Services>Survey Technology>Gravity

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Services>= ;Survey Technology>Gravity

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Introduc= tion

The Fugro Airborne Gravity System was developed, and cont= inues to evolve, in accordance with five unique design criteria:

Develop a reliable and aircraft independent gravity system. =
Measure geodetic quality GPS velocities, rather than using less accurate velocities computed from GPS positions.
Use exhaustive gravity sensor and platform modeling along with rigorous processing corrections in order to minimise filtering - "model, measure and correct - don't over filter&qu= ot;.
Provide clients with open access to all aspects of their data including in-fie= ld and final data processing and QC methods.
Evaluate the system resolution and accuracy using three independent analysis methods on data acquired in real world exploration conditions.

Aircraft independence required that the gravity system not be dependent on the flight characteristics of any specific aircraft, nor that the aircraft required any extensive aircraft airframe or system modifications. The Fugro system has been installed on six different aircraft all of which have been = used to fly successful airborne gravity surveys:

Cessna 208
Cessna 404
Cessna 421
Rockwell Aero-Commander 500 Shrike
Piper Aztec
Airship Industries R-300 airship (blimp)

This aircraft versatility and ease of gravity system installation, typically requiring le= ss than one day, simplify mob/demob operations and reduce survey costs. Measuring geodetic quality GPS velocities requires the= use of state-of-the-art dual frequency GPS receivers and the development of customised post-mission GPS processing software.

The Fugro system tracks aircraft kinematics with extreme accuracy, thus reducing the demands for extreme flight stability. The motto= of the Fugro gravity system is "model, measure and correct - don't over filter". Filtering is = minimised by applying an extensive suite of correctio= ns derived from GPS velocities, sensor and platform modeling measurements prio= r to filtering. Clients are provided open access to all aspects of their data including in-field and final data processing and QC methods. System resolut= ion and accuracy is evaluated by three independently measured analysis methods = on data acquired in real world exploration conditions.

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System Development and Operations

The Fugr= o airborne gravity system arose from a joint effort by three companies, inclu= ding a leading U.S. defense contractor specialising in advanced GPS applications. The development team incorporated both geophysicists and geodesists, led by Dr. J. (Christopher) Harrison. Dr. Harrison, was a lead scientist, along with D= r. Lucien LaCoste in the world's first non-military airborne gravity tests, conducted in a B17 in 1959¹. Additionally Drs Harrison and LaCoste pioneered research in = the understanding of cross-coupling effects in dynamic gra= vimetry measurements². Many important aspects of Dr. Harrison's extensive dynamic gravity knowledge has been incorporated = in the design of the Fugro airborne gravity system.

Flight-testing included three separate campaigns, flown over an industry sponsored gravity test range in = Texas. In additi= on, Fugro was contracted to fly a gravity test range in <= st1:State w:st=3D"on">Pennsylvania. Te= ns of thousands of line kilometres of data, flown in typical real world exploration survey conditions (rather than waiting for i= deal weather for flying) were collected, processed and analyzed over a period of four years prior to Fugro conducting its first commercial airborne gravity survey in 1995.

Safety is the paramount facto= r in Fugro's airborne gravity operations. Fugro is a founding member of the International Airborne Geophysics Safety Association (IAGSA). In addition to IAGSA guidelines F= ugro utilises it's own comprehensive safety procedures, each project is analyzed and a full job sa= fety plan activated. Fugro monitors all it's aircraft worldwide via the Inmarsat-C satellite service.

System H= ardware

Fugro uses a Lacoste & Romberg (L&R) dynamic gravity meter upgraded to the ZLS UltraSysT system. The ZLS model is a modern digital c= ontrol upgrade of the widely tested and proven LaCoste= and Romberg (L&R) dynamic gravity meter sensor. Fugro'= s airborne gravity system only uses L&R sensors with the latest generation air dampers (serial number S-88 and higher), which min= imise cross-coupling effects.

A unique three-stage vibration isolati= on platform was developed as a result of extensive vibration testing at the Bo= yd More Research Laboratory. Fugro's testing revea= led a wide range of significant vibration effects on the gravity meter sensor, at both long and short periods, which have been eliminated by careful attentio= n to platform mounting.

The gravity meter clock is synchronised to GPS time at the beginning of each fli= ght, and kept in synchronisation (equivalent of 1 mi= llisecond drift in 32 years) by an extremely stable Rubidium oscillator. The large amplitudes and short periods of aircraft vertical accelerations make tight = synchronisation of gravity and GPS data a necessity.<= o:p>

GPS data is collected using geodetic quality dual frequency receivers. The system logs full carrier pha= se (required for maximum accuracy), dual-frequency (required for ionospheric effect removal) raw GPS data for up to ni= ne satellites.

Data Pro= cessing

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 uti= lises independently measured aircraft accelerations derived from high precision G= PS navigation.

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 rem= oved, the system developers were able to concentrate on modeling and removing sub= tler 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 s= ervo loop to calculate a correction, which allows predictions of platform tilt a= s 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 traditi= onal 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 assu= ming an aircraft speed of 100 knots.

Using less filtering minimises reliance on extremely slow aircraft speeds = to produce good spatial resolution. In addition, proper modeling and correctio= n 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 model= s to correct GPS ellipsoidal elevations to geoidal elevations, thereby removing a potential long-wavelength error source (seve= ral milliGals over tens of kilometres) 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. Gauss= ian and RC filter comparison

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Resoluti= on & Accuracy

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:

Internal consistency - repeat lines
Internal consistency - crossover errors
External consistency - ground truth

A half-amplitude, half wavele= ngth point of the processing filters is used as the resolution measure. This mea= ns that a gravity anomaly of the quoted resolution width, for example 6 kilometres, would be attenuated to roughly half of it's original amplitude after final processing. The te= st survey was flown at an elevation of 610 metres = 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 f= rom very calm to conditions where it was very unpleasant to be a passenger on t= he aircraft.

The ground gravity dataset co= nsists 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 tes= ting, a single flight line was flown multiple times to test the system repeatabil= ity. 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 profi= le is 0.66 milliGals (mGals).

Figure 4. Multiple repeats of a single line

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

This emphasises the lack of long-wavelength drift errors i= n the Fugro system, which allows survey design with much wi= der tie lines spacing than other systems and producing improved survey economic= s.

Conclusi= on

The unique d= esign of the Fugro airborne gravity system provides costs savings by way of:

Aircraft independent gravity systems;
Reliable L&R gravity sensors;
Minimal tie line requirements; and
Corrections that minimise the need for extreme flight stability.

The unique design criteria also provides high quality, minimally filtere= d data that have been verified by three independently measured methods. These data have been acquired in real exploration survey conditions.=

Internal and external evaluat= ions have quantified these improvements over salt domes and thick sediments in t= he petroleum environment. Internal evaluation consistent of 6 non-averaged repeated lines and 101 network crossovers, along with external evaluation consistent of ground gravity comparison results in an accuracy of approxima= tely 0.6 mGals with 6km half-wavelength, half-amplit= ude resolution.

The Fugr= o fleet, under the command of 15 strategically located Fugro Airborne Survey offices around the world, inclu= de 16 aircraft suitable for the economic acquisition of airborne gravity data.= Fugro has acquired over 150 million kilometres of geophysical data of which 80,000 has been gravity.

Acknowledgments=

The ground gravity dataset was donated by Conoco for use in airborne gravity d= ata evaluation. (Unfortunately, due to contractual obligations, we cannot show = the ground gravity data itself.)