handbook of modern sensors

292 7 Position, Displacement, and Level
Fig. 7.44. (A) Attenuation of radio
waves in different materials. Attenuation
varies with excitation frequency
and type of material. At low frequencies
(< 1 MHz), attenuation is primarily
controlled by dc conductivity. At high
frequencies (> 1000 MHz), water is a
strong energy absorber. (B) When attenuation
limits exploration depth, power
must increase exponentially with depth.
(A)
(B)
through soils, rocks, and most man-made materials such as concrete. The exponential
attenuation coefficient, α, is primarily determined by the electrical conductivity
of the material. In simple uniform materials, this is usually the dominant factor. In
most materials, energy is also lost to scattering from material variability and to water
contents. Water has two effects: First, water contains ions which contribute to bulk
conductivity, and second, the water molecule absorbs electromagnetic energy at high
frequencies, typically above 1000 MHz. Figure 7.44 shows that attenuation varies
with excitation frequency and material. Thus, practical maximum distance increases
for the dry materials (Fig. 7.45A). An example of data presented on the radar monitor
is shown in Fig. 7.45B.
Lowering the frequency improves the depth of exploration because attenuation
primarily increases with frequency. As the frequency decreases, however, two other
fundamental aspects of the GPR measurement come into play. First, reducing the
frequency results in a loss of resolution. Second, if the frequency is too low, electromagnetic
fields no longer travel as waves but diffuse, which is the realm of inductive
electromagnetic (EM) or eddy-current measurements.