Absolute gravimetry. Gabriel Strykowski Geodynamics Department DTU-Space (formerly Danish National Space Center)

Transcription

1 Absolute gravimetry by Gabriel Strykowski Geodynamics Department DTU-Space (formerly Danish National Space Center)

2 Contents 1. Instrumentation 2. Absolute gravity measurements: applications & practicalities 3. Absolute gravity measurements in Denmark 4. A10 demonstration

3 1. Instrumentation Strykowski, G.: Absolute gravimetry Relative and absolute gravimeters until 1980 (after Torge, Gravimetry,1989)

4 1. Instrumentation Strykowski, G.: Gravity adjustment FG5-224 A FG-5 accuracy: ~2 μgal (2 days; drops) operation: can be operated in a tent but is more suited for a laboratory. FG5-108 A10 accuracy: ~5 µgal (1-1½ hr; drops) operation: outdoor instrument. A purchased by DTU Space (July 2008)

5 FG-5 Strykowski, G.: Absolute gravimetry

6 A Borowa Góra, PL Strykowski, G.: Absolute gravimetry

7 Possible A10 tent Strykowski, G.: Absolute gravimetry

8 1. Instrumentation A10 Specifications Portable Field Absolute Gravimeter Accuracy: 10 μgal (observed agreement between A10 instruments) Precision: at a quiet site, 10s drop interval, 50μGal/sqrt(Hz) [eg. About 1 μgal in 30 minutes] Operating dynamic range: World-Wide Operating temperature range: -15 C to 40 C

9 A10 Principle of Operation Vacuum Chamber Freefalling Upper Mirror Interferometer A freely falling reflective test mass is dropped in a vacuum. An optical signal is detected at the output of the interferometer. This signal is used to determine the local gravitational acceleration. Interference Detector Stationary Lower Mirror

10 Interferometry MAX λ/2 min Laser B.S. Photodiode Fringes fringe signal sweeps in frequency as test mass falls under influence of gravity sin( 2πx 2 ) 0 Michelson s interferometer x time recorded (with rubidium oscillator) at each minimum creating (time,distance) pairs at every λ/2 5

11 g Determination Fringe = l/2 x i For each x i, a measured time t i, The following function is fitted to the data x i,t i : ~ x = x + v t + i 0 0 i g ~ 0ti 2 2 ~ 2 0 i γx t ~ 3 1 ~ 4 + γ v0ti + γg0t i 6 24 ~ ( xi x0) t = ti c γ is the vertical gravity gradient (~3 µgal/cm), c the speed of light x 0 the initial position v 0 the initial velocity g 0 the initial acceleration x i, t i, i = 1,,700

12 A10 Schematic Laser is frequency-stabilized He-Ne laser (red 633 nm) Interferometer splits beam into test and reference beams The test beam bounces off falling corner cube then off stationary spring corner cube The reference beam travels straight through interferometer. Beams are recombined and interference signal (fringes) is used to track falling test mass The time intervals between the occurrence of each fringe are measured by a Rubidium oscillator

13 Drop Scatter should be as small as possible. In a lab, <20uGal for an FG5, <50uGal for an A10 The value depends on the site, but it should at least show no overall trends up or down (these data are really good, by the way...)

14 2. Absolute gravity measurements: applications & practicalities 1. Change of a fundamental reference gravity point in Denmark 2003 from: Buddinge gravity station to: Vestvolden gravity station 2. Geodynamic studies absolute control tie on relative gravity lines Scandinavian land uplift GNET 3. Absolute gravity measurements in Denmark 4. A10 demonstration

15 Strykowski, G.: Gravity adjustment 2. Absolute gravity measurements: applications & practicalities National gravity networks Gravity network constructed with ties between absolute (red dots) and relative (black dots) base stations Minimise closure errors around polygons

16 Strykowski, G.: Gravity adjustment 2. Adjustment software for relative gravity surveys Geophysical signals land uplift Measuring the ice by GPS loading Melting of glaciers in Greenland: mass loss Less mass => land uplift Earth s response is visco-elastic: - Short term elastic effect - Long term viscous effect ( år+) Elastic effect is up to 10 mm/year Measured land uplift is a function of the historical changes of ice load

17 GNET

18 Station Occupations 36 different stations occupied since 2003 epoch IfE total overlapping institutions BKG, FGI, IfE FGI, IfE, UMB FGI, IfE, UMB FGI, IfE, UMB FGI, IfE, UMB, LM after Gitlein et al. (2008)

19 Strykowski, G.: Gravity adjustment with 3 nm/s² per hpa with ECMWF Ekman and Mäkinen µgal/yr Atmospheric reduction at 11 AG stations -1.5 µgal/yr -1.5 µgal/yr decreases the standard deviation of linear gravity changes at 9 stations (0.1 to 0.4 µgal/yr) -2.1 µgal/yr decreases the linear trend at 7 stations Gitlein et al. (2008)

20 Strykowski, G.: Gravity adjustment 2. Absolute gravity measurements: applications & practicalities Geodynamics: land uplift in Svcandinavia Nordic relative gravity lines to monitor land uplift land uplift mm/year land uplift gravity lines

21 2. Absolute gravity measurements: applications & practicalities Cooperation with University of Hannover, Germany since 1986 JILAg-3 and FG5-220 (Timmen et al., 2008) Other groups that measured in DK: BKG (D) Tejn and Gedser (2005); Lantmäteriet Tejn (2008)

22 Strykowski, G.: Gravity adjustment 2. Absolute gravity measurements: applications & practicalities In-situ vertical gradient measurements for geodynamic applications Vertical gravity gradient in a building is slightly less negative than the free-air gravity gradient μgal/m BKG (D) Esbjerg airport, DK, 2007 IfE (D) Tebstrup, DK, 2003 gradient: μgal/m

23 FG-5 measurements in a tent: Smidstrup GPS antenna

24 FG-5 measurements in a tent in DK: 2005 Smidstrup GPS antenna, 2005, Ludger Timmen, IfE (D), monitors the ground water level Suldrup GPS antenna, 2005, preparing for the Installation of the wooden floor of the tent

25 BKG (D), Gedser GPS antenna, 2005, tent measurement. Eccenter point.

26 20 15 TIGO Concepcion: combined gravity signals SG-38 and FG5-227 (corrected for SG-drift, tides, air pressure, polar motion) FG5-227 measured at 402, reduced to 403 by 9 µgal FG5-227 measured at 403, corrected by µgal g [µgal] pier 403@125 cm FG5-227 measured at 403 FG5-227 measured at 403, corrected by 2.9,µGal SG-38 observed at 401, drift 1.6 µgal/year Example for strong gravity variations at a geodetic reference station Combination of SG and AG in Concepcion (Chile) after (Wilmes, 2008)

27 3. Absolute gravity measurements in Denmark

28 3. Absolute gravity measurements in Denmark

29 3. Absolute gravity measurements in Denmark