Absorption In Sea Water Research Articles

Low frequency sound absorption in the Arctic Ocean: Potential impact of ocean acidification

The principal mechanism for low frequency absorption in seawater is a boron reaction that is pH dependent; the lower the pH, the lower the absorption. Twenty seven years ago, Mellen et al. [J. Acoust. Soc. Am. 82, S30 (1987)] computed the low frequency sound absorption for the Arctic Ocean. Since the time the carbon dioxide (CO2) level in the atmosphere has been continually increasing. Experts predict that the resulting ocean acidification may increase by up to 170% this century. Acoustically, the Arctic Ocean is most sensitive to rapid change not only because the cold water readily absorbs CO2, but also because the sound channel axis is at or near the surface. The range of reduction in the low frequency sound absorption is presented based on possible future acidification scenarios, mindful that this is just one component of a complex evolution that is occurring in the Arctic.

Dispersed air flotation as a pretreatment process for seawater desalination

Pretreatment of seawater before reverse osmosis (RO) processes with dispersed air flotation involves a change in oxygen concentration in seawater. In order to understand this phenomenon, a series of experimental aeration studies was conducted. Aeration tests were performed under different operating conditions of seawater temperature, flow rate of the air and depth of the water in the aeration tank. The experimental results show that the overall mass transfer coefficient increases by increasing air flow rate. A measured increase in the overall mass transfer coefficient was observed with elevation of the temperature from 24 to 38 °C. The estimated value of activation energy of oxygen absorption in seawater is 42.5 kJ/mol. The value of the overall mass transfer coefficient is found to be affected by diffuser type and stirrer speed as these parameters influence the bubble size and the surface area available for mass transfer of oxygen from air bubbles to seawater.

The feasibility of detecting a large plume of oil drops using low‐to‐mid frequency active sonar.

The use of long range active sonar in the frequency range 1–10 kHz is considered with regard to its potential for detecting large quantities of submerged oil drops. In the scenario considered, the drops are modeled as fluid spheres of radius of order 1 cm, for which the acoustic wavelength is large compared with the oil drop radius. This means that the volume backscattering strength of a cloud of oil (of given volume fraction) increases rapidly with increasing frequency. The implied requirement for the use of high frequency sound is balanced against the preference for low frequencies in order to reduce absorption in seawater. Detection performance is simulated for a background of bottom reverberation and ambient noise. It is shown that the optimum detection frequency depends on drop size, coverage distance, and ambient noise level. At the time of writing, the authors are exploring the possibility of carrying out measurements during the summer of 2010 to test the modelling results. If available in time, results from these measurements will be presented. The possibility of detection of oil by forward scattering in a bistatic geometry is also considered.

The research on measuring the coefficient of sound absorption in turbid seawater

When naval mine-hunting sonars and side-scan surveying sonars operating at 20∼60 kHz are working in shallow coastal waters, the visco-thermal absorption of sound by suspended mud in the water may greatly decrease their detection properties. This kind of water is also characterized as turbid seawater. However, the research on measuring the coefficient of sound absorption in turbid seawater has little been done. The main difference between turbid seawater and clear seawater is that there are mud particles suspending in turbid seawater. It is also the main reason why sound absorption in turbid seawater is greater than that in clear seawater. In the paper, the coefficient of sound absorption at 20̃60 kHz in turbid seawater has been measured by the reverberation technique. Results demonstrate that if the concentration of the mud is bellow 0.11 kg/m3, the mud in turbid seawater doesn't cause additional absorption. When the concentration of the mud is between 0.14 and 0.49 kg/m3, the coefficient of sound absorption in turbid seawater is as twice at least as that in clear seawater.

Use of Seawater Scrubbing for SO2Removal from Marine Engine Exhaust Gas

The mechanism of SO2 absorption in seawater is treated. Emphasis is on applications of scrubbing of marine engine exhaust gas containing SO2. The formulated model is used to predict the influence of various parameters on SO2 absorption efficiency, e.g., seawater temperature, partial pressure of SO2, seawater salinity, and seawater alkalinity. It is found that the absorption capacity of standard seawater is approximately twice that of brackish water with close to zero salinity. The absorption capacity decreases with both decreasing salinity and alkalinity. Different scenarios in which the required water supply rate for a given SO2 cleaning efficiency is calculated. It is found that a 66% cleaning efficiency, corresponding to meeting the limits of SOx emission control areas (SECA) when operating on a fuel containing 4.5% w/w sulfur, requires a minimun water supply rate of 40–63 kg/(kW h) depending on the seawater composition in terms of salinity and alkalinity. Such data are essential in judging the operating cost of seawater scrubbing compared to alternative methods.

Fred Fisher’s high-pressure work with eyewash and epsom salts

Starting in 1957 Fred Fisher led research programs devoted to high-pressure measurements related to the physical chemistry of sound absorption in seawater due to magnesium sulfate and other salts. As he put it, he spent his professional lifetime squeezing epsom salt. His interest in the low-frequency anomalous sound absorption in the ocean below 1 kHz led to the discovery of boric acid as the cause of the low-frequency relaxation. This paper is a short review of Fred Fisher’s contributions to our knowledge of sound absorption in seawater, based in part on his carefully hand-written lecture notes and numerous low-pressure discussions.

Absorption of sound in seawater and ocean ambient noise, the scientific passions of Fred Fisher

Fred Fisher made seminal contributions to ocean acoustics in the understanding of the absorption of sound in seawater and ocean ambient noise. Laboratory data and long-range sound propagation data revealed excess acoustic absorption in seawater. Fred Fisher spent much of his scientific career, beginning with his Ph.D dissertation, teasing out the contributions of various seawater components to sound absorption, and his work on this topic set the standard for understanding and modeling these phenomena. Ambient noise is an important aspect of underwater signal detection and is the focus of recent concerns about disturbance of marine organisms. Fred Fisher made important contributions to ambient noise studies by conducting measurements of vertical directionality, thereby testing models for ambient noise production. The value of archival ambient noise data and recent increases in ambient noise will be discussed.

Acoustic propagation in the subsurface sound channel

Experimental data obtained in the kilohertz frequency band for the sound propagation in the subsurface channel formed by the wind-caused mixing of subsurface waters are discussed. The data were obtained in different years in the northeastern region of the Atlantic Ocean, where the subsurface waters down to the depths of 40–70 m are mixed by both wind waves and the swell that arrives from distant ocean areas. The hydrological conditions in the subsurface waters of this region are characterized by a good reproducibility. The spatial structure of the sound field and the attenuation of sound propagating in the subsurface channel are analyzed. The origin of the additional attenuation (in comparison with the absorption in sea water) is discussed. The data of our experiments are compared with those obtained by other experimenters and with the calculations performed using the computer code by Avilov. The necessity of improving the computer codes to allow for the scattering of sound beyond the channel under the influence of the swell, whose parameters are unrelated to the wind regime at the experimental site, is emphasized.

Long-range sound propagation in the northeastern Atlantic

Experimental data on the long-range propagation of explosion-generated signals are analyzed. The experiments were performed in the northeastern Atlantic under the conditions of a two-axis underwater sound channel. The sound field in the upper channel was governed by the vertical redistribution of the ray structure and sound energy under the influence of a smooth increase in the depth of the channel’s axis along the propagation path. The explosions were produced in the upper sound channel at a depth of 200 m, which was constant along the path. The time structure of the sound field is analyzed for the upper channel (a reception depth of 200 m) and for deeper layers lying somewhat below the boundary between the upper and lower sound channels (a reception depth of 1200 m). The deviation of the decay law obtained for the sound field level in the upper channel from the cylindrical law is used to estimate the attenuation coefficient. The low-frequency (several hundreds of hertz) attenuation coefficients experimentally determined with allowance for the sound field redistribution agree well with the calculated sound absorption in seawater. The attenuation coefficients determined by the differential method also agree well with the absorption calculated by the formulas proposed earlier. The analysis of the time structure of the sound field near the boundary between the upper and lower channels reveals a permanent insonification of this horizon by weak water-path signals propagating with the velocity typical of the signals traveling in the upper channel.

Catalytic Oxidation of S(IV) in Seawater Slurries of Activated Carbon

Flue gas desulfurization (FGD) by means of SO2 absorption in seawater is a well-known process with a main drawback; due to the low S(IV) oxidation rate at the low pH values of the absorption tower effluent, a large oxidation basin is required. Laboratory reactor tests, in which a commercially available activated carbon was used, showed a significant catalytic effect. It is shown that the kinetic equation for the catalytic oxidation of S(IV), in seawater slurries of activated carbon, is first-order with respect to S(IV) and zero-order with respect to oxygen. The dependence of the kinetic constant with respect to pH, temperature, and catalyst size particles were also obtained.

Long-range sound propagation in the shallow-water part of the Sea of Okhotsk

Experimental data on the long-range propagation of explosion-generated sound signals in the shallow-water northern part of the Sea of Okhotsk are analyzed. The propagation conditions in this region are characterized by a fully-developed underwater sound channel that captures the rays crossing the channel axis at angles lower than 3°. The experimental data reveal a small increase in the duration of the sound signal in proportion to the range with the proportionality factor lower than 0.00025 s/km. The frequency dependence of attenuation exhibits a pronounced minimum whose position on the frequency axis is close to the critical frequency of the first “water” mode (about 160 Hz). The increase in the attenuation coefficient at lower frequencies is confirmed by the field calculations performed with the wave-field computer code and is explained by the sound energy loss in the bottom sediments. At frequencies higher than 200 Hz, as in the Baltic Sea, the most probable reason for the attenuation to exceed the absorption in sea water is sound scattering by internal waves.

A Kinetic Study of the Oxidation of S(IV) in Seawater

Flue gas desulfurization by means of SO2 absorption in seawater is a well-known process. However, it can be optimized if the oxidation kinetics of dissolved S(IV) is known. Laboratory reactor experiments in which the pH and temperature were controlled, determined the oxidation kinetics of S(IV) in seawater. This study conclusively shows that the order with respect to S(IV) is one and that the order with respect to oxygen is zero. The kinetic constant depends greatly on the temperature and on the pH; consequently, the activation energy and a relationship between the kinetic constant and the hydrogen proton concentration (2 < pH < 6) were also obtained.

A simplified formula for viscous and chemical absorption in sea water

A simplified expression is presented for predicting the absorption of sound in sea water, retaining the essential dependence on temperature, pressure, salinity, and acidity of the more complicated formula on which it is based [R. E. Francois and G. R. Garrison, “Sound absorption based on ocean measurements. Part II: Boric acid contribution and equation for total absorption,” J. Acoust. Soc. Am. 72, 1879–1890 (1982)]. The accuracy of the simplified formula is demonstrated by comparison with the original one for a range of oceanographic conditions and frequencies between 100 Hz and 1 MHz.

The pH of the North Atlantic Ocean: Improvements to the global model for sound absorption in seawater

At frequencies below 1 kHz, sound absorption coefficients in the ocean are a function of pH, and at higher frequencies they are dependent upon MgSO4. The pH dependent terms are attributable to relaxation of B(OH)3 and MgCO3 species, and the ensemble effect has been approximated (Mellen et al., 1987a) as α = α1(MgSO4) + α2(B(OH)3) + α3(MgCO3), where α is the absorption coefficient in decibels per kilometer, and αn = (S/35)An[ƒ2ƒn/(ƒ2 + ƒn2)], where S is the salinity, An is the amplitude of each component and depends on pH, ƒ is the frequency, and ƒn is the relaxation frequency. Overall accuracy of ±15% in a requires that pH be known to 0.05 pH units. The presently used oceanic pH field for sound absorption models is derived from a combination of Geochemical Ocean Sections Study (GEOSECS) data and Soviet data from the 1978 Gorshkov atlas for the North Atlantic where GEOSECS data are absent. We compare the North Atlantic fields with the well‐constrained Transient Tracers in the Ocean North Atlantic CO2 data set and find large differences. We further show that sufficiently strong correlations exist between CO2 system variables and other more commonly available hydrographic properties and that improved renditions of the pH field in other regions of the ocean are possible once equivalent local correlations are established, thus leading to greatly improved estimates of the sound absorption field.

A cylindrical resonator method for the investigation of low-frequency sound absorption in sea water

In this paper, a cylindrical resonator technique developed for the investigation of low-frequency sound absorption in sea water and synthetic media is described. The lowest usable measuring frequency has been extended down to about 3 kHz. Due to the low residual losses of the resonator system and to the high reproducibility of the energy decay time measurement, it was possible not only to measure excess absorption at normal sea water concentration and at low pH, but also, under certain conditions, to determine both the relaxation frequency and maximum absorption per wavelength of a single definite relaxation with acceptable accuracy, even though absorption was being measured mostly at two frequencies only. As examples, some experimental results are presented. An error analysis of the data is given.

Effect of pressure on sound absorption in seawater at 0°C

Earlier measurements of the effect of pressure on sound absorption by magnesium sulfate in seawater by Hsu at 0 °C in the laboratory were limited to pressures of just over 300 bars. The pressure was limited by the strength of the 100‐liter titanium sphere, a spare ballast tank from the Woods Hole submarine ALVIN. To obtain data at higher pressures at 0 °C, measurements were recently made in 0.5 M magnesium sulfate solutions in a cylindrical reesonator. Because the magnesium sulfate relaxation frequency is independent of pressure, measurements are necessary at only one frequency. This was done in the 100‐kHz region and a sodium sulfate solution was used to correct for wall losses in the cylinder. Mode interference problems as well as a shutdown in the building air circulation system, which gave rise to temperature control problems, degraded the data. Nevertheless, enough data were obtained at higher pressures (600 to 1300 bars) to be useful in assessing the effect of pressure on seawater sound absorption a...

A comparison between low-frequency sound absorption in seawater and high-frequency sound absorption in serum albumen

Although large scale (&amp;gt; 1000 km), low-frequency (&amp;lt;20000 Hz) sound absorption measurements in seawater [Mellen et al., J. Acoust. Soc. Am. 74, 987–993 (1983)] might appear to have nothing in common with high-frequency sound absorption in serum albumen [Barnes et al., J. Acoust. Soc. Am. 83, 2393–2404 (1988)], the results show some intriguing similarities that may be due to a common chemistry. Specifically, both show a pH dependence, a buffered reaction involving a carboxyl group, and the need of calcium to promote the principal reaction. However, boric acid, which is essential for seawater absorption, is apparently not a factor in serum absorption. A comparison is presented in the hope of stimulating a dialogue with the bioacoustics community to explore a possible linkage.

Sound absorption in Arctic regions

Sound absorption in seawater involves relaxations of magnesium sulfate, boric acid, and magnesium carbonate. Absorption varies with both region and depth, due mainly to the pH dependence of the boric acid relaxation. The nominal seawater pH range is 7.7–8.3 and magnitudes can vary over nearly a factor of 4 at low frequencies. A global model for World Ocean prediction has been developed, employing contour charts of the pH factor at several depths. Profiles for integration of loss along ray paths are generated by algorithm. The pH range in the Arctic is nominally 8.0–8.3 and similar contour charts are provided for the pH factor in these regions. Although long-range propagation is generally limited by under-ice scattering, absorption can be the dominant loss mechanism under ice-free conditions and for ranges less than 50 km at sonar frequencies with an ice cover. Predicted absorption spectra are presented and compared with the limited data available. [Work supported by NUSC/NL.]

Slow relaxations in sea water: T‐jump measurements

Sound absorption in sea water involves chemical equilibria with rather slow relaxation rates. The low‐frequency range of the resonator method is limited by practical considerations of size and experimental conditions must usually be altered to place the relaxation frequency within the measurement window. For relaxations involving acids or bases, the temperature‐jump pH method permits rate measurements below a limit set by indicator response (∼ 105/s). Recent T‐jump studies confirm unambiguously that the low‐frequency (∼ 1 kHz) relaxation mechanism in sea water is an acid‐base exchange between boric acid and carbonate [P. Mallo et al., J. Phys. Chem. (in press) (1984)]. Further investigations of the boric acid exchange in similar Lewis systems, e.g., ammonia, reveal single relaxations with comparable rates. The magnesium carbonate ion‐pair equilibrium—a minor component of sea water absorption at mid‐frequencies (∼ 10 kHz)—shows more complex behavior with several identifiable relaxations in the measurement range. The magnesium sulfate relaxation (∼ 100 kHz) is unsuited to the method because of both speed and lack of pH dependence. Visiting scientist; home address: 117 Boston Post Road, Old Lyme, CT 06320.

Pressure dependence of sound absorption in synthetic seawater at 0 °C

Measurements have been made of sound absorption in Lyman and Fleming seawater at 1 and 307 atm. Results for maximum absorption per wavelength (αλ)max/106 are 45.8 ± 0.4 at 1 atm and 28.9 ± 0.6 at 307 atm. The relaxation frequency of ∼39 kHz appears to be independent of pressure as had been observed earlier at 25 °C. Our results are in excellent agreement with the predictions of Francois and Garrison for 0 °C for both sound absorption and relaxation frequency. Based on these results together with the earlier ones at 25 °C reported by Hsu and Fisher [J. Acoust. Soc. Am. 74, 564–569 (1973)], we find a temperature dependence for both absorption and relaxation frequency different from that reported by Francois and Garrison. These differences will be discussed. The following equation represents our results for absorption (s = ± 0.8): (αλ)max/106 = 45.6 + 0.652(t/°C) − 0.0533(P/atm). [Work supportedby the Office of Naval Research, Code 400R.]