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	Title      OBS-2003      Gravity meter base readings
	By O.M.	Date 05 November 2014	Version 0.1

INTRODUCTION

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Clean-up of http://www.geo.uib.no/eworkshop/OBS-2003/gravitymeter-basereadings/

DATA FORMAT DIFFERENCE BETWEEN HIGH-RESOLUTION AND MARINE MODE

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Referring to the UltraSys format description dated June 14, 1999: ultrasys-data-formats.html

Gravity data on this survey was logged in high-resolution mode - normally, marine mode should be used. Be aware of the gravity data format difference between high-resolution and marine mode.

The hard disk data has the same structure in both these cases:

   EXAMPLE:

   IIIIIIIIIIYYYYDDDHHMMSSGGGGG.GGSSSSS.SSCCCC.CCBBBBBB.BVVVVVVV.
   LLLLLLL.XXXXXXX.#######.-------------etc---------#######.HHHHHHHH
   PPPPP.AAA.AAA---etc----AAA.AAA.

The important detail is the gravity value (GGGGG.GG) which in marine mode is given in mGal or counter units, as selected by the operator. In high-resolution mode, the gravity value is always given in counter units, updated at 10 sec intervals. But the "Video Gravity Display" will give gravity in mGal units - independant of mode setting.

GRAVITY REFERENCE POINT, REALFAGBYGGET

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Gravity reference station Realfagbygget (A), Bergen. Click to enlarge. Link to gravity station description (PDF).

Gravity reference station Realfagbygget (B), Bergen. Click to enlarge. Link to gravity station description (PDF).

• Gravity value Realfagbygget: 9.81 932 372 ms^-2.
• Link to gravity station description (PDF).
• Lat/Lon datum: NGO 1948 ("Norges Geografiske Oppmåling 1948").
• Height reference level: NN54 ("Norsk null av 1954").
• Some information about the NGO 1948 datum.

Lat/Long, WGS84 datum

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Realfagbygget gravity reference station, 60.38440º N, 5.32948º E (60° 23′ 3.8″N, 5° 19′ 46.1″E). Datum = WGS84. Map source: http://kart.finn.no/ Click to enlarge.

Height - MASL

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Map source: http://www.bergenskart.no/bergen/ Select "Turkart". Click to enlarge.

Map source: http://kart.finn.no/ Click to enlarge.

In the Realfagbygget gravity station description, height is stated to be 20.8 meter, with reference datum "NN54" which means "Norsk null av 1954" - a Norwegian national reference established in 1954. (The Norwegian mean sea level reference is updated at regular intervals, feed this news article into Google translate: "From 1 January 2015, 37 cm is added to Oslo mean sea level").

If this value is uncertain we can contact Bergen municipality, mapping office and get information about reference points they have in the vicinity. Using a laser leveling surveying tool is should be fairly easy to determine the height of the gravity reference point.

GPS determination of height value

This is an idea for determining the height of the gravity reference point in Realfagbygget, measured in the GPS system. There are details in the relationship between "Mean Sea Level", Geiod and the WGS84 ellipsoid that need careful consideration, as outlined in this ESRI article.

• Data log over some hours, with GPS in 2nd floor Realfagbygget, above gravity reference point in basement. We can extract height from $GPGGA telegram (format description is here), calculate mean and standard deviation s, and exclude readings that are more then 1 s from the mean, and calculate new mean. Using a tape measure we can get the height difference from GPS to gravity reference point.
• GPS is very ordinary: GlobalSat BU-353. The idea is to collect data over a long period and calculate the average height, excluding values that are more then 1 standard deviation from the mean, and calculate a new mean.

BASE READING 20 MAY 2003, DOKKEN, BERGEN

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NOTE: This base reading is divided into two parts:

1. Still reading (readings of a stable marine gravity meter, when ship is quayside) done 20 May 2003
2. Tying the point on the quay where the ship lay moored, to the reference station in Realfagbygget. This was done at a later date, 20 October 2004.

Still reading marine gravity meter

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From video display:

  Day:HH:MM    Gravity   Spring tension    Cross coupling    Raw beam     Total correction
               [mGal]    [Counter units]   [Counter units] 	[mV]      [Counter units]
  ------------------------------------------------------------------------------------------------------
  140:20:06    11968.4      11968.5             0.0             4.0         0.1
  140:20:15    11967.5      11967.0             0.0             4.0         0.6
  140:20:34    11967.2      11967.0             0.0            -1.0         0.0

Tying Dokken to Gravity station in Realfagsbygget, Bergen

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• Gravity value Realfagsbygget: 9.81 932 372 ms-2.
• Link to gravity station description (PDF).
• All measurements on 20 October, 2004

Using land gravity meter:

   Location                                     Reading       Time (local)
   -----------------------------------------------------------------------
   Realfagbygget, Bergen                        5392,125      13:22
   -- " --                                      5392,215      13:34
   -- " --                                      5392,180      13:40
   Dokkeskjærskaien, bollard no. 167
  (labelled no. 10 on side that faces the sea) 	5395,941      14:16
   -- " --                                      5395,912      14:20
   -- " --                                      5395,979      14:23
   -- " --                                      5395,950      14:25
   Realfagbygget                                5392,108      14:54
   -- " --                                      5392,205      14:57
   -- " --                                      5392,190      14:59

BASE READING 30 JUNE, 2003, MARINEHOLMEN, BERGEN,

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NOTE: This base reading also is divided into two parts:

1. Still reading (readings of a stable marine gravity meter, when ship is quayside) done 30 June 2003
2. Tying the point on the quay where the ship lay moored, to the reference station in Realfagbygget. This was done at a later date, 3 November 2004.

Still reading marine gravity meter

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From video display:

  Day:HH:MM    Gravity   Spring tension    Cross coupling    Raw beam     Total correction
               [mGal]    [Counter units]   [Counter units] 	[mV]      [Counter units]
  ------------------------------------------------------------------------------------------------------
   181:08:41   11956.6      11956.3              0.0            6.0            0.4
   181:09:05   11956.6      11956.3              0.0            2.0            0.3
   181:09:25   11956.5      11956.3              0.0           -2.0            0.1

Tying Marineholmen to Gravity station in Realfagsbygget, Bergen

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• Gravity value Realfagsbygget: 9.81 932 372 ms-2.
• Link to gravity station description (PDF).
• All measurements on 3 November, 2004

Using land gravity meter:

   Location                                     Reading       Time (local)
   -----------------------------------------------------------------------
   Realfagbygget, Bergen                        5392,360      11:01
   -- " --                                      5392,310      
   -- " --                                      5392,323
   Marineholmen, close to bollard
   shown in fig. 2                              5394,380      11:24
   -- " --                                      5394,380      
   -- " --                                      5394,395
   Realfagbygget                                5392,320      11.55
   -- " --                                      5392,275      
   -- " --                                      5392,338

L&R LAND GRAVITY METER, G-MODEL, S/N: G-936

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Calibration table

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Converting the counter reading to milliGals

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Excerpt from p.1-9 of G-936 Instruction Manual is given below.

  If the counter reading is 2654.32, look at the calibration table for your meter. Remember that each meter has its own unique table.

                          Portion of calibration table
     _____________________________________________________________________________
    /                                                                             \
   /    Counter reading           Interval factor           Cumulative value       \

            2500   ..............     1.00794    .............   2519.42 
    +---->  2600   ..............     1.00799    .............   2620.21  <---+
    |       2700   ..............     1.00805    .............   2721.01      |
    |       2800   ..............     1.00811    .............   2821.82      |
    |                                                                         |
    |    Divide the reading into two parts:                                   |
    |                                                                         |
    +---->  2600.00                               2620.21  <------------------+
            + 54.32                                +54.75  <----+
            -------                               -------       |
            2654.32                               2674.96       |
                                                                |
         Interval factor x reading within interval              |
                                                                |
                               1.00799 x 54.32 = 54.75  <-------+

Here is link to newer version of the Model-G manual where the conversion example is on page 1-9. If readings are within a separate interval in the calibration table, instructions are provided on page 1-10:

• http://www.ifg.tu-clausthal.de/fileadmin/grav/gdmanual.pdf

More information of relevance:

• http://crack.seismo.unr.edu/ftp/pub/louie/class/field/Gravimeter-Manual.pdf