Statistics and vertical directionality of low-frequency ambient noise at the North Pacific Acoustics Laboratory site

Abstract

We examine statistical and directional properties of the ambient noise in the 10-100 Hz frequency band from the NPAL array. Marginal probability densities are estimated as well as mean square levels, skewness and kurtoses in third octave bands. The kurotoses are markedly different from Gaussian except when only distant shipping is present. Extremal levels reached approximately 150 dB re 1 micro Pa, suggesting levels 60dB greater than the mean ambient were common in the NPAL data sets. Generally, these were passing ships. We select four examples: i) quiescent noise, ii) nearby shipping, iii) whale vocalizations and iv) a micro earthquake for the vertical directional properties of the NPAL noise since they are representative of the phenomena encountered. We find there is modest broadband coherence for most of these cases in their occupancy band across the NPAL aperture. Narrowband coherence analysis from VLA to VLA was not successful due to ambiguities. Examples of localizing sources based upon this coherence are included. kw diagrams allow us to use data above the vertical aliasing frequency. Ducted propagation for both the quiescent and micro earthquake (T phase) are identified and the arrival angles of nearby shipping and whale vocalizations. MFP localizations were modestly successful for nearby sources, but long range ones could not be identified, most likely because of signal mismatch in the MFP replica.

Ocean tomography with acoustic daylight

“…[5] The Billboard Array site was close to shipping lanes, the known migration routes of marine mammals, and epicenters of micro-earthquakes associated with convergence of the North American and Pacific Ocean plates [Worcester and Spindel, 2005;Baggeroer et al, 2005]. In individual data segments, localized, transient noise sources either of biological origin or due to nearby shipping and microearthquakes usually dominate over contributions of distributed noise sources, such as distant shipping and breaking water waves on the ocean surface.…”

Emergence of the Green's Functions from Noise and Passive Acoustic Remote Sensing of Ocean Dynamics

“…In individual data segments, localized, transient noise sources either of biological origin or due to nearby shipping and micro-earthquakes usually dominate over contributions of distributed noise sources, such as distant shipping and breaking water waves on the ocean surface. Statistics of the noise field at the experimental site exhibit strong deviations from Gaussianity when the field is dominated by localized noise sources (Baggeroer et al, 2005). Therefore, the contributions of localized noise sources can be suppressed, and diffusivity of the noise field can be effectively enhanced by excluding data segments with non-Gaussian noise from time averages.…”

“…To investigate the feasibility of passive acoustic tomography of the ocean, we re-examined noise data obtained by the NPAL Group during the 1998-1999 Billboard Array Experiment (Worcester and Spindel, 2005;Baggeroer et al, 2005). The data have been made available for this study by Dr. P. F. Worcester.…”

“…Theoretical predictions have been verified using low-frequency noise records from the North Pacific Acoustic Laboratory (NPAL) Billboard Array (Worcester and Spindel, 2005;Baggeroer et al, 2005 C. Spindel, and P. F. Worcester). Our theoretical approach is based on the flow reversal theorem (Godin, 1997), which is an extension of the reciprocity principle to moving media.…”

Estimating the horizontal and vertical direction-of-arrival of water-borne seismic signals in the northern Philippine Sea

“…T-phases (T being an abbreviation for tertiary, following P, primary, and S, secondary, phases) are waterborne seismic waves created by P and S wave components propagating through and refracting, diffracting, and scattering from the ocean bottom into the water column. First described by Tolstoy et al (1949), T-phases are frequently recorded by passive acoustic sensor systems in the deep ocean (e.g., Baggeroer et al, 2005). A definitive review of T-phase research is found in Okal (2008).…”