Page 493 - The ROV Manual - A User Guide for Remotely Operated Vehicles 2nd edition

488 CHAPTER 18 Ancillary Sensors
(a)
FIGURE 18.13
(b)
(a) Handheld coil manually passed over cable and (b) submerged pole-mounted coil.
geomagnetism are not well understood, but the quantitative components are generally classed into three broad categories:
1. Declination (also termed “variation”): The horizontal vector difference between the Earth’s geographic axis (Ng/Sg) and the Earth’s magnetic axis (Nm/Sm).
2. Inclination (or “dip”): As further depicted in Figure 18.15, the Earth’s magnetic field has a vertical component as well. Just as a bar magnet’s lines of magnetic flux curve into the magnet at the poles, the flux lines curve into the Earth at the magnetic North/South poles. The lines are generally horizontal at the equator and vertical at the poles.
3. Intensity: The Earth’s magnetic field intensity varies with its proximity to the magnetic poles with (generally) its highest intensity at the poles and its lowest intensity at the equator.
As shown in Figure 18.15, the pole on a bar magnet typically attracts to its opposite polarity;
therefore, the Earth’s Magnetic North Pole is considered the southern pole of the Earth’s magnetic field (with the North Pole of a compass needle seeking the Earth’s North Pole). The two vector quan- tities of the Earth’s magnetic field are depicted in Figure 18.16 for (i) declination and (ii) inclination.
Magnetometers are classed in two general categories based upon their ability to sense vector quantities:
1. Scalar: Magnetometers able to (only) sense the magnitude of the local magnetic field
2. Vector: Magnetometers that sense magnitude as well as the directional component of the local
magnetic field