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Geometrics G-882 Marine Magnetometer

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

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

G-882 sensor orientation

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:

The sensor head should be oriented so that the earth's field vector arrives at an angle of from 15° to 75° to the optical axis of the sensor.
The earth’s field vector is vertical at the poles, between 50º and 60º in the mid latitudes and horizontal at the magnetic equator.
(See CSAZ program on Magnetometer CD). Adjusting the sensor for the polar and mid-latitude regions is simple, by orienting the sensor
either at 45º (rotating the main tube 1/8th turn for polar regions) or 0º (no rotation required at mid-latitudes) respectively.

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

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:

G-882 Tow cable wiring diagram
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
8-pin male subsea connector, on dry end of tow cable. Geometrics P/N: 21-236-411.
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

G-882 Tow cable assembly. Source: G-882Man_RevC.pdf, page 4

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

Stock as of 18 Sept 2008:

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

24825-04                G-882 SHIP KIT

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

G-882 S/N: 882150 repair log

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.


  • 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.

Location of gravitymeter
RV "Håkon Mosby": Gravitymeter - which is the navigation system reference point - is placed at frame 40. Frame distance is 60cm.


Click to see larger version.
Magnetometer winch onboard RV "G.O. SARS" (click to enlarge).


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.


  • 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.")

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

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

Click to open PDF document
Magwinch foundation, as measured on RV "G.O. SARS". Click to open PDF document.

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

Mag winch foundation, on RV "G.O SARS" (click to enlarge).

Winch foundation, manufacturer's drawing

Detail from Karmøy Winch drawing no.: M361804.DWG

Winch pictures, 22 March 2010

Click to see large images.

Click to see large image.
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Mag winch slipring box

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

Click images to see larger versions

Click to see large image.
Click to see large image.
Click to see large image.
Click to see large image.

GEOMETRICS G-801 (phased out)

Click to see larger image
  • 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 1 (click to enlage)

Rear view 2 (click to enlage)



Contact information

  • Prof. R. Mjelde, Dept. of Earth Science University of Bergen, Norway, tel. +47 5558 3600
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Page last modified on July 26, 2019, at 11:30 AM
Electronics workshop
Department of Earth Science - University of Bergen