gradiometry has been used on a variety of petroleum and mineral exploration plays. As explorers focus on increasingly deeper targets with ever more subtle geophysical signatures, there is a growing need to accurately gauge the accuracy and resolution of airborne gravity gradiometer systems. In this article we present various noise estimates derived from surveys over the R.J. Smith Airborne Gravity Gradiometry Test Range in Western Australia.

The Bass Strait FALCON airborne gravity gradiometer (AGG) survey was flown over an area of the Gippsland Basin in Bass Strait in July 2002. The survey, centred on the gas-producing Marlin and Snapper fields, coincided with a contemporary, detailed marine gravity survey. These marine gravity data, as well as pre-existing data from satellite gravity and sparse marine gravity surveys, were used in comparisons with the FALCON data. The vertical gravity gradient data outlined a major Eocene channel. Interpretation, including modelling of representative seismic lines, revealed the particular usefulness of these data for mapping shallow faults. Conventional vertical gravity from the FALCON survey was better at the longer wavelength features, typically the major tectonic elements such as basin bounding faults, and relative sediment thickness. Comparisons with the marine and satellite gravity data showed that the FALCON gravity reproduced all the information available in the other surveys at wavelengths up to the survey size. At shorter wavelengths, the FALCON data had higher sensitivity than all other datasets, including the detailed marine gravity. Incorporation of longer wavelengths from the satellite or other gravity into the FALCON gravity proved successful in improving the longer wavelength information and regional context. As a result of this study, the standard FALCON processing has been improved to allow such incorporation on all surveys where separately acquired sparse gravity data are available.

Summary In standard processing of fixed-wing FALCON ® airborne gravity gradiometer (AGG) data a low-pass filter with a cut-off wavelength of 300m is typically applied. The choice of cut-off wavelength presents a compromise between seeking to minimize the noise and seeking to maintain the spatial resolution in the data. However, for AGG surveys with traverse line spacing equal to or less than 100m the cut-off wavelength can be reduced and the resolution improved. Analysis of survey results from the Kauring AGG Test Site in West Australia shows that for a survey of 100m traverse line spacing the cut-off wavelength can be reduced to 100m, providing a resolution of 50m. Comparison with the high resolution ground gravity data suggests an RMS error of 7.5Eo for the vertical gravity gradient, GDD, component at a cut-off wavelength of 100m. The corresponding noise amplitude density of 2.4Eo√km is in line with previously published estimates. The 200% improvement in resolution is accompanied by 30% higher RMS noise levels in the vertical gravity gradient, GDD, data.

BHP Billiton commenced exploration surveying with the world's first fully operational, airborne gravity gradiometer in October 1999. This gradiometer (called Einstein), together with a later one called Newton, was developed in conjunction with Lockheed Martin by BHP Billiton's FALCON project. FALCON data are acquired by Sander Geophysics Ltd., flying a Cessna Grand Caravan to survey specifications typical of aeromagnetic surveys. The first FALCON survey was flown over a portion of the Bathurst mining camp in New Brunswick, Canada in order to compare system performance with existing extensive and detailed ground-gravity data. The ground-gravity data, supplied courtesy of Noranda Minerals Exploration Ltd., were upward continued to the flying height and vertically differentiated to provide vertical gravity gradient data suitable for comparison with the airborne data. The two data sets compare very well and the results demonstrate that the FALCON airborne gravity gradiometer is capable of detecting sources with a vertical gravity gradient signal of greater than 10 Eo and a full-width at half-maximum amplitude of 500 m.

In April 2000 BHP conducted six FALCON airborne gravity gradiometer (AGG) test surveys over the Cannington Ag-Pb-Zn ore body in NW Queensland, Australia. The purpose of the test surveys was to demonstrate the capabilities of the AGG instrument by comparison with known detailed ground gravity data, to investigate the effects of source distance on signal strength by flying surveys at various altitudes, and to estimate AGG instrument noise levels in survey conditions. The processed FALCON gD data compare very favorably with the upward-continued residual ground gravity data, capturing most geological features and clearly delineating the Cannington ore body. Analysis of the effects of source distance on signal strength indicates that with a nominal flying height of 120m, the FALCON AGG instrument can detect the gravity anomaly from a deposit with the size and geometry of Cannington through 130m overburden. By repeating a survey using the same acquisition parameters we can make an estimate of the RMS noise of the instrument under survey conditions. Analysis of repeat survey readings show that the FALCON AGG instrument attained a noise level of 10 Eotvos RMS in the bandwidth from 0.0 Hz to 0.125 Hz during the Cannington test flights.

King George is a high priority magnetic anomaly that was identified within regional aeromagnetic data. The anomaly is located in 20-30m of water in the Spencer Gulf, South Australia, adjacent to the Moonta-Wallaroo mining field. Regional geology indicates that this area is highly prospective for Iron Oxide Copper-Gold (IOCG) style deposits. IOCG deposits are expected to have a high gravity signature with possible association of magnetic anomalism, the latter being dependent on magnetite content. In March 2000, the Falcon airborne gravity gradiometer (AGG) system was flown over the King George anomaly, previously inaccessible to conventional gravity measurement techniques. The survey showed a 7 mGal gravity anomaly coincident with the 10,000nT magnetic anomaly, making the anomaly to a high-priority drill target. Modelling of the airborne gravity and magnetic data indicated that two closely spaced bodies 200m below the surface produced the observed anomaly. Vertical gravity gD was used during the modelling exercise. The Falcon AGG system measures the quantities GNE and GUV from which vertical gravity gradient GDD and vertical gravity gD are derived. To verify the gravity model, the GNE and GUV responses were also computed and compared with actual quantities measured by the Falcon AGG system. A good match between the measured and the modelled components was obtained.

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.

Airborne geophysical data provide valuable information of the Earth’s subsurface and may be acquired for varying purposes including geotechnical, groundwater, mineral and petroleum exploration, environmental and geodesy applications. Airborne geophysical data is inherently rapid in its acquisition and has the capacity for significant cost savings while allowing access to areas too difficult or expensive to reach using traditional methods. Recent advances in gravity gradiometric and electromagnetic technologies in particular have led to significant improvement in sensitivity, resolution, depth of investigation and interpretability of airborne geophysical data. Since inception in 1999, the FALCON Airborne Gravity Gradiometer (AGG) has undergone continual and significant development with noise levels being halved since 2004. Further dramatic gains in both sensitivity and spatial resolution have been made possible through the adaptation of this technology to use in light helicopter platforms. Airborne electromagnetic technology is more mature, however recent significant increases in system power and innovation to the receiver platform technology have led to improved signal to noise ratios and enhanced target detectability. The introduction of 3 component receiver coils to helicopter platforms have also increased the understanding and interpretability of detected conductors.