Page 496 - The ROV Manual - A User Guide for Remotely Operated Vehicles 2nd edition

FIGURE 18.17
Earth’s magnetic field intensity (in both nT and γ).
18.2 Metal object detection 491
Table 18.2 Common Magnetic Anomalies
Object Near Distance
Far Distance
1 Ton automobile 1000 Ton ship Light aircraft
10 inch file
5 inch screwdriver
12 inch pipeline
Well casing and wellhead
30 ft/40 nT
100 ft/300700 nT 20 ft/1030 nT
5 ft/50100 nT
5 ft/510 nT
25 ft/50200 nT 50 ft/200500 nT
100 ft/1 nT
1000 ft/0.30.7 nT 50 ft/0.52.0 nT 10 ft/510 nT
10 ft/0.51.0 nT 50 ft/1250 nT 500 ft/25 nT
at a known distance to derive the magnetic field “gradient” (expressed in nT/m) between the sensors, thus allowing the vector solution (Figure 18.19). The higher the gradient, the larger the anomaly for any given offset with homogeneous magnetic materials.
The Proton Precision Magnetometer (PPM) operates on the principle of the protons in certain fluids naturally aligning along the Earth’s lines of magnetic flux. Within the sensor of a PPM, an electrically conducting coil surrounds the reservoir of some hydrogen-rich fluid (typically mineral spirits, diesel, or some other hydrocarbon). When the coil is nonenergized, the protons assume their natural state aligned along the lines of ambient magnetic flux. But when the coil is energized, the protons immediately snap to the local lines of flux with reference to the coil. Once the energy is
60,000
60,000
50,000
40,000
65,000
50,000
60,000
30,000
50,000
40,000