BASE READING REPORT

June - September 2008

This report covers base readings performed in the period from 29 June 2008 to 30 August 2008.

Click to see photo album of all pictures

List of timestamps on all photos

GRAVITY METER

Sensor: Lacoste & Romberg
Electronics: ZLS
Serial number: S-38
Owner: DTU Space, Dept. of Geodynamics

SURVEYS

IMR = Institute of Marine Research

Survey No.	Area	PI / Cruise leader	Start Date / Location	End Date / Location	Vessel
1	Norwegian Sea	RBP / RBP	29 June 2008 Tromsø, Rica pier	17 July 2008 Tromsø, Breivika	RV "G.O Sars"
2	OBS Part 1	RM / AK	23 July 2008 Tromsø, IMR pier depot	5 August 2008 Tromsø, IMR pier depot	RV "Håkon Mosby"
3	OBS Part 2	RM / RM	7 August 2008 Tromsø, IMR pier depot	14 August 2008 Longyearbyen, Bykaia	RV "Håkon Mosby"
4	SVALEX-2008	RM / RM	19 August 2008 Longyearbyen, Bykaia	30 August 2008 Longyearbyen, Bykaia	RV "Håkon Mosby"

During the summer field season of 2008, Dept. of Earth Science had four marine surveys where the gravity meter was used, on two different vessels. Gravity ties were performed on three occations - one in Tromsø (combining several port of calls for two different vessels), and two in Longyearbyen.

The first section of the report details each port of call, providing information on exact location and estimated height difference between marine gravity meter sensor and assumed position of land gravity meter used during gravity ties.

The second part describes each gravity tie.

PORT OF CALLS

Survey no. 1, start: "G.O. Sars", 29 June 2008, Tromsø, Rica Pier

Unfortunately, it was not possible to log gravity data at 10 seconds interval on external computer and merge these data with time and position information. This means that merging of gravity and navigational data must be a separate pre-processing task. The ship's computer system logs time and position at regular intervals, here are these data:

SARS-navdata-2008-105.rar Δ

Note that time is GPS. During the survey period, GPS is ahead of UTC by ... seconds.

Gravity tie point Tromsø harbour, 29 June 2008

RV "G.O.Sars 29 June 2008, 18:00 UTC.
Estimated distanse between top of port hole and land gravity meter: 40 cm.

This picture for comparison only: Estimated distance top of port hole to hatch lower part: 80 cm.

Sketch of gravitymeter location on G.O. Sars. Source: http://www2.geo.uib.no/OBS-JanMayen-2006/

The important dimension in this sketch is the distance from the gravity meter sensor to porthole top: 160 cm.

We estimate that land gravitymeter sensor would be 40 cm above top of porthole, which means that:

On 29 June 2008, 18:00 UTC, the height between gravity tie point on wharf (assumed land gravity meter posistion) and position of marine gravitymeter sensor was 160+40 = 200 cm.

STILL READINGS

Inspection of raw gravitymeter data reveals that readings are not stable before 18:23 UTC. So if deemed necessary, height should be adjusted. Tide data are available from Tromsø harbour.

Get raw gravity data from this file:

http://www2.geo.uib.no/gravitymeter/uploads/Main/gravity-data-GOS-July2008.zip

Choose file:

2008_181.dat

See hard disk format description (Marine Mode).

YYYYDDDHHMMSS GRAVITY SPRING_TENSION CROSS_COUPLING AVG_BEAM
------------------------------------------------------------
20081811828 0 9773.92 9773.40    .00     9.5
20081811829 0 9773.91 9773.40    .00    10.7
20081811830 0 9773.91 9773.40    .00    11.9
20081811831 0 9773.94 9773.40    .00    13.1

Survey no. 1, finish: "G.O. Sars", 17 July 2008, Tromsø, Breivika

Gravity tie point Tromsø harbour, 17 July 2008

RV "G.O. Sars" 17 July 2008, 12:15 UTC. Photo to determine height difference between tie point and gravity meter. Bollard is 2nd from S.

We estimate that land gravitymeter sensor would be 40 cm below top of porthole, which means that:

On 17 July 2008, 12:15 UTC, the height between gravity tie point on wharf (assumed land gravity meter position) and position of marine gravitymeter sensor was 160-40 = 120 cm.

STILL READINGS:

  DOY:Time     Dig. gravity     ST       CC      Raw      TC
-------------------------------------------------------------
199:12:11:10      9775.5      9775.8     0.0     -6.0    -0.3 
199:12:12:40      9775.4      9775.8     0.0     -3.0    -0.4
199:12:13:20      9775.4      9775.8     0.0     -5.0    -0.5 

where DOY = Day of year.

Still readings, screen photo some minutes later:

Still readings, G.O.Sars, 17 July 2008

Survey no. 2, start: "Håkon Mosby" 24 July 2008, Tromsø

All Tromsø visits at Institute of Marine Research Pier Depot, Stokkevollveien 1.

RV "Håkon Mosby" gravity tie point Tromsø harbour (same position used in all visits to Tromsø)''

Ship position, 24 July 2008, 15:10 UTC

24 July 2008, 15:10 UTC: Height from gravitymeter sensor to upper deck is 5.7 m. Assumed land gravitymeter position 1.8 m below, so height between marine gravitymeter sensor and land gravitymeter should be 5.7-1.8 = 3.9 m.

Survey no. 2, finish: "Håkon Mosby" 6 August 2008, Tromsø

As part of comprehensive gravity tie procedure, the position near IMR pier depot was measured three times with land gravity meter:

08:10 UTC

Tromsø, IMR pier depot, 6 August 2008, 08:10 UTC

Tromsø, IMR pier depot, 6 August 2008, 08:10 UTC: Position of land gravitymeter estimated 0.5 m below upper deck, which is 5.7 m above marine gravity sensor. So land gravity meter is 5.7-0.5 = 5.2 m above marine gravity sensor.

09:12 UTC

Tromsø, IMR pier depot, 6 August 2008, 09:12 UTC

Tromsø, IMR pier depot, 6 August 2008, 09:12 UTC: Position of land gravitymeter estimated 0.1 m below upper deck, which is 5.7 m above marine gravity sensor. So land gravity meter is 5.7-0.1 = 5.6 m above marine gravity sensor.

10:00 UTC

Tromsø, IMR pier depot, 6 August 2008, 10:00 UTC

Tromsø, IMR pier depot, 6 August 2008, 10:00 UTC: Position of land gravitymeter estimated 0.2 m below upper deck, which is 5.7 m above marine gravity sensor. So land gravity meter is 5.7-0.2 = 5.5 m above marine gravity sensor.

Survey no. 3, finish: "Håkon Mosby" 14 August 2008, Longyearbyen

Bykaia, 14 Aug 2008, 08:05 UTC

Longyearbyen, Bykaia, 14 August 2008, 08:05 UTC: Height between gravity meter sensor and top deck is 5.7 m. From this we estimate height from sensor to assumed position of land gravity meter to: 5.7-1.4 = 4.3 m (ref. picture above).

Survey no. 4, start: "Håkon Mosby" 19 August 2008, Longyearbyen

Bykaia, 19 August 2008, 13:35 UTC

Bykaia, 19 August 2008, 13:35 UTC: Estimated distance between upper deck and assumed land gravimeter position is 2.0 m. Upper deck is 5.7 m above marine gravity sensor, so distance between marine gravity sensor and land gravitymeter is 5.7-2.0 = 3.7 m.

Survey no. 4, part 1: "Håkon Mosby" 22 August 2008, Longyearbyen, Kullkaia

Longyearbyen, Kullkaia, 22 August 2008, 13:57 UTC

22 August 2008, 13:57 UTC, Longyearbyen, Kullkaia: Estimated distance between upper deck and assumed land gravitymeter position: 1.4 m. Height difference between marine gravitymeter sensor and upper deck is 5.7 m, so distance between marine gravitymeter and assumed land gravitymeter is 5.7-1.4 = 4.3 m.

Note: Refer to earlier base readings to obtain the gravity difference between Longyearbyen airport reference site, and Kullkaia:
-> SVALEX-2007
-> SVALEX-2004

GRAVITY TIES

Calibration table, land gravity meter

Calibration table, land gravity meter S/N G-936 (click to enlarge)

Calibration table, land gravity meter S/N G-936 (click to enlarge)

Tromsø gravity reference

Tromsø gravity reference S (click to enlarge).

Note: There is a small discrepancy in the gravity values given in these documents - 0.07 mgal - for unknown reasons.

Norwegian Mapping Authority S-point - old police station, now art museum.
Gravity: 982552.53 mgal

Tromsø S-point reference

Longyearbyen gravity reference - official

Unfortunately the Longyearbyen P reference point has been altered when new terminal was built in 2007. Measurements were made some meters to the east of original point.

Longyearbyen P-point reference now within hanger area

Longyearbyen gravity references - home brewed

By repeatedly measuring the difference between Longyearbyen official reference point, and two other points near the harbour area, we are trying to establish two new reference points.

Previous measurements:

Nor-Cargo quay

Arial view, reference point "A", Longyearbyen, Svalbard

Gravity sensor location, Ref. A

Main quay ("Bykaia")

Harbour Office, Bykaia, Longyearbyen, Svalbard.
Reference point on the right side of building.

Gravity sensor location, Ref. B

6 August 2008, Tromsø

Date: 6 August 2008
Land gravitymeter used: LaCoste & Romberg, Mod. G, S/N G-936

Location                    Time [UTC]    Counter reading
---------------------------------------------------------
IMR pier depot                 08:09         6016,10
  -"-                          08:10         6016,095
  -"-                          08:12         6016,10

Tromsø ref. point S            08:36         6015.75
  -"-                          08:38         6015.73
  -"-                          08:39         6015.74

Rica pier, 7th bollard from S  08:47         6015.57
  -"-                          08:48         6015.56
  -"-                          08:50         6015.56

Tromsø ref. point S            08:56         6015.74
  -"-                          08:58         6015.75
  -"-                          08:59         6015.74

IMR pier depot                 09:10         6016,11
  -"-                          09:11         6016,14
  -"-                          09:12         6016,11

Breivika, 3rd bollard from S   09:26         6018.94
  -"-                          09:27         6018.92
  -"-                          09:28         6018.91

Tromsø ref. point S            09:44         6015.74
  -"-                          09:45         6015.74
  -"-                          09:46         6015.735

IMR pier depot                 09:55         6016,10
  -"-                          09:57         6016,11
  -"-                          09:58         6016,105

15 August 2008, Longyearbyen

Date: 15 August 2008
Land gravitymeter used: LaCoste & Romberg, Mod. G, S/N G-936

Location                    Time [UTC]    Counter reading
---------------------------------------------------------
Longyearbyen ref.point P       09:49         6418.205
  -"-                          09:51         6418.204
  -"-                          08:12         6418.205

City quay ("Bykaia")           10:12         6420.35
  -"-                          10:14         6420.35
  -"-                          10:16         6420.35

19 August 2008, Longyearbyen

Date: 19 August 2008
Land gravitymeter used: LaCoste & Romberg, Mod. G, S/N G-936

Location                    Time [UTC]    Counter reading
---------------------------------------------------------
Nor-Cargo quay                 09:46         6421.63
  -"-                          09:51         6421.63
  -"-                          09:52         6421.63

City quay ("Bykaia")           10:05         6421.13
  -"-                          10:12         6421.13
  -"-                          10:13         6421.13

Longyearbyen ref.point P       10:33         6419.01
  -"-                          10:35         6419.01
  -"-                          10:36         6419.01

City quay ("Bykaia")           10:50         6421.15
  -"-                          10:52         6421.15
  -"-                          10:53         6421.15

Nor-Cargo quay                 13:15         6421.63
  -"-                          13:16         6421.63
  -"-                          13:17         6421.63

26 August 2008, Longyearbyen

Difference between City quay ("Bykaia") "reference point" and ship's position at this quay.

Date: 26 August 2008
Land gravitymeter used: LaCoste & Romberg, Mod. G, S/N G-936

Location                    Time [UTC]    Counter reading
---------------------------------------------------------
Ship's position                19:30         6420.690
  -"-                          19:31         6420.690

City quay ("Bykaia")           19:39         6421.095
  -"-                          19:41         6421.095