Ground Impedance Models: A Review

A semi-empirical impedance model based on a single parameter (effective flow resistivity) for the characteristic impedance of fibrous materials has proved remarkably successful for predicting outdoor ground effects. This model allows for the effects associated with the porosity of the ground. Surface roughness with scales much smaller than the incident wavelengths influences reflection from hard ground in a manner similar to an effective impedance and changes the impedance of soft ground compared with a smooth surface of the same material. Models for the impedance of outdoor ground surfaces, based on more rigorous theory for rigid-porous materials with rough surfaces, are reviewed. Examples of data are given for which the simple single parameter model provides erroneous predictions so that more sophisticated multi-parameter models are required. A three-parameter model is being used increasingly for incorporation into time-domain codes. The extent to which this is an adequate model for ground surface impedance is discussed.