Ground Impedance Measurement Over a Range of 20 m

In the course of aircraft signal measurements on the military airport at Jever, Germany, the surface impedance of the grass-covered ground was determined from c. 60 Hz to 2.2 kHz. Complex transfer functions were measured over ranges of up to 20 m using (quasi-)white noise and a numerical inversion of the spherical wave reflection factor. The relatively long range enables to determine an effective surface impedance, which is an average over a patch of the ground within the measurement set-up. The resulting normalised impedance shows a real part, i.e. the resistance, decreasing from around 6 to around 2 at frequencies from around 100 Hz to around 1.2 kHz and increasing again above c. 1.2 kHz. Its imaginary part, i.e. the reactance, decreases from around 8 to around 2 in the same frequency range and increases again above c. 1.2 kHz. The increase beyond c. 1.2 kHz may be due to the influence of turbulence.

A comparatively simple two-parameter model has been fitted to the measured impedance. It results in an effective flow resistivity σ _e = 100 kPas/m² and an effective rate of porosity decrease with depth of α _e = 9 m⁻¹.