To further improve the accuracy-resolution, a number of avenues are possible:

• Slowing down the aircraft platform
• Better mapping of the aircraft platform accelerations;
• Reducing the aircraft accelerations
• Improving the gravimeter platform

With the N-GRAV system we chose to use a GT-1A gravimeter and to focus on the first three options above. NRG recently purchased a dedicated Pilatus PC6 aircraft renowned for its ability to fly slowly. To compensate for the slow survey speed the aircraft has been fitted with long-range fuel tanks markedly improving the aircrafts endurance. The aircrafts ability to operate from extremely short bush strips and exceptional safety record make it the ideal platform for survey in remote locations. Improved GPS processing allows for better mapping of aircraft movement. The large wingspan of the PC6 reduces the effect of troublesome high frequency turbulence. With the above advancements, the N-Grav system has been able to improve the accuracy-resolution specs to 0.5 mgals @ 2.5 km resolution.

Application
Modern airborne gravity systems provide an ideal method of rapidly evaluating regional basin structure in the petroleum industry especially in frontier environments where areas of interest may be vast with little or no background information available to assist with area/basin prioritisation.

As a regional dataset gravity is a useful lithostructural mapping tool, especially for deeper structure and intrusives, where it provides an ideal compliment to magnetic data. By combining techniques it is possible to create a 3 dimensional view of crustal structure ideal for identifying wide-scale processes responsible for economic deposit formation.

Technical background The following summary has been taken from Berzhitzky et al (2002), Gabell et al (2004) and Wooldridge (2004): The GT-1A was developed by the Joint Stock Company Gravimetric Technologies in the Russian Federation for both civilian and military markets. The post-processing software was developed by the Lomonosov Moscow State University’s Department of Mechanics and Mathematics. The GT-1A is an airborne, single vertical sensor, GPS-INS scalar gravimeter with a Schuler-tuned three-axis platform. The gravity sensor is a custom-designed accelerometer mounted inside a gyro-stabilised unit. Inputs from fibre optic gyro, inclinometers, angle sensors and dual frequency GPS are used to drive servo motors which maintain the sensor in a vertical position. This virtually eliminates the effects of horizontal accelerations in the measured signal. The entire assembly is mounted on a rotation table, maintaining the sensor orientation at the same heading.

Gravity data is sampled at ~18 Hz, sub-sampled to 2 Hz and integrated with dual frequency DGPS data to remove effects of vertical aircraft acceleration and Eötvös effect. Final gravity is reduced using a non-stationary adaptive Kalman filter using: raw gravity, aircraft vertical velocity (DGPS phase information), and platform misalignment errors. Filter length is user-defined according to resolution requirements

References
Berzhitzky, V.N., Bolotin, Y.V., Golovan, A.A., Ilyin, V.N., Parusnikov, N.A., Smoller, Y.L., Yurist, S.S. (2001). GT-1A inertial gravimeter system – Results of flight tests. MSU Faculty of Mechanics and Mathematics, 2002

Gabell, A., Olsen, D., Tuckett, H. (2004). The GT-1A mobile gravimeter. ASEG 2004 Airborne Gravity Workshop.