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MAGNETOMETER

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CESIUM MAGNETOMETER GEOMETRICS G-882

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• Manufacturer: Geometrics Inc., USA
• Model: G-882
• Specifications: PDF
• Manual: G-882 Manual Rev H -- 2020 - Check for latest edition
• Warranty information:
  • Serial Number: 882150
  • Shipment no.: 020475, dated 01/18/2007

Specifications, excerpt from page 6 of the G-882 manual rev. D. (local copy - not latest manual version).

. Geometrics mod. G-882 Magnetometer specifications (from page 6 of the | Rev D manual).

G-882 sensor orientation

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Ref. G-882 User Manual, p. 31, section 4.3.1 "Sensor Positioning in Relation to the Dip Angle of the Survey Area".

Geometrics recommend that the sensor be oriented such that Earth’s magnetic field lines (H field) are centered in its active zone.

Quoting from p. 29 of the manual:

Picture below shows sensor orientation suitable for operation in the 22.5° - 67.5° region.

For the 67.5°-90° field inclination angle regions, roll the sensor to an angle of 45° with respect to the Earth’s surface.

Maps on p. 34 in the manual shows magnetic field inclination and intensity for the survey area.

G-882 wiring diagram

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This information is provided in case the mag fish tow cable should be used for other purposes.

The tow cable wiring diagram is given on p. 107 of the G-882 User Manual:

P2

This connector is either on the "wet side" of the deck cable, or on the winch slipring assembly. It's an 8-pin female subsea connector, Geometrics P/N: TBD

JP2

8-pin male subsea connector, on dry end of tow cable. Geometrics P/N: 21-236-411.

JP1

8-pin female subsea connector, on wet end of tow cable. Geometrics P/N: 21-236-410.
Picture (from p. 80 in User Manual) shows this connector.

G-882 tow cable

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The wet side of the cable consist of a cable clevis assembly attached to towing point on mag fish, and a connector.

Details about clevis assembly and connector will be provided later.

Tow cable specifications:

• Supplier: Geometrics
• P/N 50030-50: Tow cable w/floatation, specify markings in 5m intervals from wet end
• P/N 60-453-103: Floatation cable wire, included in 50030-50
• Length: 250 meter (821 ft)
• Breaking strength: 1814 kg (4000 lbs.)
• Maximum working load: 363 kg (800 lbs.)
• Minimum bend radius: 23 cm (9 in.)
• Outer diameter (incl. flotation material): 18.2 mm
• No. of conductors: 6
  • Power wires: 2 each AWG 14 conductors
  • Twisted pairs: 2 each AWG 20 insulated conductors twisted together to form a pair
• Connector, wet end: 8-conductor female subsea connector, Geometrics P/N: 21-236-410
  • Mating connector: Geometrics P/N: 21-236-411
• Connector, dry end: 8-conductor male subsea connector, Geometrics P/N: 21-236-411
  • Mating connector: Geometrics P/N: 21-236-410

Geometrics recommends a winch if the length of the tow cable exceeds 120 m (400 ft).

Use "Kellems grip" (aka "Chineese fingers" to attach the tow cable a winch.

G-882 spare parts

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Stock as of 18 Sept 2008:

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MFR P/N     QTY  UNIT   DESCRIPTION
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24780-01     1   ea     Power/data junction box (JB) for G-882
24810-02     2   ea     AC/DC Power Unit, 95 - 230 Vac, 50/60 Hz for 24780-01 G-882 JB
                        30V 1.9A output
21-201-019   2   ea     SP06A-14-5P (SR)
                        Connector that mates "Onboard cable" JB input connector               
21-201-020   2   set    SP06A-14-5S (SR)
                        Connector that mates "Power" JB input connector
21-236-410   2   ea     MCIL8F 8 PIN FEM PIGTAIL SUBC.
                        Connector that mates connector on ship's side of tow cable
28-602-002  10   ea     Cotter pin, 1/16 x 3/4" 316 98355A020
                        Small, for top cover linch pins
28-602-001  10   ea     Cotter pin, 1/8 x 1 3/4" 316 SS
                        Large, for nose tow pin
21-236-420   5   ea     Locking sleeve MCDLSF, for connector on ship's side of tow cable
21-236-421   5   ea     Locking sleeve MCDLSM, for connector on ship's side of tow cable
21-236-412   5   ea     Dummy conn 8 pin female
21-236-413   5   ea     Dummy conn 8 pin male

Shipping crate:
25969-01                G-882 SHPNG CRT W/CRDL PNTD WHITE

Kits:
24825-04                G-882 SHIP KIT

Deck cable:
60-453-101              CABLE, FM071003-2CP 6-WIRE GREEN

G-882 S/N: 882150 repair log

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Repairs can either be done at Geometrics, or their U.K. representative Geomatrix.

Here is a (Word format) Packing/Proforma Invoice template that could be useful.

2008, June

In 2007 survey operation the sensor values were suddenly reduced to noice. Fault finding at Geomatrix U.K. confirmed that the Cesium light source was defective. It was replaced, and the unit then worked as expected.

RV "H. MOSBY": DISTANCE FROM SENSOR TO NAV. REFERENCE POINT

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• Total magnetometer tow cable length is 250m
• Drum size: Dia. inner = 508 mm, drum width 600mm.
• Tow cable outer diameter (incl. flotation material): 18.2mm
• We have one cable layer left on drum during deployment. This means 600mm / 18.2mm = 33 turns, each having a length of 2*PI*(254mm + 9.1mm) => 2*PI*0,2631m = 1.653m, yielding a total length of 55m. So estimated length of deployed cable is (250-55) = 195m.
• Distance from magnetometer winch to navigation system reference point - which is the position of the gravitymeter: Gravitymeter is placed at frame 40. Frame distance is 0.6m. Thus, gravitymeter is 40 x 0.6m = 24m from the stern.

On RV "Håkon Mosby", the distance from magnetometer towfisk to navigation system reference point is thus (195m + 24m) = 219 meter.

MAGNETOMETER WINCH

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Magnetometer winch specifications

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We have two identical magnetometer winches. The oldest was delivered in 1996, and the newest in 2009. Each winch is furnished with slip-ring box and hydraulic motor.

Specifications, 2009 mag winch:

• Manufacturer: Karmøy Winch AS
  • Ref. Karmøy Winch drawing no. 262039. Winch has name plate with the same number on (262039).
• Drum size: Dia. inner/outer: 508 / 1100 mm x drum width 600 mm.
• Drum capacity: 3000 meter w/ Ø=31mm dia. cable.
• Hydraulic pressure: 160 bar.
• Hydraulic oil flowrate: 10 liter/minute (yields 30 meter/minute).
• Force: 300 kg on inner layer.
• Brake: Yes (Norwegian: "Pal bremse påmontert vinsjen".)
• Winch adapted for slipring ("Vinsjen tilpasses for slepering i lager-ende.")
• Winch delivered sandblasted, primed and painted ("Sandblåste, metalliserte og malte.")
• Weight (approx.): 520 kg

Winch foundation, measured on RV "G.O. SARS"

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Click to open drawing (PDF format) made of "as-built" magwinch foundation on RV "G.O. SARS":

Mag winch foundation on RV "G.O. SARS", facing aft - click to enlarge:

Winch foundation, manufacturer's drawing

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Winch pictures, 22 March 2010

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Click to see large images.

Mag winch slipring box

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The Geometrics G-882 has a 5-conductor tow cable with layout as shown below (excerpt from page 108 of G-882 Manual).

Power lines have max 32 Vdc, 0.75 Amp at start-up and 0.5 Amp thereafter. RX/TX are low voltage/current signal lines.

Geometrics G-882 Magnetometer tow cable layout (excerpt from page 108 of G-882 Manual)

Click images to see larger versions

GEOMETRICS G-801 (phased out)

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• Manufacturer: GeoMetrics, U.S.A.
• Model: G 801
• Type: Proton precession
• Sensitivity and Repetition Rate (unit conversion: 1 gamma = 10^-5 Oersted = 1 nanotesla)
  • Marine
    • 0.25 gamma: 6.0 seconds
    • 0.5 gamma: 3.0 seconds
    • 1.0 gamma: 3.0 seconds
  • Airborne
    • 0.25 gamma: 1.0 seconds
    • 0.5 gamma: 0.8 seconds
    • 1.0 gamma: 0.5 seconds
• Range, overall: 20.000 to 100.000 gammas

Rear view 2 (click to enlage)

MISC.

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Links

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• Wikipedia:
  • Magnetometer
  • Earth's Magnetic Field
• Tromsø Geophysical Observatory
• Swedish Institute of Space Physics
• Geomagnetic Observatories

Contact information

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• Prof. R. Mjelde, Dept. of Earth Science University of Bergen, Norway, tel. +47 5558 3600