The origins and nature of microseisms in the frequency range 4 to 100 c/s

Abstract

Microseisms in the frequency band 4 to 100 c/s, commonly encountered in seismic prospecting, have been investigated in various parts of England, urban and rural. A diurnal variation in background level, indicating man-made sources, was found everywhere within 2 miles of a major road or community. The main sources were found to be traffic, heavy machinery, aircraft, wind and rain, with traffic prevailing. In rural areas at night-time a steady background of about 10 -6 cm/s r.m.s. particle velocity was found, with occasional superimposed activity lasting 4 to 30 s. The latter has been shown mostly to be local and of geophysical origin, the precise nature of which is unknown. Various possible causes are discussed, both of this and of the minimum background.

Overview of Seismic Noise and it's Relevance to Personnel Detection

“…Wind noise was evident at frequencies of 10-70 Hz (maximum frequency analyzed). High-frequency wind noise attenuated with depth more rapidly than lower-frequency wind noise: 10-Hz wind noise had decreased by less than 1 dB at 2 m, while 70-Hz wind noise was 9 dB down at 1 m and ~18 dB down at 2 m, all referenced to the level of wind noise at 0.5 m. Wilson (1953) identified the wind-related component of seismic noise at a Cambridge, England, site by comparing ground motion data from a geophone near a tree to that from a geophone in a nearby field. Wind-induced tree movement caused ground motion of 3 × 10 -5 cm/s in the frequency range 4-100 Hz.…”

“…Although prior cultural noise measurements are suspect today unless the type and intensity of cultural activity at a site have not changed significantly, two examples of diurnal variation in cultural noise are presented. Wilson (1953), from measurements of seismic noise levels near Cambridge, England, reported that the daytime level was 10-20 times the nighttime level of 1.5 × 10 -6 cm/s rms for the frequency range 4-100 Hz. Kanai and Tanaka (1961) measured seismic noise amplitudes in Tokyo over 24 hours; the daytime maximum was 0.4-0.5 µm, while at night the maximum was 0.1-0.2 µm and the nighttime noise spectra shifted to lower frequencies.…”