Cp

a io0

10-2

10-4

0 102 104 106 108 1010 1012 Frequency, Hz

FIGURE 6.10 Skin depth plotted as a function of frequency. One skin depth is the depth at which the signal is attenuated by the amount 1/e. The area in gray is the electromagnetic induction region, where attenuation is primarily influenced by the conductivity (o) and is minimally affected by the permittivity (e).

0 102 104 106 108 1010 1012 Frequency, Hz

FIGURE 6.10 Skin depth plotted as a function of frequency. One skin depth is the depth at which the signal is attenuated by the amount 1/e. The area in gray is the electromagnetic induction region, where attenuation is primarily influenced by the conductivity (o) and is minimally affected by the permittivity (e).

under pavement

FIGURE 6.11 Field features that can interfere with electromagnetic measurements.

under pavement

FIGURE 6.11 Field features that can interfere with electromagnetic measurements.

Furthermore, structures containing metal are always more problematic than nonmetallic objects; a building constructed from some types of stone may not create any noticeable interference, but a building constructed with a lot of steel-reinforcing bar or from rocks containing a lot of magnetite may cause serious interference for any EM system. A practical guideline is to avoid making measurements any closer to an object than five times the height of the object. A simple field test consisting of running a profile line perpendicular to the potentially interfering feature can determine the limit of influence of overhead structures.

0 0

Post a comment