
553Problems and Application Limits of Numerical Simulations
It has been pointed out in References 8,9,40, and 43 that attenuation starts
to decrease at a critical frequency that is inversely proportional to the earth’s
resistivity and the conductor’s height. This phenomenon is caused by nega-
tive conductance and corresponds to the transition between the TEM mode
propagation, called “earth-return wave,” and the transverse magnetic (TM)
mode propagation, called “surface wave,” as discussed by Kikuchi in 1957
[9]. When earth-return admittance is neglected, attenuation increases monot-
onously as the frequency increases. The wave-propagation velocity ...