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

• Published in: The Journal of the Acoustical Society of America 2013-10, Vol.134 (4), p.3282-3298
• Main Authors: Freeman, Simon E, D'Spain, Gerald L, Lynch, Stephen D, Stephen, Ralph A, Heaney, Kevin D, Murray, James J, Baggeroer, Arthur B, Worcester, Peter F, Dzieciuch, Matthew A, Mercer, James A
• Format: Article
• Language: English
• Subjects: Acoustics - instrumentation; Equipment Design; Fourier Analysis; Models, Theoretical; Motion; Oceanography - instrumentation; Oceanography - methods; Oceans and Seas; Philippines; Reproducibility of Results; Scattering, Radiation; Seawater; Sound; Sound Spectrography; Time Factors; Transducers
• ISSN: 0001-4966; 1520-8524
• DOI: 10.1121/1.4818843

Conventional and adaptive plane-wave beamforming with simultaneous recordings by large-aperture horizontal and vertical line arrays during the 2009 Philippine Sea Engineering Test (PhilSea09) reveal the rate of occurrence and the two-dimensional arrival structure of seismic phases that couple into the deep ocean. A ship-deployed, controlled acoustic source was used to evaluate performance of the horizontal array for a range of beamformer adaptiveness levels. Ninety T-phases from unique azimuths were recorded between Yeardays 107 to 119. T-phase azimuth and S-minus-P-phase time-of-arrival range estimates were validated using United States Geological Survey seismic monitoring network data. Analysis of phases from a seismic event that occurred on Yearday 112 near the east coast of Taiwan approximately 450 km from the arrays revealed a 22° clockwise evolution of T-phase azimuth over 90 s. Two hypotheses to explain such evolution-body wave excitation of multiple sources or in-water scattering-are presented based on T-phase origin sites at the intersection of azimuthal great circle paths and ridge/coastal bathymetry. Propagation timing between the source, scattering region, and array position suggests the mechanism behind the evolution involved scattering of the T-phase from the Ryukyu Ridge and a T-phase formation/scattering location estimation error of approximately 3.2 km.