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

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0 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

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1 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

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2 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 –2.645 ± 0.007 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 5th_{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. 2006

980 533 984.4

1.7 6/8 Nov. 2006

980 533 984.1

2.2 8/9 Nov. 2006

980 533 984.4

0.5

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3

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.

C

2. Relative gravity measurements

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4 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

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5 Reference

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6

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7 STATION: TURIN

City:

Turin

Country:

Italy

Location:

INRIM

Particularity:

Situation:

Gravity Laboratory

Remarks:

Date:

5/6 June 2006

Code number:

Latitude:

45.0170 degrees

Longitude:

7.64270 degrees

Elevation:

236 m

Gradient:

-2.645 µgal/cm

Reference height:

12.85 mm

Meter:

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.

1.675

0.566

0.250

0.147

0.353

0.077

0.156

0.043

0.0

0.0 Phase:

61.6

33.5

56.6

159.5

83.2

62.1

39.1 -139.5

0.0

0.0 Polar motion correction

Air pressure correction

X-coordinate:

0.1232

arc seconds

Nominal air pressure:

985.22 mbar

Y-coordinate:

0.3271

arc seconds

Barometric admittance factor:

0.3 µgal/mbar

Gravity

Set gravity mean:

9 80 533 984.42

Microgal

Set std. dev.:

1.71 microgal

Mean std. dev.:

18.85 microgal

Number of sets:

21 Number of drops per set:

100 Drop interval:

10 seconds

Set interval:

60 minutes

Nominal/datum height:

130.0 cm

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8 Project file

Site Code: IMGC

Lat: 45.01700 Long: 7.64270 Elev: 236.00 m Reference Height: 12.85 cm

Datum Height: 130.00 cm Gradient: -2.645 µGal/cm

Nominal Air Pressure: 985.22 mBar Barometric Admittance Factor: 0.30 Polar Motion Coord: 0.1232 " 0.3271 " 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 0.000000 0.002427 1.000000 0.0000 DC 0.002428 0.249951 1.160000 0.0000 Long 0.721500 0.906315 1.154250 0.0000 Q1 0.921941 0.974188 1.154240 0.0000 O1 0.989049 0.998028 1.149150 0.0000 P1 0.999853 1.216397 1.134890 0.0000 K1 1.719381 1.906462 1.161720 0.0000 N2 1.923766 1.976926 1.161720 0.0000 M2 1.991787 2.002885 1.161720 0.0000 S2 2.003032 2.182843 1.161720 0.0000 K2 2.753244 3.081254 1.07338 0.0000 M3 3.791964 3.937897 1.03900 0.0000 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): 1.675 0.566 0.250 0.147 0.353 0.077 0.156 0.043 0.000 0.000 0.000 Phase (deg): 61.6 33.5 56.6 159.5 83.2 62.1 39.1 -139.5 0.0 0.0 0.0 Instrument Data Meter Type: FG5 Meter S/N: 216 Factory Height: 116.40 cm Rubidium Frequency: 10000000.01020 Hz Laser: WEO100 (187) ID: 632.99117754 nm ( 0.40 V) IE: 632.99119473 nm ( -0.08 V) IF: 632.99121259 nm ( -0.45 V) IG: 632.99123023 nm ( -1.18 V) IH: 632.99136890 nm ( -1.78 V) II: 632.99139822 nm ( -1.53 V) IJ: 632.99142704 nm ( -1.30 V) Modulation Frequency: 8333.420 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: 980533984.42 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

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9

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): -11.69 µGal Ocean Load: 0.04 µGal

Polar Motion: -1.50 µGal Barometric Pressure: 1.33 µGal Datum Height: -1.98 µGal Reference Xo: -0.00 µGal

Uncertainties Sigma Reject: 3.00 Earth Tide Factor: 0.000

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

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10 Set File

Source Data Filename: tu20060605 g Acquisition Version: 1.082300 g Processing Version: 6.060320

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14

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15 STATION: TURIN

City:

Turin

Country:

Italy

Location:

INRIM

Particularity:

Situation:

Gravity Laboratory

Remarks:

Date:

6/8 June 2006

Code number:

Latitude:

45.0170 degrees

Longitude:

7.64270 degrees

Elevation:

236 m

Gradient:

-2.645 µgal/cm

Reference height:

12.85 mm

Meter:

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.

1.675

0.566

0.250

0.147

0.353 0.077 0.156 0.043

0.0

0.0 Phase:

61.6

33.5

56.6

159.5

83.2

62.1

39.1 -139.5

0.0

0.0 Polar motion correction

Air pressure correction

X-coordinate:

0.1232

arc seconds

Nominal air pressure:

985.22 mbar

Y-coordinate:

0.3271

arc seconds

Barometric admittance factor:

0.3 µgal/mbar

Gravity

Set gravity mean:

9 80 533 984.06

Microgal

Set std. dev.:

2.20 microgal

Mean std. dev.:

19.39 microgal

Number of sets:

36 Number of drops per set:

100 Drop interval:

10 seconds

Set interval:

60 minutes

Nominal/datum height:

130.0 cm

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16 Project file

Micro-g Solutions g Processing Report File Created: 09/20/06, 14:49:27

Project Name: tu20060606 g Acquisition Version: 1.082300 g Processing Version: 6.060320 Company/Institution: Operator: Station Data Name: TURIN Site Code: IMGC

Delta Factors

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Date: 06/07/06 Time: 08:38:15 DOY: 158 Year: 2006 Time Offset (D h:m:s): 0 0:0:0 Gravity: 980533984.06 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:

Gravity Corrections

Earth Tide (ETGTAB): -24.83 µGal Ocean Load: -0.06 µGal

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18 Set File

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19

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23

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24 STATION: TURIN

City:

Turin

Country:

Italy

Location:

INRIM

Particularity:

Situation:

Gravity Laboratory

Remarks:

Date:

8/9 June 2006

Code number:

Latitude:

45.0170 degrees

Longitude:

7.64270 degrees

Elevation:

236 m

Gradient:

-2.645 µgal/cm

Reference height:

12.85 mm

Meter:

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.

1.675

0.566

0.250

0.147

0.353

0.077

0.156

0.043

0.0

0.0 Phase:

61.6

33.5

56.6

159.5

83.2

62.1

39.1 -139.5

0.0

0.0 Polar motion correction

Air pressure correction

X-coordinate:

0.1232

arc seconds

Nominal air pressure:

985.22 mbar

Y-coordinate:

0.3271

arc seconds

Barometric admittance factor:

0.3 µgal/mbar

Gravity

Set gravity mean:

9 80 533 984.37

Microgal

Set std. dev.:

0.53 microgal

Mean std. dev.:

20.46 microgal

Number of sets:

12 Number of drops per set:

200 Drop interval:

5 seconds

Set interval:

60 minutes

Nominal/datum height:

130.0 cm

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25 Project file

Micro-g Solutions g Processing Report File Created: 09/20/06, 15:07:16

Project Name: tu20060608 g Acquisition Version: 1.082300 g Processing Version: 6.060320 Company/Institution: ECGS Operator: O. Francis Station Data Name: TURIN Site Code: IMGC

Delta Factors

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26

Date: 06/08/06 Time: 21:57:43 DOY: 159 Year: 2006 Time Offset (D h:m:s): 0 0:0:0 Gravity: 980533984.37 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:

Gravity Corrections

Earth Tide (ETGTAB): -63.40 µGal Ocean Load: -0.13 µGal

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27 Set File

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