Estimation of surface noise source level from low-frequency seismoacoustic ambient noise measurements

Abstract

The extraction of noise source levels from ambient noise measurements requires accounting for the seismoacoustic propagation from the surface generated noise sources to the field measurement positions. At frequencies below 100 Hz, the waveguide nature of the environment strongly influences the distribution of ambient noise. Only when these propagation effects are considered can measurements from different experiments taken in different environments be combined to ascertain the global properties of surface noise sources. Here, such a previous analysis of a shallow water experiment [Schmidt et al., in Natural Mechanisms of Surface Generated Noise, edited by B. Kerman (Reidel, Dordrecht, The Netherlands, 1988)] is used and the results are combined with Kibble-white and Ewans' lower-frequency results [J. Acoust. Soc. Am. 78, 981– 994 (1985)], the latter also containing a summary of previous experimental results obtained by others. These data are treated in the same way by accounting for the environments using a wave theory model of distributed noise [W. A. Kuperman and F. Ingenito, J. Acoust. Soc. Am. 67, 1988– 1996 (1980)]. When the particular environments are used, the spread in the reprocessed noise levels is substantially reduced, showing more consistency in the frequency dependence of the noise sources, suggesting that there are only a few (or possibly only one) dominant natural noise source mechanisms that primarily contribute to ocean ambient noise below 100 Hz. The importance of appropriately including propagation factors in processing noise data for estimating source levels is therefore strongly demonstrated.

Synthetic array processing of ocean ambient noise for higher resolution seabed bottom loss estimation

“…(3), OASN is used because none of the simplifications made in the preceding derivations are assumed, and this methodology has been used in previous ocean noise and propagation studies. 8 For these simulations, the acoustic frequency is 3500 Hz and the water depth is 200 m. The sound speed in the water column is 1500 m/s from the surface to 50 m depth and then is linearly downward refracting to 1490 m/s at the seabed. The seabed has a 0.75 m layer over a half-space.…”

Beaufort Ambient Seismo-Acoustics Beneath Ice Cover (BASIC)

“…At the Arctic seaflocr site scattering processes are probably insignificant because of the great depth of the sediments. In shallow (shelf depths) water the Stoneley modes are directly excited by the surface sources and dominate the microseism spectrum (Schmidt and Kuperman, 1988).…”

Very low frequency seismo-acoustic noise below the sea floor (0.2-10 Hz)

“…Both of these studies involve a microseism data set collected on land and cannot fully account for the propagation of seismic interface waves and modes within the water column and crust. A study of the propagation effects within a shallow water waveguide over a viscoelastic medium is presented in Schmidt and Kuperman (1988). They emphasize the importance of considering the propagation mechanisms when estimating the noise field source strength.…”