Ambient noise in the natural surf zone: wave-breaking frequencies

Ambient noise in the surf zone, in the frequency range 120 Hz to 5 kHz, was recorded using a broad-band hydrophone, located approximately 1 m above bottom and 1-2 m below the mean sea surface. The predominant source of this noise is breaking waves. Analysis of simultaneous land-based video observations of the sea surface in the region of the hydrophone, along with wave height data, reveals quantitative correlation between wave-breaking events and the hydrophone signal. In energetic surf, locally breaking waves appear as discrete events in the ambient noise spectra. Distant breaking events do not appear to be detected, as distinct events above the ambient background noise, by the hydrophone. The noise events associated with local breakers are characterized by an asymmetry in the time envelope: low frequencies (less than 500 Hz) are observed leading the breaking crest, followed by a broader range of frequencies peaking in intensity with the passage of the wave crest above the hydrophone, and then decreasing abruptly at all frequencies. Low frequencies are generally not observed trailing the breaking wave. The detection by the hydrophone of breaking waves in the immediate vicinity implies that ambient noise in heavy surf provides a means of studying breaking-wave statistics in the surf zone in situ: in particular, the frequency of occurrence of local breaking     

The April 1992 Cape Mendocino Earthquake Sequence: Seismo-Acoustic Analysis Utilizing Fixed Hydrophone Arrays

The oceanic T-waves of earthquakes associated with the 1992 Cape Mendocino earthquake sequence were recorded and analyzed using fixed hydrophone arrays located throughout the north-east Pacific Ocean. The T-waves of these events were well recorded with high S/N ratios and strong acoustic energy present over a 0–64 Hz bandwidth. The smallest event recorded by the hydrophone arrays from the sequence had a local magnitude of 2.4. The hydrophone records of the three largest shocks in the sequence (ML 6.9, 6.2, 6.5) exhibited both T-waves and lithospheric phases from these events. Low-pass filtering (2 Hz) of the lithospheric phases yielded a clear P-wave arrival for epicentral distances of <10°, but no apparent S-wave. A seafloor cable-break was detected immediately after the second M>6 aftershock, possibly the result of a submarine slide. The direct P-wave hydrophone records from the second large aftershock showed a relatively high-amplitude, high-frequency arrival, consistent with seismic analyses which used this information to infer rupture direction. The rupture direction was toward the location of the cable break, thus rupture directivity possibly played a role in initiating the slide event. Modelling of the T-wave propagation path, using the Parabolic Equation model, produced estimates of the acoustic transmission loss from epicenter to receiver. The transmission loss to the most distant phones is typically 10-20 dB , and can be as large as 50–70 dB for acoustic propagation paths that cross the continental margin. The amount of acoustic energy each earthquake released into the ocean at the seafloor–water interface was estimated applying the transmission loss and instrument response to the recorded T-wave signals. This acoustic source power level was calculated for 41 events with magnitudes over a recorded range of 2.4ML6.9, with 17 of these events having their seismic moment estimates available through the NEIC. Ground displacement spectra were estimated from the acoustic power spectra and showed no indication of a corner frequency. Thus empirical analyses relating source level to magnitude and seismic moment were necessary to quantitatively derive an earthquake's size from hydrophone records. The results of indicator variable regression analyses suggest that T-wave source level increases linearly with the event's local magnitude and seismic moment. Furthermore, the source power level versus magnitude relationships for oceanic and continental earthquakes are significantly different, probably illustrating differences in the seismic and acoustic propagation paths from hypocenter to the hydrophone receivers. The results indicate that acoustic measurements provide a reasonable estimate of magnitude and seismic moment of an oceanic earthquake that was not detected by land-based seismic networks.     

Cambridge deep Ocean geophone

We describe recent mechanical andeelectronic modifications to the Cambridge Ocean Bottom Hydrophone system, enabling it to record in addition three geophone channels from a deployed, disposable geophone package. Examples of data from seismic refraction experiments show good correspondence between records of ground motion detected by the hydrophone and the vertical geophone. Seismic signals are undistorted by noise from instrument related sources. Clear examples of P to S conversions just below the receiver are observed. Improved recording conditions are achieved by deploying the geophones in a small pressure vessel as far away as possible from the main instrument package.     

Infrasonic seismic and acoustic measurements in the deep ocean

The authors compare the signal-to-noise ratios obtained on bottomed seismometers, bottomed hydrophones, and buried seismometers from near-surface explosions in the Ngendei Expedition. The data were recorded in 5.5-km-deep water in the south central Pacific Ocean with a triaxial borehole seismograph and four triaxial ocean-bottom seismographs having externally mounted hydrophones. At ranges less than 35 km, the data indicate that the ocean bottom seismometer is a superior signal detector than the ocean-bottom hydrophone, and that the subbottom seismometer is superior in performance to the ocean-bottom seismometer. Above 4 Hz, the seismometer appears to have a 10-dB signal-to-noise advantage over the hydrophone for surface explosions at ranges less than 30 km     

A high-resolution seismic profiling system using a deep-towed horizontal hydrophone streamer

A seismic reflection profiling system utilising a surface air gun source and a deep-towed horizontal hydrophone streamer has been developed for high resolution studies in the deep ocean. The instrument is deployed on a conventional armoured single conductor cable at depths of up to 6 km. Seismic data from the 30 m long streamer is wide-band frequency modulated up the towing cable to the ship together with a high frequency monitor from a 3.5 kHz echo-sounder mounted on the instrument package. The geometry of the system allows an order of magnitude improvement in spatial resolution compared with that obtained from standard surface source/receiver configurations. The summed hydrophones of the streamer attenuate cable-generated mechanical noise, and the 3.5 kHz sea-surface and bottom reflected returns provide receiver positioning information. The system has been successfully deployed at depths of 5 km in the Vema Fracture Zone in the North Atlantic, and although initially difficulties were experienced in balancing the streamer, subsequent profiles across the transform fault show details of sub-bottom structure which on conventional surface records are generally masked by diffraction hyperbolae.     

Reflection profiling in the deep ocean using a near bottom hydrophone

A 20 km long high resolution seismic reflection profile was carried out approximately 300 km southwest of Bermuda. The data were collected using a small airgun sound source and a single hydrophone receiver towed 100 m above the seafloor at a depth of 5400 m. Comparisons with nearby conventional seismic reflection profiles show the considerable improvement of resolving power provided by this method, particularly of the basement morphology beneath the 700 m thick sediment column. The data were recorded digitally and selected data examples show the enhancement provided by filtering, stacking, source deconvolution and corrections for hydrophone motion. The precise picture of basement topography that results from this data is compared with deep tow bathymetry profiles of the present day mid-Atlantic Ridge flanks, and is seen to be remarkably similar.     

Vertical synthetic aperture sonar for ocean acoustic tomography

This paper presents the result of a first attempt to achieve a vertical synthetic aperture in the ocean for SOFAR multipath identification. The experiment was conducted during the deployment of a tomographic array in the Mediterranean Sea. Drifting the hydrophone up or down from a ship while listening to the transmitted signal created a powerful synthetic aperture at 400 Hz. Wide-band phase-coded signals, typically used in ocean tomography, were found suitable for this application. The displacement length was 100 m and the hydrophone velocity 1 m/s. The obtained resolution of 1° enabled all the rays in the tested middle range configuration to be resolved and identified. Most of them could not have been resolved with a static hydrophone. Several Doppler processing methods are presented. The narrowband approximation leading to fast algorithms is discussed. Phase time series of individual paths obtained with an array-like wave separation method show that the phase coherence of the different multipaths is nearly perfect. An angle/velocity calibration method and a first rough inversion are finally presented     

Seabed characterization through a range of high-resolution acoustic systems – a case study offshore Oman

This study uses three acoustic instruments (different in their operating frequencies, 13, 3.5, and 6–10 kHz, and deployment type, hull-mounted, surface-towed and deep-towed) to investigate and characterize the acoustic response of seafloor NE of Oman in a frequency-independent manner. High-resolution control was achieved by having selected areas of our acoustic transects ground-truthed by sampling and/or sea-floor photography. On the regional scale, the greatest degree of change in backscatter amplitude was correlated with major changes of seabed morphology and lithology. However, small-scale roughness had the biggest effect on amplitude on the local scale, i.e. within each area of specific seafloor type. The study also shows that seafloor reflection amplitude changes are far more easily detected by deep-towed instrument than by surface-towed or hull-mounted systems. Whilst there are significant changes in bioturbation types and density along the transects, the suite of instruments deployed was not able to pick up the effect of the bioturbation on acoustic signals.     

Deep structure of the transition zone from the Baltic shield to the Barents Sea basin based on seismic data

Results of the analysis and interpretation of the records of 17 ocean bottom seismometers designed at the Shirshov Institute of Oceanology, Russian Academy of Sciences (a three-component geophone and a hydrophone), installed with an interval of 10–20 km along a profile in the transition zone from the Baltic shield to the Barents Sea basin are presented. The studies were carried out in 1995 from R/V Professor Kurentsov. An air gun with a chamber volume of 80 1 was used as the source of seismic waves with a shooting interval of 250 m. The longest range of records of deep refracted and wide-angle reflected waves (up to 300 km) was reached with the hydrophones. Two-dimensional seismic modeling allowed us to refine the earlier versions of the seismic cross section of the earth’s crust and uppermost mantle in the study region. New data confirmed that, in the central area of the Barents Sea, the “granitic-metamorphic” layer of the crust with a seismic velocity of 6.2 km/s typical of the Baltic Shield is absent. In this region, a thin consolidated crust with a seismic velocity of 6.8 km/s is covered with a thick (more than 25 km) sedimentary layer. In this layer, a local low-velocity zone probably exists, which causes a strong attenuation of the “crustal” waves.     

Multidisciplinary geophysical measurements on the ocean floor usingdecommissioned submarine cables: VENUS project

To perform geophysical and multidisciplinary real-time measurements on the ocean floor, it has been attempted to reuse decommissioned submarine cables. The VENUS project reuses the TPC-2, which is one of these systems and runs across the entire Philippine Sea Plate between Guam Island and Okinawa Island. The VENUS system comprises an ocean floor observatory, a submarine cable, and a land system. The major components of the ocean floor observatory are geophysical instruments and a telemetry system. There are seven scientific instrument units including broadband seismometers and a hydrophone array. Digital telemetry using the old analog telephone cable obtains high data accuracy and real-time accessibility to data from a laboratory on land. The bottom-telemetry system and a part of sensor units were installed at a depth of 2157 m on the landward slope of the Ryukyu (Nansei-Syoto) Trench on August 29, 1999. The data from the hydrophone array and tsunami gauge have been correctly transmitted to the data center. The rest of the scientific instruments will be deployed by deep-tow equipment and a remotely operated vehicle. Using a decommissioned submarine cable will greatly reduce construction costs compared to using a new cable system     

光纤水听器阵列应用于海洋地震勘探的试验*

为了检验光纤水听器阵列在海洋地震勘探中应用的性能, 在南海, 使用1024基元的光纤水听器阵列与进口的360道压电水听器海洋地震勘探仪, 采用相同的作业方式在同一位置进行海洋地震勘探调查, 对两个设备采集的海洋地震勘探资料进行对比。试验结果表明, 光纤水听器阵列采集的数据主频范围达到10~120Hz, 宽于压电水听器海洋地震勘探仪的主频范围10~80Hz; 地震剖面分辨率优于压电水听器海洋地震勘探仪。光纤水听器阵列在经过适应性改进后完全能够满足海洋地震勘探的性能要求。     

Ensuring very shallow-water sediment properties: case study from Capo Granitola harbour,Sicily (Italy)

We present high-resolution Vp models of the Capo Granitola harbor, Sicily (Italy) obtained by first arrival traveltime tomography. Seismic data were collected along four hydrophone arrays on the sea-bottom and via a Watergun as seismic source, in order to plan dredging operations in the harbor. Using a hydrophone spacing of 2.5 m and shot spacing of 5 m, very high resolution quality data were recorded. Seismic tomography expands existing knowledge of the harbour subsoil with a penetration of about 20 m, illuminating the Lower Pleistocene bedrock (Marsala calcarenites) that corresponds to high-Vp regions (Vp?>?4.5 km/s). Low Vp (1.8–4.5 km/s) deposits belonging to terraced calcarenites (Upper Pleistocene in age) are also well imaged; they are about 8 m thick and lie below loose sand deposits (Vp?=?1.5 km/s). The substratum has an articulated morphology; Vp images unravel small steps in the basement probably related to structural discontinuities (e.g., faults). Processing data with 3D techniques enables images of the structure and the thickness of the lithotypes to be reconstructed, thus leading to large-scale, realistic estimates of the total quantity of material to be excavated or dredged. Tomographic profiles permit clear discrimination of the soft sediment above the basement and thus allow the determination of the total volume of sediment above the seismic bedrock, estimated at about 265,000 m³.     

A fixed receiver for recording multichannel wide-angle seismic data on the seabed

Previous experiments to record seismic data at wide angle on the continental shelf have generally been unsuccessful in determining velocity structure in the lower crust; either the lines were too short or shot-receiver density too sparse to identify lower crustal arrivals. In contrast, deep normal incidence profiles show good structural resolution in the crust and uppermost mantle. A sea-bottom multichannel instrument has been developed to record datasets containing closely spaced traces, in order to improve the resolution of reversed wide-angle experiments on the continental shelf.The Pull-up Multichannel Array (PUMA) is a 1200 m, 12-channel hydrophone array for remotely recording seismic data on the seabed. It consists of 12 short hydrophone sections linked by 100 m-long passive sections. A pressure case is attached to the array at one end, in which recording electronics, cassette tape recorders and a battery power supply are housed. The PUMA is designed for deployment in water depths less than 200 m from a research ship and is moored to buoys for recovery.The instrument, which was successfully used in an experiment west of Lewis, Outer Hebrides, UK (Powell and Sinha, 1987) was specifically designed to provide a reliable determination of the velocity structure of the crust and uppermost mantle over part of the BIRPS WINCH deep normal incidence profile. Because the traces are closely spaced it is easy to correlate phases across the record section and to monitor changes in amplitude. A velocity structure for the continental crust and uppermost mantle has been devised from these data, using amplitude modelling.     

Using ocean ambient noise for array self-localization and self-synchronization

Estimates of the travel times between the elements of a bottom hydrophone array can be extracted from the time-averaged ambient noise cross-correlation function (NCF). This is confirmed using 11-min-long data blocks of ambient noise recordings that were collected in May 1995 near the southern California coast at an average depth of 21 m in the 150-700 Hz frequency range. Coherent horizontal wavefronts emerging from the time derivative of the NCF are obtained across the array's aperture and are related to the direct arrival time of the time-domain Green's function (TDGF). These coherent wavefronts are used for array element self-localization (AESL) and array element self-synchronization (AESS). The estimated array element locations are used to beamform on a towed source.     

High-resolution seismic studies of gas hydrates west of Svalbard

 A strong bottom-simulating reflection (BSR) with high-amplitude variations is detectable in high- resolution reflection seismic profiles west of Svalbard. Above the BSR, anomalously high velocities up to 1840 m/s, calculated from high-frequency ocean-bottom hydrophone (HF-OBH) data, indicate the existence of gas-hydrated sediments. Below the BSR, a low-velocity layer, interpreted as gas-bearing sediments, shows thickness variations from 12 to 25 m. In addition, two other low-velocity layers clearly containing free gas are detected within the classic hydrate stability zone (HSZ) where, a theoretical viewpoint, free gas cannot exist. Received: 6 August 1997 / Revision received: 26 January 1998     

An Ocean Bottom Hydrophone instrument for seismic refraction experiments in the deep ocean

Tests of a new Ocean Bottom Hydrophone (obh) instrument have recently been completed at Woods Hole Oceanographic Institution. This instrument is designed to float 3 m above the seafloor at depths of up to 6100 m for periods of up to 10 days and continuously records the output of a single hydrophone on a four-channel 0.064 cm/s (1/40 in./s) analog magnetic tape recorder. This instrument has an acoustic transponder and release system and is designed primarily for multiple deployments as a fixed ocean bottom receiver for seismic refraction work.Contribution No. 4174 of the Woods Hole Oceanographic Institution.     

Beam patterns of an underwater acoustic vector hydrophone located away from any reflecting boundary

A vector hydrophone is composed of two or three spatially collocated but orthogonally oriented velocity hydrophones plus an optional collocated pressure hydrophone. A vector hydrophone may form azimuth-elevation beams that are invariant with respect to the sources' frequencies, bandwidths and radial location (in near field as opposed to the far field). This paper characterizes the spatial matched filter beam patterns (a.k.a. fixed or conventional or maximum signal-to-noise ratio beam patterns) and the minimum variance distortionless response (MVDR) beam patterns associated with a single underwater acoustic vector hydrophone distant from any reflecting boundary.     

Time-frequency signal analysis of hydrophone data

The acoustic spectrum of a transiting aircraft, when received by a hydrophone located beneath the sea surface, changes with time due to the acoustical Doppler effect. The traditional method for analysing signals whose frequency content changes with time is the short-time Fourier transform that selects only a short segment of the signal (or window of data) for spectral analysis at any one time. The short-time Fourier transform requires the frequency content of the signal to be stationary during the analysis window, otherwise the frequency information will be smeared by the transformation. Recently, joint time-frequency distributions, which highlight the temporal localisation of a signal's spectral components, have been used to analyse nonstationary signals whose spectra are time dependent. In this paper, the short-time Fourier transform and the Wigner-Ville time-frequency distribution are applied to time-series data from a hydrophone so that the instantaneous frequency of the propeller blade rate of a turbo-prop aircraft can be estimated at short time intervals during the aircraft's transit over the hydrophone. The variation with time of the estimates of the Doppler-shifted blade rate is then compared with the corresponding temporal variation predicted using a model that assumes the sound propagates from the airborne acoustic source to the subsurface receiver through two distinct isospeed media (air and water) separated by a plane boundary (the air-sea interface). The results for five transits are presented in which the altitude of the aircraft ranged from 350 to 6050 ft with the speed of the aircraft varying from 232 to 245 kn     

Self-noise measurements using a moored sonobuoy with a suspended hydrophone

Moored sonobuoys are used for refraction seismic measurements at sea, although the range of operation is limited by the self noise caused by the hydrophone suspension.In this paper the main causes of self noise are discussed and a useful deployment scheme of a moored sonobuoy with suspended hydrophone is described.Some design criteria, in particular an equation for calculation of the optimal length of the expandable rubber band-which is situated between the surface float and the damping-body-are discussed.Finally, it is shown from noise measurements during the F. S. Valdivia cruise in 1975, that self noise level in the band 2 Hz to 20 Hz is 180 mPa to 280 mPa at a fully developed windsea with a characteristic wave height of more than 4 meters and a waterstream velocity about 0.5 m s¹.During these measurements the hydrophone was attached to the cable without rubber band. It is concluded that at a normal seastate the self noise level can be reduced to the order of ocean floor noise.     

A digital acoustic recording tag for measuring the response of wild marine mammals to sound

Definitive studies on the response of marine mammals to anthropogenic sound are hampered by the short surface time and deep-diving lifestyle of many species. A novel archival tag, called the DTAG, has been developed to monitor the behavior of marine mammals, and their response to sound, continuously throughout the dive cycle. The tag contains a large array of solid-state memory and records continuously from a built-in hydrophone and suite of sensors. The sensors sample the orientation of the animal in three dimensions with sufficient speed and resolution to capture individual fluke strokes. Audio and sensor recording is synchronous so the relative timing of sounds and motion can be determined precisely. The DTAG has been attached to more than 30 northern right whales (Eubalaena glacialis) and 20 sperm whales (Physeter macrocephalus) with recording duration of up to 12 h per deployment. Several deployments have included sound playbacks to the tagged whale and a transient response to at least one playback is evident in the tag data.