The gravity method was the first geophysical technique to be used in oil and gas exploration. Despite being eclipsed by seismology, it has continued to be an important and sometimes crucial constraint in a number of exploration areas. In oil exploration the gravity method is particularly applicable in salt provinces, overthrust and foothills belts, underexplored basins, and targets of interest that underlie high-velocity zones. The gravity method is used frequently in mining applications to map subsurface geology and to directly calculate ore reserves for some massive sulfide orebodies. There is also a modest increase in the use of gravity techniques in specialized investigations for shallow targets. Gravimeters have undergone continuous improvement during the past 25 years, particularly in their ability to function in a dynamic environment. This and the advent of

Historical development of the gravity method in exploration

We develop a new method for interpretation of tensor gravity field component data, based on regularized focusing inversion. The focusing inversion makes its possible to reconstruct a sharper image of the geological target than conventional maximum smoothness inversion. This new technique can be efficiently applied for the interpretation of gravity gradiometer data, which are sensitive to local density anomalies. The numerical modeling and inversion results show that the resolution of the gravity method can be improved significantly if we use tensor gravity data for interpretation. We also apply our method for inversion of the gradient gravity data collected by BHP Billiton over the Cannington Ag-Pb-Zn orebody in Queensland, Australia. The comparison with the drilling results demonstrates a remarkable correlation between the density anomaly reconstructed by the gravity gradient data and the true structure of the orebody. This result indicates that the emerging new geophysical technology of the airborne gravity gradient observations can improve significantly the practical effectiveness of the gravity method in mineral exploration.

Three-dimensional regularized focusing inversion of gravity gradient tensor component data

The analytic signal concept can be applied to gravity gradient tensor data in three dimensions. Within the gravity gradient tensor, the horizontal and vertical derivatives of gravity vector compo ...

Analytic signals of gravity gradient tensor and their application to estimate source location

BHP Billiton's FALCON airborne gravity gradiometer is a derivative of the Gravity Gradient Instrument (GGI) developed by Bell Aerospace (now Lockheed Martin) between 1975 and 1990. The basis of the GGI design is an accelerometer complement consisting of four accelerometers equi-spaced on a circle with their sensitive axes tangential to the circle. This configuration rejects both common mode acceleration and rotations about the axis perpendicular to the plane of the complement. The complement remains intrinsically sensitive to rotation rates about axes in the plane of the complement and is sensitive to the acceleration environment to the extent that there is imbalance in the accelerometer sensitivities. Rotation of the complement about the perpendicular axis moves the gradient signal to twice the rotation frequency, away from the effects of low frequency accelerometer bias changes. The GGI is mounted in a high-performance inertial stabilised platform to reduce rotation of the instrument so that its sensitivity to this motion does not represent a significant noise source. The GGI accelerometers are designed for very low noise, requiring hard evacuation, high pendulosity, low spring constant and attention to the constrainment loop. Accelerometer pairs are aligned with precision and their sensitivities and frequency responses are matched. The scale factor (sensitivity) and alignment of the sensitive axis of each accelerometer are adjusted by compensation feedback loops to minimise accelerometer imbalance by monitoring the response of the system to specific stimuli. The requirements of survey operations were taken into account during development of the system and the result is an instrument which requires limited preparation, is largely automated during surveys, places few restrictions on flight planning and has been operated in harsh ambient conditions. Data processing is streamlined and data quality can be checked immediately after a flight.

https://www.tandfonline.com/doi/pdf/10.1071/ASEG2001ab068

FALCON gravity gradiometer technology

We present a case study of applying 3D inversion of gravity gradiometry data to iron ore exploration in Minas Gerais, Brazil. The ore bodies have a distinctly high-density contrast and produce well-defined anomalies in airborne gravity gradiometry data. We have carried out a study to apply 3D inversion to a 20  km2 subarea of data from a larger survey to demonstrate the utility of such data and associated inversion algorithm in characterizing the deposit. We examine multiple density contrast models obtained by first inverting Tzz; then Txz, Tyz, and Tzz jointly; and finally all five independent components to understand the information content in different data components. The commonly discussed Tzz component is sufficient to produce geologically reasonable and interpretable results, while including additional components involving horizontal derivatives increases the resolution of the recovered density model and improves the ore delineation. We show that gravity gradiometry data can be used to deli...

3D inversion of airborne gravity gradiometry data in mineral exploration: A case study in the Quadrilátero Ferrífero, Brazil

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.

Performance of airborne gravity gradiometers

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.

Airborne gravity gradiometry: Terrain corrections and elevation error

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.

Conforming Falcon‡ gravity and the global gravity anomaly

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.

Noise and Repeatability of Airborne Gravity Gradiometry

From late April to the end of July 2000, a 39,000 line km airborne gravity gradient survey was completed over the EkatiO mine property in the NWT, Canada. This was the world?s first airborne gravity gradient survey for the purpose of detecting kimberlite pipes. Preliminary data processing was done on site at the EkatiO diamond mine. Subsequent drilling of gravity anomalies in the year 2000 has resulted in the discovery of two new kimberlite pipes. More anomalies will be drilled in 2001. The AGG data shows that more than half of the known kimberlite pipes have associated gravity anomalies. Some pipes with a diameter as small as 100 m or less can be detected in the AGG data. The AGG data has a 300 m resolution with an average RMS noise of 7.6 Eotvos in the derived vertical gradient. Laser profilometer data and differential GPS data were also acquired in the survey to construct a detailed digital elevation model for terrain correction. Besides detecting kimberlite pipes, the AGG data is also useful for mapping details of geological structures. This is complementary to the magnetic data acquired simultaneously with the AGG data.

Mark Dransfield

Papers

Performance of airborne gravity gradiometers

Airborne gravity gradiometry: Terrain corrections and elevation error

Conforming Falcon‡ gravity and the global gravity anomaly

Noise and Repeatability of Airborne Gravity Gradiometry

Detecting kimberlite pipes at Ekati with airborne gravity gradiometry