XTRACT™ AIRBORNE GRAVITY
New Resolution Gravity
The early 2000s saw the introduction of a new generation of gyro-stabilised total field gravity systems which incorporated high definition DGPS information to correct for vertical aircraft accelerations. The key advantage of these systems over predecessors was the ability to minimise horizontal accelerations and accurately map the aircrafts vertical movement by incorporating phase information from the differential GPS data.
As a result a significant improvement in accuracy-resolution was achieved in more turbulent conditions.
The new systems resulted in an improvement in accuracy-resolution from around 1 mgal @ 6 km resolution to approximately 0.6 mgal @ 4 km resolution using a standard C208 (Grand Caravan) aircraft platform.
As results are limited by the GPS ability to measure aircraft accelerations, 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
- Improved GPS processing.
Resolution: NRG 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 aircraft's 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, our gravity system has been able to improve the accuracy-resolution specs to 0.6 mgals @ 2.5 km resolution.
ACCURACY
Due to the down-line resolution limit, airborne gravity datasets often have a degree of over-correlation as line spacing is tightened. The additional data can be used to spectral filter uncorrelated noise in the data resulting in an improvement in accuracy. This encourages the use of tighter line spacing.
Bouguer Gravity Image
Bouguer gravity images illustrate the use of spectral filtering tight line spaced data (2 km) to remove uncorrelated noise thus improving accuracy.Left: Raw Bouguer gravity
Right: Grid filtered Bouguer gravity.
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.
Although basin identification is easily accomplished due to the large long-wavelength gravity anomalies produced, any form of detailed basin mapping requires much higher resolution data. As a result any incremental improvement in accuracy-resolution has a significant effect on the applicability of the data.
