Airborne Gravity
Introduction
- System
Development & Operations - Data
Processing - Resolution
and Accuracy - Conclusion
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In keeping with Fugro’s philosophy of measuring acceleration
effects and system noise, correcting rather than over filtering,
Fugro’s ability to acquire high quality dynamic airborne
gravity data utilizes independently measured aircraft accelerations
derived from high precision GPS navigation.
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figure 1. Comparison
of the GPS-derived vertical accelerations and the gravity
meter vertical accelerations.
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The use of extremely accurate GPS navigation allows
removal of the major first-order noise sources in airborne gravity,
namely aircraft vertical accelerations and the Eötvös effect.
(figure 1) With these effects removed, the system developers
were able to concentrate on modeling and removing subtler effects,
such as horizontal acceleration, (see figure 2) extended
and long period cross coupling.
figure 2. Horizontal acceleration correction
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To calculate horizontal acceleration corrections, Fugro
has developed a proprietary algorithm that uses the differential
equation of the platform servo loop to calculate a correction,
which allows predictions of platform tilt as a function
of vehicle acceleration history. Knowledge of the platform
tilt enables accurate computation of both components of
the off-level component. These more subtle effects have
traditionally been dealt with by applying a series of RC
filters which can greatly attenuate or even eliminate signals
at wavelengths of geological interest. The advantage gained
from the "model, measure and correct – don’t over filter"
approach, in terms of less anomaly attenuation at short
wavelengths, is shown in figure 3. The traditional filter
descriptions are read as 3x2x6.6 is 3 passes of two-way
RC filtering, 6.6 second time constant; 3x2x6.6 + 3x2x20
is a cascade of 3x2x6.6 followed by 3x2x20. RC time constant
filters responses are converted to wavelength assuming an
aircraft speed of 100 knots.
Using less filtering minimizes reliance on extremely
slow aircraft speeds to produce good spatial resolution.
In addition, proper modeling and correction of long-period
cross-coupling effects allows for a survey layout with fewer
tie-lines being required. The long wavelength accuracy of
the system is reinforced by the use of the global geoid
models to correct GPS ellipsoidal elevations to geoidal
elevations, thereby removing a potential long-wavelength
error source (several milliGals over tens of kilometers)
in free-air corrections.
Fugro provides clients with full access to their
data as well as the processing and QC methods used by Fugro.
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figure 3. Gaussian and RC filter comparison
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