Suspended Hydrophone Research Articles

Seismicity of the Incoming Plate and Forearc Near the Mariana Trench Recorded by Ocean Bottom Seismographs

AbstractEarthquakes near oceanic trenches are important for studying incoming plate bending and updip thrust zone seismogenesis, yet are poorly constrained using seismographs on land. We use an ocean bottom seismograph (OBS) deployment spanning both the incoming Pacific Plate and the forearc to study seismicity near the Mariana Trench. The yearlong deployment in 2012–2013 consisted of 20 broadband OBSs and 5 suspended hydrophones, with an additional 59 short period OBSs and hydrophones recording for 1 month. We locate 1,692 earthquakes using a nonlinear method with a 3D velocity model constructed from active source profiles and surface wave tomography results. Events occurring seaward of the trench occur to depths of ~35 km below the seafloor, and focal mechanisms of the larger events indicate normal faulting corresponding to plate bending. Significant seismicity emerges about 70 km seaward from the trench, and the seismicity rate increases continuously towards the trench, indicating that the largest bending deformation occurs near the trench axis. These plate‐bending earthquakes occur along faults that facilitate the hydration of the subducting plate, and the lateral and depth distribution of earthquakes is consistent with low‐velocity regions imaged in previous studies. The forearc is marked by a heterogeneous distribution of low magnitude (<5 Mw) thrust zone seismicity, possibly due to the rough incoming plate topography and/or serpentinization of the forearc. A sequence of thrust earthquakes occurs at depths ~10 km below seafloor and within 20 km of the trench axis, demonstrating that the megathrust is seismically active nearly to the trench.

Potential for use of passive acoustic monitoring of piranhas in the Pacaya–Samiria National Reserve in Peru

Abstract Passive acoustic monitoring (PAM) can be an effective tool in the identification of fishes and mapping of their temporal and spatial distribution patterns and thereby aids in ecosystem management in remote locations. However, to date measurements of the acoustic properties of piranha have been primarily made in aquaria on captive specimens obtained through the aquarium trade. Data on the sound production of wild piranha taken under natural field conditions would enhance PAM applications. Piranha captured as part of routine annual monitoring by Operation Wallacea within the Pacaya–Samiria National Reserve in Peru were auditioned for sound production prior to release. Auditioning was done by gently holding a fish underwater in the river near a suspended hydrophone, thus recordings included the piranha sound as well as natural ambient sound. Seventy‐nine per cent of the 129 auditioned piranha, including Pygocentrus nattereri, Serrasalmus maculatus, Serrasalmus cf. sanchezi and an unidentified Serrasalmus spp. complex, produced sounds consisting of 2 to 23 barks in a sequential series. Sound production by S. maculatus and S. cf. sanchezi are reported for the first time. Bark characteristics exhibited high variation within bark series of individual fish. Piranha could not be distinguished by single variables but did exhibit significant multivariate differences. The relationship of several variables to fish size also differed significantly among species. Measurements of within‐fish variation and bark series pattern attributes were found to be useful for discrimination of sounds among piranha species. We demonstrate that closely related species of piranha can be distinguished by their sounds under natural acoustic conditions based on multivariate analyses, suggesting that passive acoustic monitoring (PAM) can be an effective tool for ecosystem management in the Amazon. More broadly, our study also suggests (a) the need to report more detailed statistical descriptions of fish sounds, including measures of within‐fish variation, (b) the importance of describing characteristics of sound series produced by fishes in addition to those of individual sound types and (c) the need to deposit museum voucher specimens to anchor specific sounds to specific individuals.

Passive ocean acoustic tomography using ships as sources of opportunity recorded on an irregularly spaced free-floating array: A feasibility study

This presentation investigates the practical feasibility of using an adaptive volumetric array, such as a series of free-floating buoys with suspended hydrophones, which record ships as acoustic sources of opportunity in coastal waters for performing acoustic thermometry or other environmental inversions in near-shore environments in a totally passive manner. Ships are tracked using the Automatic Identification System (AIS). Numerical simulations using a standard normal mode propagation model were first used to test limitations of the proposed approach with respect to frequency band, drifting receiver configuration, signal to noise ratio, precision, and accuracy of the inversion results, along with sensitivity to environmental and position mismatch. Performance predictions using this model are compared with experimental results using at-sea data collected off the coast of New London, CT, in Long Island Sound during August of 2015.

Influence of short time scale water column fluctuations on broadband signal intensity

The KAM11 experiment was conducted in 100 m of water off the Western side of Kauai, Hawaii, in June/July, 2011 during which two identical bottom mounted tripods, separated by 1 km, transmitted reciprocal chirp sequences. A monitoring hydrophone was suspended at a depth of 25 m from the R/V Kilo Moana approximately midway between the tripod stations. Impulse response measurements at the suspended hydrophone show intermediate arrivals between direct-path/bottom-bounce and surface-bounce arrivals unique to only one tripod source. Focusing and defocussing of the intermediate arrivals show the influence of fluctuations in the sound speed profile over time scales from seconds to hours. Short term intensity variations arise from surface driven vertical undulations in the water column. Over longer time scales changes in the thermocline govern the evolution of the intermediate arrival. Data model comparison is conducted with ray tracing and parabolic equation modeling. [Work supported by ONR 322OA.]

Two-dimensional acoustic field measurements in a complex, shallow-water waveguide

A shallow-water acoustics modal mapping experiment (MOMAX) was conducted in March 1997 in the vicinity of the East Coast STRATAFORM site. An acoustic modal field was established in a shallow-water environment using fixed and drifting sources radiating several pure tones in the range 50–300 Hz. The field was then densely sampled on a horizontal plane at midwater depth using freely drifting buoys equipped with suspended hydrophones and a radio telemetric link to the ship for data acquisition. GPS and acoustic navigation were used on each buoy for precise spatial tracking to a maximum range of 10 km. An overview of the experiment is presented emphasizing buoy configuration and drift paths, signal processing, and acoustic field mapping. Specific buoy deployments will be examined with regard to the lateral variability of the acoustic field using data obtained from the two-dimensional buoy drift tracks. [Work supported by ONR.]

Preliminary report on the summer ’96 joint China–U.S. internal wave/acoustic wave experiment in the Yellow Sea

Simultaneous observations of internal wave activity and acoustic wave propagation in 70-m water in the Yellow Sea were made in the late summer of 1996. The primary objective of this experiment was to validate the predicated modal coupling and fluctuations induced by shallow-water internal waves. The Yellow Sea provides an ideal environment for such research because it has a very flat and homogeneous bottom and a very strong, sharp thermocline. The environment lends itself to relatively simple models for both the acoustic field and the internal wave field. Propagation was measured over distances up to 55 km in the frequency range of 50 Hz–5 kHz. The receivers were three moored and two suspended hydrophone arrays. Internal wave activity was monitored using thermistor chains, SAR satellite imagery, and high-frequency sonar. Propagation data were taken as a function of range, as a function of time at a fixed range, and as a function of azimuth in the presence of differing levels of internal wave activity. Supporting environmental data obtained during the experiment include ADCP, bottom profile and surface wave-height spectra, and bottom coring. Preliminary data on internal wave field, sound propagation, reverberation, and bottom acoustic parameters will be presented. [Work supported by ONR and the Chinese Academy of Sciences.]

Amplitude fluctuations of signals received from cw sources towed at long ranges in the deep ocean

In April 1974, three cw sources (9.807, 110, and 262 Hz) were towed for 16 days in the Atlantic along a track from Antigua to the Grand Banks of Newfoundland. Data from these sources were recorded near Bermuda, Puerto Rico, and Antigua on both bottom and suspended hydrophones. A 3-h running average was subtracted from the amplitude data in order to remove long-term deterministic trends. The remaining fluctuations were analyzed in fixed time segments to determine the correlation time, the density function and the associated first four moments. The variate difference technique was used to estimate the degree of randomness of the fluctuations. Finally, the effects of range, range-rate, source frequency, and environment on the fluctuation statistics are discussed.

Time reversal of SOFAR arrivals

Ray theory techniques are used to investigate a time-reversal phenomenon seen during the KIWI-ONE (Phase II) propagation experiment. Shots detonated along a track between New Zealand and South America were received by a deep suspended hydrophone. For ranges less than 3000 km, the standard SOFAR energy crescendo and abrupt termination is apparent, but for longer ranges, more than half of the received energy arrives after the strong axial arrival. This unexpected time reversal is shown to be cause by a broad minimum layer in the sound-speed field beyond 3000 km. Computer ray traces show that this broad minimum layer can produce the 1 sec of time reversal seen at 5300 km, while a more nearly classic SOFAR profile measured at the receiver exhibits negligible reversal. Traces simulating KIWI shot signatures were performed through a range-dependent sound-speed field with a linear segmented bottom. The calculated arrival structure agrees well with the data in both time and level. Fine structure of the arrival-time distribution is shown to correspond to discontinuities in the gradient in the broad minimum layer. [Supported by ONR 412.]

Precise Tracking of Suspended Hydrophone Motion

A Doppler tracking system has been developed to enable continuous real-time measurement of the motion of suspended hydrophones. The system consists of three bottom-moored CW beacons, a phase detector for each beacon signal, and a computer processing scheme. Displacements as small as 1 cm have been detected in ship-mounted, ship-suspended, sonobuoy-deployed, and bottom-moored hydrophones at ranges up to 10 km. The system was used to coherently phase detect low-frequency acoustic transmissions by using displacement information to correct the Doppler shift due to hydrophone motion; however, its applicability extends to any environment where highly accurate mooring motion must be measured. An analysis of the combined effects of surface and forward volume scattering shows that these phenomena do not compromise the tracking system. Random phase fluctuations in beacon signals attributable to these effects were estimated to be less than 0.2 rad. An analysis of hydrophone motion derived from data obtained during two ocean tests of the system is presented in this paper.

Dynamic Response of Suspended Underwater Systems

A method is presented for analyzing the steady-state or stationary random heave response of suspended underwater systems. The nonlinear continuous system is replaced by a discretized pseudolinear system by means of an equation difference minimization technique. The resulting pseudolinear system is solved by an iterative scheme. An example of the application of the proposed method of analysis to a suspended hydrophone system is given. The example shows that the nonlinear pressure drag forces have a very significant effect on the over-all response of the system. The nature of the response is discussed and some practical conclusions are given.

Flutter Noise in Suspended Hydrophones

Telephone wires have long been known to “sing” in a wind, creating aeolian tones attributed to periodic vortex shedding reinforced by the vibrational resonance of the wire. When a hydrophone is suspended from a cable in a water current, a similar quasi-sinusoidal noise, caused by cable flutter transmitted to the hydrophone, may be expected at the hydrophone terminals. Observations of what was apparently noise of this type were made in Chesapeake Bay at current speeds up to 1.3 knots. Strong sinusoidal noise voltages, at frequencies between 10 and 30 cps, depending upon current speed, were measured at the hydrophone terminals. The amplitude of this noise varied irregularly with cable length and cable tension but often was equivalent to a sound pressure level of +35 db above 1 dyne/cm2. Several remedies were tried and found effective: using a streamlined cable fairing, reducing cable tension by suspending the hydrophone by rubber bands just above its attachment to the cable, and using a hydrophone less sensitive to acceleration. Noise of this type should be expected in all low-frequency work with suspended hydrophones but apparently can be reduced greatly by one or more simple control methods.