Absolute and Relative Gravity Measurements at the Istituto Nazionale di Ricerca Metrologica (INRIM) of Turin (Italy) in June 2006

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1 Absolute and Relative Gravity Measurements at the Istituto Nazionale di Ricerca Metrologica (INRIM) of Turin (Italy) in June 2006 Final Report September 2006 Prof. Dr. Olivier Francis University of Luxembourg Faculty of Sciences, Technology and Communication Campus Kirchberg 6, rue Coudenhove-Kalergi L-1359 Luxembourg Grand-Duché de Luxembourg Tel. : , olivier.francis@uni.lu 0

2 Foreword Absolute gravity measurements were carried out at the Istituto Nazionale di Ricerca Metrologica (I.N.RI.M.) in Turin on the invitation of Dr. A. Germak and Dr. G. D Agostino. The objective is to compare with the observations of the IMGC-02 absolute gravimeter of the INRIM. The absolute gravimeter FG5#216 from the European Center for Geodynamics and Seismology (ECGS) was operated by Olivier Francis from the University of Luxembourg and Gilles Celli from the ECGS. Relative gravity measurements were carried out with the spring gravimeter Scintrex CG5#008. The vertical gravity gradient was measured at the absolute gravimeter site and a network was observed in the Force Laboratory of the INRIM. 1. Absolute Gravity measurements The absolute gravity observations were performed in the absolute gravity room. A picture of the set-up of the absolute gravimeter FG5#216 on the site is shown in Figure 1. Figure 1. The FG5#216 from the ECGS in the absolute gravity laboratory of the INRIM in Turin. 1

3 1.1 Data processing Raw data from the absolute gravimeters consist of vectors of time and position of the falling object during the drops. To obtain the gravity value, a linear equation representing the equation of motion is fit to the raw data including the gravity gradient which has been measured with relative meters. The data processing follows the protocol adopted during absolute gravimeters comparisons at the BIPM in Sèvres (Francis and van Dam, 2003). Geophysical corrections are applied to the raw gravity data: Earth tides using modelled tidal parameters, atmospheric pressure effect using a constant admittance, and the polar motion effect using pole positions from IERS. The g-soft version 6.0 software from Micro-g Solutions Inc. was used for the processing. All the text outputs as well as some figures are compiled in the annexes of this report for future reference. 1.2 Vertical Gravity Gradient The vertical gravity gradient was measured with a relative spring gravimeter Scintrex CG5. This gradient is needed to linearize the equation of motion but also to transfer the measured absolute gravity value from the reference height around 1.3 m to the floor. The final value of the linear vertical gravity gradient is ± microgal/cm at the pier where the absolute gravimeter measurements were performed. 1.3 Results of the absolute gravity measurements The FG5#216 stayed on the pier in the gravity lab from the 5 th to the 9 th of June. A total of 8100 drops were taken in three different sessions. The results are summarized in table 1. A time interval of 10 second was chosen for the two first ones (21 and 36 sets of 100 drops). A time interval of 5 second was tried for the last session (12 sets of 200 drops) to avoid the aliaising of the microseismic noise. This last choice proves to be efficient: the mean value of gravity stayed unchanged but one can observe a drastic improvement in the mean set standard deviation which dropped from about 2 microgal to 0.5 microgal. Table 1. Results of the absolute gravity measurements at the pier of the gravity laboratory of the INRIM in Turin. Date Gravity value at 1.30 m /microgal Mean Set Standard Deviation /microgal 5/6 Nov /8 Nov /9 Nov The time interval of the last set is 5 second. During the reprocessing of the data, we processed the complete set of data. Then we selected one data over two, equivalent to a time interval of 10 second. By comparing Figures 2, we can clearly see the effect of the aliaising (projection of the high frequency noise in the low frequency) of the microseismic noise on the results: for the 10 second interval, the mean set standard deviation significantly increases. The 2

4 superspring is intended to reduce the aliaising effect but at noisy site, the efficiency is somewhat limited. The good news is that the mean value is practically not affected. A B Figures 2. Absolute gravity values of the last session. A. time interval of 5 second and 200 drops per set; B. time interval of 10 second and 100 drops per set; C. time interval of 5 second and 100 drops per set. 2. Relative gravity measurements C During our stay at the INRIM, we had the opportunity to perform some relative gravity measurements between three different sites in the Force laboratory. A picture of the different locations is displayed in Figures 2. 3

5 Site A Site B1 Site B2 Figures 3. Sites of the relative network in the Force Laboratory. Due to an accidental power supply disconnection, the batteries of the Scintrex were empty and the gravimeter cooled down a few hours before the measurements. After 1 hour of warming up, the gravimeter was ready to take observations. In consequence, we observed a huge linear drift in the gravity data. Fortunately, it did not affect the precision of the measurements. The results of the gravity ties are given in Table 2. The R.MS. is the formal error of the least square adjustment of the observations taking the precision of the measurements (which is estimated using the standard deviation of the raw data over the integration time). Table 2. Relative ties in the Force laboratory of the INRIM in Turin. Site gravity difference /microgal RMS /microgal A 0 B B

6 Reference Francis O., van Dam T.M., Processing of the Absolute data of the ICAG01, Cahiers du Centre Européen de Géodynamique et de Séismologie, vol. 22, 45-48,

7 5-6 June

8 STATION: TURIN City: Turin Country: Italy Location: INRIM Particularity: Situation: Gravity Laboratory Remarks: Date: 5/6 June 2006 Code number: Latitude: Longitude: Elevation: Gradient: Reference height: Meter: degrees degrees 236 m µgal/cm mm FG5 S/N: 216 Ocean loading correction (µgal, Greenwich degree) Wave M 2 S 2 K 1 O 1 N 2 P 1 K 2 Q 1 M f M m S sa Ampl Phase: Polar motion correction Air pressure correction X-coordinate: arc seconds Nominal air pressure: mbar Y-coordinate: arc seconds Barometric admittance factor: 0.3 µgal/mbar Gravity Set gravity mean: Microgal Set std. dev.: 1.71 microgal Mean std. dev.: microgal Number of sets: 21 Number of drops per set: 100 Drop interval: Set interval: Nominal/datum height: Author: O. Francis Date: September 20, seconds 60 minutes cm University of Luxembourg 7

9 Project file Site Code: IMGC Lat: Long: Elev: m Reference Height: cm Datum Height: cm Gradient: µgal/cm Nominal Air Pressure: mbar Barometric Admittance Factor: 0.30 Polar Motion Coord: " " Earth Tide (ETGTAB) Selected Potential Filename: C:\Program Files\Micro-g Solutions Inc\gWavefiles\Etcpot.dat Delta Factor Filename: F:\ABSOLU\DATA\2006\Turin\torino.dff Delta Factors Start Stop Amplitude Phase Term DC Long Q O P K N M S K M M4 Ocean Load ON, Filename: F:\ABSOLU\DATA\2006\Turin\torino.olf Waves: M2 S2 K1 O1 N2 P1 K2 Q1 Mf Mm Ssa Amplitude (µgal): Phase (deg): Instrument Data Meter Type: FG5 Meter S/N: 216 Factory Height: cm Rubidium Frequency: Hz Laser: WEO100 (187) ID: nm ( 0.40 V) IE: nm ( V) IF: nm ( V) IG: nm ( V) IH: nm ( V) II: nm ( V) IJ: nm ( V) Modulation Frequency: Hz Processing Results Date: 06/06/06 Time: 00:53:12 DOY: 157 Year: 2006 Time Offset (D h:m:s): 0 0:0:0 Gravity: µgal Set Scatter: 1.71 µgal Measurement Precision: 0.37 µgal Total Uncertainty: 0.37 µgal Number of Sets Collected: 21 Number of Sets Processed: 21 Set #s Processed: 1,2,3,4,5,6,7,8,9,10,11,12,13,14,15,16,17,18,19,20,21 8

10 Number of Sets NOT Processed: 0 Set #s NOT Processed: Number of Drops/Set: 100 Total Drops Accepted: 2061 Total Drops Rejected: 39 Total Fringes Acquired: 700 Fringe Start: 7 Processed Fringes: 613 GuideCard Multiplex: 4 GuideCard Scale Factor: 250 Acquisition Settings Set Interval: 60 min Drop Interval: 10 sec Number of Sets: 24 Number of Drops: 100 Gravity Corrections Earth Tide (ETGTAB): µgal Ocean Load: 0.04 µgal Polar Motion: µgal Barometric Pressure: 1.33 µgal Datum Height: µgal Reference Xo: µgal Uncertainties Sigma Reject: 3.00 Earth Tide Factor: Average Earth Tide Uncertainty: 0.00 µgal Ocean Load Factor: 0.00 Average Ocean Load Uncertainty: 0.00 µgal Barometric: 0.00 µgal Polar Motion: 0.00 µgal Laser: 0.00 µgal Clock: 0.00 µgal System Type: 0.00 µgal Tidal Swell: 0.00 µgal Water Table: 0.00 µgal Unmodeled: 0.00 µgal System Setup: 0.00 µgal Gradient: 0.00 µgal ( 0.00 µgal/cm) 9

11 Set File Source Data Filename: tu g Acquisition Version: g Processing Version: Set Time DOY Year Gravity Sigma Error Uncert Tide Load Baro Polar Datum Refxo Temp Pres Accept Reject 1 14:53: :53: :53: :53: :53: :52: :53: :53: :53: :53: :53: :53: :53: :53: :53: :53: :53: :53: :53: :53: :53:

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15 6-8 June

16 STATION: TURIN City: Turin Country: Italy Location: INRIM Particularity: Situation: Gravity Laboratory Remarks: Date: 6/8 June 2006 Code number: Latitude: Longitude: Elevation: Gradient: Reference height: Meter: degrees degrees 236 m µgal/cm mm FG5 S/N: 216 Ocean loading correction (µgal, -Greenwich degree) Wave M 2 S 2 K 1 O 1 N 2 P 1 K 2 Q 1 M f M m S sa Ampl Phase: Polar motion correction Air pressure correction X-coordinate: arc seconds Nominal air pressure: mbar Y-coordinate: arc seconds Barometric admittance factor: 0.3 µgal/mbar Gravity Set gravity mean: Microgal Set std. dev.: 2.20 microgal Mean std. dev.: microgal Number of sets: 36 Number of drops per set: 100 Drop interval: Set interval: Nominal/datum height: Author: O. Francis Date: September 20, seconds 60 minutes cm University of Luxembourg 15

17 Project file Micro-g Solutions g Processing Report File Created: 09/20/06, 14:49:27 Project Name: tu g Acquisition Version: g Processing Version: Company/Institution: Operator: Station Data Name: TURIN Site Code: IMGC Lat: Long: Elev: m Reference Height: cm Datum Height: cm Gradient: µgal/cm Nominal Air Pressure: mbar Barometric Admittance Factor: 0.30 Polar Motion Coord: " " Earth Tide (ETGTAB) Selected Potential Filename: C:\Program Files\Micro-g Solutions Inc\gWavefiles\Etcpot.dat Delta Factor Filename: F:\ABSOLU\DATA\2006\Turin\torino.dff Delta Factors Start Stop Amplitude Phase Term DC Long Q O P K N M S K M M4 Ocean Load ON, Filename: F:\ABSOLU\DATA\2006\Turin\torino.olf Waves: M2 S2 K1 O1 N2 P1 K2 Q1 Mf Mm Ssa Amplitude (µgal): Phase (deg): Instrument Data Meter Type: FG5 Meter S/N: 216 Factory Height: cm Rubidium Frequency: Hz Laser: WEO100 (187) ID: nm ( 0.33 V) IE: nm ( V) IF: nm ( V) IG: nm ( V) IH: nm ( V) II: nm ( V) IJ: nm ( V) Modulation Frequency: Hz Processing Results 16

18 Date: 06/07/06 Time: 08:38:15 DOY: 158 Year: 2006 Time Offset (D h:m:s): 0 0:0:0 Gravity: µgal Set Scatter: 2.20 µgal Measurement Precision: 0.37 µgal Total Uncertainty: 0.37 µgal Number of Sets Collected: 36 Number of Sets Processed: 36 Set #s Processed:1,2,3,4,5,6,7,8,9,10,11,12,13,14,15,16,17,18,19,20,21,22,23,24,25,26,27,28,29,30,31,32,33,34,35, 36 Number of Sets NOT Processed: 0 Set #s NOT Processed: Number of Drops/Set: 100 Total Drops Accepted: 3500 Total Drops Rejected: 100 Total Fringes Acquired: 700 Fringe Start: 7 Processed Fringes: 613 GuideCard Multiplex: 4 GuideCard Scale Factor: 250 Acquisition Settings Set Interval: 60 min Drop Interval: 10 sec Number of Sets: 36 Number of Drops: 100 Gravity Corrections Earth Tide (ETGTAB): µgal Ocean Load: µgal Polar Motion: µgal Barometric Pressure: 2.45 µgal Datum Height: µgal Reference Xo: µgal Uncertainties Sigma Reject: 3.00 Earth Tide Factor: Average Earth Tide Uncertainty: 0.00 µgal Ocean Load Factor: 0.00 Average Ocean Load Uncertainty: 0.00 µgal Barometric: 0.00 µgal Polar Motion: 0.00 µgal Laser: 0.00 µgal Clock: 0.00 µgal System Type: 0.00 µgal Tidal Swell: 0.00 µgal Water Table: 0.00 µgal Unmodeled: 0.00 µgal System Setup: 0.00 µgal Gradient: 0.00 µgal ( 0.00 µgal/cm) 17

19 Set File Source Data Filename: tu g Acquisition Version: g Processing Version: Set Time DOY Year Gravity Sigma Error Uncert Tide Load Baro Polar Datum Refxo Temp Pres Accept Reject 1 15:08: :08: :08: :08: :08: :07: :08: :08: :08: :08: :08: :08: :08: :08: :08: :08: :08: :07: :08: :08: :08: :08: :08: :08: :08: :08: :08: :08: :08: :08: :08: :08: :08: :08: :07:

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24 8-9 June

25 STATION: TURIN City: Turin Country: Italy Location: INRIM Particularity: Situation: Gravity Laboratory Remarks: Date: 8/9 June 2006 Code number: Latitude: Longitude: Elevation: Gradient: Reference height: Meter: degrees degrees 236 m µgal/cm mm FG5 S/N: 216 Ocean loading correction (µgal, -Greenwich degree) Wave M 2 S 2 K 1 O 1 N 2 P 1 K 2 Q 1 M f M m S sa Ampl Phase: Polar motion correction Air pressure correction X-coordinate: arc seconds Nominal air pressure: mbar Y-coordinate: arc seconds Barometric admittance factor: 0.3 µgal/mbar Gravity Set gravity mean: Microgal Set std. dev.: 0.53 microgal Mean std. dev.: microgal Number of sets: 12 Number of drops per set: 200 Drop interval: Set interval: Nominal/datum height: Author: O. Francis Date: September 20, seconds 60 minutes cm University of Luxembourg 24

26 Project file Micro-g Solutions g Processing Report File Created: 09/20/06, 15:07:16 Project Name: tu g Acquisition Version: g Processing Version: Company/Institution: ECGS Operator: O. Francis Station Data Name: TURIN Site Code: IMGC Lat: Long: Elev: m Reference Height: cm Datum Height: cm Gradient: µgal/cm Nominal Air Pressure: mbar Barometric Admittance Factor: 0.30 Polar Motion Coord: " " Earth Tide (ETGTAB) Selected Potential Filename: C:\Program Files\Micro-g Solutions Inc\gWavefiles\Etcpot.dat Delta Factor Filename: F:\ABSOLU\DATA\2006\Turin\torino.dff Delta Factors Start Stop Amplitude Phase Term DC Long Q O P K N M S K M M4 Ocean Load ON, Filename: F:\ABSOLU\DATA\2006\Turin\torino.olf Waves: M2 S2 K1 O1 N2 P1 K2 Q1 Mf Mm Ssa Amplitude (µgal): Phase (deg): Instrument Data Meter Type: FG5 Meter S/N: 216 Factory Height: cm Rubidium Frequency: Hz Laser: WEO100 (187) ID: nm ( 0.33 V) IE: nm ( V) IF: nm ( V) IG: nm ( V) IH: nm ( V) II: nm ( V) IJ: nm ( V) Modulation Frequency: Hz Processing Results 25

27 Date: 06/08/06 Time: 21:57:43 DOY: 159 Year: 2006 Time Offset (D h:m:s): 0 0:0:0 Gravity: µgal Set Scatter: 0.53 µgal Measurement Precision: 0.15 µgal Total Uncertainty: 0.15 µgal Number of Sets Collected: 12 Number of Sets Processed: 12 Set #s Processed: 1,2,3,4,5,6,7,8,9,10,11,12 Number of Sets NOT Processed: 0 Set #s NOT Processed: Number of Drops/Set: 200 Total Drops Accepted: 2330 Total Drops Rejected: 70 Total Fringes Acquired: 700 Fringe Start: 7 Processed Fringes: 613 GuideCard Multiplex: 4 GuideCard Scale Factor: 250 Acquisition Settings Set Interval: 60 min Drop Interval: 5 sec Number of Sets: 12 Number of Drops: 200 Gravity Corrections Earth Tide (ETGTAB): µgal Ocean Load: µgal Polar Motion: µgal Barometric Pressure: 2.78 µgal Datum Height: µgal Reference Xo: µgal Uncertainties Sigma Reject: 3.00 Earth Tide Factor: Average Earth Tide Uncertainty: 0.00 µgal Ocean Load Factor: 0.00 Average Ocean Load Uncertainty: 0.00 µgal Barometric: 0.00 µgal Polar Motion: 0.00 µgal Laser: 0.00 µgal Clock: 0.00 µgal System Type: 0.00 µgal Tidal Swell: 0.00 µgal Water Table: 0.00 µgal Unmodeled: 0.00 µgal System Setup: 0.00 µgal Gradient: 0.00 µgal ( 0.00 µgal/cm) 26

28 Set File Source Data Filename: tu g Acquisition Version: g Processing Version: Set Time DOY Year Gravity Sigma Error Uncert Tide Load Baro Polar Datum Refxo Temp Pres Accept Reject 1 16:27: :27: :27: :27: :27: :27: :27: :27: :27: :27: :27: :27:

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