The IMGC-02 Absolute Gravimeterpersonalpages.to.infn.it/~biolcati/presgrav.pdf · 2014. 2. 10. ·...

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


IMGC-02

Completely developed at Turin, Italy as a prototypeUsed both for research and measurement session


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


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


Main parts - scheme

1) launch system 2) measure system3) front-end electronics 4) vacuum system


Main parts - picture

1 launch system

2 measure system

3 front-endelectronics

4 vacuum system


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


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


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


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


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.


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


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


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


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.


The IMGC-02 Absolute Gravimeterpersonalpages.to.infn.it/~biolcati/presgrav.pdf · 2014. 2. 10. ·...

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