bamako project mali airborne gt-1a gravity survey for ... aquisition... · project: bamako area,...

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


GT-1A APG-2009-11

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


GT-1A APG-2009-11

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


GT-1A APG-2009-11

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


GT-1A APG-2009-11

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.


GT-1A APG-2009-11

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


GT-1A APG-2009-11

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


GT-1A APG-2009-11

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


GT-1A APG-2009-11

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.


GT-1A APG-2009-11

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.


GT-1A APG-2009-11

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;


GT-1A APG-2009-11

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


GT-1A APG-2009-11

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.


GT-1A APG-2009-11

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


GT-1A APG-2009-11

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.


GT-1A APG-2009-11

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


GT-1A APG-2009-11

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.


GT-1A APG-2009-11

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


GT-1A APG-2009-11

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 (Abs 035421) B 16:13 1741.31


Table 9. Gravity Tie-in Observations using Lacoste & Romberg ground meter


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


GT-1A APG-2009-11

Figure 8. Repeat Line Location


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


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.


GT-1A APG-2009-11

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.


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




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.


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


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


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;


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.


GT-1A APG-2009-11

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


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


GT-1A APG-2009-11


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


GT-1A APG-2009-11

Appendix II – Survey Flight Path


GT-1A APG-2009-11


GT-1A APG-2009-11

Appendix III – Geophysical Maps


GT-1A APG-2009-11


GT-1A APG-2009-11

Appendix IV – Final Line Data Description


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


GT-1A APG-2009-11

Appendix V – Project Metadata


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


GT-1A APG-2009-11

Appendix VI – Summary of Daily Activity


GT-1A APG-2009-11

Appendix VII – Survey Line QC Parameters


GT-1A APG-2009-11

Appendix VIII – Repeat Line QC Parameters


GT-1A APG-2009-11

Appendix IX – Reference Measurements Statistics


GT-1A APG-2009-11

Appendix X – AUSPOS GPS Processing Reports


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


GT-1A APG-2009-11

Appendix XII – Gravity Reference Stations

bamako project mali airborne gt-1a gravity survey for ... aquisition... · project: bamako area,...

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