Geomagnetic Measurements Workshop:
DIflux:
Absolute Measurement of the Geomagnetic
Field orientation in Space
prepared for the
XIVth IAGA WORKSHOP
ON GEOMAGNETIC OBSERVATORY INSTRUMENTS,
DATA ACQUISITION AND PROCESSING September 13 - 23, 2010
by
Jean RASSON
Royal Meteorological Institute of Belgium
XIVth IAGA WORKSHOP
ON GEOMAGNETIC OBSERVATORY INSTRUMENTS,
DATA ACQUISITION AND PROCESSING September 13 - 23, 2010
1
Magnetic Observations Basics
The Diflux Concept
Selected DIflux Topics
XIVth IAGA WORKSHOP
ON GEOMAGNETIC OBSERVATORY INSTRUMENTS,
DATA ACQUISITION AND PROCESSING September 13 - 23, 2010
2
Magnetic Observations Basics
•
•
•
•
Geomagnetic field
Magnetic conditions in Observatory environment
Instrumentation: Absolutes, Variometers
The baseline concept
XIVth IAGA WORKSHOP
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3
Geomagnetic field
Components and coordinate systems
The Earth Geomagnetic Field is a
vector field’
Frame of reference: Geographic North
and direction of local gravity
Sets of components used:
XYZ, HDZ, FDI
Components recorded:
XYZ, HNHEZ, FHEV
XIVth IAGA WORKSHOP
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DATA ACQUISITION AND PROCESSING September 13 - 23, 2010
4
Magnetic conditions in Observatory environment
The Magnetic Observatory is constructed so that:
• It is not situated on a local magnetic anomaly
• All buildings intended for magnetic measurements
are non-magnetic
XIVth IAGA WORKSHOP
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DATA ACQUISITION AND PROCESSING September 13 - 23, 2010
5
Magnetic conditions in Observatory environment
• Only natural magnetic field is present
• Low magnetic spatial gradient
Consequently:
1. Magnetic field is highly homogeneous: magnetic field lines are parallel
2. Magnetic field differences between pillars are small (ΔF ~ 1 nT)
3. Magnetic field changes are supposed to be identical inside the
Observatory space
Additionally:
• Stable pillars
• Availability of targets with known azimuths
Note: Over time, these conditions may degrade. Check them by performing absolutes on more than
one pillar!
XIVth IAGA WORKSHOP
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DATA ACQUISITION AND PROCESSING September 13 - 23, 2010
6
Instrumentation: Absolutes
•
Absolute instruments measure one or more
components of the Geomagnetic Field in
absolute units or the direction of the field with
respect to geographic North and direction of
gravity
•
DIFlux Theodolite for measurement of the
Declination angle D and Inclination angle I
•
Proton Precession Magnetometer measures
Total Field, F in nT.
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DATA ACQUISITION AND PROCESSING September 13 - 23, 2010
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Instrumentation: Variometers
A variometer measures the variation
of the components of the
magnetic field.
The variometer measures the field
from an unknown level called the
baseline.
XIVth IAGA WORKSHOP
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DATA ACQUISITION AND PROCESSING September 13 - 23, 2010
8
The Magnetic Observatory Baseline Concept
Fact: Absolute component instruments (Diflux) cannot yet measure
continuously. (Total field instrument can)
How to obtain a continuous Geomagnetic recording with absolute quality?
1. Record the variation of the components about a value close to its
mean: the baseline. This is done by the variometer
2. Measure from time to time (*) the value of the baseline with an
absolute instrument
* ideally the baseline is a constant. Practically it changes slowly and barely over time
XIVth IAGA WORKSHOP
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DATA ACQUISITION AND PROCESSING September 13 - 23, 2010
9
The Magnetic Observatory Baseline Concept
Exemple
Take the measurements made at the same time:
• Absolute declination measurement D -------------
• Variometer measurement δD ------------------------
defines the baseline D0 :
D = D0 + δD
Note 1: This equation can be written for any component
Note 2: Variometer measurements dD and dI in nT are converted
to angular units δD and δI by:
 dD 

H 


D  atan
XIVth IAGA WORKSHOP
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DATA ACQUISITION AND PROCESSING September 13 - 23, 2010
 dI 

F 
 
I  atan
10
The DIflux concept for
measuring the Geomagnetic
Field Orientation
• Introduction
• Theodolite basics
• Fluxgate basics
• Diflux measurement protocol
• Levelling of (Zeiss) Theodolite
• Geographic North: the reference in
horizontal plane
• Precautions required for obtaining
high accuracy
XIVth IAGA WORKSHOP
ON GEOMAGNETIC OBSERVATORY INSTRUMENTS,
DATA ACQUISITION AND PROCESSING September 13 - 23, 2010
11
The DIflux Concept for measuring the Geomagnetic Field Orientation
• DIflux is an instrument able to measure the value of the
geomagnetic declination D and inclination I.
• The instrument consists of a non-magnetic theodolite and a fluxgate
sensor mounted on the telescope, so that optical and magnetic axes
are parallel.
XIVth IAGA WORKSHOP
ON GEOMAGNETIC OBSERVATORY INSTRUMENTS,
DATA ACQUISITION AND PROCESSING September 13 - 23, 2010
12
The DIflux Concept
Non-magnetic theodolites (are hard to find)
XIVth IAGA WORKSHOP
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DATA ACQUISITION AND PROCESSING September 13 - 23, 2010
13
The DIflux Concept
• Fluxgate basics : A fluxgate sensor is very sensitive to its orientation
with respect to magnetic field direction. The highest sensitivity is
when sensor magnetic axis and magnetic field are almost ┴ (orthogonal
= perpendicular = 2 axis form a 90° angle).
XIVth IAGA WORKSHOP
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The DIflux Concept
•
Determining a plane P ┴ to the geomagnetic vector is
the same as measuring its direction (our task)
• The fluxgate sensor will give null reading when ┴ to
geomagnetic vector
• So we can find the plane P ┴ to geomagnetic vector:
two alignements of the sensor giving a null reading will
define it
Here are the 2 usual alignements inside P:
1. We require the sensor to be horizontal : Declination
measurement -----------------------------------------------------
2. We require the sensor to be in the geomagnetic
meridian : Inclination measurement (top drawing)
Note : this gives a total of 4 (D) + 4 (I) different positions
XIVth IAGA WORKSHOP
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005°07’35’’
Use of Angle Units
•
Degrees, minute and seconds:
– Most theodolite circles are divided in DMS.
•
Grades:
– Some Zeiss theodolites circles are divided in grades
•
Radian:
– Computation using software (Excel) and compilers
•
•
•
1 sec of arc = 1/200000 rad or 1 mm seen from 200 m
distance
1 sec of arc movement of magnetic field direction ~ 0.2
- 0.3 nT
1 sec of arc change of declination depends on value of
H.
 dD 

H 


D  atan
XIVth IAGA WORKSHOP
ON GEOMAGNETIC OBSERVATORY INSTRUMENTS,
DATA ACQUISITION AND PROCESSING September 13 - 23, 2010
16
Levelling of (Zeiss) Theodolite: finding the reference in Vertical plane
• Coarse levelling using circular bubble level
• Accurate levelling using two linear bubble levels
• For best levelling use automatic vertical index
• During normal D and I measurements accurate levelling is not
necessary as the build-in vertical compensator can correct for minor
tilt of the theodolite.
• A precise levelling however makes the measurements faster.
• During measurements with elevated telescope, where readings of
both circles are needed (sun and star observations) extremely
careful levelling is needed.
XIVth IAGA WORKSHOP
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DATA ACQUISITION AND PROCESSING September 13 - 23, 2010
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Geographic North: the reference in horizontal plane
The sequence for Declination measurement positions is as follows:
 Measurement of Azimuth mark sensor up
 Measurement of Azimuth mark sensor down
 D1 : Telescope horizontal towards E, sensor up
 Test NOW for magnetism of Observer
 D2 : Telescope horizontal towards W, sensor down
 D3 : Telescope horizontal towards E, sensor down
 D4 : Telescope horizontal towards W, sensor up
 Measurement of Azimuth mark sensor up
 Measurement of Azimuth mark sensor down
XIVth IAGA WORKSHOP
ON GEOMAGNETIC OBSERVATORY INSTRUMENTS,
DATA ACQUISITION AND PROCESSING September 13 - 23, 2010
18
Diflux measurement
protocol
XIVth IAGA WORKSHOP
ON GEOMAGNETIC OBSERVATORY INSTRUMENTS,
DATA ACQUISITION AND PROCESSING September 13 - 23, 2010
19
Diflux measurement protocol
XIVth IAGA WORKSHOP
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DATA ACQUISITION AND PROCESSING September 13 - 23, 2010
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Diflux measurement protocol: Inclination
Note: from the D measurement we obtain the direction of the magnetic
meridian.
Note: We can orient the theodolite around the vertical axis so that the
horizontal axis is ┴ to the magnetic meridian: the telescope will then
always swing in the magnetic meridian.
The sequence for I measurement positions is then as follows:

I5 : Telescope in magnetic meridian pointing towards N, sensor up

I6 : Telescope in magnetic meridian pointing towards S, sensor down

I7 : Telescope in magnetic meridian pointing towards N, sensor down

I8 : Telescope in magnetic meridian pointing towards S, sensor up
XIVth IAGA WORKSHOP
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DATA ACQUISITION AND PROCESSING September 13 - 23, 2010
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Diflux measurement protocol
I5 : Telescope in magnetic meridian pointing towards N, sensor up
I6 : Telescope in magnetic meridian pointing towards S, sensor down
I7 : Telescope in magnetic meridian pointing towards N, sensor down
I8 : Telescope in magnetic meridian pointing towards S, sensor up
Note: in fact we get two determinations of I:
IH = (I5 + I8)/2 which is the inclination at the “sensor up” position and
IL= (I6 + I7)/2 at the “sensor down” position.
ΔI = IH – IL is called the “I gradient”
We will come back later on ΔI
XIVth IAGA WORKSHOP
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DATA ACQUISITION AND PROCESSING September 13 - 23, 2010
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Residual Method
The residual method utilize the digital output of the Diflux to
eliminate errors and improve the accuracy of the
measurements.
In each of the 4 D and 4 I positions one does not try to adjust
the reading to zero but only close to zero (below 10 nT).
On the minute one reads the circles as usual and also the
digital output of the DIflux.
Using to the digital reading from the Diflux, one can calculate
the circle readings at zero-output of the Diflux.
XIVth IAGA WORKSHOP
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DATA ACQUISITION AND PROCESSING September 13 - 23, 2010
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Residual Method, advantages
•
The observer can step back from the Diflux at the
time of measurement and reduce the effect of
magnetic contamination from the observer.
•
During active period and also at high latitude it is
difficult to adjust to 0.0 nT on the minute. This is
not needed using the residual method.
•
Using difficult to read theodolites like Zeiss 020,
one does not have to estimate the value of the
fraction of minute of arc. One simply set the circle
to a major division line and use the digital reading
to calculate the seconds.
XIVth IAGA WORKSHOP
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DATA ACQUISITION AND PROCESSING September 13 - 23, 2010
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Precautions required for obtaining high accuracy
•
•
Obtain highest precision for target readings and horizontality of telescope
for D measurements
”Clean” environment
– Absolute house is the ”holy” place in the observatory and should be
absolutely clean.
– Check area around absolute house
•
”Clean” observer
– Use the Diflux to check for magnetic material. Shoes are often very magnetic.
•
Keep observer away from theodolites at the time of reading.
– Residual method is highly recommended
•
•
•
Keep good timing < 1 sec
Use instantaneous variometer data for reduction not filtered minute
values
Keep list af all available data from the absolute observation
– Baseline values, Absolute values, Temperatures, DI-constants, Observer ID
and more
•
Continuous quality check of DI-Theodolite, Observer and Variometer
XIVth IAGA WORKSHOP
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DATA ACQUISITION AND PROCESSING September 13 - 23, 2010
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Measuring the Geomagnetic Field Orientation: Selected DIflux Topics
• Fluxgate electronics basics
• Analysing DIflux measurements : Azimuth, Site and magnetisation
errors
• DIflux magnetic hygiene check
• Specs of commercial and other available devices
• Troubleshooting of the ZEISS 010/15/20
• Optics cleaning
XIVth IAGA WORKSHOP
ON GEOMAGNETIC OBSERVATORY INSTRUMENTS,
DATA ACQUISITION AND PROCESSING September 13 - 23, 2010
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Selected DIflux Topics: Fluxgate electronics basics
XIVth IAGA WORKSHOP
ON GEOMAGNETIC OBSERVATORY INSTRUMENTS,
DATA ACQUISITION AND PROCESSING September 13 - 23, 2010
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Selected DIflux Topics: Fluxgate electronics basics
XIVth IAGA WORKSHOP
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DATA ACQUISITION AND PROCESSING September 13 - 23, 2010
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Selected DIflux Topics: Analysing DIflux measurements : Azimuth, Site and
magnetisation errors )
•
•
•
•
The readings of the four D-positions gives:
– Mean Declination
D = ( A1 + A2 + A3 + A4 ) /4 – π
rad
– Magnetisation error
S = ( A1 – A2 + A3 - A4 ) /4 *H
rad
– Elevation or Site error
ε = ( A1 – A2 – A3 + A4 +- 2π ) /4*H*/Z rad
– Azimuth error
δ = ( -A1 – A2 + A3 + A4 ) /4 rad
Similar formulas for the four I-positions gives:
– The inclination in the two UP positions (IH)
– The inclination in the two DOWN positions (IL)
– Magnetisation error S
– Elevation or site error ε
When you plot the collimation errors it is often convenient to convert them to
equivalent nT so that S, ε and δ can be compared:
– Magnetisation error
S = ( A1 – A2 + A3 - A4 ) /4 *H nT
– Elevation or Site error
Z*ε = ( A1 – A2 – A3 + A4 +- 2π ) /4*H nT
– Azimuth error
H*δ = ( -A1 – A2 + A3 + A4 ) /4*H nT
Please be careful using these formulas as the sequence of positions 1,2,3,4 may be
different that the one your are using.
XIVth IAGA WORKSHOP
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DATA ACQUISITION AND PROCESSING September 13 - 23, 2010
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Selected DIflux Topics :
Analysing DIflux measurements : Azimuth, Site and magnetisation errors
• All three instrument errors ε, δ and S are
eliminated by averaging the four position
readings for D and for I
• The size of the three instrument errors should be kept
small to make the measurements faster and convenient
• However, large errors will not influence the accuracy of the
measurements as long as they remain unchanged during
the measurement sequence.
XIVth IAGA WORKSHOP
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DATA ACQUISITION AND PROCESSING September 13 - 23, 2010
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Selected DIflux Topics: DIflux magnetic hygiene check
1. The best magnetic check is done by
participating in the international
IAGA workshops for intercomparing
magnetometers
2. Another check can be done by a
second Diflux, installed in the I
measuring position. DUT should be
rotated in front of fluxgate and the
extrema subtracted. !! Attention: the
fluxgate sensor is magnetic
3. Inspection of the vertical I gradient
ΔI gives indication of “non-hygienic”
Diflux. (ΔI = IH – IL)
Note: magnetic elements on the telescope do not
introduce errors as long as they remain constant
during the whole measurement protocol.
XIVth IAGA WORKSHOP
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Selected DIflux Topics
Specs of Theodolites
Circle
•
•
•
•
•
•
Zeiss THEO 010:
Zeiss THEO 015
Zeiss THEO 020
3T2KP-NM
YOM MG2KP (Russia)
Ruska theodolite
Vertical index
+- 1’’
+- 0.3’’
+- 3’’
+- 1’’
+- 6’’
+- 1’’
+-1”
+-1”
+- 1’’
??
30” Bubble levels
Sys. errors
none
eccentricity*
eccentricity*
none
none
none
• *eccentricity errors are also removed using 4 positions
XIVth IAGA WORKSHOP
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DATA ACQUISITION AND PROCESSING September 13 - 23, 2010
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Selected DIflux Topics: Troubleshooting
of the ZEISS 010/15/20
 Caution: do not troubleshoot a Zeiss
theo if you are not at ease with fine
mechanics
 Caution: always use the correctly
sized screwdriver
Special tools for repair
of Zeiss theos
XIVth IAGA WORKSHOP
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Selected DIflux Topics: Troubleshooting of the ZEISS 010/15/20
•
•
•
•
If footscrews are loose you will have random horizontal circle reading errors and problems for levelling
Therefore check regularly the footscrew play and adjust if necessary
Footscrew play adjustement differs from theo type and series
Below it is adjusted by tightening a screw, visible and accessible for a single specific position of the
footscrew
XIVth IAGA WORKSHOP
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DATA ACQUISITION AND PROCESSING September 13 - 23, 2010
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Selected DIflux Topics: Troubleshooting of the ZEISS 010/15/20
•
•
Footscrew play adjustement differs from theodolite type and series
Here it is adjusted by tightening the footscrew, after inserting a pin in the holes, visible and accessible for
a single specific position of the footscrew
XIVth IAGA WORKSHOP
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Selected DIflux Topics: Troubleshooting of the ZEISS 010/15/20
Horizontal axis clamp adjustment-------------------
Vertical axis clamp adjustment ↓
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Selected DIflux Topics: Optics cleaning
• Optics cleaning is difficult, so avoid dirtying them in the first place:
never touch or contaminate the lenses and optical glass surfaces of
the theodolite. (!! greases and lubricants)
• Valid tool for optics cleaning is cotton held on a brass needle. A
fresh cotton tip is OK for easily accessible places.
• Golden rule: never reuse a cotton wad for optics cleaning.
• Cotton should be used dry (moderate dirt)
• Otherwise the correct fluid for optics cleaning is ether.
XIVth IAGA WORKSHOP
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DATA ACQUISITION AND PROCESSING September 13 - 23, 2010
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Special Issues of Repeat Survey Measurements:
Precise levelling using Zeiss/UOMZ pendulum
• Zeiss 010/015/020 and UOMZ 3T2KP theos have automatic vertical
circle index levelling (index is automatically levelled by a pendulum )
• It is possible to use this level for levelling the theodolite.
XIVth IAGA WORKSHOP
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DATA ACQUISITION AND PROCESSING September 13 - 23, 2010
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Special Issues of Repeat Survey Measurements:
Precise levelling using Zeiss/UOMZ pendulum
•
•
•
•
When use pendulum levelling?
When very high levelling accuracy is required (elevated target
like North Star, Cruz del Sur or Sun at noon)
When bubble level is malfunctioning (f.i. direct sunshine heats
the bubble and this causes drifts in the level)
So, before sunshots or starshots
XIVth IAGA WORKSHOP
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DATA ACQUISITION AND PROCESSING September 13 - 23, 2010
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Special Issues of Repeat Survey Measurements:
Precise levelling using Zeiss/UOMZ pendulum
•
•
•
•
•
•
•
•
Procedure (theo should already be leveled within a few ’):
Select mentally two footscrews (1 and 2) and position
horizontal telescope along the line they form: call it horizontal
direction A
Set telescope perfectly horizontal (VC reads 90°00’00”)
Rotate 180° around vertical
Read VC. (say VC reads 90°00’48”). Levelling error in direction
A is 48/2=24”
Using Horizontal axis slow motion screw bring VC to 90°00’24”
Using footscrew 1 bring VC to 90°00’12”
Using footscrew 2 bring VC to 90°00’00”. Theo is now perfectly
levelled in direction A
XIVth IAGA WORKSHOP
ON GEOMAGNETIC OBSERVATORY INSTRUMENTS,
DATA ACQUISITION AND PROCESSING September 13 - 23, 2010
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Special Issues of Repeat Survey Measurements:
Precise levelling using Zeiss/UOMZ pendulum
•
•
•
•
•
•
Rotate 90° around vertical. This is horizontal direction B
Set telescope perfectly horizontal (VC reads 90°00’00”)
Rotate 180° around vertical
Read VC. (say VC reads 90°00’20”). Levelling error in direction B is
20/2=10”
Using Horizontal axis slow motion screw bring VC to 90°00’10”
Using footscrew 3 bring VC to 90°00’00”. Theo is now perfectly
levelled
XIVth IAGA WORKSHOP
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DATA ACQUISITION AND PROCESSING September 13 - 23, 2010
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Acknowledgements
• Thank you for your attention,
• Thanks to CEA and Dr Dongmei Yang for
the invitation to this Workshop,
• and thanks to our Masters Kring E.
Lauridsen and Daniel Gilbert, for teaching
us so many things.
XIVth IAGA WORKSHOP
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