The nature of high-frequency Earth noise spectra

The spectra of earth noise in the frequency range 0.2 to 100 cps are presented for a number of widespread locations within the United States and for other selected North American and foreign sites. The data are analyzed as a function of physiographic environment. Some of the spectral anomalies are correlated with human activity and certain natural sources. The analysis suggests that a substantial portion of high-frequency earth noise originates at the earth's surface and follows a propagation path at, or near, the surface.