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where a is the attenuation constant in the wave equation (Balanis, 1989), o is the subsurface electrical conductivity, ^ is the subsurface magnetic permeability, ra is angular frequency, and e is the subsurface electrical permittivity. A plot of the skin depth versus frequency is provided in Figure 6.10.

6.4.4 Interference Effects

The physical surroundings of a survey and the proximity of interfering features should be a consideration for any survey. Some of these features are shown in Figure 6.11. Overhead or underground power lines can give off a signal that interferes with commercial systems, although most systems have 60 Hz filters that eliminate most of the power line related noise. There are no foolproof rules of thumb for eliminating these noise sources. However, they should be avoided whenever possible.

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