Noise source level density due to surf. I. Monterey Bay, CA

Ambient noise measurements made in Monterey Bay, CA, in 1981 were reduced by estimations of wave-breaking noise and the residual noise was combined with modeled transmission loss (TL) to estimate the spectral source level of surf-generated noise. A Hamilton geoacoustic model of the coastal environment was derived and used in a finite-element parabolic equation propagation-loss model to obtain TL values. Estimates of both the continuous, or local, and discrete components of wave-breaking noise intensity were subtracted from the total measured noise field to determine the contribution due to surf only. Surf breaking on a uniform 12.5-km linear section of beach near Ft. Ord was found to be the dominant source of surf-generated noise. Estimated noise source level densities for heavy surf at Ft. Ord beach varied from 138 dB ref. 1 μPa Hz^-1/2 m at 1 m from the source at 50 Hz to 107 dB at 1 kHz, with a slope of about -5 dB per octave. Although these results must be considered as preliminary, since they are based on a small number of measurements, they may he useful for prediction of ambient noise in other littoral regions     

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     

A low-density extended acoustic sensor for low-frequency arrays

A novel PVDF hydrophone is described which has a sensitivity of -183 dBV/μPa or better and can be made in lengths from 4 cm to more than 1 m, and in diameters from 8 mm to >40 mm. It is based on a piezoelectric PVDF cable which allows considerable design freedom because of its very great mechanical flexibility and high longitudinal piezoelectric coefficient. The design centres on a helical structure which translates a hydrostatic pressure field into a longitudinal compressive strain in the cable and thus makes use of the very sensitive mode. Excellent pressure stability can be achieved, with devices showing a pressure dependency of 0.04 dB/MPa. The device has a flat frequency response from less than 1 Hz to several kHz and a density in the region of 1800 kg/m³, making it suitable for application in low-frequency sonar     

Underwater sounds near a fuel receiving facility in western Hong Kong: relevance to dolphins

Western Hong Kong is home to two species of marine mammals: Indo-Pacific humpbacked dolphins (Sousa chinensis) and finless porpoises (Neophocaena phocaenoides). Both are threatened in many parts of their range in southeast Asia [for example, International Biological Research Institute Reports 9 (1997), 41; Asian Marine Biology 14 (1997) 111]. In 1998, when the new Hong Kong International Airport opened in western Hong Kong, small tankers (about 100 m long, cargo capacity about 6300 metric tons) began delivering fuel to the Aviation Fuel Receiving Facility (AFRF) just off Sha Chau Island, north of the airport. Calibrated sound recordings were taken over a 4-day period from a quiet, anchored boat at distances 80-2000 m from aviation fuel delivery activities at the AFRF. From the recordings, 143 sections were selected for analysis. Narrowband spectral densities on the sound pressures were computed, and one-third octave band levels were derived for center frequencies from 10 to 16,000 Hz. Broadband levels, viz. 10-20,000 Hz. were also computed. The results showed that the Sha Chau area is normally noisy underwater, with the lowest broadband levels measured corresponding to those expected during a storm at sea (sea state 6). This background noise is believed to come largely from heavy vessel traffic in the Urmston Road to the north and east of Sha Chau and from vessels in the Pearl River Estuary to the West. The sound levels from the AFRF tankers are comparable to the levels measured from similar- and smaller-sized supply vessels supporting offshore oil exploration. The strongest sounds recorded were from a tanker leaving the AFRF at distance 100 m from the hydrophone, for which the one-third octave band level at 100 Hz was 141 dB re 1 microPa (spectrum level 127 dB re 1 microPa2/Hz) and the 10-20,000 Hz broadband level was 146 dB. At distances of 100 m or more and frequencies above 300 Hz, the one-third octave band levels were less than 130 dB (spectrum level 111 dB re 1 microPa2/Hz) and decreased with increasing frequency and distance. At distances greater than about 500 m, AFRF-associated sounds were negligible, masked by the generally high noise level of the area and attenuated by poor transmission in the very shallow water (<10 m). Because it is believed that humpbacked dolphins and finless porpoises are not very sensitive to sounds below 300 Hz, the Airport Authority Hong Kong (AA) stipulated that dedicated terminal vessels not radiate underwater sounds at spectrum levels greater than 110 dB re 1 microPa2/Hz at frequencies above 300 Hz and distances greater than 300 m. The spectrum levels at 300 Hz and higher frequencies of sounds from the tankers arriving, departing, or off-loading at AFRF were less than 110 dB re 1 microPa2/Hz even at distances of 200 m or less. The AA stipulation was met. However, it is presently unknown whether the generally strong noise levels of western Hong Kong inhibit acoustically based feeding and communication, or result in increased stress or permanent shifts in hearing thresholds.     

Ambient noise data logger buoy

A buoy for measuring wind speed and the ambient noise sound pressure level from 10 to 1500 Hz with 1-Hz resolution is described. The measurement buoy was deployed in a remote fjord in southeastern Alaska from October to December, 1989. The results from the data collected show that, for a wind speed of 5 kn, the measured ambient noise level at 900 Hz lies well below the Knudsen curve for open-ocean, wind-generated noise. As the wind speed increases from 5 to 10 kn, the measured ambient noise level approaches the Knudsen curve, increasing at 4 dB/kn compared to 1 dB/kn for the Knudsen curve. Above 10 kn, the measured ambient noise level matches the Knudsen curve     

台风期间南海北部岛礁及下坡海底对水下噪声的影响研究

The correlation of ambient noise with wind speed, and the depth dependence of ambient noise are both investigated, where the ocean noise data were recorded by a vertical line array in the northern South China Sea. It is shown that the correlation coefficients increase with increasing hydrophone depth during typhoon periods when the frequency ≥ 250 Hz, which opposes the generally accepted knowledge that the correlation coefficients of noise level and wind speed decrease with increasing depth during non-typhoon periods. Particularly at frequencies of 250 Hz, 315 Hz and 400 Hz, the correlation coefficients increase by more than 0.05 at depths ranging from 155 m to 875 m. At the three frequencies, the average noise levels also increase with increasing depth during typhoon periods. It is suggested that these differences are attributed to the wind-generated noise in shallow waters and the effect of "downslope enhancement" to sound propagation. During typhoon periods, the surf breaking and surf beat upon the shores and reefs are strengthened, and the source levels are increased. The wind-generated noise in shallow waters interacts with the downslope sea floor, with the noise-depth distribution changed by a "downslope enhancement" effect promoting noise propagation.     

Acoustic interaction of humpback whales and whale-watching boats

The underwater acoustic noise of five representative whale-watching boats used in the waters of west Maui was measured in order to study the effects of boat noise on humpback whales. The first set of measurements were performed on 9 and 10 March, close to the peak of the whale season. The ambient noise was relatively high with the major contribution from many chorusing humpback whales. Measurements of boat sounds were contaminated by this high ambient background noise. A second set of measurements was performed on 28 and 29 April, towards the end of the humpback whale season. In both sets of measurements, two of the boats were inflatables with outboard engines, two were larger coastal boats with twin inboard diesel engines and the fifth was a small water plane area twin hull (SWATH) ship with inter-island cruise capabilities. The inflatable boats with outboard engines produced very complex sounds with many bands of tonal-like components. The boats with inboard engines produced less intense sounds with fewer tonal bands. One-third octave band measurements of ambient noise measured on 9 March indicated a maximum sound pressure level of about 123 dB re 1 microPa at 315 Hz. The maximum sound pressure level of 127 dB at 315 Hz was measured for the SWATH ship. One of the boats with outboard engines produced sounds between 2 and 4 kHz that were about 8-10 dB greater than the level of background humpback whale sounds at the peak of the whale season. We concluded that it is unlikely that the levels of sounds produced by the boats in our study would have any grave effects on the auditory system of humpback whales.     

Tidal asymmetry in suspended sand transport on a macrotidal intermediate beach

Time-series of nearbed horizontal flow velocities and suspended sediment concentrations obtained from a colocated electromagnetic current meter (EMCM) and optical backscatter sensor (OBS), respectively, are used to examine the relative importance of steady and fluctuating components to the total sediment transport over a full tidal cycle on a macrotidal, intermediate beach (Spurn Head, UK). Fluctuating sediment fluxes are decomposed into gravity and infragravity contributions using co-spectral techniques. The relative importance of the oscillatory (gravity and infragravity) and steady (mean) transport components to the total sediment transport is analysed throughout the tidal cycle.

A continuum of 34 discrete suspended sediment-cross-shore velocity co-spectra are computed over a full tidal cycle for the OBS and EMCM measurements 0.10 m above the bed. These net transport spectra vary greatly both with cross-shore location and tidal state. In particular, a marked asymmetry in transport processes is evident between the flood and ebb tides, with high levels of sediment resuspension and transport occurring on the ebbing tide approximately two hours after high water (just seaward of the breakpoint). At this time the dominant transport was directed offshore (co-spectral peak, 0.04 kg/m²/s) at incident wave frequency.

Typical patterns are observed in transport spectra outside the surf zone and within the inner surf zone. Outside the narrow surf zone cross-shore transport spectra show weak offshore transport (co-spectral peak = 0.002 kg/m²/s) associated with bound long waves and stronger onshore transport (co-spectral peak = 0.006 kg/m²/s) at incident wave frequencies. Conversely, co-spectra computed within the inner surf zone show the offshore sediment fluxes (spectral peak = 0.010 kg/m²/s) at infragravity frequencies to be greater in magnitude than the corresponding onshore transport (co-spectral peak = 0.008 kg/m²/s) occurring at incident wave frequencies.     

An impact sound source useful for Arctic remote sensing

The impact on an ice surface of an energetic projectile, such as a rifle bullet, which creates an acoustic impulse in the water whose properties are useful for Arctic remote sensing, is discussed. The source level of these impulses is typically between 220 and 230 dB Re 1 μPa at 1m and most of their energy is within the band 500 Hz to 3 kHz. Possible remote sensing applications are reviewed together with available experimental data. These include the measurement of ice properties and water depth, as well as bottom imaging. It is concluded that the source described has potential for Arctic acoustic remote sensing applications where probability and ease of deployment are important and the required number of measurements is not too large     

Behavioral responses of a harbor porpoise (Phocoena phocoena) to playbacks of broadband pile driving sounds

The high under-water sound pressure levels (SPLs) produced during pile driving to build offshore wind turbines may affect harbor porpoises. To estimate the discomfort threshold of pile driving sounds, a porpoise in a quiet pool was exposed to playbacks (46 strikes/min) at five SPLs (6 dB steps: 130–154 dB re 1 μPa). The spectrum of the impulsive sound resembled the spectrum of pile driving sound at tens of kilometers from the pile driving location in shallow water such as that found in the North Sea. The animal's behavior during test and baseline periods was compared. At and above a received broadband SPL of 136 dB re 1 μPa [zero-peak sound pressure level: 151 dB re 1 μPa; t₉₀: 126 ms; sound exposure level of a single strike (SEL_ss): 127 dB re 1 μPa² s] the porpoise's respiration rate increased in response to the pile driving sounds. At higher levels, he also jumped out of the water more often. Wild porpoises are expected to move tens of kilometers away from offshore pile driving locations; response distances will vary with context, the sounds' source level, parameters influencing sound propagation, and background noise levels.     

A comparison of four methods for bubble size and void fractionmeasurements

We compare the performance of four different bubble-sensing techniques in a range of environment from the surf zone to the open ocean: a remote sensing method using high-frequency backscatter, two in situ methods using an acoustical resonator and a pulse propagation sensor, and a bulk method using electrical conductivity. Comparisons between the techniques show general consistency within the appropriate operational bubble density ranges, although spatial variability in bubble clouds introduces substantial variance. Each technique has its strengths and limitations. Our acoustical resonator is suitable for bubble concentrations with air fractions greater than approximately 10 ^-9 and the pulse propagation sonar for air fractions from 10 ^-6; the upper limit for both is constrained by attenuation and the validity of the Foldy scattering approximation. Both sensors can be implemented to encompass a wide frequency range with high resolution, corresponding to resonant bubble radii of 10~1200 μm. For air fractions higher than ~5×10^-4, bulk measurement using electrical conductivity provides a measure of air fraction. Sufficient overlap in operational air-fraction range exists between in situ acoustical techniques and conductivity measurement to permit comparison and demonstrate consistency in the measurement. Single- and multi-frequency backscatter sonars may be used for low air fractions (<1×10^-5) and provide a continuous vertical profile from a deployment beneath the active surface zone, but are subject to masking by dense bubble clouds and are unable to resolve high air fractions close to the surface. This study suggests that the best approach is to use a combination of sensors to probe the bubble field     

海上风电打桩水下噪声测量及其对大黄鱼的影响

海上风电场建设期风机打桩会产生高强度的水下噪声,研究水下冲击打桩噪声的监测方法、特性分析及对海洋生物的影响是非常重要的。采用自容式水下声音记录仪,多点同步测量了福建省兴化湾海上风电场二期工程建设期单次完整的水下冲击打桩噪声,从时频域特性进行了分析,并利用最小二乘法拟合得到了打桩声源级和声暴露级。结果表明：水下冲击打桩噪声是典型的低频、高强度的脉冲信号,单个脉冲持续时间约90~100 ms,峰值声源级约209.4±2 dB,声暴露级约197.7±2 dB;主要能量分布在50 Hz~1 kHz频段,750 m测量点的该频段声压级相比海洋环境背景噪声,提高了约40~50 dB。水下冲击打桩噪声频域能量分布与大黄鱼的听觉敏感频段相重叠,对大黄鱼影响程度和范围较大,实际工程应用中宜采用声暴露级作为评价指标。     

Sonar array characterization, experimental results

Measurements in the Levantine Sea with a seismic-type array [i.e., the high-frequency array (27 wavelengths at 348 Hz), the mid-frequency array (27 wavelengths at 175 Hz), and the low-frequency array (21 wavelengths at 58 Hz)] were found to have on average results within 1 dB of the theoretical signal gain. Observed signal gain degradations for peak-tracked and short integration times (1 min) had standard deviations from 2 to 3 dB and were caused by the combination of coherent multipaths, array shape, and array motion. The relative motion of source and receiver (5-8 kn) was an important cause of the average degradation at longer integration times (5 min). Equivalent plane wave beam noise levels were measured as a function of frequency, time, bearing, and aperture length. The beam noise level results show contributions from distant surface-ship-generated noise and natural environmental background noise. These results showed resolved distant shipping with median beam noise levels consistent with array noise gain 1-2 dB greater than the theoretical value for incoherent isotropic noise. The beam noise cumulative probability distribution function versus equivalent plane wave levels differed significantly from log-normality. Beam noise surfaces (beam noise levels versus time and bearing) show a higher density of ships for the high-frequency array when compared to the low-frequency array. Beam-to-beam cross correlations were found be sharply peaked and beam autocorrelation functions versus time showed zero crossing times on the order of 9-10 min. Significant space-time noise fade durations were observed at lower frequencies     

UDATS-II: data acquisition in a client-server environment

Continuous data acquisition from current meters and other sensors deployed in the surf zone makes special demands on the capabilities of systems. This paper describes a new system that allows continuous digital recording of data at 10 Hz, with the simultaneous processing of the data to estimate statistical moments. This new design uses a real-time multitasking operating system in a client-server model to achieve the objective of continuous operation while maintaining a robust distributed system. This paper explores some of the alternatives and then details the hardware and software design of the new system. Data collected during the Duck94 field experiment are presented as examples of the capabilities of the new system     

Ambient noise measurements in the northwest Indian Ocean

Ambient noise measurements were made at seven different locations during the first four months of 1977. The measurement systems included: two types of towed arrays, a bottomed array, and systems with sensors distributed throughout the water column. The noise in the frequency regime dominated by shipping was found to be extremely high with a spectrum level of 92 dB//1 /spl mu/Pa at 50 Hz. These high levels were attributed to the high-density shipping. Horizontal directionality of the noise varied from site to site, ranging from highly directional to nearly nondirectional. The character of the directionality was highly dependent on the site position relative to shipping lanes. The noise showed very little dependence on depth.     

Sediment transport and morphology at the surf zone of Presque Isle, Lake Erie, Pennsylvania

A four-year investigation of surf zone sedimentation at Presque Isle, Pennsylvania, was undertaken in preparation for the design of a segmented breakwater system. Sediment transport calculations were based on hind-cast annual wave power statistics and “calibrated” by known accretion rates at the downdrift spit terminus. 30,000 m³ of sediment reaches the peninsula annually from updrift beaches. The transport volume increases downdrift due to shoreface erosion and retreat of the peninsular neck. At the most exposed point on Presque Isle (the lighthouse) the annual transport is 209,000 m³. East of the lighthouse is a zone of net shoreface accretion as the longshore transport rate progressively decreases.

The downdrift variation in sediment supply, combined with increasing refraction and attenuation of the dominant westerly storm waves produce a systematic change in prevailing surf zone morphology. Storms produce a major longshore bar and trough along the exposed peninsular neck. The wave energy during non-storm periods is too low to significantly alter the bar which consequently becomes a permanent feature. The broad shoreface and reduced wave energy level east of the lighthouse produce a morphology characterized by large crescentic outer bars, transverse bars, and megacusps along the beach. At the sheltered and rapidly prograding eastern spit terminus the prevalent beach morphology is that of a ridge and runnel system in front of a megacuspate shore.

The morphodynamic surf zone model developed for oceanic beaches in Australia is used as a basis for interpretation of shoreface morphologic variability at Presque Isle. In spite of interference by major shoreline stabilization structures, and differences between oceanic and lake wave spectra, the nearshore bar field at Presque Isle does closely correspond to the Australian model.     

The sandy beach macrofauna of Gulf of Gabès (Tunisia)

This study describes the macrofauna of the three beaches situated in central Gulf of Gabès (Tunisia): Ouderef, Gabès and Zarrat. The Gulf of Gabès has the largest tides in the Mediterranean and the beaches showed a wide intertidal zone. The beaches were sampled once during the spring low tides of June 2005. A transect was extended at each beach, from above the drift line to below the swash line at five sampled levels; at each level six 0.05 m² replicates were taken to a depth of 30 cm and sieved through a 1-mm mesh, and the organisms collected and preserved. The three beaches showed a different physical environment. Sediment type was medium sand at the steeper Ouderef beach, fine sands at Gàbes beach, and very fine sands at the flatter Zarrat beach. The total number of species collected was 31: 12 crustaceans, 10 polychaetes, four molluscs and five insects. The supralittoral and mediolittoral zones were very different. The supralittoral zone was dominated by Talitrus saltator and insects. The most abundant mediolittoral species were the amphipod crustacean Bathyporeia guilliamsoniana at Ouderef beach (23069 ind.m⁻¹), the surf clam Donax trunculus at Gabès beach (60711 ind.m⁻¹) and the spionid polychaete Scolelepis mesnili at Zarrat beach (18345.6 ind.m⁻¹).     

VLF propagation loss to a buried seismometer

The propagation loss from a shallow underwater explosion to a buried vertically polarized seismometer over the frequency band between 3 and 15 Hz was measured during the 1983 Ngendei Experiment. The data was recorded in 5.5-km-deep water in the southwest Pacific with a triaxial borehole seismograph emplaced 50 m within the basaltic basement. It is found that the average power decays as r^-3 (r is slant range) beyond 30 km and that the propagation loss is minimal between 6-9 Hz. At shorter ranges, the propagation loss is more complicated and exhibits a stronger frequency dependence. Power in the first water wave is estimated for both buried and ocean-bottom sensors. The ocean-bottom power is approximately 7 dB higher than that recorded by the buried sensor, and both exhibit similar decay rates     

Large aperture digital acoustic array

A digital array of 120 acoustic channels 900 m in length has been constructed to study low-frequency (20-200 Hz) ambient noise in the ocean. The array may be deployed vertically or horizontally from the research platform FLIP and the array elements are localized with a high-frequency acoustic transponder network. The authors describe the instrumentation, telemetry, and navigation systems of the array during a vertical deployment in the northeast Pacific. Preliminary ambient noise spectra are presented for various array depths and local wind speeds. Ambient noise in the frequency band above 100 Hz or below 25 Hz increases with local wind speed. However, in the frequency band 25-100 Hz, ambient noise is independent of wind speed and may be dominated by shipping sources     

Microbial Biomass Dynamics Along a Trophic Gradient at the Atlantic Barrier Reef off Belize (Central America)

Abstract. Recent findings indicate that heterotrophic bacteria and not phytoplankton are the most numerous biomass components even in the euphotic zone of oligotrophic, open oceans. In this study it was hypothesized that the microbial biomass components change within a few hundred meters as oligotrophic water flows across the reef and becomes enriched with nutrients. Along a trophic gradient, four stations at the Atlantic Barrier Reef off Belize (Central America) were sampled for microbial biomass components. Phytoplankton biomass (measured as chlorophyll a) ranged from the most oligotrophic station (St. 1) to the most eutrophic station (St. 4) from 6.9–415.5 μg CI"' (assuming a C:chl a ratio of 30): heterotrophic bacterial biomass increased 4-fold (from 10.1–46.4μg C 1^-1), heterotrophic nanoflagellate (HNAN) biomass increased from 4.6-19ug C 1^-1, and cyanobacteria from 0.9-4.5 μg C-1^-1. Production estimates derived from seawater cultures revealed a 5-fold increase in bacterial production from the oligotrophic station (3.7 ug C 1^-1 d^-1) to the eutrophic St. 4 (17.8ug C-1-d1^-1)- Cyanobacterial production rose from 1.1–3.5ug C-1–d^-1 and HNAN production from 0.65-1.13 μg C-1^-1 -d^-1. While cyanobacteria contributed between 13 and 20% to the autotrophic plankton component in the oligotrophic waters, their contribution dropped to about 1 % at the eutrophic stations.