BAMAKO PROJECT
MALI
AIRBORNE GT-1A GRAVITY SURVEY
for
UTS Geophysics
ACQUISITION AND PROCESSING REPORT
Survey flown Jan. 2009 - Apr. 2009
by
UTS Geophysics
and
GT-1A Gravity data acquired and processed
by
110 Middle Road #05-03, Chiat Hong Blg Singapore 188968 Tel: +65 6333 4866 Fax: +65 6333 4966
APG Project No.: APG-2009-11
UTS Project No.: B075
GT-1A
Project: Bamako Area, Mali Airborne GT-1A Gravity Survey 2
GT-1A APG-2009-11
1. Introduction ........................................................................................................... 6
2. Project Crew .......................................................................................................... 8
3. Summary of Survey Parameters .......................................................................... 8
3.1. Survey Area Parameters ....................................................................................... 8
3.2. Flight Plan ............................................................................................................. 9
3.3. Job Safety Plan ................................................................................................... 10
3.4. Daily Activity Report ........................................................................................... 10
4. Airborne Data Acquisition Equipment and Specifications ............................... 11
4.1. GT-1A Mobile Gravimeter System ..................................................................... 12
4.1.1. Gravimeter Sensor .............................................................................................. 14
4.1.2. Gravimeter Platform ............................................................................................ 14
4.1.3. Automated Operation .......................................................................................... 14
4.1.4. Dynamic Range ................................................................................................... 14
4.1.5. Operating Software ............................................................................................. 15
4.2. GT-1A Control and Display Unit (CDU) and Logging Computer ...................... 15
4.3. Uninterruptable Power Supply (UPS) ................................................................ 15
4.4. Dual Frequency GPS .......................................................................................... 15
4.5. Navigation System .............................................................................................. 16
5. Ground Data Acquisition Equipment ................................................................. 17
5.1. GPS Base Stations .............................................................................................. 17
6. Gravimeter Calibrations and Monitoring ........................................................... 18
6.1. GT-1A Reference Measurements ....................................................................... 18
6.2. Tying to the International Gravity Standardization Net (1971) ......................... 20
6.3. Repeat Line ......................................................................................................... 21
7. Data Processing .................................................................................................. 23
7.1. Field Data Processing Equipment ..................................................................... 23
7.2. Field Data Processing – Quality Control ........................................................... 23
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7.2.1. Navigation Tolerance .......................................................................................... 23
7.2.2. Gravimeter Data ................................................................................................... 24
7.2.3. GPS post processing proprietary software GTNAV .......................................... 25
7.2.4. Acceleration QC software GTQC20 .................................................................... 25
7.2.5. RMS errors of modelled anomaly from proprietary software GTGRAV ........... 25
7.2.6. QC Estimate of Errors for Repeat Lines ............................................................ 26
7.3. Final Data Processing ........................................................................................ 26
7.4. QC Estimate of RMS Errors of Cross-overs ...................................................... 27
7.5. Proprietary software GTGRAV ........................................................................... 27
7.6. Geosoft Oasis montaj ......................................................................................... 29
7.7. Final Delivered Products .................................................................................... 31
8. References ........................................................................................................... 32
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APPENDICES
APPENDIX I Flight Planned Lines
APPENDIX II Survey Flight Path
APPENDIX III Geophysical Maps
APPENDIX IV Final Line Data Description
APPENDIX V Project Metadata
APPENDIX VI Summary of Daily Activity
APPENDIX VII Survey Line QC Parameters
APPENDIX VIII Repeat Line QC Parameters
APPENDIX IX Reference Measurements Statistics
APPENDIX X AUSPOS GPS Processing Reports
APPENDIX XI Reference, Green & Lane “Estimating Noise Levels in AEM Data”
APPENDIX XII Gravity Reference Stations
FIGURES
Figure 1. Location of Project Area.......................................................................7
Figure 2. Flight Plan of Project Area....................................................................9
Figure 3. Survey Aircraft ZK-FNZ at Bamako International Airport....................11
Figure 4.. GT-1A Gravimeter Installation in ZK-FNZ...........................................13
Figure 5. Ashtech Z-Xtreme GPS Receiver.......................................................15
Figure 6. Thales Z-Max GPS Receiver..............................................................17
Figure 7. GPS & IGSN71 Station Locations at Bamako International Airport....18
Figure 8. Repeat Line Location..........................................................................22
Figure 9. GT-1A Gravity Data Processing Flow.................................................24
TABLES
Table 1. General Information.............................................................................6
Table 2. Survey Area Specifications..................................................................8
Table 3. Project Area Co-ordinates....................................................................9
Table 4. GT-1A Gravimeter Specifications.......................................................12
Table 5. Base GPS Locations..........................................................................17
Table 6. Reference Point – Rover GPS Antenna Positions.............................19
Table 7. Gravimeter GSE Offsets....................................................................19
Table 8. Gravity Station Locations...................................................................20
Table 9. Gravity Tie-in Obs using Lacoste & Romberg ground meter.............20
Table 10. Gravity Corrections using Lacoste & Romberg ground meter...........21
Table 11. Repeat Line Co-ordinates..................................................................21
Table 12. Survey Altitude Statistics...................................................................23
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TABLES CONT’D
Table 13. Survey Speed Statistics......................................................................23
Table 14. QC Repeat Line Analysis Noise Estimates........................................ 26
Table 15. Estimates of Intersection Errors (100 sec Filter Length).................... 27
Table 16. Delivered Final Line Data Files...........................................................31
Table 17. Delivered Final Repeat Line Data Files..............................................31
Table 18. Delivered Final Grid Data Files...........................................................31
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1. Introduction
This report summarises the acquisition and processing of gravity data from the Bamako Block
airborne GT-1A gravity and magnetic project flown by UTS Geophysics (UTS) with an aircraft
operated by Kiwi Air and the GT-1A gravity data acquired and processed by Airborne
Petroleum Geophysics (Asia Pacific) Pty Ltd (APG).
APG Project Number APG-2009-11
Project Area Bamako
Country Mali
Base of Survey Operations Bamako International Airport
Total Line Kilometres
Presented
Trimmed to survey boundary: 7,939.79
(survey boundary defined section 3.2)
Survey Duration Dates 22rd
January 2008 – 15th April 2009
Client UTS Geophysics Pty.Ltd
Client Representative Fiona Wedenig (Project Manager)
Mark Devenish (Field Project Manager)
Client Address 11 Fauntleroy Ave
Perth Airport
Western Australia 6104
Table 1. General Information
The GT-1A gravimeter (Berzhitzky et. Al., 2002) was developed by JSC STC Gravimetric
Technology of Moscow, Russia (GT) with funding assistance from, firstly, World Geoscience
Corporation, and subsequently from Fugro Airborne Surveys. Canadian Micro Gravity (CMG)
has an exclusive marketing arrangement with GT and supplies the system worldwide. APG is
a wholly owned subsidiary of CMG and operates the GT-1A gravimeter on airborne
geophysical surveys.
The GT-1A is a small, lightweight “INS-GPS” system that is operated independently from any
other equipment carried by a fixed wing survey aircraft.
The GT-1A is different from existing, commercially available total field gravimeters by:-
Its small size and weight and low power requirement;
Its ease of operation, with no on-board operator required;
A vertically constrained accelerometer which minimises cross-coupling, allowing
measurements during turns and thus providing short lead-ins for survey lines;
Dual dynamic ranges of +/- 500 Gals and +/- 250 Gals, respectively, allowing high quality
data to be collected even in moderate turbulence;
Monitoring or variations in the geometry of the gravimeter and the GPS antenna;
Advanced data processing routines that remove the effects of changes in the system
geometry.
It is the system’s ability to operate in a broad range of conditions with high levels of
productivity that make the GT-1A unique.
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The Bamako Block project area is located 30km north of the city of Bamako in southwest Mali.
The survey area covers approximately 67,200km². For the duration of the project, survey
operations were based out of Bamako.
Figure 1. Location of Project Area
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2. Project Crew
The following APG personnel were employed on this project:
Field Operations
GT-1A Operator Ryan Olson
GT-1A Data Processor Igor Epof
Base Operations
Operations & Logistics Manager Wayne Hewison
Final Data Processing Matthew Gray
Data Processing Manager Helen Tuckett
3. Summary of Survey Parameters
3.1. Survey Area Parameters
Parameter Specification
Total Flight Planned Line Kilometres 8,404.01
Total Flight Planned Line Kilometres
excluding Lead In/Out 7,940.01
Lead In/Out Kilometres Traverses 5km, Ties 5km (West) & 6km (East)
Traverse Line Spacing 10,000 metres
Traverse Line Direction 000-180 degrees (North/South)
Traverse Line Numbers L100010 – L100420 (42 traverses)
Traverse Line Kilometres 6,720.01 (includes 5 km lead in/out)
Tie Line Spacing 50,000 metres
Tie Line Direction 090-270 degrees (East/West)
Tie Line Numbers T190010 – T190040 (4 ties)
Tie Line Kilometres 1684.00 (includes 5 & 6 km lead in/out)
Survey Flying Height 1045 metres constant height above MSL
Survey Flying Speed Approx 65 metres/second
(approx. 235 km/hour or 130 knots)
Survey Navigation Tolerance Not > 5000m for more than a distance of 1,000m.
Repeat Line Kilometres Approx. 30 km
Repeat Line Frequency Pre-survey, then once a week
Repeat Line Acceptability RMS < 1.00 mGal
Automatic Reflight Specification Number of Satellites ≤ 4 and PDOP ≥ 7.0 for a
duration of 5 minutes or more
Line X-Over StdDev ≥ 1.5 mGal
Table 2. Survey Area Specifications
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3.2. Flight Plan
The flight plan for the survey area was prepared by UTS and included a lead in/out of 5 kms
for the survey lines, and 5 & 6 km for the tie-lines. These extensions were to allow for aircraft
manoeuvring whilst turning onto and off the survey lines. The total flight planned line
kilometres, including the lead in/out was 8,404.01 km.
Due to the nature of the 100 second Kalman filter used in the GT-1A post processing
software, gravity data can be affected by aircraft manoeuvring for at least half the wavelength
of the filter, depending on the severity of the manoeuvre. Therefore, if the aircraft is travelling
at an average speed of 65 metres/second, then half a wavelength will be approximately 3,250
metres.
The survey boundary co-ordinates (excluding lead in/out) are as follows:
WGS 84, UTM Zone 29N
Point No. Easting (m) Northing (m)
1 469900.0 1580000.0
2 880000.0 1580000.0
3 880000.0 1430000.0
4 469900.0 1430000.0
5 469900.0 1580000.0
Table 3. Project Area Co-ordinates
Figure 2. Flight Plan of Project Area
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3.3. Job Safety Plan
On this project APG adopted and worked under the UTS Occupational Safety and Health
Management System.
3.4. Daily Activity Report
A report of daily activity covering the dates January 22nd, 2008 to April 15th, 2009, may be
found in Appendix VI, “Summary of Daily Activity”. The report covers crew mobilisation and
de-mobilisation, equipment installation, production figures, flight duration times, production
kilometres and estimates of weather conditions for each flight.
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4. Airborne Data Acquisition Equipment and Specifications
A PAC750-XL single turbine engine aircraft operated by Kiwi Air Ltd was used for the project
and installed with the GT-1A gravimeter system equipment. Aircraft registration is ZK-FNZ.
The PAC750-XL has a maximum speed of 168 knots with an endurance of 10 hours.
Figure 3. Survey Aircraft ZK-FNZ at Bamako International Airport
The aircraft gravimeter installation was completed January 25 at Bamako International airport.
During the course of the following two days, gravimeter auto-calibrations were successfully
completed. Gravimeter test-flights were delayed until February 28 due to delays with the
issuance of flight permits.
Gravimeter S/N004 pre-survey test flights were conducted out of Bamako International Airport
between February 28 – March 1 using a repeat-line established by APG in 2006. All lines from
the first test flight (Flt:904) were scrubbed due to gravimeter instability produced from
excessive aircraft manoeuvring. A second test flight (Flt:905) produced two acceptable repeat
lines, while two lines were scrubbed due to turbulence. The repeatability of the free air
anomaly for the two accepted lines was 0.56mGal RMS. As an additional check, the results
were compared with four previous repeat-lines acquired during November 2008 (see
Appendix VIII). Based on these results, GT-1A S/N004 was deemed ready to commence
production on the project area.
A GT-1A main sensor failure occurred after flight 8 which resulted in gravimeter S/N004 being
replaced with gravimeter S/N010 for the remainder of the project. A single installation
confirmation test of gravimeter S/N010 (Flt:914) was completed prior to re-commencement of
survey acquisition (x4 repeat lines). The four lines produced a repeatability of the free air
anomaly of 0.63mGal RMS compared to all previous repeat-lines. The results confirmed that
gravimeter S/N010 was ready to re-commence survey operations. A QC summary of all
repeat lines flown during the survey is presented in section 7.2.6.
Survey flight operations were conducted from Bamako International Airport, with the first
production flight commenced on March 3, 2009. The final survey flight concluded on April 13,
2009. The APG/UTS crews demobilised on April 15th.
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4.1. GT-1A Mobile Gravimeter System
Specifications of the GT-1A Gravimeter system installation are:
Survey Aircraft
Model PAC750-XL
Registration ZK-FNZ
Operating Company
Kiwi Air Limited
PO Box 2087
Gisborne, 4040
New Zealand
Gravimeter
Type GT-1A
Serial Number S/N004 & S/N010
Measurement Range 9.76 to 9.84 m/s²
Dynamic range Coarse Channel ± 0.50 g
Dynamic range Fine Channel ± 0.25 g
Drift over 24 hours < 5.00 mGal
Drift over 24 hours (corrected) < 0.03 mGal
Sampling Interval
300Hz,
With accelerometer data recorded at 18.75Hz and
platform data 3.125Hz
Ultimate angles – roll and pitch ±45º
Latitude measurement range 75º S to 75º N
Operating ambient temperature +5ºC to +50ºC
Gravimetric anomaly evaluation error
(RMS) mGal under “ideal”¹ conditions
Over bandwidth of 0.0100 Hz = 0.6
Over bandwidth of 0.0125 Hz = 1.0
Weight 150 kg
Dimensions Ø 600 x 920 mm H
Power consumption 150 W
System readiness time from cold start 48 hours
Gravimeter GPS Receivers
Type Thales and Ashtech dual-frequency receivers
Model Z-Max Z-Xtreme
Accuracy 0.5cm + 0.5ppm (horiz)
1.0cm + 0.5ppm (vert)
1.0cm + 1.0ppm (horiz)
2.0cm + 1.0ppm (vert)
Sampling interval 2Hz
Table 4. GT-1A Gravimeter Specifications
¹Gravimetric anomaly evaluation error (RMS) under the following conditions:
vertical accelerations up to 0.5g;
correct gravimeter and GPS antennae installation on aircraft and at base stations;
use of dual frequency GPS receivers with a data acquisition rate of at least 2 Hz;
visibility of more than 6 satellites;
PDOP not more than 2.5;
GPS base line length less than 100km;
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The GT-1A is an airborne, single sensor, vertical scalar, GPS-INS gravimeter with a Schuler-
tuned three-axis inertial platform. It consists of three basic units.
The gravimeter main sensor unit weighs 50 kg. Together with the rotation turntable they
measure 40 x 40 x 70 cm. The shock-mount arrangement has a cut-off frequency of 5 Hz and
is 60cm in diameter by 22 cm high, giving an overall height to the gravimeter of 92cms.
The main unit houses most of the electronics in the top third, while the main gravity sensor is
held vertically by the inertial platform in the bottom; the turntable provides the platform’s
azimuth axis control.
Connections to the gravimeter include a 27V dc power source drawing approximately five
amps. A GPS RS-232 serial link providing velocity and co-ordinate data are used to assist in
aligning the platform vertical and a second serial connection to a data acquisition unit.
This data acquisition computer acts as the control and display unit for the gravimeter, as well
as collecting and storing data. The gravimeter also contains a microprocessor, input/output
interfaces, and secondary power supplies, all within the electronics bay at the top of the main
unit.
Figure 4. GT-1A Gravimeter Installation in ZK-FNZ
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4.1.1. Gravimeter Sensor
The vertical accelerometer, or gravity sensing element (GSE) has an axial design with a
reference mass on a spring suspension, a photoelectric position pickup and a moving-coil
force feedback transducer. The GSE suspension design minimises the effect of cross-
coupling, an undesirable effect which contaminates gravity measurements with components
of horizontal accelerations induced by aircraft motion. This feature allows the GT-1A to
collect data in the presence of large horizontal accelerations, such as during aircraft turns or
during periods of turbulence.
4.1.2. Gravimeter Platform
The GSE, with a bandwidth of 100Hz, is installed on the gyro-stabilised platform contained in
a double-axis gimbal suspension. The platform also holds two horizontal accelerometers, a
dynamically tuned gyro with a vertical angular momentum, a fibre optic gyro for azimuth
control, and two gravimeter calibration devices (GCD). The GCD’s are designed to calibrate
the gravimeter by means of inclination without dismounting on the GSE. In operation, the
GSE is limited to 45 degrees in both the pitch and roll axes.
The GSE is located in a double-loop constant-temperature environment on the inertial
platform. Additional elements installed on the platform, plus the current regulator of a code-to
current converter within the gyro control circuit, are individually temperature controlled.
4.1.3. Automated Operation
The Gravimeter is fully automated – no operator is required on board the aircraft while
collecting data on survey lines. All systems including stabilisation servo systems, temperature
control systems and gyroscopic correction systems are controlled by the built-in
microprocessor. The computer also takes control of actuation, reference measurements,
balancing, and measurements during survey mode. A vertical gyro correction system using
GPS-derived information on heading, latitude and aircraft speed provides vertical gyro
stability. An optimal Kalman filter is implemented in control algorithms for both the
stabilization servo system and the vertical gyro correction system.
4.1.4. Dynamic Range
Two Dynamic ranges are measured and recorded simultaneously: +/- 250 Gals (0.25g) and
+/- 500 Gals (0.50g). The smaller range data, which also has a finer resolution, is used
during periods of calm flying conditions, while the coarse range allows measurements in more
turbulent conditions.
Data is acquired through short periods of accelerometer saturation in severe turbulence by
the automatic application of a reduced-order Kalman filter, enabling platform misalignment to
be computed and hence controlled.
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4.1.5. Operating Software
The Gravimeter is operated by programs running on an external control and display computer,
which can be a data acquisition computer with a serial port. The easy-to-use Windows-based
programs include calibration and diagnostic functions.
The calibration program has two modes for carrying out automatic GSE calibration. A 3-hour
measurement period provides GSE calibration coefficients, while over a period of 5.5 hours
the program determines the above coefficients plus the non-perpendicular angles between
the GSE and the platform surface. This program is normally run once before each survey
project and does not need to be run at the completion of the project.
4.2. GT-1A Control and Display Unit (CDU) and Logging Computer
The GT-1A gravimeter data acquisition CDU is a rugged computer with IBM PC architecture.
It executes a proprietary program for gravimeter system control, data acquisition and
recording. Control commands are provided to the GT-1A microprocessor via software menu
items. During system operation the CDU displays operational information on the main screen.
The operator initiates data recording prior to take-off and stops the recording when the aircraft
returns from its survey flight.
Raw gravimeter data are recorded on the CDU as a “G-“ file containing horizontal and vertical
acceleration data at 18.75 Hz and as an “S-“ file containing platform misalignment information
at 3.125Hz.
4.3. Uninterruptable Power Supply (UPS)
Ground power (240 V AC) is supplied to the gravimeter via the UPS, which also acts as a
transformer and converts AC power to 27 V DC. The UPS also provides backup power from
internally mounted gel-cell batteries for up to 15 minutes in the case of a power failure.
4.4. Dual Frequency GPS
The gravimeter measures total accelerations – a combination of inertial and gravity
accelerations. In order to separate gravity accelerations from the total, an Ashtech Z-Xtreme
dual frequency GPS was initially used to record raw GPS data at a frequency of 2 HZ.
Ashtech Z-Xtreme GPS Receiver Specifications
GPS satellite tracking channels: 12 channels L1 CA/PL1 & PL2
Z-Tracking & Multipath mitigation
Typical post processed accuracy (up to several hundred
kilometres
depending on satellite geometry): 1cm + 1 ppm (horizontal)
2cm + 1 ppm (vertical)
Figure 5. Ashtech Z-Xtreme
GPS Receiver
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The GPS data were post-processed, and the vertical (inertial) acceleration calculated,
allowing the gravity acceleration to be derived once the data are integrated with
accelerometer data from the gravimeter.
The GPS data were recorded on an internal PCMCIA format disk, however data are also
provided to the gravimeter microprocessor in real time for system timing and synchronisation,
and to assist with real-time control of the inertially stabilised platform in which the gravity
sensing element is housed. This GPS is completely independent from the GPS system used
for aircraft navigation.
4.5. Navigation System
Survey navigation utilised the aircraft GPS system for real time position. During on-line
survey operations, the aircraft was controlled by the autopilot system, which provided two
axis, roll and pitch inputs to track on pre-programmed GPS lines. The autopilot model was S-
Tec System 55.
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5. Ground Data Acquisition Equipment
The ground data acquisition equipment used on this project consisted of the following
systems:
5.1. GPS Base Stations
The primary base GPS station, Base 1, consisted of a Thales Z-Max dual frequency GPS
receiver, with integrated antenna and powered by an internal 12 V battery pack. The backup
base GPS station, Base 2, was also a Thales Z-Max Dual Frequency GPS, with the same
configuration as the primary Base. Both GPS base stations were located in a clear area with
an unobstructed view of the sky at Bamako International Airport.
Thales Z-Max GPS Receiver Specifications
GPS Satellite tracking channels: 24 parallel channels,
L1 C/A code & carrier,
L1/L2 P-code, full wavelength carrier,
Z-Tracking & Multipath mitigation.
Typical post processed accuracy (up to several hundred kilometres
depending on satellite geometry): 0.5cm + 0.5ppm (horizontal)
1.0cm + 0.5ppm (vertical)
Figure 6.Thales Z-Max
GPS Receiver
The base GPS location co-ordinates were determined by submitting at least 6 hours of static
data to the AUSPOS online GPS Processing Service offered by Geoscience Australia. See
Appendix X for the AUSPOS GPS processing reports.
Base 1 was the primary GPS station that was used for all flights.
The calculated base GPS antenna positions and the flights that these positions were used for
were:
Base
No.
UTS
Flight
No.
Latitude Longitude
Antenna
Height
Ellipsoid
Antenna
Height
Geoid
AUSPOS
GPS Report
No.
1
1 - 17
T1 – T5
904-906
12º32’16.6892” -7º57’09.8631” 401.008m 370.351m 336949
2 - 12º32’14.9342” -7º57’12.0336” 399.793m 369.135m 336949
Table 5. Base GPS Locations
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Figure 7. GPS and IGSN71 Station Locations at Bamako International Airport
6. Gravimeter Calibrations and Monitoring
6.1. GT-1A Reference Measurements
A “reference location” for the aircraft was determined at the beginning of the project (GT-1A
Ref Loc 1). This position was where the aircraft parked between survey flights and was
marked on the airport apron. During the survey, it was necessary to move the initial reference
location due to tarmac maintenance. A second reference location was established (GT-1A Ref
Loc 2) approximately 70m from the initial position. Details of these positions are provided in
Table 6.
At the start and end of each survey flight, the aircraft was parked at the “reference location”
and a gravimeter reference measurement of at least 15 minutes duration was recorded. The
position of the Gravity Sensing Element (GSE) when the aircraft is parked at the reference
location is referred to as the “reference point”.
The reference point Rover GPS antenna position was determined by submitting at least 6
hours of static data to the AUSPOS online GPS Processing Service offered by Geoscience
Australia. See Appendix X for reports of AUSPOS GPS processing.
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The calculated position of the aircraft GPS antenna when parked at the reference location
was:
Ref Loc.
UTS Flight No.
Latitude Longitude Antenna Height
Ellipsoid
Antenna Height Geoid
AUSPOS GPS
Report No.
1 1 - 12
904,905,914 12º32’20.1135” -7º57’05.7551” 405.192m 374.536m 336957
2
13 – 17 T1-T5
12º32’18.6452” -7º57’07.5545” 404.189m 373.533m 338226
Table 6. Reference Point – Rover GPS Antenna Positions
The GT-1A reference location is tied in to absolute gravity station BAMAKO-035421 at
Bamako International Airport. Appendix XII provides details of the absolute gravity station.
Section 6.2 below outlines the gravity tie-in procedures for this survey.
The pre- and post-flight reference data are used by APG proprietary software GTGRAV to
correct for in-flight gravimeter drift. There is no need to correct for drift between flights, as the
relative free air anomaly is assumed to be equal to zero at the reference point by the post
processing software. Thus the instrument is effectively reset for each flight. The reference
measurement data also give an indication of the stability of the instrument over the period of
the survey. These data are presented in Appendix IX, “Reference Measurements Statistics”.
Instrument drift of greater than 1mGal was observed for flights 9-17, T1-T5 and test flight 906.
Standard meter drift corrections were applied to all lines with the exception of L100133,
L100150, L100170, L100180, L100190, T190010 and T190032. In these instances no drift
correction was applied in GTGRAV. Subsequent first-order levelling was considered more
effective to remove the long-term instrument drift in these cases. This conclusion was based
on comparative tie-line intersection statistics using the two methods.
Gravimeter GSE Offsets
The reference point / Rover GPS antenna offsets were:
X +0.1m (positive starboard)
Y -0.8m (positive forward)
Z +0.6m (positive upward)
The reference point / reference location offset:
-1.70m (positive upward)
Table 7. Gravimeter GSE Offsets
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6.2. Tying to the International Gravity Standardization Net (1971)
The GT-1A data processing stream produces free air gravity anomalies relative to the
reference location. The relative free air anomalies were corrected to absolute free air
anomalies tied to an IGSN71 gravity base station. The closest IGSN71 base station to GT-1A
reference location 1 is a distance of 1175m (BAMAKO-035421). This gravity station is located
next to terminal buildings within Bamako International Airport, and has an estimated accuracy
of <0.1 mGal. Details of the IGSN71 station BAMAKO-035421 are presented in Appendix XII.
The geographic co-ordinates of the BAMAKO-035421 station provided by the International
Gravimetric Bureau (IGB) were deemed inaccurate. The IGB co-ordinates place the station
14km north of the international airport, which is in disagreement with the description of the
physical location of the station. The location of BAMAKO-035421 was re-surveyed with a
differential GPS to produce located co-ordinates which were used for the gravity tie. The
revised location co-ordinates are provided in Table 8.
The gravity tie was performed during December 2008 and referenced to an older parking
position 60 metres from GT-1A reference location 1 used during the current survey. The
accuracy of the tie therefore is considered approximate due to the different GT-1A reference
positions used during the survey.
Reference Location Ref Latitude Longitude East (m) North (m) Height Geoid
(m)
Bamako(Abs 035421) A 12.32.38.17 -007.56.31.36 614937.97 1386932.70 399.80
Bamako (GT-1A ref) B 12.32.19.03 -007.57.06.98 613865.19 1386340.52 373.98
Table 8. Gravity Station Locations
The gravity tie-in was performed on the 7/12/2008 using portable Lacoste & Romberg model
“G” gravity meter (S/N-360). An A-B-A-B-A loop covering a 1.2 hour period was conducted
between the GT-1A reference location “B” and the absolute station “A”, both located at
Bamako International Airport. The results are presented in Table 9.
Date Reference Location Ref Avg Time
(Local) Avg Meter Reading
7/12/2008 Bamako (GT-1A ref loc) A 15:18 1739.24
7/12/2008 Bamako (Abs 035421) B 15:36 1741.20
7/12/2008 Bamako (GT-1A ref loc) A 15:57 1739.27
7/12/2008 Bamako (Abs 035421) B 16:13 1741.31
7/12/2008 Bamako (GT-1A ref loc) A 16:28 1739.26
Table 9. Gravity Tie-in Observations using Lacoste & Romberg ground meter
Project: Bamako Area, Mali Airborne GT-1A Gravity Survey 21
GT-1A APG-2009-11
The absolute gravity value for the GT-1A reference location “A” was determined using both
manual calculations and the Geosoft Oasis Montaj v7.0 executable GRDRIFT.GX. All gravity
measurements were appropriately scaled using supplied meter constants, and then tide
corrected. The Geosoft Gx was used to calculate standard drift corrections. The final step was
the calculation of drift corrected absolute gravity values for the ground meter data. The results
are presented in Table 10.
Date Ref
Location Time
(Local) Meter
Reading Tide Corr
Corrected Meter
Drift Closure
Absolute Gravity
7/12/2008 A 15:18 1739.24 -0.0364 1845.587 0.0 978190.160
7/12/2008 B 15:36 1741.20 -0.0294 1847.674 - 978192.225
7/12/2008 A 15:57 1739.27 -0.0198 1845.635 0.48 978190.160
7/12/2008 B 16:13 1741.31 -0.0109 1847.810 - 978192.331
7/12/2008 A 16:28 1739.26 -0.0024 1845.642 0.007 978190.160
Table 10. Gravity Corrections using Lacoste & Romberg ground meter
The final absolute value for “B” using the ground meter is 978192.29 mGal being the average
of all “B” absolute values (standard deviation: 0.06 mGal).
After calculation of the absolute value for the GT-1A reference location, the relative free air
values were corrected for the 1.7m difference in vertical position between the tarmac and the
reference point. The free air correction of 0.30764mGal/m (3.0764μms-2
/m) * 1.7 was used to
calculate a correction value of 0.523 mGal. Using the absolute information calculated for the
GT-1A reference point, relative free air values for the survey area were corrected to absolute
free air anomalies, relative to the geoid, by the addition of a constant offset of 40.93 mGal
(409.3 μms-2
) from each value.
6.3. Repeat Line
During the survey, system repeatability was assessed using a 26km repeat line located 15km
North of Bamako. The repeat line was flown 18 times during the course of the survey and
processed in the same manner as the survey lines. Additional processing information and
statistics can be found in Section 7.2.6 and Appendix VIII.
WGS 84, UTM Zone 29N
Point No. Easting (m) Northing (m)
1 608462 1417015
2 608444 1443843
Table 11. Repeat Line Co-ordinates
Project: Bamako Area, Mali Airborne GT-1A Gravity Survey 22
GT-1A APG-2009-11
Figure 8. Repeat Line Location
Project: Bamako Area, Mali Airborne GT-1A Gravity Survey 23
GT-1A APG-2009-11
7. Data Processing
7.1. Field Data Processing Equipment
A laptop computer was used in the field processing centre for the purpose of data retrieval
from the aircraft, GPS data download, preliminary processing and quality control checks on
the acquired data. The suite of post processing software consisted of the commercial
products:
Waypoint GrafNav GPS processing
Geosoft Oasis montaj Geophysics processing
and APG proprietary software products:
TIMELAG and GTNAV GPS Processing software
GTQC20 and GTGRAV Free Air Anomaly software
Section 7.2.2 covers the data processing software in more detail.
7.2. Field Data Processing – Quality Control
7.2.1. Navigation Tolerance
Flight path for the gravity acquisition system was recorded at 2Hz in WGS 84 latitude and
longitude. These were converted to East and North and presented in the UTM Zone 29 North
co-ordinate system. On occasion it was difficult for the aircraft to maintain good aircraft
dynamics in pitch and roll over sections of the survey lines due to high turbulent conditions
caused by thermal heating over the desert landscape. The problem was compounded by the
necessity to conduct survey operations during daylight hours. Generally smooth air conditions
are a requirement for collecting good quality gravity data.
The Navigation Tolerance for the project was set at no greater than 5000m over a continuous
distance of 1000m or more along line. No lines exceeded this specification.
An important aspect of an airborne gravity survey is the maintenance of altitude. A well flown
gravity survey should be returning a mean Std Dev value of 5 metres or better. The mean
Standard Deviation for the survey was 1.94 metres which indicates excellent height holding
from the autopilot. The height tolerance for this survey was set at 1045 +/-50m over a
continuous distance of 1000m or more along line. No lines exceeded this specification.
GPS Altitude metres above the Ellipsoid
Min Max Mean Mean Std Dev
1023.0m 1069.9m 1044.6m 1.94m
Table 12. Survey Altitude Statistics
The average flying speed for the survey was 66.81 m/second (240.52 km/hour), which
equated to a half wavelength of 3.34km using a 100 second filter.
Survey Speed (m/s)
Min (Avg) Max (Avg) Mean (All) Mean Std Dev (All)
63.45 68.60 66.81 0.57
Table 13. Survey Speed Statistics
Project: Bamako Area, Mali Airborne GT-1A Gravity Survey 24
GT-1A APG-2009-11
7.2.2. Gravimeter Data
The data processing sequence to produce the relative free air gravity anomaly is shown in
Figure 9.
Figure 9. GT-1A Gravity Data Processing Flow
On a daily basis, the GPS data are processed, quality controlled and integrated using APG
proprietary software program GTNAV. The resulting processed GPS data are imported into
an Oasis montaj “flight” QC database where the QC results are examined.
The raw GT-1A gravimeter data files are quality controlled and examined for missing records
and the number of saturations using the APG proprietary software program GTQC20.
An in-flight drift correction is determined from the pre and post flight reference measurements.
This is applied to the raw gravimeter acceleration data. The GT-1A gravimeter system
parameters are examined during the modelling of a typical 100 second filtered Free Air gravity
anomaly. At this time other filters lengths may be applied, dependent on line spacing and
data quality.
The processing sequence produces a large number of QC parameters. The parameters listed
in the following sections are the most reliable indicators of the data quality, and are examined
on a daily basis as the data from each flight are processed. Where the value of a parameter
exceeds its nominal value, closer inspection is made of the data. Where the occurrence is
outside the survey boundary, the value is ignored. Where the occurrence is on line within the
survey boundary, this is noted, but unless there are multiple parameters not meeting
specifications the data may not necessarily be rejected.
Project: Bamako Area, Mali Airborne GT-1A Gravity Survey 25
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7.2.3. GPS post processing proprietary software GTNAV
Parameters examined and their nominal specifications were:
SVs > 6 The number of satellites used in processing the
current epoch.
PDOP < 2.5 The position dilution of precision.
RMS < 1.0 L1 Root Mean Square.
RMS Velocity < 0.05 Root Mean Square of Velocity Differential Solution.
Type = 1 Differential Phase Velocity Solution Flag, Type = 1
indicates an acceptable solution.
Alpha 1 < 0.0011636 radians Estimate of gyro platform x axis misalignment errors.
Alpha 2 < 0.0011636 radians Estimate of gyro platform y axis misalignment errors.
7.2.4. Acceleration QC software GTQC20
Parameters examined and their nominal specifications were:
Fine channel saturations No saturations = 0, Saturations = 1.
Coarse channel saturations No saturations = 0, Saturations = 1.
Lost, missing, extra or corrupted records Flagged
Gravimeter hardware status problems Flagged
Flight condition problems, mostly associated to turbulence, are the major cause of the
gravimeter instrument saturating. Acceptable gravity data can still be recovered in most
cases where the dynamic range has been exceeded. However the quality of the resulting
output depends on both the number of times the instrument has saturated and the distribution
of the saturations along the line. When saturation points are many and frequent the resulting
Free Air anomaly will be “over-smoothed” and poor quality.
For the Bamako Block Gravity Survey, the number of saturations on line was generally
sufficient to render the fine channel data unsuitable for processing. Only coarse channel data
was used to produce the final free air data set. Those evidenced by coarse channel
saturations were mostly due to thermal turbulence within the survey area.
7.2.5. RMS errors of modelled anomaly from proprietary software GTGRAV
Both the peak-to-peak and RMS residual errors are calculated by APG’s proprietary program
GTGRAV when the gravity anomaly is modelled. When a single line has an RMS error of less
than 1000 mGal*sec, then the result is considered to be acceptable. This is a further internal
test of data quality. There were no lines that exceeded the 1000 mGal*sec RMS tolerance.
Lines with a number of coarse channel saturations did not necessarily result in large RMS
residual errors. This indicates that acceptable gravity anomalies may still be recovered from
data with a large number of saturations. However, while the gravity anomaly can be
recovered, the greater the number of saturations, the more likely it is that the data have been
“over-smoothed” by the Kalman filter.
Project: Bamako Area, Mali Airborne GT-1A Gravity Survey 26
GT-1A APG-2009-11
7.2.6. QC Estimate of Errors for Repeat Lines
The repeat line data were processed in the same manner as the survey flight lines, with the
same QC parameters being examined. The Repeat Line QC summary appears in Appendix
VIII.
These lines were trimmed back to a 26.83 km section of the original 30km line. They were re-
sampled (and re-oriented if necessary) so that all data samples coincided as closely as
possible.
For QC purposes, the errors of the 100 second filtered Free Air data were then estimated
using the method outlined in Green and Lane (2003); see Appendix XI, “Reference”. An RMS
error was calculated for each line from the comparison of all the valid repeat lines with each
other. In addition to a noise estimate for each line, an overall noise estimate for all lines is
produced.
The results presented in Table 14 below are considered to be the best estimates of the noise
present in the repeat lines.
Repeat
Line
Number
UTS
Flight
Number
Date
Flown
RMS Noise
Estimate
(mGal)
Comments
90400401 904 28 Feb 09 - Scrubbed – Unstable gravimeter
90400402 904 28 Feb 09 - Scrubbed – Unstable gravimeter
90400403 904 28 Feb 09 - Scrubbed – Unstable gravimeter
90400404 904 28 Feb 09 - Scrubbed – Unstable gravimeter
90400501 905 01 Mar 09 0.54 OK
90400502 905 01 Mar 09 0.58 OK
90400503 905 01 Mar 09 - Scrubbed - Turbulence
90400504 905 01 Mar 09 - Scrubbed - Turbulence
90401001 5 07 Mar 09 0.23 OK
90401002 5 07 Mar 09 Scrubbed - Turbulence
91001401 914 26 Mar 09 0.84 OK
91001402 914 26 Mar 09 0.65 OK
91001403 914 26 Mar 09 0.72 OK
91001404 914 26 Mar 09 0.73 OK
91001601 10 29 Mar 09 0.74 OK
91001602 10 29 Mar 09 Scrubbed - Turbulence
91002301 T4 7 Apr 09 Scrubbed - Turbulence
91002401 14 8 Apr 09 0.88 OK
Overall RMS of accepted lines: 0.68
Table 14. QC Repeat Line Analysis Noise Estimates
7.3. Final Data Processing
The final data processing stream involves APG proprietary software GTGRAV, where the
modelling of the Free Air anomaly is refined and iterated a number of times. The filter length
of 100 seconds was set in this program to calculate the final Free Air anomaly data for this
project.
From GTGRAV the data are imported into the Geosoft Oasis montaj environment, where each
stage of levelling is performed, final corrections and adjustments are applied to the data, and
the final products are produced.
Project: Bamako Area, Mali Airborne GT-1A Gravity Survey 27
GT-1A APG-2009-11
7.4. QC Estimate of RMS Errors of Cross-overs
Cross-over (intersection) errors were generated at three stages of the gravity processing.
The first two stages of raw data intersection and 1st order levelling (removal of offset and
slope) intersection errors were examined in the field during the course of the survey to
determine and required re-flights. The third set of cross-over errors, are generated from the
final levelled data. These errors and those produced after the 1st order levelling stage are the
better indicators of data quality.
The errors were estimated by calculating the standard deviation of each intersection, then
calculating the mean of the standard deviations for each line and multiplying the resulting
value by 1/√2, this distributes the error between the line and the tie line.
Appendix VII, “Survey Line QC parameters” details the intersection statistics for each line at
each stage of cross-overs in the processing.
Raw Intersections 1st
Order Intersections Final Intersections
3.44 mGal 0.59 mGal 0.54 mGal
Table 15. Estimates of Intersection Errors (100 sec Filter Length)
7.5. Proprietary software GTGRAV
This GTGRAV program models the free air gravity anomaly, and includes the following
corrections:
a) Static correction based on pre-flight and post-flight reference measurements to remove
drift within each flight;
b) Eötvös correction:
Eötvös Correction =
cos22
e
M
n
N
e vhR
v
hR
v
Where; 22 sin1/ eaRN and
23
22
2
sin1
1
e
eaRM
c) Subtraction of Theoretical Gravity. The GRS80 formula was used to calculate
Theoretical, or Normal gravity ( ), in mGal:
2sin
4
1sin1 2
4
2 ffe
d) Free air correction (in mGal), using the standard formula:
hg fa 3086.0
Project: Bamako Area, Mali Airborne GT-1A Gravity Survey 28
GT-1A APG-2009-11
e) Note: The GTGRAV software assumes that the free air anomaly at the reference point is
zero – that data produced for survey lines are therefore Free Air anomalies relative to this
Reference Point, and with respect to the ellipsoid.
Where: ev = Velocity in the East (x) direction (m/s)
nv = Velocity in the North (y) direction (m/s)
NR = Radius of curvature in the prime vertical (m)
MR = Radius of curvature in the meridian (m)
h = Altitude above the WGS84 ellipsoid (m) ω = Angular velocity of the Earth (rad/s)
= 2 x 7.2921157x10-5
e = Normal Gravity at the Equator
= 978032.7 mGal
= Latitude (rad)
a = WGS84 Semi-major axis (m)
= 6378137.0000 m
2e = WGS84 First eccentricity squared
= 6.6943799901413 x 10-3
f * = gravity flattening
=
a
ab
= 5.3024 x 10-3
f 4 = fmf2
5
2
1 2
m =
a
a
2
Project: Bamako Area, Mali Airborne GT-1A Gravity Survey 29
GT-1A APG-2009-11
7.6. Geosoft Oasis montaj
The following processes were applied in the Geosoft Oasis montaj environment:
f) 1st Order Tie line levelling correction, based on cross-over values, removing offset and
slope;
g) Micro-levelling of the relative free air data using Fourier method.
h) Conversion of GPS height (Ellipsoid) to GPS height (Geoid) by calculation of the NValue
using the Geosoft Oasis montaj GEOID.GX and subtracting from GPS height (Ellipsoid).
GPSHtGEOID = GPSHtELLIPSOID – NValue m;
i) Conversion of Free Air anomaly (Ellipsoid) to Free Air anomaly (Geoid) by calculating an
NValue Free Air correction value for each record.
gFA GEOID = gFA ELLIPSOID – (NValue * 0.3086) mGal;
j) Correction of the relative free air anomaly at the “Reference Point” (GT-1A gravimeter
sensor height) to the height of the “Reference Location” (point on the ground) by adding
an offset of 0.525 mGal, being the Free Air correction of 0.3086 * 1.7m (GT-1A sensor
height above the ground. See Table 8: Gravimeter GSE Offsets).
gFA GEOID = gFA GEOID + 0.525 mGal;
k) Conversion of the relative Free Air anomaly to final absolute Free Air anomaly values.
The relative Free Air values were corrected to the absolute Free Air values, relative to the
geoid, by the addition of 40.93 mGal to each value:
gAFA GEOID = gFA GEOID + 40.93 mGal;
l) Calculation of a normalised Simple Bouguer correction for the density of 1.0 g/cm³ using
formula:
gCBsim = 0.04191088 * (Density * ground elevation,) mGal;
A high resolution Digital Elevation Model (Shuttle Radar Mission Topography) SRTM90 data was used in this process for the ground elevation (metres above the geoid).
m) Calculation of the Bullard (earth curvature) correction using the formula:
gBULL = ((1.464 * (Terrain height/1000)) - (0.3533 * (Terrain height/1,000)2) + (0.000045 * (Terrain / 1,000)
3)) mGal;
n) Calculation of a normalised Terrain correction for density 1.0 g/cm³ using Geosoft Oasis
montaj Gravity and Terrain correction software. The method involves the combination of the Aircraft GPS height above the Geoid and the terrain elevation and if necessary, a regional terrain elevation grid that extends kilometres beyond the survey boundary.
GRREGTER.gx – Creation of a regional terrain correction grid
GRTERAIN.gx – Calculation of the terrain correction;
o) Filtering of the Simple Bouguer corrections, Bullard correction and the Terrain correction
was performed using a 1D along line filter matching the GTGRAV Kalman filter length (100 seconds). This process was undertaken to “match” the frequency content of the final absolute free air anomaly data;
Project: Bamako Area, Mali Airborne GT-1A Gravity Survey 30
GT-1A APG-2009-11
p) Calculation of the Simple Bouguer anomaly for the density of 2.67 g/cm³ using the
formula:
gBsim DEN = gAFA GEOID – (gCBsim DEN – gBULL mGal);
q) Calculation of the complete Bouguer anomaly for the density 2.67 g/cm³ using the formula:
gBcom DEN = gBsim DEN + (terrain correction * Density).
The 100 second Final Absolute Free Air Anomaly and Final Absolute Complete Bouguer Anomaly grids are shown in Appendix III “Geophysical Maps”. Note: In the gravity processing stream the tie lines are only used in first-order leveling of the survey lines. The Final Free Air channel for the tie lines equals TLevFA100 – (NValue*0.3086) + 0.525 mGal (Geoid and Reference location adjustment) +40.93 mGal (Correction to absolute free air values). The tie lines are not carried through into the Bouguer correction and Anomaly channels. For the tie lines these channels contain nulls.
Project: Bamako Area, Mali Airborne GT-1A Gravity Survey 31
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7.7. Final Delivered Products
Final processed survey line data are provided as an ASCII .XYZ file. The final processed grid
data are provided as ERMapper grids and Geosoft binary grids. The final acquisition and
processing report, “APG-2009-11 Acquisition and Processing Report.pdf”, is provided as a
bound hardcopy product and as an Adobe .PDF digital copy. All digital products are provided
on a DVD-ROM and can be found at the end of the hardcopy report.
Data and channel descriptions for the final line data files are described in Appendix IV.
Metadata for the APG-2009-11 project appears in Appendix V.
Line Data File Name Data Type Description
APG-2009-11-FINAL-DATA.gdb Geosoft .gdb Final Survey Line Data – 100 secs filter
APG-2009-11-FINAL-DATA.XYZ ASCII XYZ Final Survey Line Data – 100 secs filter
APG-2009-11-FINAL-README.txt Text Final Survey Line Data – Readme text
Table 16. Delivered Final Line Data Files
Line Data File Name Data Type Description
APG-2009-11-REPEAT-DATA.gdb Geosoft .gdb Gravity Repeat Line Data – 100 secs
filter
APG-2009-11-REPEAT-DATA.XYZ ASCII XYZ Gravity Repeat Line Data – 100 secs
filter
APG-2009-11-REPEAT-README.txt Text Gravity Repeat Line Data – Readme
text
Table 17. Delivered Final Repeat Line Data Files
Grid Name Units Cell
Size Grid Type Description
APG_2009_11_FinalFA100.ers APG_2009_11_FinalFA100 APG_2009_11_FinalFA100.grd
mGal mGal
2500m 2500m
ERMapper Header ERMapper Grid Geosoft Binary Grid
Final Absolute Free Air Anomaly, 100 secs filter
APG_2009_11_CBgr100_267.ers APG_2009_11_CBgr100_267 APG_2009_11_CBgr100_267.grd
mGal mGal
2500m 2500m
ERMapper Header ERMapper Grid Geosoft Binary Grid
Final Complete Bouguer Anomaly, Density 2.67 g/cm³
Table 18. Delivered Final Grid Data Files
Project: Bamako Area, Mali Airborne GT-1A Gravity Survey 32
GT-1A APG-2009-11
8. References
Berzhitzky, V. N., Bolotin, Y.V., Golovan, A. A., llyin, V. N., Parusnikov, N. V., Smoller, Y.L., and
Yurist, S. S, 2002. GT-1A Inertial Gravimeter System – Results of Flight Tests. Report by Z A O
Scientific and Technological Enterprise Gravimetric Technologies and Lomonosov Moscow State
University Faculty of Mechanics and Mathematics.
Green, A., and Lane, R., 2003. Estimating Noise Levels in AEM Data. Extended Abstract, ASEG
16th Geophysical Conference and Exhibition, February 2003, Adelaide.
See Appendix XII.
Moritz, H., 1980. Geodetic reference system 1980. Journal of Geodesy, 54. 395-405
Project: Bamako Area, Mali Airborne GT-1A Gravity Survey
GT-1A APG-2009-11
Appendix I – Flight Planned Lines
Project: Bamako Area, Mali Airborne GT-1A Gravity Survey
GT-1A APG-2009-11
Project: Bamako Area, Mali Airborne GT-1A Gravity Survey
GT-1A APG-2009-11
Flight plan co-ordinates and line distances
FROM TO Total
Line No. X Y X Y Distance (m)
L100010 880000 880000 1425000 1585000 160000
L100020 870000 870000 1425000 1585000 320000
L100030 860000 860000 1425000 1585000 480000
L100040 850000 850000 1425000 1585000 640000
L100050 840000 840000 1425000 1585000 800000
L100060 830000 830000 1425000 1585000 960000
L100070 820000 820000 1425000 1585000 1120000
L100080 810000 810000 1425000 1585000 1280000
L100090 800000 800000 1425000 1585000 1440000
L100100 790000 790000 1425000 1585000 1600000
L100110 780000 780000 1425000 1585000 1760000
L100120 770000 770000 1425000 1585000 1920000
L100130 760000 760000 1425000 1585000 2080000
L100140 750000 750000 1425000 1585000 2240000
L100150 740000 740000 1425000 1585000 2400000
L100160 730000 730000 1425000 1585000 2560000
L100170 720000 720000 1425000 1585000 2720000
L100180 710000 710000 1425000 1585000 2880000
L100190 700000 700000 1425000 1585000 3040000
L100200 690000 690000 1425000 1585000 3200000
L100210 680000 680000 1425000 1585000 3360000
L100220 670000 670000 1425000 1585000 3520000
L100230 660000 660000 1425000 1585000 3680000
L100240 650000 650000 1425000 1585000 3840000
L100250 640000 640000 1425000 1585000 4000000
L100260 630000 630000 1425000 1585000 4160000
L100270 620000 620000 1425000 1585000 4320000
L100280 610000 610000 1425000 1585000 4480000
L100290 600000 600000 1425000 1585000 4640000
L100300 590000 590000 1425000 1585000 4800000
L100310 580000 580000 1425000 1585000 4960000
L100320 570000 570000 1425000 1585000 5120000
L100330 560000 560000 1425000 1585000 5280000
L100340 550000 550000 1425000 1585000 5440000
L100350 540000 540000 1425000 1585000 5600000
L100360 530000 530000 1425000 1585000 5760000
L100370 520000 520000 1425000 1585000 5920000
L100380 510000 510000 1425000 1585000 6080000
L100390 500000 500000 1425000 1585000 6240000
L100400 490000 490000 1425000 1585000 6400000
L100410 480000 480000 1425000 1585000 6560000
L100420 470000 470000 1425000 1585000 6720000
Total flight line distance (including 5km lead in/out): 6720.01 km
T190010 465000 886000 1434000 1434000 421000
T190020 465000 886000 1481000 1481000 842000
T190030 465000 886000 1528000 1528000 1263000
T190040 465000 886000 1575000 1575000 1684000
Total tie line distance (including 6km lead in/out): 1684.00 km
Project: Bamako Area, Mali Airborne GT-1A Gravity Survey
GT-1A APG-2009-11
Appendix II – Survey Flight Path
Project: Bamako Area, Mali Airborne GT-1A Gravity Survey
GT-1A APG-2009-11
Project: Bamako Area, Mali Airborne GT-1A Gravity Survey
GT-1A APG-2009-11
Appendix III – Geophysical Maps
Project: Bamako Area, Mali Airborne GT-1A Gravity Survey
GT-1A APG-2009-11
Project: Bamako Area, Mali Airborne GT-1A Gravity Survey
GT-1A APG-2009-11
Project: Bamako Area, Mali Airborne GT-1A Gravity Survey
GT-1A APG-2009-11
Appendix IV – Final Line Data Description
Project: Bamako Area, Mali Airborne GT-1A Gravity Survey
GT-1A APG-2009-11
The final line data file variable names, units, datum and description of each variable are presented in
the following table:
Variable Units Datum Description
Line Integer - Line Number
Flt Integer - Flight Number
Date YYYY/MM/DD - Date
GPSTime HH:MM:SS.ss - GPS time since start of GPS day
Fid Integer - Fiducial
UTMZone - - Universal Transverse Mercator Zone
Latitude Deg.Min.sec.ss WGS 84 Latitude
Longitude Deg.Min.sec.ss WGS 84 Longitude
X Metres WGS 84, UTM
Zone 29 North East
Y Metres WGS 84, UTM
Zone 29 North North
GPSHt Metres - GPS Height above the Geoid
RawFA100 mGal - Raw Relative Free Air Anomaly, 100
Second Filter, Unlevelled
TLevFA100 mGal - 1
st Order Tie Line Levelled Relative Free
Air Anomaly, 100 Second Filter
FinalFA100 mGal - Final Absolute Free Air Anomaly,
100 Second Filter
SRTM Metres - Shuttle Radar Topography Mission Data
relative to the geoid. (SRTM90)
BlldC mGal - Bullard (Earth Curvature) Correction
NBgrC mGal - Normalised Bouguer Correction, Density
1.00g/cm³
TerrainC mGal - Normalised Terrain Correction, Density
1.00g/cm³
Cbgr100_267 mGal -
Final Absolute Complete Bouguer
Anomaly, 100 Second Filter, Density
2.67g/cm³
OverLap - - Overlap flag on broken lines,
1 = overlap section, 0 = No overlap
Project: Bamako Area, Mali Airborne GT-1A Gravity Survey
GT-1A APG-2009-11
Appendix V – Project Metadata
Project: Bamako Area, Mali Airborne GT-1A Gravity Survey
GT-1A APG-2009-11
Name:
Project APG-2009-11 and UTS Project B075
GT-1A Airborne Gravity Survey
Bamako Block, Mali
Start Date: 3 March 2009
End Date: 13 April 2009
Operators: Petroma
Contractor: UTS Geophysics Limited
Processors: Airborne Petroleum Geophysics (Asia Pacific) Pte Ltd
Software:
GPS Processing – Waypoint GrafNav
GT-1A Gravity Processing – GTNAV, GTQC20, GTGRAV
and Geosoft Oasis montaj
Vessel Type: Aircraft (fixed wing) PAC750-XL Operated by Kiwi Air Ltd
Geodetic Datum: WGS 84
Projection: UTM Zone 29 North
Accuracy: 10cm or better
Location Method: Post Processed Dual Frequency GPS
Traverse Spacing: 10,000 metres
Tie Spacing: 50,000 Metres
Traverse Direction: 0/180 degrees
Tie Direction: 90/270 degrees
Traverse Kilometres: 6,299.14
Tie Kilometres: 1,640.35
Total Kilometres: 7,939.79
Height above ground
level: Constant height 1045 metres above sea level
West longitude: -009º 16’ 41.36”
East longitude: -005º 29’ 56.43”
North latitude: 014º 17’ 29.95”
South latitude: 012º 54’ 43.17”
Gravity Datum: IGSN71 (International Gravity Standardization Net 1971)
Terrain Correction
Computation Method: Geosoft Oasis montaj: GRTERAIN.gx
Gravity Network Ties: Bamako 035421
Equipment Details:
Gravimetric Technology GT-1A mobile gravimeter S/N004 & S/N010
Ashtech Z-Xtreme and Thales Z-Max Dual Frequency GPS
receivers.
APG Crew: Ryan Olson (APG) - GT-1A Operator
Igor Epof (APG) - GT-1A Data Processor
Any Other Information Spatial Resolution – filter half-width using mean survey velocity of
66.81 metres/second, 100 second filter is approx. 3.34 km
Project: Bamako Area, Mali Airborne GT-1A Gravity Survey
GT-1A APG-2009-11
Appendix VI – Summary of Daily Activity
Project: Bamako Area, Mali Airborne GT-1A Gravity Survey
GT-1A APG-2009-11
Appendix VII – Survey Line QC Parameters
Project: Bamako Area, Mali Airborne GT-1A Gravity Survey
GT-1A APG-2009-11
Appendix VIII – Repeat Line QC Parameters
Project: Bamako Area, Mali Airborne GT-1A Gravity Survey
GT-1A APG-2009-11
Appendix IX – Reference Measurements Statistics
Project: Bamako Area, Mali Airborne GT-1A Gravity Survey
GT-1A APG-2009-11
Appendix X – AUSPOS GPS Processing Reports
Project: Bamako Area, Mali Airborne GT-1A Gravity Survey
GT-1A APG-2009-11
Appendix XI – Reference
Green, A., and Lane, R., 2003. Estimating Noise Levels in AEM Data. Extended Abstract,
ASEG 16th Geophysical Conference and Exhibition, February 2003, Adelaide
Project: Bamako Area, Mali Airborne GT-1A Gravity Survey
GT-1A APG-2009-11
Appendix XII – Gravity Reference Stations