Data Processing
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
| 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. |
figure 3. Gaussian and RC filter
comparison |
