Post on 03-Feb-2021
The IMGC-02 Absolute Gravimeter
Emanuele Biolcati
Istituto Nazionale di Ricerca MetrologicaIstituto Nazionale di Geofisica e Vulcanologia
Cosenza - October 15, 2013
Emanuele Biolcati (INRiM-INGV) IMGC-02 AG Cosenza - October 15, 2013 1 / 18
Introduction
Absolute gravimeters,ballistic or atomic, canexploit:- free fall motion- rise and fall motion
Common units are:1µGal = 10−8m s−2
Common features:
repeatibility of about 5 µGal
extended uncertainty of about 15 µGal
relative uncertainty of 10−9
Emanuele Biolcati (INRiM-INGV) IMGC-02 AG Cosenza - October 15, 2013 2 / 18
IMGC-02
Completely developed at Turin, Italy as a prototypeUsed both for research and measurement session
Emanuele Biolcati (INRiM-INGV) IMGC-02 AG Cosenza - October 15, 2013 3 / 18
Operating principle - idea
The idea:
find a reference point
throw up a test object in the vacuum
measure the vertical distance between theobject and the reference point
measure the time of the flight
reconstruct the trajectory
calculate g
s(t) =1
2gt2
Emanuele Biolcati (INRiM-INGV) IMGC-02 AG Cosenza - October 15, 2013 4 / 18
Operating principle - real life
The real life:
find a reference point realized with a quasi-inertial system cutting highfrequency noise
throw up a test object in the vacuum with a verticality of 50 µrad
measure the vertical distance using interferometer and laser
measure the time of the flight using an atomic clock
reconstruct the trajectory starting from 700 asymmetric time-spacecoordinates
calculate g using dedicated model and applied corrections
Emanuele Biolcati (INRiM-INGV) IMGC-02 AG Cosenza - October 15, 2013 5 / 18
Main parts - scheme
1) launch system 2) measure system3) front-end electronics 4) vacuum system
Emanuele Biolcati (INRiM-INGV) IMGC-02 AG Cosenza - October 15, 2013 6 / 18
Main parts - picture
1 launch system
2 measure system
3 front-endelectronics
4 vacuum system
Emanuele Biolcati (INRiM-INGV) IMGC-02 AG Cosenza - October 15, 2013 7 / 18
Launch system
test object
corner-cube prism (10 µrad)container of Aluminumtotal mass of 78 gaccurately balancedoptical center ≡ center of mass
throw, catch, center4 iron springsrecirculating ball slidesverticality adjusted up to 50 µradforce of 70 Npath of 20 mm
Emanuele Biolcati (INRiM-INGV) IMGC-02 AG Cosenza - October 15, 2013 8 / 18
Measure system
quasi inertial system i.e. long periodseismometer (20 s) with reference mirror
Mach-Zender interferometer
Aluminum structure
optical fiber with adjustable mirrors to setthe verticality of the beam
photo-multiplier to detect the fringe signal
quad-cell detector to monitor parasiticmovements of the test objects
Emanuele Biolcati (INRiM-INGV) IMGC-02 AG Cosenza - October 15, 2013 9 / 18
Front-end electronics and vacuum system
Front-end electronicsStandard reference system of time and space:
Rubidium oscillator at ν = 10 MHzHe-Ne laser λ = 632 nm
PXI computer → recording and processing dataauxiliar instruments
barometer → pressure correctionvacuometer → check the launch chamber pressuretermometer
trigger units and power supplies
Vacuum system
rotative mechanical vacuum pump → 10−3 mbarturbo-mulecolar pump → 10−6 mbarglass vacuum chamber
Emanuele Biolcati (INRiM-INGV) IMGC-02 AG Cosenza - October 15, 2013 10 / 18
Data processing
The main algorithm for each drop:
1 start from a single array of N time values Ti2 obtain space-time coordinates (Ti ,Si ) as Si = N · λ/23 find the apex position and invert the fall branch coordinates
4 fit the trajectory using lest square method
5 apply the correction and extract g.
0
100
200
300
400
500
600
700
0 0.05 0.1 0.15 0.2 0.25 0.3 0.35
stat
ion
num
ber
time / s
0
0.05
0.1
0.15
0.2
0 0.05 0.1 0.15 0.2 0.25 0.3 0.35
dist
ance
/ m
time / s
Emanuele Biolcati (INRiM-INGV) IMGC-02 AG Cosenza - October 15, 2013 11 / 18
Some open issues
To achieve the requested accuracy, some issues mustbe monitored and investigated.
Trajectory. Asymmetry of the parabola,decalage effect.
Physical model. Linear or non-linear? Residualfriction of the air? Drift effect?
Height. The extracted g is referred to the ofthe trajectory? Best reference height? (warning:vertical gravity gradient ' 0.300 µGal/mm)Drop goodness. Is the drop vertical? Is itaffected by rotation or shift during the flight(Coriolis force)?
Noise. Electronics, human, floor recoil,temperature, humidity.
Emanuele Biolcati (INRiM-INGV) IMGC-02 AG Cosenza - October 15, 2013 12 / 18
Corrections and uncertainty
For each drop, the following effects are corrected (if necessary) and accounted inthe uncertainty budget.
Instrumental effectsDrag, outgassing, magnetic field,electrostatic field, air gap modulation,index of refraction, fringe timing, finitevalue of speed of light, radiationpressure → negligible
temperature gradient, self-attraction,laser beam verticality and divergence,clock delay, reference height →u < 1 µGal
retroreflector balancing →u ' 3.6 µGal
Site dependent effects
Polar motion, floor recoil → negligible
Atmospheric pressure, tide, oceanloading, standard deviation →u < 2 µGal
Coriolis force u ' 1.5 µGal
⇓⇒ combined uncertainty:
u ' 4.5 µGal
Emanuele Biolcati (INRiM-INGV) IMGC-02 AG Cosenza - October 15, 2013 13 / 18
Example of measurement - I
For each drop, a dedicated softwareprocess data on fly giving
parameters from the least squarefit
reference height
plot of the signal amplitude (tomonitor verticality of the drop)
plot of all processed drops versustime after application ofChauvenet criterion
Emanuele Biolcati (INRiM-INGV) IMGC-02 AG Cosenza - October 15, 2013 14 / 18
Example of measurement - II
tide correction values calculatedusing FORTRAN software
local atmospheric pressurecorrection values
values coming from fit about thefriction effect
list of enabled corrections
average and last drop residualscoming from the fit
Emanuele Biolcati (INRiM-INGV) IMGC-02 AG Cosenza - October 15, 2013 15 / 18
Conclusion
The IMGC-02 Absolute Gravimeter is used for:
measurements of g with an accuracy of few parts in 109
for geophysical analysis: seismic, vulcanology. etc.for metrological purposes: as the Watt balance, relativistic correction,International Comparisons, etc.
detailed study on the parasitic effects can influence the measurements
development of new and more transportable Absolute Gravimeters
Thanks for the attention.
Emanuele Biolcati (INRiM-INGV) IMGC-02 AG Cosenza - October 15, 2013 16 / 18