In-situ seismic observations identified that volcanic activity of Ioto (formerly Iwojima), a volcanic island offshore Japan, increased in early September 2018. Observations of discolored nearshore waters and a splash reported by a local flyover provided evidence for a connection between undersea eruptions and recorded seismic activity. However there remain uncertainties as to when the undersea eruption series commenced and how much of the in-situ seismic activity recorded on the island was associated with volcanic earthquakes versus undersea eruptions. During this period, a large number of underwater acoustic (hydroacoustic) signals were recorded by the Comprehensive Nuclear-Test-Ban Treaty (CTBT) International Monitoring System (IMS) hydroacoustic station HA11, at Wake Island (U.S. Territory), in the northwestern Pacific Ocean with signals with directions of arrival consistent with sources located at Ioto. The analysis presented here interprets signal features of the remote hydroacoustic recordings provided by HA11 in order to attempt to distinguish between volcanic earthquake signals and undersea eruption signals originating from Ioto. Histograms of hydroacoustic events interpreted as originating from Ioto correlate well with the in-situ seismic observations at Ioto in the early stage of volcanic activity. The results presented suggest that around 75% of the signals detected at HA11 with directions of arrival consistent with Ioto as their origin could be associated with undersea eruptions, supporting the conclusion that the IMS hydroacoustic stations can contribute to volcanic event remote monitoring.
Underwater acoustic signal waveforms recorded during the 2015 Chile earthquake (Mw 8.3) by the hydrophones of hydroacoustic station HA03, located at the Juan Fernandez Islands, are analyzed. HA03 is part of the Comprehensive Nuclear‐Test‐Ban Treaty International Monitoring System. The interest in the particular data set stems from the fact that HA03 is located only approximately 700 km SW from the epicenter of the earthquake. This makes it possible to study aspects of the signal associated with the tsunamigenic earthquake, which would be more difficult to detect had the hydrophones been located far from the source. The analysis shows that the direction of arrival of the T phase can be estimated by means of a three‐step preprocessing technique which circumvents spatial aliasing caused by the hydrophone spacing, the latter being large compared to the wavelength. Following this preprocessing step, standard frequency‐wave number analysis (F‐K analysis) can accurately estimate back azimuth and slowness of T‐phase signals. The data analysis also shows that the dispersive tsunami signals can be identified by the water‐column hydrophones at the time when the tsunami surface gravity wave reaches the station.
The point scattering model offers a parameterization of the reverberation probability density function (pdf) in terms of the coefficient of excess (kurtosis) and a coherent component represented by a harmonic process with random phase. In this paper the potential utility of this parametrization is investigated in the context of seafloor characterization. The problem of separating out the effect of each parameter is discussed. Computer simulations are used to verify model predictions on the reverberation quadrature, envelope, and phase pdf. As part of the verification study, the scatterer density was determined from the kurtosis of the reverberation quadrature pdf. A statistical analysis of this procedure points to reduced estimate accuracy with decreasing kurtosis. Additional computer simulations show that the chosen pdf family, developed under the assumption of a Poisson scatterer distribution, is flexible enough to fit reverberation data generated by non-Poisson scatterer distributions exhibiting a degree of clustering or regularity. A computer experiment demonstrates how this parametrization can be used in conjunction with a simple sonar geometry to generate acoustic signatures for seafloor classification. In addition, real reverberation data collected by a Sea Beam sonar system in two different seafloor areas are interpreted according to the chosen parametrization. PACS numbers: 43.30.Hw, 43.30.Gv, 43.20.Fn INTRODUCTION Acoustic reverberation is usually modeled as a Gaussian stochastic process. Equivalently, its envelope and phase are assumed to have a Rayleigh and uniform probability density function (pdf), respectively. However, real reverberation data often exhibit deviations from this nominal model. •'2 More general and flexible pdf families are then needed to describe the statistical behavior of reverberation. 3 If rever- beration is viewed as interference, incorporating its non-Gaussian nature into the design of a detection/estimation processor will lead closer to optimum performance. If reverberation is viewed as an information-bearing signal, deviations from normal statistics convey useful information about the scatterer distributions in the medium and ultimatelyabout the nature of the medium.In this paper, we adopt the latter viewpoint and concentrate on the problem of seafloor characterization. We focus our attention on a reverberation pdf family supported by the point-scattering model. 4-•ø This pdf family is parametrized by (1) the coefficient of excess, that is associated with the average number of scatterers contributing to the return and (2) a "coherent component" coefficient. Under certain conditions it is possible to relate these parameters to seafloor characteristics such as rms roughness and correlation area. TM However, the point-scattering theory has been developed for random (Poisson) scatterer distributions. Therefore, direct analytical connections to seafloor parameters cannot be obtained when non-Poisson distributions are present. In general, scatterers in the ocean exhibit clustering...
Abstract-Cardioid or triplet towed arrays are utilized in sonar applications to resolve left/right ambiguity by placing a null on the ambiguous direction. This leaves an uncalibrated residual signal on the opposite direction from which the desired signal is arriving. Here the mechanism of cardioid beamforming is presented and analytical expressions for the calibration of continuous wave (CW) and linear frequency-modulated (LFM) signals are derived. The validity of these expressions is verified using simulated and real data. The same data sets are also used for comparing the performance between the standard beamformer and a modified version designed to suppress endfire singularities. The effect of correlated vs. uncorrelated intratriplet noise is assessed using a simulation scenario with a point target.
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