EXPERIMENTS, MODERN FIELD DATA AND THE LONG- LONG-TERM GEOLOGICAL RECORD
PS87 (ARK-XXVIIII/4) Data management
9. Marine Geophysics Data management
9.3 Gravity measurements
Graeme Eagles1, Bernard Coakley3 1AWI, 3UoAF
Objectives
Polarstern’s permanently-installed Bodenseewerk KSS-32 gravimeter measured relative gravity throughout leg PS87 as part of its ongoing reconnaissance gravity surveying effort.
Data were logged at all locations except in Danish and Russian waters where permission to do so was not sought or granted. Two sets of onshore tie measurements were made at the very beginning and very end of the leg, in Tromsø and in Bremerhaven. Together with meas-urements made earlier in Tromsø, these measmeas-urements allow us to convert the shipboard data to absolute gravity values and correct them for instrument drift.
Work at sea
Tie and drift measurements in Tromsø & Bremerhaven
Measurements were made before the leg with the portable LaCoste and Romberg gravity meter G-1031 next to the ship at the port of Tromsø, and at the IGSN absolute gravity points at the Nordnorsk Kunstmuseum (formerly the Police Station) and Tromsø Museum (Figs. 9.3.1—
9.3.3). A tie measurement was also completed at the Nordnorsk Kunstmuseum in Tromsø on July 4th, 2014 at the start of the preceding cruise leg. A further tie was completed with measurements next to the ship and at AWI’s building D after Polarstern’s return to Bremerhaven, using the portable LaCoste and Romberg gravity meter G-877. No measurement was possible at the quayside location in Bremerhaven before this season’s activities, as Polarstern had been in dry dock and the ship’s gravity meter not logging for long periods.
The KSS-32 instrument logged continuously during the periods of tie measurements in order to use the G-1031 and G-877 measurements for estimating dockside tie values.
The measurements were used to determine an acceleration of g=982555.59 mGal next to Polarstern in the harbour in Tromsø, and of g=981356.76 mGal next to Polarstern in Bremerhaven. Once corrected for the differences in height between the quayside and the ship’s KSS-32 gravimeter, the absolute accelerations at the ship’s gravimeter was determined to be 982556.46 mGal in Tromsø and 981347.22 mGal in Bremerhaven. The corresponding ship’s gravimeter readings during the measurement were 2337.51 scu (scale units) in Tromsø and 1132.90 scu in Bremerhaven.
9.3 Gravity measurements
Fig. 9.3.2: Top left: Bolt Q measurement point at Tromsø Museum. Top Right: Bolt R measurement point at Tromsø Museum. Bottom Right: Office with ante-room where Bolt R is located. Bottom left:
Outside view of the Tromsø Museum. (Photos: W. Geissler)
Fig. 9.3.1: Gravity tie point outside Nordnorsk Kunstmuseum in central
Tromsø. (Photo: W.
Geissler).
9. Marine Geophysics
Fig. 9.3.3: Tie point at Kai 25, Breivika, next to Polarstern on August 5 2014 (Photo: W. Geissler)
Details of the tie measurements made at stations with known absolute g values are given below:
Measurement 1 (Table 9.3.1).
Instrument : LaCoste & Romberg gravimeter, G-1031 with feedback
Location: Tromsø, Nordnorsk Kunstmuseum, BGI008019 (former Police Station)
Date: 5. August 2014
Conditions: Calm, no wind, no precipitation Abs. gravity: 982552.47 mGal
Present : W. Geissler, W. Jokat
Tab. 9.3.1: Gravity measurements at Nordnorsk Kunstmuseum. The average value from all measurements is 6216.94 mGal. The scaling factor for transfer from scale units (scu) to mGal, was 1.02123.
UTC Counter
[scu] Type Counter
[mGal] feedback
[mGal] Total. gravity [mGal]
07:09 6088 Feedback 6222.01 -5.028 6216.98
07:11 6089 Feedback 6223.03 -6.050 6216.98
07:13 6090 Feedback 6224.05 -7.082 6216.97
07:14 6087 Feedback 6220.99 -4.088 6216.90
07:15 6086 Feedback 6219.97 -3.070 6216.90
?? 6084.59 Manual 6218.52 -1.630 6216.89
Five measurements were done with the feedback unit and on the plate. One measurement was completed manually. The temperature inside the instrument was a constant 53.5°C.
9.3 Gravity measurements
Measurement 2 (Table 9.3.2)
Instrument : LaCoste & Romberg gravimeter, G-1031 with feedback Location : Tromsø, Tromsø Museum Bolt Q (Tromsø Q)
Date : 5. August 2014 Conditions : Indoors, calm Abs. g : 982551.448 mGal Present : W. Geissler, W. Jokat
Five measurements were done with the feedback unit and on the plate. One measurement was completed manually. The temperature inside the instrument was a constant 53.5°C.
Tab. 9.3.2: Gravity measurements at Tromsø Museum Bolt Q. The average value from all measurements is 6215.82 mGal. The scaling factor for transfer from scale units (scu) to mGal, was 1.02123.
08:00 6083.505 Manual 6217.42 -1.621 6215.80
08:01 6085 Feedback 6218.94 -3.060 6215.88
08:03 6086 Feedback 6219.97 -4.081 6215.88
08:04 6087 Feedback 6220.99 -5.142 6215.85
08:05 6084 Feedback 6217.92 -2.155 6215.77
08:06 6083 Feedback 6216.90 -1.134 6215.77
Measurement 3 (Table 9.3.3)
Instrument : LaCoste & Romberg gravimeter, G-1031 with feedback Location : Tromsø, Tromsø Museum Bolt R (Tromsø R)
Date : 5. August 2014
Conditions : Indoors, instrument read unstable accelerations at start Abs. gravity : 982552.140 mGal
Present : W. Geissler, W. Jokat
Three measurements were done with the feedback unit and on the plate. One measurement was completed manually. The temperature inside the instrument was a constant 53.5°C.
Tab. 9.4.3: Gravity measurements at Tromsø Museum Bolt R. The average value from all measurements is 6216.56 mGal. The scaling factor for transfer from scale units (scu) to mGal, was 1.02123.
08:23 6084.202 Manual 6218.13 -1.614 6216.52
08:26 6085 Feedback 6218.94 -2.342 6216.60
08:27 6086 Feedback 6219.97 -3.366 6216.60
08:28 6082 Feedback 6215.88 0.633 6216.51
9. Marine Geophysics
Measurement 4 (Table 9.3.4)
Instrument : LaCoste & Romberg gravimeter, G-877 with feedback Location : Building D, Alfred Wegener Institute, Bremerhaven Date : 8. October 2014
Conditions : Indoors, instrument read unstable accelerations Abs. gravity : 981356.72 mGal
Present : H. Kirk
Six measurements were done manually with the instrument on the plate. No measurement was successfully completed with feedback. The temperature inside the instrument was a constant 49.8°C.
Tab. 9.4.4: Gravity measurements at AWI Building D in Bremerhaven. The average value from all measurements is 5018.85 mGal. The scaling factor for transfer from scale units (scu) to mGal, was 1.02462.
As described below, harbour values were determined using the different tie possibilities that the various absolute stations offer.
Measurement 5.1: Before visiting the tie stations in Tromsø (Table 9.3.5.1) Instrument : LaCoste & Romberg gravimeter, Model G-1031 with feedback Location : 69.67962°N, 18.99687°E, Kai 25 Breivika, next to Polarstern Date : 5. August 2014
Conditions : calm, no wind, no precipitation
Present : W. Geissler, W. Jokat, O. Hüttebraucker
Four measurements were done with the feedback unit and on the plate. Four measurements were completed manually. The temperature inside the instrument was a constant 53.5°C. The KSS-32 gravimeter inside the ship was determined to be situated 2.01 m below the LaCoste &
Romberg instrument on the quayside.
9.3 Gravity measurements
Tab. 9.3.5.1: Gravity measurements at Kai 25, Breivika, next to Polarstern. The average value from all measurements is 6220.10 mGal. The scaling factor for transfer from scale units (scu) to mGal, was 1.02123.
06:18 6084 Feedback 6217.92 2.128 6220.05
06:20 6085 Feedback 6218.94 1.201 6220.15
06:23 6086 Feedback 6219.97 0.170 6220.14
06:25 6087 Feedback 6220.99 -0.844 6220.14
06:29 6087.78 Manual 6221.78 -1.730 6220.05
06:35 6087.73 Manual 6221.73 6221.73
06:36 6087.73 Manual 6221.73 6221.73
06:39 6087.71 Manual 6221.71 -1.664 6220.05
Measurement 5.2: After visiting the tie stations in Tromsø (Table 9.3.5)
Instrument : LaCoste & Romberg gravimeter, Model G-1031 with feedback Location : 69.67962°N, 18.99687°E, Kai 25 Breivika, next to Polarstern Date : 5. August 2014
Conditions : calm, no wind, no precipitation
Present : W. Geissler, W. Jokat, O. Hüttebraucker
Four measurements were done with the feedback unit and on the plate. Four measurements were completed manually. The temperature inside the instrument was a constant 53.5°C. The KSS-32 gravimeter inside the ship was determined to be situated 2.36 m below the LaCoste instrument on the quayside.
Tab. 9.3.5.2: Gravity measurements at Kai 25, Breivika, next to Polarstern. The average value from all measurements is 6220.10 mGal. The scaling factor for transfer from scale units (scu) to mGal, was 1.02123.
06:18 6084 Feedback 6217.92 2.128 6220.05
06:20 6085 Feedback 6218.94 1.201 6220.15
06:23 6086 Feedback 6219.97 0.170 6220.14
06:25 6087 Feedback 6220.99 -0.844 6220.14
06:29 6087.78 Manual 6221.78 -1.730 6220.05
06:35 6087.73 Manual 6221.73 6221.73
06:36 6087.73 Manual 6221.73 6221.73
06:39 6087.71 Manual 6221.71 -1.664 6220.05
9. Marine Geophysics
Measurement 6: After visiting the tie station at Building D in Bremerhaven (Table 9.3.6) Instrument : LaCoste & Romberg gravimeter, Model G-877 with feedback
Location : Lloydwerft Bremerhaven, Quayside of Polarstern Date : 8. October 2014
Conditions : calm weather, busy harbour activities Present : H. Kirk
Eight measurements were completed manually and seven with feedback, all with the instrument on the plate. The temperature inside the instrument was a constant 49.7°C. The KSS-32 gravimeter inside the ship was determined to be situated 1.50 m below the LaCoste instrument on the quayside. Continuous display whilst operating in feedback mode showed the measurements to be unstable by <0.1 mGal during measurement period.
Tab. 9.3.6: Gravity measurements at the Lloydwerft in Bremerhaven, next to Polarstern. The average value from manual measurements is 5018.90 mGal and the average from feedback measurements is 5018.13 mGal. The scaling factor for transfer from scale units (scu) to mGal, was 1.02462.
06:39 4906 Feedback 5021.20 -3.050 5018.15
06:39 4905 Feedback 5020.17 -2.035 5018.14
06:39 4904 Feedback 5019.15 -1.030 5018.12
06:39 4903 Feedback 5018.12 0.000 5018.12
06:39 4902 Feedback 5017.10 1.020 5018.12
06:39 4901 Feedback 5016.07 2.050 5018.12
06:39 4900 Feedback 5015.05 3.070 5018.12
Using these tie points, and correcting for the height differences between the quayside and the KSS-32 gravimeter inside the ship, absolute gravity at the quay in Tromsø was determined as g=982556.30 mGal before the tie measurements and g=982556.40 mGal afterwards.
The corresponding ship’s gravimeter measurement readings were 2337.82 and 2337.94 CU.
Quayside absolute gravity in Bremerhaven at the end of the leg was determined as 981357.22 mGal for ship’s gravimeter readings of 1132.9 scu.
9.3 Gravity measurements