Radio waves decrease exponentially and soon become undetectable in energy absorbing materials, depicted in Figure 1. GPR signals decay exponentially in soil and rock. The exponential attenuation coefficient, a, is primarily determined by the ability of the material to conduct electrical currents. In most materials energy is also lost to scattering from material variability and to water being present. Attenuation increases with frequency as depicted in Figure 2.
Attenuation varies with excitation frequency and material.
This family of graphs depicts general trends. Lowering frequency improves depth of exploration because attenuation primarily increases with frequency. As frequency decreases, however, two other fundamental aspects of the GPR measurement come into play. First, reducing frequency results in a loss of resolution. Second, if frequency is too low, electromagnetic fields no longer travel as waves but diffuse which is the realm of inductive EM or eddy current measurements.