Use of Hydroacoustic Phases for the Detection of Oceanic Events: Observations and Numerical Modeling

摘要

Hydroacoustic monitoring of the Comprehensive Nuclear Test Ban Treaty (CTBT) requires the ability to detect and locate phenomena that give rise to acoustic signals. An improved understanding of the coupling of seismic energy to acoustic energy is necessary to improve location estimates of earthquakes and volcanic eruptions. Using known variations in near-source bathymetry, the authors demonstrated that scattering of seismic to acoustic energy at a rough seafloor yields the approximate time-frequency characteristics of T-phases excited both at shallow regions within the SOFAR channel, as well as at abyssal depths far below the sound channel. The modeling predicts that T-phases are generated most efficiently at shallow depths, and consist mainly of low order acoustic modes. Abyssal phases, which are excited at depths of several kilometers, consist of high order acoustic modes that interact weakly with the seafloor along much of the transmission path but can have significant amplitude for paths with few bathymetric obstacles. An improved understanding of the accuracy needed for various input parameters to long-range acoustic propagation models, such as bathymetry and temperature data, is essential both in predicting acoustic shadow regions and in estimating source locations. They found that long-range acoustic propagation models predict wider and deeper shadow zones behind islands for high-resolution bathymetric datasets than for the more coarsely gridded ETOPO data. Seasonal variations in modal group velocity can reach up to 4mm/sec, which could translate to a time difference of 9 sec. over a 5000km path. This is only a small source of error compared to the large errors routinely made in picking T-phase arrivals from earthquakes.