FALCON Airborne Gravity Gradiometer Research Articles

Mathematical properties and physical meaning of the gravity gradient tensor eigenvalues

The eigenvalues of the gravity gradient tensor can be expressed as functions of two parameters: a magnitude and a phase. The decomposition gives physical meaning to the eigenvalues: the magnitude measures the amount of curvature and the phase is related to the type of source. Their understanding allows to propose new quantities to interpret. As an example, a modified phase eigenvalue offers the interpreter an enhanced version of the vertical gravity gradient which is demonstrated with model data and applied to FALCON airborne gravity gradiometer data from the Perth Basin, Australia

Mathematical properties and physical meaning of the gravity gradient tensor eigenvalues

The eigenvalues of the gravity gradient tensor can be expressed as functions of two parameters: a magnitude and a phase. The decomposition gives physical meaning to the eigenvalues: The magnitude measures the amount of curvature, and the phase is related to the type of source. Their understanding allows one to propose new quantities to interpret. As an example, a modified phase eigenvalue offers the interpreter a subtle enhanced version of the vertical gravity gradient, which is evaluated with model data and applied to FALCON airborne gravity gradiometer data from the Perth Basin, Australia.

Applying advanced gravity and magnetic inversion methods to expand the Platreef PGE-Ni-Cu resource in the Bushveld Complex

There are many potential field inversion algorithms available, and all are sufficiently capable of generating a model that explains supplied geophysical observations. The challenge is extracting a model that provides real geological insight. Here we present applications of two different styles of advanced inversions to a deep exploration program at the Platreef PGE-Ni-Cu deposit in the Bushveld Igneous Complex of South Africa. The initial approach was to apply generalised focussing constraints to a 3D magnetic vector inversion, an approach chosen to manage the effect of expected strong remanent magnetisation. This resulted in successful prediction and drill definition of inferred resources within a deep, west-dipping extension to the shallow-dipping “Flatreef” deposit. Later, a detailed 3D model of geological constraints based on drilling and mapping was constructed and used to tightly constrain inversions of gravity data derived from a FALCON airborne gravity gradiometer survey. The resulting 3D density model accurately predicted a continuation of the Flatreef host rocks to shallower levels than previously anticipated. This facilitated further drill-definition of additional inferred resources within a southern extension of the Flatreef deposit. Key to the success of the inversions at accurately targeting mineralisation at depths of 700–1300 m depth, was the inclusion and integration of all available information to ensure that predictions were consistent with prior observations.

Interpreting the direction of the gravity gradient tensor eigenvectors: The main tidal force and its relation to the curvature parameters of the equipotential surface

Rotating the gravity gradient tensor about a vertical axis by an appropriate angle allows one to express its components as functions of the curvatures of the equipotential surface. The description permits the identification of the gravity gradient tensor as the Newtonian tidal tensor and part of the tidal potential. The identification improves the understanding and interpretation of gravity gradient data. With the use of the plunge of the eigenvector associated with the largest eigenvalue or plunge of the main tidal force, it is possible to estimate the location and depth of buried gravity sources; this is illustrated in model data and applied to FALCON airborne gravity gradiometer data from the Canning Basin, Australia.

Interpreting the direction of the gravity gradient tensor eigenvectors: Their relation to curvature parameters of the gravity field

Rotating the gravity gradient tensor about a vertical axis by an appropriate angle allows one to express its components as functions of the curvatures of the equipotential surface. The description permits the identification of the gravity gradient tensor as the Newtonian tidal tensor and part of the tidal potential. The identification improves the understanding and interpretation of gravity gradient data. With the use of the plunge of the eigenvector associated with the largest eigenvalue or plunge of the main tidal force, it is possible to estimate the location and depth of buried gravity sources; this is developed in model data and applied to FALCON airborne gravity gradiometer data from the Canning Basin, Australia.

Targeting oil and gas in the Perth Basin using an airborne gravity gradiometer

In recent years there has been renewed interest in the hydrocarbon potential of the Perth Basin in Western Australia. It is close to the regional capital city and the gas pipeline that runs between Dampier in the north and Bunbury in the south. Recent discoveries of gas by AWE have shown that there is a working hydrocarbon system within at least the northern and central parts of the basin. In most parts of the basin, modern seismic data is relatively scarce. In the current low oil price environment, explorers are looking for cost-effective ways of exploring and targeting seismic acquisition. Airborne gravity gradiometry is such a technique. It has been widely used in frontier basins to understand the basin architecture (Bain et al., 2013, Roberts et al., 2015), sedimentary structure (Feijth et al., 2015 and Kovac et al. 2013) and planning of seismic acquisition (Moore et al., 2012). The Black Swan geophysical survey was conducted by CGG to assist oil and gas producer, Empire Oil and Gas, in identifying target areas for hydrocarbon exploration. The main tools employed were the Falcon Airborne Gravity Gradiometer (AGG), magnetic and digital terrain data. The survey was flown east-west with a nominal flying height of 100 m with 1 000-m line spacing, using a flight line to tie line ratio of 10:1. Structural interpretation has been carried out in two phases. Regional interpretation provided an overview of major regional structures and aimed to analyse the linkage between segments within the exploration blocks. It was derived from regional, publicly available gravity and magnetic data. Detailed structural interpretation was derived from the AGG (Airborne Gravity Gradiometer) data in order to improve current understanding of the tectonic pattern within Empire Oil’s exploration blocks. Depth to magnetic basement was calculated using publicly available government data. The survey identified areas containing large structural leads and trends for targeting future gas exploration activities, including infill 2D seismic acquisition.

Geophysical Responses over the Cannington Ag-Zn-Pb Deposit-Queensland

SUMMARY The Cannington deposit is a high grade Ag-Zn-Pb deposit found in 1990 by BHP Minerals drilling an isolated 1,000 nT aeromagnetic feature. Following the discovery of Cannington, numerous airborne, ground and borehole surveys have been carried out which overall, provided some assistance at better defining the ore system but did not lead to the discovery of new major deposit in the area. While Cannington possessed a clear magnetic response, the presence of a thick conductive cover made the use of EM and electrical techniques challenging. BHP used Cannington as a test ground for a variety of new techniques including a ground SQUID EM sensor, modified airborne EM technology (higher power and lower base frequency) and over 10 years after discovery, the first ever Falcon airborne gravity gradiometer survey in Australia.

3D geophysical model of the Glyde Basin, Northern Territory, based on curvatures derived from airborne gravity gradient data

SUMMARY This paper presents automatic 3D geophysical model generation based on equivalent pseudodepth slicing of the shape index of the equipotential surfaces derived from airborne gravity gradient data. The method is carried out in three steps. First, the pseudodepth slices of the vertical gravity gradient and the magnitude of the differential curvature components are generated. Second, the equivalent pseudodepth slices of the shape index are generated. Finally, 3D interpolation is carried out to obtain the final model. The method is applied to FALCON airborne gravity gradiometer data from the Glyde Basin, Northern Territory and compared to an independently interpreted, integrated 3D geological Earth model.

Automatic generation of 3D geophysical models using curvatures derived from airborne gravity gradient data

A method for automatic 3D model generation derived from airborne gravity gradient data was illustrated. The proposed method computed a volumetric distribution of curvatures that form a 3D image of possible density distributions. The method relies on spectral analysis and the equivalence of the power spectra of the classical mean and differential curvatures of equipotential surfaces to create pseudodepth slices of a quantity that describes the geometry of the surfaces: the shape index. The method was carried out in three steps. First, the pseudodepth slices of the vertical gravity gradient and the magnitude of the differential curvature components were generated. Second, equivalent pseudodepth slices of the shape index were generated. Finally, 3D interpolation was carried out to obtain the final model. The synthetic model data indicated that the vertical density contrasts were well modeled. A 3D model derived from FALCON airborne gravity gradiometer data from the Canning Basin, Australia, was compared to an independently interpreted integrated 3D earth geologic model.

Application of curvatures to airborne gravity gradient data in oil exploration

We apply equipotential surface curvatures to airborne gravity gradient data. The mean and differential curvature of the equipotential surface, the curvature of the gravity field line, the zero contour of the Gaussian curvature, and the shape index improve the understanding and geologic interpretation of gravity gradient data. Their use is illustrated in model data and applied to FALCON airborne gravity gradiometer data from the Canning Basin, Australia.

Integrated Interpretation of FALCON Airborne Gravity Gradiometer, Magnetic and Seismic data acquired over the Chirete Block, Argentina

Integrated Interpretation of FALCON Airborne Gravity Gradiometer, Magnetic and Seismic data acquired over the Chirete Block, Argentina

KING GEORGE: Measured and modelled AGG response over an IOCG terrane

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.

Falcon airborne gravity gradiometer survey results over the Cannington Ag-Pb-Zn deposit

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.

FALCON test results from the Bathurst Mining camp

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.

Falcon Gravity Gradiometer Technology

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.

Falcon Test Results from the Bathurst Mining Camp

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 Eö and a full-width at half-maximum amplitude of 500 m.