Space-time characteristics of broadband seismic noise: Results of the onshore component of SAMSON.

Abstract

In Oct. and Nov., 1990, data from an array of 37 three-component seismometers (16 broadband, 6 5 s, 15 1 s) deployed on the coastal plain of northeastern North Carolina and southeastern Virginia were collected as a part of the multidisciplinary SAMSON (Sources of Ambient MicroSeismic Oceanic Noise) experiment. The array extended from a few hundred meters to 170 km from the coastline. Large variations in the broadband noise level are observed as a function of time with the power during the noisiest time periods being more than 100 times higher than during the quiet times. Many of these noise variations are directly related to regional environmental changes (e.g., hurricane Lily and a ‘‘nor-easter’’) and changes in ocean wave heights observed near Duck, NC); however, some seismic noise variation is uncorrelated with local environmental variables, indicating much more distant sources. The correlation with the ocean wave heights is better at higher frequencies (0.2–0.4 Hz) than at lower. Looking more closely, the ‘‘double-frequency’’ (0.15–1 Hz) and ‘‘single frequency’’ (0.05–0.15 Hz) microseisms show interesting patterns. At certain times, the variations of the power in the two spectral bands are independent of each other, but at other times the power increases occur across the entire 0.05–1 Hz frequency band. When broadband increases in the noise power are observed, the higher frequency noise increase always precedes the lower frequency noise increase, and the noise peak in 0.09–0.2 Hz band typically lags the peak in the 0.4–1.0 Hz band by about 7 h. In several instances, ocean wave heights display similar reddening of the spectra with time, indicating an offshore origin for the low-frequency microseismic noise and reflecting the longer time required by the regional offshore storm systems to generate low-frequency waves. The high-frequency (0.2–1 Hz) band is dominated by a number of peaks lasting about 12 h, primarily during daylight hours, and is also correlated with the local sea state. The character of these peaks indicates a regional offshore source in response to increases in wind-speed during daylight hours. Preliminary frequency–wave-number analysis indicates clear directionality for noise associated with the noise peaks that are not correlated with the local sea state. This indicates a dominant source at a large distance from which seismic waves travel essentially as plane waves across the array. The seismic noise field, however, becomes highly heterogeneous when the noise increase is correlated with the local sea state and is presumably of regional origin. In that case, the energy arrives from many azimuths along the coast line.