Jungyul Na

Sediments in the East China Sea

This paper describes measurements of sediments during the 2000-2001 Asian Seas International Acoustic Experiment in the East China Sea. A number of techniques were used to infer properties of these sediments, including gravity and piston cores, subbottom profiling using a water gun, long-range sediment tomography, and in situ measurement of conductivity. Historical data from echosounder records and cores showed two regions of surficial sediments in the experimental area: a silty area to the west and a sandy area to the east. The tomography, cores, and water-gun measurements confirm the two surficial sediment regions seen in the historical data and also indicate that the subbottom structure at the experimental site consists of a thin (0-3 m thick) layer of sandy sediment directly beneath the sea floor. Below this layer, there is an extensive package of sediment with relatively uniform acoustic attributes. Core analysis shows that the surface sediment layer varies in compressional wave speed from a low near 1600 m/s in the west side of the experiment area to 1660 m/s in the east side of the experiment area. Long-range sediment tomography inversions show a similar spatial variation in the surface layer properties. In addition, the layer thickness as determined from tomography is consistent with the estimates from subbottom profiling.

Annual and Seasonal Variations of the Sea Surface Heat Fluxes in the East Asian Marginal Seas

Based on the twice-daily marine atmospheric variables which were derived mostly from the weather maps for 18 years period from 1978 to 1995, the surface heat flux over the East Asian marginal seas was calculated at 0.5°×0.5° grid points twice a day. The annual mean distribution of the net heat flux shows that the maximum heat loss occurs in the central part of the Yellow Sea, along the Kuroshio axis and along the west coast of the northern Japanese islands. The area off Vladivostok turned out to be a heat-losing region, however, on the average, the amount of heat loss is minimum over the study area and the estuary of the Yangtze River also appears as a region of the minimum heat loss. The seasonal variations of heat flux show that the period of heat gain is longest in the Yellow Sea, and the maximum heat gain occurs in June. The maximum heat loss occurs in January over the study area, except the Yellow Sea where the heat loss is maximum in December. The annual mean value of the net heat flux in the East/Japan Sea is −108 W/m2 which is about twice the value of Hirose et al. (1996) or about 30% higher than Kato and Asai (1983). For the Yellow Sea, it is about −89 W/m2 and it becomes −75 W/m2 in the East China Sea. This increase in values of the net heat flux comes mostly from the turbulent fluxes which are strongly dependent on the wind speed, which fluctuates largely during the winter season.

Acoustic tomography for monitoring the Sea of Japan: a pilot experiment

A pilot experiment was conducted in the Sea of Japan (also called the East Sea) in September-October 1999, to assess the possibility of using acoustic tomographic techniques for monitoring water mass structure and dynamics. Acoustic m-sequence signals at various frequencies between 250 and 634 Hz were transmitted from bottom-mounted acoustic sources in shallow water off the coast of Vladivostok to vertical-array receiving systems deployed off the north coast of Ulleung-Do island (S. Korea), 558 km to the south. The data are analyzed for temporal correlation, time spread, and transmission loss and are interpreted in terms of a tomographic system for monitoring the East Sea.

240-kHz bistatic bottom scattering measurements in shallow water

High-frequency bistatic sediment scattering experiment was conducted in the shallow waters off the east coasts of Korea. Acoustic data were taken as a function of grazing angle (30/spl deg/, 45/spl deg/, and 60/spl deg/), scattered angle (30/spl deg/, 45/spl deg/, and 60/spl deg/), and bistatic (azimuthal) angle (0/spl deg/, 60/spl deg/, and 120/spl deg/). Besides a flat bottom it was artificially raked so as to produce directional ripples. The measured scattering strengths for a flat bottom were compared to model predictions of D.R. Jackson et al. (1986). The surface reverberation component is seen to dominate over the volume scattering part at the frequency of 240 kHz. Compared to the flat bottom case, the scattering strengths for directional ripples showed lower and higher variation depending on the ripples orientation.

High-frequency bistatic sea-floor scattering from sandy ripple bottom

To obtain the bistatic scattering function on the sandy ripple bottom, high-frequency bistatic sea-floor scattering measurements were made in the shallow waters off the east coast of Korea. A sand ripple field was present at the site, with wavelength generally in the 10-20-cm range. The mean ripple orientation relative to the direction of wave propagation was estimated to be roughly 20/spl deg/-30/spl deg/. Field experiments were made to measure forward (in-plane) and out-of-plane scattering from the ripple bottom. The measured scattering strengths were compared to the predictions of the APL-UW bistatic scattering model. Overall, forward-scattering strength measurements showed favorable comparison with the model predictions. The global scattering characteristics for the ripple bottom gave an augmented out-of-plane scattering.

Diel variation in high-frequency acoustic backscatter from Cochlodinium polykrikoides

The integrated backscatter power (IBP) from Cochlodinium polykrikoides was measured every 15 min by a 5-MHz acoustic system during a 5-day cultivation with an irradiation cycle. IBP increased by 0.6 dB in 5 days, but varied by 0.83 dB during the irradiation cycle. The daily increase and diel variation in IBP were postulated to be affected by an increase in cell numbers and a diel variation in cell biovolume or density via photosynthesis, respectively. Cell division/separation might also affect a total variation in IBP. This study suggests that high-frequency acoustics may be a potential tool for investigating phytoplankton cell functions.

Infrasonic Noise Measurements in Ocean Using Screened Hydrophones

Effects of screening a hydrophone with open-cell foams are determined to reduce the level of infrasonic background noise, which is the self-noise probably induced by the presence of flow around the hydrophone, in shallow and deep seas. The result obtained in shallow sea with bottom-mounted geometry shows that the level of background noise could be reduced by more than 20 dB below 10 Hz if the hydrophone is screened. Also, the result of ship noise measurement conducted in deep sea with drifting geometry shows that a maximum background noise reduction of 15 dB could be achieved in the infrasonic band without affecting the true acoustic signals.

A Passive Fathometer Technique for Bottom Profiling Using Ambient Noise

A passive fathometer technique using ambient noise can be used for estimating water depth and the thickness of the sub-bottom layer. This technique uses the correlation of sea surface-generated noise estimated for signals received by a vertical hydrophone array via a beamforming process. In this paper, the nested vertical line array (N-VLA) was used for effective broadband beamforming, and the beamforming result for each sub-array was applied to the passive fathometer technique. The estimated results are presented here and compared to the sediment information obtained by chirp sonar profiles and coring analysis.

Shift of the interference extrema of low-frequency acoustic propagations near the axis of a deep sound channel

Broadband interference patterns measured from acoustic propagations near the axis of a deep sound channel are interpreted. Analyses using mode theory for the waveguide with bilinear sound speed profiles show that the increase in sound speed without gradient variation shifts the positions of intensity maxima to higher frequencies in a fixed range whereas the increase in the gradient shifts the maxima to lower frequencies. Analytic results imply that the frequency shift of intensity extrema appearing in the measurements could be explained by the increase in the sound speed gradient above the axis of the deep sound channel.

Observations of 5‐MHz acoustic backscattering from Cochlodinium polykrikoides blooms in coastal waters.

Measurements of 5‐MHz volume‐backscattering strength of harmful algal blooms were made in summer 2008 at sites off the coast of Geumo island in southern Korea. The most abundant phytoplankton in the blooms was the chain‐forming, toxic dinoflagellate Cochlodinium polykrikoides, which accounted for about 70% of total abundance. The measured backscattering strengths are compared with the abundance of phytoplankton estimated using an optical microscopy from water samples collected simultaneously with the acoustic measurements. The results show that the scattering strengths are highly correlated with dinoflagellate concentrations, suggesting that the acoustic method may be a useful tool for detecting the harmful algal blooms in their initial stage. Finally, the measurements are compared with the scattering strengths predicted by a fluid‐sphere scattering model using a single‐cell size of C. polykrikoides and the equivalent cell size of a sphere the same size as the chain‐forming cells. The results imply that t...