Terrestrial Gravity Measurements with the mobile … · Terrestrial Gravity Measurements with the mobile Atom ... - People - Plate tectonics - Volcanology . Graphic by GOCE - Geodesist

Geodätische Woche - 9th October 2014

Terrestrial Gravity Measurements with the mobile Atom Interferometer GAIN

Christian Freier

Matthias Hauth Vladimir Schkolnik

Achim Peters

AG Optische Metrologie Humboldt-Universität zu Berlin

Graphics by IQO Hannover


Gravity Variations on Earth

Gravitational effects Magnitude

Geographical gravity variations

Global scale ~ 10-3 g

Regional scale ~ 10-6 g

Basic tidal effect ~ 10-7 g

Man-made changes

New buildings ~ 10-8 g

Trains, people, … ~ 10-9 g

Geological effects

Tectonic plate movements ~ 10-9 g

Atmospheric pressure ~ 3 x 10-10 g / mbar

Free air gravity gradiant ~ 3 x 10-9 g / cm

- Buildings

- People

- Plate tectonics

- Volcanology

Graphic by GOCE

- Geodesist g ≅ 9.81m

s2≅ 109μGal


State-of-the-Art Gravimeters

Relative Absolute

Spring/ Mass

Super-conducting

Falling Corner Cube

Noise [g/Hz] 2 x 10-9 2 x 10-10 ~ 4 x 10-8

Drift [g/Day] 1 x 10-8 1 x 10-11 -

Accuracy [g] - - ~ 2 x 10-9

Unique feature Small, Robust,

Flexible Extremely high

precision Absolute Value

Example


State-of-the-Art Gravimeters

Relative Absolute

Spring/ Mass

Super-conducting

Falling Corner Cube

Noise [g/Hz] 2 x 10-9 2 x 10-10 ~ 4 x 10-8

Drift [g/Day] 1 x 10-8 1 x 10-11 -

Accuracy [g] - - ~ 2 x 10-9

Unique feature Small, Robust,

Flexible Extremely high

precision Absolute Value

Example

One gravimeter for all purposes !?


Atom Interferometry

Interferometry Sequence

F1 – Ground State F2 – Excited State


Atom Interferometry

Population of excited state after pulse sequence




Atom Interferometry

Population of excited state after pulse sequence



𝑃𝐹2 = 𝑃 + 𝐶2 cos ∆𝜙

Acceleration term: ∆𝜙 ∝ 𝑘 𝑔 𝑇2

∆𝜙 = 𝜙3 − 2𝜙2 + 𝜙1


Electronics and optics Physics-package

Mobile Atom Interferometer GAIN (Gravimetric Atom Interferometer)

Targeted Performance:

Sensitivity

10−8 g Hz

Accuracy: 5 × 10−10 g

• Mobile setup

• Atomic fountain ℎ ≅ 0.7m 𝑇 ≤ 0.3𝑠

• Rubidium 87

• Diode Laser System @ 780nm


0.7 Hz

Experimental Sequence

~ 108 Rb87 atoms trapped within 0.6s

1. Trap loading & launch

2. State selection

3. Interferometry sequence

4. Detection via fluorescence


0.7 Hz



2. State selection






2. State selection



0.7 Hz




2. State selection



0.7 Hz




2. State selection



0.7 Hz


E F’=3

F’=2

F’=1 F’=0

F=2

F=1

Dete

ctio

n

Repum

per


F2

F1+F2

Inte

nsi

ty /

a.u

.

Time / a.u.

𝑃𝐹2 = 𝑃 + 𝐶2 cos ∆𝜙 ∆𝜙 ∝ 𝑘 𝑔 𝑇2


2. State selection



0.7 Hz


1st Mobile Measurement

550km Transport by Road

Berlin Adlershof

Geodetic Observatory Wettzell

openstreetmap.org



• Geodetic Observatory Wettzell (BKG)

• Transport from Berlin (small truck)

• 3 day set-up time at new site

• 2 weeks of gravity recordings

• Simultaneous operation of super-conducting gravimeter SG30 on site

SG30 GAIN

Measured by GAIN:

• Tidal gravity variation

• Excellent agreement with model

• Reliable operation with few gaps in data set

Published in proc. of Proceedings of the International School of Physics Enrico Fermi Course CLXXXVIII on Atom Interferometry



• Upper Graph: without air pressure correction

• Lower Graph: with air pressure correction

• Excellent agreement, no observable drift


GAIN (blue) SG30 (black)








• Allan Standard Deviation GAIN vs. SG30

• Blue: previous SG30 scale factor

• Green: SG30 scale factor recalibrated using this dataset








• Allan Standard Deviation GAIN vs. SG30

• Blue: previous SG30 scale factor

• Green: SG30 scale factor recalibrated using this dataset


Performance Overview

Performance Targeted Achieved Status

Sensitivity

intrinsic noise only •Fulfilled by laser phase noise

including realistic vibrational conditions

•Limited by vibration isolator performance

Stability

Bias Stability 5 . 10-10 g <5 . 10-10 g •Fulfilled

Absolute accuracy 5 . 10-10 g 1 . 10-8 g •analysis of systematic effects in progress


Transport – Measurement - Transport

Gravity measurements:

• Sensitivity of 0.6 . 10-9g after one hour

• Allan standard deviation in the low 10-10g level

• Demonstrated mobile, continuous gravity recordings

• Sensitivity comparable to best spring-based gravimeters but without long-term drift

• Recalibration of superconducting gravimeter scale factor

Mobile setup: completed

Summary


Outlook

Onsala Space Obsorvatory (Chalmers University of Technology) • Absolute accuracy of gravity value - further compensation of systematics

• Measurements at other locations (e.g. Onsala Space Observatory)

• Simultaneous recording of both g and the gravity gradient

• Tests for Non-Newtonian gravitational forces

Setup Improvements

Pumped Storage Reservoir in Hornberg,De


http://www.physik.hu-berlin.de/qom

Thank you for your attention

Questions ?

Special thanks to the staff of the IfE and the BKG for their cooperation


Active Vibration Isolation Stage

Freier, Measurement of Local Gravity using Atom Interferometry, Humboldt Universität zu Berlin (2010), Diplomarbeit


Tilt and Coriolis Compensation

back-coupling into the Raman-fibre

Tilt-Compensator

Raman beams

• Raman retro-reflector on piezo tip/tilt stage with sub-μrad precision and closed-loop actor

• Active Raman beam alignment during

gravimeter measurements

• Suppression of Coriolis effects during interferometer sequence

Coriolis Effect suppression

Hauth et al., Applied Physics B, 113 (2013) 49


Atom Interferometry

Raman Pulses

| g , 0ħk >

| e, + 2ħk >

| i, + 1ħk >

Laser 1 Laser 2 v

v

Π-Pulse

v

v

Π/2-Pulse

mirror beamsplitter

DE


Mobile atom interferometer GAIN (Gravimetric Atom Interferometer)

Determing gravitational acceleration


Mobile atom interferometer GAIN (Gravimetric Atom Interferometer)

Laserfrequencies

permanently correct for the dopplereffect

Over a time T this chirp

causes a phase shift Φchirp

Calculation of g

Φfringe

Φchirp + Φfringe = k g T²


Table of Systematic Effects

without

correction

with

correction

Wavefront Distortions -50 0 +- 50

Sync Vibrations 90 0 +- 80

Coriolis Effects 100 0 +- 15

RF Groupdelays 100 0 +- 10

AC-Stark Shift (1PLS) 1000 0 +- 5

Self Gravitation 19 0 +- 5

Quadratic Zeeman shift 0 0 +- 5

Rb Vapor Pressure 5 0 +- 3

Ref-Frequenz -10 0 +- 2

AC-Stark Shift (2PLS) 0 0 +- 2

vertical alignment 0 0 +- 1

Total Uncertainty 97

Systematic Effect Magnitude [nm/s²] uncertainty

[nm/s²]




2. State selection



0.7 Hz

Documents

Terrestrial Gravity Measurements with the mobile … · Terrestrial Gravity Measurements with the mobile Atom ... - People - Plate tectonics - Volcanology . Graphic by GOCE - Geodesist