Page 495 - The ROV Manual - A User Guide for Remotely Operated Vehicles 2nd edition

490 CHAPTER 18 Ancillary Sensors
FIGURE 18.16
(a)
Horizontal component
Angle of inclination
Earth
(b)
(a) Earth’s magnetic field declination and (b) vector components.
Magnetic field intensity is measured in quantitative units based upon the flux intensity. The SI unit for magnetic flux density is the Tesla (symbol T) with the commonly accepted unit for geo- physical application as the nanotesla (symbol nT). An older convention for geomagnetic intensity is the gamma (symbol γ) with 1 γ 5 1 nT. The reader will notice in Figure 18.17 that the magnetic field of the Earth is very nonhomogeneous. The Earth’s magnetic field varies in intensity from approximately 25,000 nT at the magnetic equator to approximately 70,000 nT at the magnetic poles with the lines of equal intensity (termed “isogonics”) running generally parallel to the equator. Therefore, the general approach to anomaly detection in the Earth’s magnetic field is to run the magnetometer parallel to the Earth’s isogonics and note any perturbations. There you will find a magnetic field attached to some magnetized item. The intensity of the anomalous field surrounding a man-made object within the Earth’s magnetic field will follow a simple rule of thumb as “1 ton of iron is 1 nT at 100 ft (33 m).” Some typical maximum magnetic anomalies for common items are shown in Table 18.2.
In Figure 18.18, a dipole magnet is inducing an anomaly in the Earth’s magnetic field. The search magnetometer is run parallel to the magnetic field lines to identify the anomaly as it is approached and then crossed in a dipole manner.
All magnets are dipoles having both north and south poles. A magnet oriented to the horizon will produce a full dipole reading in the Earth’s magnetic field. But if the magnet is oriented verti- cally, a reading in one direction (only) will result (i.e., an anomalous rise or fall (depending upon the pole’s orientation as “up” or “down”) in the total field intensity).
With a simple magnetometer, only field intensity can be measured without some means of deriving the vector portion of the field. A gradiometer, on the other hand, is an array of magnetometers offset
Total field intensity
Vertical component