English version of an article in WDC News No. 145

May 31, 2014

Release Note for Long-term Seafloor Geomagnetic Data Observed in the West
Philippine Basin
Hiroaki Toh
Data Analysis Centre for Geomagnetism and Space Magnetism, Graduate School of Science, Kyoto
University, JAPAN
Since June of 2006, long-term geomagnetic observation has been conducted at a site hereafterreferred to as ‘WPB’ in the West Philippine Basin. The seafloor observation is based on thecollaboration among Data Analysis Centre for Geomagnetism and Space Magnetism, Kyoto
University and Department of Deep Earth Structure and Dynamics Research, Japan Agency for
Marine-Earth Science and Technology. Almost continuous record of approximately 6 years has
been obtained so far. The 1.7 years of the raw record up from June, 2009 through February, 2011
has been further processed/corrected to be ready for public release on and after June 1, 2014. The
intent of this short note is to briefly outline the contents of the seafloor geomagnetic data.	  Table 1 summarizes information of the seafloor site and the observed geomagnetic data.
Table 1 Outline of the seafloor site and data
Site Location: 19.324N, 135.110E, Depth=5700m (WGS84)
Site Code: WPB (West Philippine Basin)
Observation Period: June 7, 2009 - February 14, 2011 (UTC)Sampling Interval: 1 minute
Reported Geomagnetic Components: XYZF
Resolution: 0.01nT
Absolute Precision for the Geomagnetic Total Force: 0.2 nT (Toh and Hamano, 1997)
Absolute Precision for the Geomagnetic Three Components: 1 nT for Z and 5 nT for X and Y
Data Format: IAGA2002
Magnetic Sensors Used: Fluxgate for vector measurements, Overhauser for scalar measurements
Figure 1 shows the site location of WPB. The precise position of WPB at the seafloor
with respect to the world-standard geodetic system, WGS84, was determined by connecting theresearch vessel’s GPS position with results of acoustic ranging conducted from the vessel at thetime of installation/recovery of the seafloor instrument (SFEMS; Toh et al., 2004, 2006). The
seafloor position was also confirmed by the acoustic navigation system of an ROV used for thepin-point delivery of the SFEMS to the desired position. Precision of the acoustic site
determination is, therefore, around 10 m. The site code, ‘WPB’, was given to stand for 'West
Philippine Basin'.
1English version of an article in WDC News No. 145

The seafloor site, WPB, is geoscientifically unique in the sense that:(1) It locates around the centre of a local maximum of the geomagnetic secular variation due to thewestward drift of the equatorial dipole (Toh et al., 2009).
(2) It locates on an old seafloor aged approximately 49 Ma (Salisbury et al., 2006).
(3) Stagnant slabs are present beneath WPB (Fukao et al., 2009).
The first feature will be useful for research on the Earth’s core dynamics and structure of theD”-layer, while the second fact may be of use for study of evolution of the oceanic lithosphere.
The last issue enables detection of electrical anomalies in the mantle transition zone using thegeomagnetic data from WPB.
Time precision of the seafloor instrument is approximately 10-7 ppm, which has beenfurther corrected using clock calibration data to yield one-minute values in UTC. The clock
calibration was conducted before installation as well as after recovery by comparing theinstrument’s master clock with the research vessel’s GPS clock.
The least count of the geomagnetic components is as small as 10 pT. However, theabsolute precision of the geomagnetic total force is 0.2 nT (see Toh and Hamano, 1997) while thatof the vertical component is 1 nT. As for the horizontal geomagnetic components, their absolute
precision is as large as 5 nT. The difference in precision of vector field measurements stems fromthat of tilt and orientation measurements as described below.
The geomagnetic vector field was corrected for temporal variations of tilt. The
instrument, ‘SeaFloor ElectroMagnetic Station (SFEMS; Toh et al., 1998; Toh et al., 2006)’,installed at WPB is capable of measuring temperature and two horizontal components of tilt and thegeoelectric field, in addition to the 3-component geomagnetic field. Precision of temperature, tilt
and the geoelectric field is 0.01 degrees, 3 arc seconds and approximately 60 nV/m, respectively.
Temperature correction was not conducted, since both the temperature coefficient of the tilt meterused and the actual temperature variations (±0.05 deg within a year) were small. It is well-known
that correction for attitude variation of vector magnetometers is essential not only to improve theabsolute precision of the observed geomagnetic field but also to detect the geomagnetic secularvariation. Because the tilt meter equipped with SFEMS is as precise as 3 arc seconds (Toh et al.,2006), the absolute precision of the corrected vertical geomagnetic component is as good as 1 nT or
better in mid-latitudes where WPB is located. However, that of the horizontal geomagnetic
components is still as large as 5 nT due to larger errors in the instrument’s orientation at the seafloor.
A small fibre optical gyro (FOG), originally developed for automated tunnel drillers, was equippedwith SFEMS, whose standard error of mean was as large as 10 arc seconds. Furthermore, it turns
out that the FOG generates magnetic noise and is power-consuming when activated, which prevents
us from monitoring orientation variation continuously. We, therefore, ended up with intermittent
orientation measurements at the seafloor once in three months. The final orientation error comesfrom discrepancy between the FOG’s reference frame and the vector magnetometer’s frame. Thissums up to a total orientation precision of slightly better than 30 arc seconds. The large orientation
2English version of an article in WDC News No. 145

error and lack in continuous orientation monitoring are the major factors that weaken the meaningof the observed horizontal components. Detailed discussion on the seafloor instrument, thecorrection method, the error estimation briefly outlined above is given in Toh et al. (1997, 1998 and2006).
Figure 2 shows temporal variation of the vertical geomagnetic component observed atWPB for as long as 600 days. It is evident from the figure that the vertical geomagnetic
component at WPB is presently increasing, which is in good agreement with prediction by global
models such as IGRF. Further analysis has revealed that the observed geomagnetic secularvariation for the vertical component can be explained by the westward drift of the equatorial dipole
terms (g11 and h11) alone. It can be interpreted by a combination of the lack of contribution fromthe non-dipole geomagnetic field and the presence of a regional maximum of the geomagnetic
secular variation due to the equatorial dipole around WPB.
The contents of the seafloor geomagnetic data have been outlined as above. It is finally
noted that the gemagnetic data will also be open to public shortly via Japan Agency for
Marine-Earth Science and Technology (http://www.jamstec.go.jp/j/database/).

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English version of an article in WDC News No. 145

Fig. 1 (Top) Location of the seafloor site, WPB. It is on the West Philippine Basin as deep as
5,700 m and as old as approximately 49 Ma. (Bottom) WPB among the existinggeomagnetic observatories on the globe. NWP is another seafloor geomagnetic observationsite now being operated in the northwest Pacific.

Fig. 2 Observed secular variation revealed in the vertical component of the geomagnetic field at
WPB for as long as 600 days. It is evident that the geomagnetic vertical component ispresently increasing in the northwest Pacific, and most of the increase can be explained by
the westward drift of the equatorial dipole.
References
Fukao, Y., Obayashi, M., and Nakakuki, T., Stagnant slab: a review. Annual Review of Earth andPlanetary Sciences, 37, 19-46, 2009.
Salisbury, M.H., Shinohara, M., Suetsugu, D., Arisaka, M., Diekmann, B., Januszczak, N., andSavov, I.P., LEG 195 synthesis: site 1201—A geological and geophysical section in the West
Philippine Basin from the 660-km discontinuity to the mudline. In Shinohara, M. et al. (Eds.),
Proc. ODP, Sci. Results, 195, 1-27, 2006.
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English version of an article in WDC News No. 145

Toh, H. and Hamano, Y., The first realtime measurement of seafloor geomagnetic total force Ocean Hemisphere Project Network, J. Japan Soc. Mar. Surv. Tech., 9, 1-23, 1997.
Toh, H., Goto, T. and Hamano, Y., A new seafloor electromagnetic station with an Overhauser
magnetometer, a magnetotelluric variograph and an acoustic telemetry modem, Earth Planets
Space, 50, 895-903, 1998.
Toh, H., Hamano, Y., Ichiki, M. and Utada, H., Geomagnetic observatory operates at the seafloor in
the Northwest Pacific Ocean, Eos, Trans. Amer. Geophys. Union, 85, 467/473, 2004.
Toh, H., Hamano, Y. and Ichiki, M., Long-term seafloor geomagnetic station in the Northwest
Pacific: A possible candidate for a seafloor geomagnetic observatory, Earth Planets Space, 58,697-705, 2006.
Toh, H., Hamano, Y., Goto, T., and Utada, H., Long-term seafloor electromagnetic observation in
the northwest Pacific may detect the vector geomagnetic secular variation. Data Science
Journal, 9, IGY100-IGY109, 2010.

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