Airborne Gravity

Fugro has tested the gravity system over a ground gravity test range located in a petroleum exploration area. The system resolution and accuracy have been evaluated using three independent analysis methods:

A half-amplitude, half wavelength point of the processing filters is used as the resolution measure. This means that a gravity anomaly of the quoted resolution width, for example 6 kilometers, would be attenuated to roughly half of it’s original amplitude after final processing. The test survey was flown at an elevation of 610 meters using a spacing of 2 km flight lines and 16 km tie-lines. In order to mimic real world exploration conditions the data were acquired in turbulence ranging from very calm to conditions where it was very unpleasant to be a passenger on the aircraft.

The ground gravity dataset consists of approximately 11,500 nearly uniformly distributed stations in a 165 x 50 km area (1.4 stations / km²⁾.

Internal Evaluation

As part of the evaluation testing, a single flight line was flown multiple times to test the system repeatability. An example of six repeats of this line is included as figure 4. This data has been filtered to 6 km (half-wavelength, half-amplitude) resolution, not averaged. The standard deviation of these repeats around the mean profile is 0.66 milliGals (mGals).

Figure 4. Multiple repeats of a single line

An additional internal accuracy measure is provided by crossover errors produced from a map survey using tie and traverse lines. The RMS crossover error (101 crossovers) before adjustment with 6 km filtering corresponds to a single-measurement accuracy of 0.69 mGals.

This emphasizes the lack of long-wavelength drift errors in the Fugro system, which allows survey design with much wider tie lines spacing than other systems and producing improved survey economics.