LOW-FREQUENCY ACOUSTIC GROUND IMPEDANCE MEASUREMENT TECHNIQUES

The low frequency (below 150 Hz) acoustic ground impedance was determi ned on a large uncultivated field using an impedance meter, the probe microphone method, a modified Biot-Stoll type calculation, and the pha se gradient method The impedance measured with the impedance meter has lower real and imaginary parts than the impedance determined from the probe microphone measurements. The modified Biot-Stoll type descripti on of acoustic to seismic transfer based upon seismic and acoustic mea surements gives an impedance which is consistent with probe microphone measurements. The impedance measured by the phase gradient method is largely consistent with the other measurements, but shows much more st ructure. This structure is believed to be caused by unwanted reflectio ns during the measurement procedure rather than by actual structure in the ground impedance undetected by the other methods.