Page 27: of Offshore Engineer Magazine (Dec/Jan 2013)

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Geology & Geophysics to more than 1° at latitudes over 80°.” Cable compass delays the period of the event (12° in radians and alpha-beta fltering

We will see later that magnetic events x 9km). with declination changes of more than Declinometer declinations are an

Observatory data 1° are common in the Arctic. We can enormous improvement over gridded also expect these events to fuctuate in INTERMAGNET is a “global network model declinations for this event, but the frequency as we progress through the of observatories, monitoring the Earth’s scale and rapidity of this event exposed 11-year solar cycle (Fig. 1). At the time magnetic feld” (www.intermagnet.org). several defciencies in the system. The this was written, the Earth was progress- The INTERMAGNET observatory compass birds used on the GeoArctic ing into the peak of a solar cycle. nearest to line 6325 is Deadhorse (DED), internally record a magnetic azimuth

In the absence of a positioning tail about 350km away. Point Barrow every 2 seconds, but an average of the buoy due to ice, as explained earlier, observatory (BRW) is farther away. Fig. available readings are passed upstream. special care must be taken with declina- 3 is a map of the Beaufort showing the The average is taken when the compasses tion to adjust the compass-to-compass location of the event (line 6325), DED are polled, but it is the last average (and positioning traverse along the streamer and BRW. not the current average) that is passed by measuring the combined geomagnetic The declination measured at DED on upstream at the time of the polling. On feld. November 1, 2012 is plotted in Figure 4, this project the compasses were polled where the horizontal axis is measured in every 16 to 18 seconds. In effect then,

Declinometer seconds of the day. This also shows the the compass data is one and a half poll-

ION Geophysical has developed a decli- peak of the event at about 55,000 sec into ing intervals old when it is fnally avail- nometer to observe magnetic declina- the UTC day. The WMM modeled grid able for use by the navigation system, tion in real time on a seismic vessel. for DED this day is 21.10°. The EMM or about 25 seconds. Recall that this is a

The declinometer consists of a fuxgate modeled grid for DED this day is 20.47°. rapidly varying event, changing 2°/ min. magnetometer and an inertial measure- The day begins quietly, but most of the sustained for more than an hour, peak to ment unit (IMU) disciplined by dual- rest of the day is dominated by the mag- trough to peak.

antenna GPS. A calibration maneuver netic storm with a difference of almost On the declinometer side, an alpha- is required to measure and compensate 10° in declination at its peak. beta flter is applied to the raw 3Hz

To assess the simultaneity of the peak for the hard and soft iron effects that a events at DED and the declinometer, the steel vessel imparts on a magnetometer.

DED, November 1, 2012 30 data between 50,000sec and 60,000sec

The measurements from the declinom- were cross correlated resulting in a peak eter are then passed to the navigation 28 at about 1 min. of lag. BRW and the decli- system and applied when processing the nometer were also cross correlated result- positioning data. Details can be found in 26 ing in a peak (less well defned) at about patent application US 20120134234 (see 24 6 min. of lag, consistent with the lag references).

between BRW and DED of about 5 min.

22

How common is an event of the size

Observed Magnetic Event 20

While shooting seismic line 6325 in the measured on line 6325 on November 1,

Beaufort Sea on November 1, 2012, the 2012 (henceforth JD306), DED 1-min- 18 02468 seismic vessel GeoArctic experienced a ute declination data are available from 4 x 10 signifcant magnetic event. The decli-

July 26, 2012, onward. To answer the

Seconds of the day nometer recorded hours of disruption question of the frequency of events, the

Fig. 4: Declination (adjusted for baseline) and a change in magnetic declination authors parsed the DED 1 minute records measured at DED on the day of the event of 12° in 6 minutes at the event’s peak from July 26, 2012, to July 13, 2013 analyzed.

Source: ION.

about 55,000 seconds into the day (the last available at the time of writ- (about 15:16 UTC) as illustrated in Fig. ing this paper). Fig. 5 shows the range

DED declaration range: 2. The blue dots in Fig. 2 are the raw in declination for each of the 353 days 7/26/2012 to 7/13/2013 declinometer observations that come at plotted against the sequential day of 25 about 3Hz. These are particularly noisy the period. JD306, the day of the event data during this period. The red line is analyzed in this paper, is plotted in red. 20 an alpha-beta fltering of the raw obser-

Its declination range is 9.49°, a bit less vations that is used to correct the cable than observed by the declinometer on the 15 compasses from magnetic azimuth to vessel (12°).

true azimuth. The green line at about

In this 353-day period at DED there are 10 24° is the declination determined by 19 days with declination ranges equal to the World Magnetic Model (WMM) for or larger than that of JD306, one as much 5 the time and position of the vessel. The as 25°. There are 54 days with declina-

Declination range in degrees Observed declination 0

GeoArctic was towing a 9km-long cable tion ranges larger than half that of JD306. 0 100 200 300 400 at the time of this event. If a mod-

For the GeoArctic in the Beaufort that

Sequential day of the period eled declination instead of the decli- would be four or fve days/month with a nometer data were used to correct the cable “wag” of almost 1km-long if not for

Fig. 5: Range in declination for 353 days compasses, the tail of the cable would real-time declination corrections from the of DED data. JD306 is plotted in red.

Source: ION.

“wag” as much as almost 2km during declinometer.

oedigital.com December 2013 | OE 29 028_1213_G&G- ION.indd 29 11/22/13 4:20 PM