Airborne Gravity Survey

Gravity surveys measure small variations in the Earth’s gravitational field caused by differences in the density of subsurface rocks. Every rock type has a characteristic density. Dense bodies, such as mafic intrusions, massive sulphides or uplifted basement, produce local increases in gravity. Lighter materials, such as sedimentary basin fill, salt, or altered and fractured zones, produce local decreases. The magnetic method responds only to magnetic minerals and the radiometric method only to the uppermost layer of the ground. The gravity method responds to density contrasts at all depths, and it does not depend on a rock’s magnetic or radioactive properties.

The basic purpose of airborne gravity surveys is to map these density variations over large areas, quickly and consistently, including terrain that is difficult or impossible to cover on the ground. The gravity signals of geological interest are extremely small compared with the accelerations the aircraft itself experiences in flight. Recovering them requires very precise measurement and careful processing. Aircraft motion and turbulence, the Eötvös effect of the aircraft’s velocity over a rotating Earth, flight altitude, GNSS positioning accuracy, latitude and surrounding topography all affect the measurements. Unless each is corrected thoroughly, they can mask or distort the geological signal. Modern strapdown inertial gravimeters, combined with high-precision GNSS and rigorous levelling, terrain and Bouguer corrections, now make it possible to produce high-quality gravity data from both fixed-wing aircraft and helicopters.

A few of the benefits that could be expected from the interpretation of gravity surveys include:

• Gravity maps outline the major geological framework of an area, including basins, basement highs, faults and major structural trends, making the method a powerful reconnaissance tool.
• Density contrasts can identify intrusions, buried igneous bodies and contacts between rock units, including those that are non-magnetic or hidden beneath cover.
• Gravity data can be used to estimate sediment thickness and depth to basement, which is critical in hydrocarbon exploration and basin analysis.
• High-density targets such as massive sulphide, iron oxide copper-gold (IOCG), iron ore and chromite deposits can produce direct gravity responses.
• Combined with magnetic and radiometric data, gravity improves geological interpretation and supports 3D modelling and inversion of the subsurface.

Airborne gravity can provide regional density coverage at a fraction of the time and cost of an equivalent ground gravity survey. It is especially cost-effective in remote, rugged or inaccessible terrain where ground stations would be slow, expensive or unsafe to acquire. It is also most effective when flown together with magnetic and radiometric surveys from the same aircraft.