The southernmost Okinawa Trough back-arc basin is an active and young basin formed just after the collision of the Philippine Sea Plate against the Eurasian continental margin. The back-arc extension ...occurs intensively because of the southward or southeastward migration of the southernmost Ryukyu Arc, or the roll-back of the Philippine Sea Plate. To better understand the active tectonics and volcanism of the southernmost Okinawa Trough, we have conducted deep-tow sub-bottom profiler and side-scan sonar surveys across the back-arc basin. Our results show that the volcanism of the southernmost Okinawa Trough is distributed in the southern half of the back-arc basin and occurs along some linear or branched zones roughly parallel to the trough axis. Volcanic seamounts are obviously located along the central depression of the basin and their sizes show lateral variation. On the other hand, the northern half of the southernmost Okinawa Trough back-arc basin has almost no volcanic activity, but contains more brittle normal faults. It is noted that gas plumes out of seafloor are generally associated with hydrothermal mounds or activities, instead of volcanic seamounts. We suggest that the more complete rifting of the southernmost Okinawa Trough back-arc is limited to the east of ~122o30′E. To the west of ~122o30’E, the back-arc extension could be still influenced by the inherited NE-SW trending structures of the continental crust created during the former Taiwan orogeny in this area.
•The volcanism in the southernmost Okinawa Trough is distributed in the southern half of the basin.•The volcanism occurs in linear or branched patterns roughly parallel to the trough axis.•Gas plumes out of seafloor are generally associated with hydrothermal activities.•The more complete back-arc rifting of the Okinawa Trough is limited to the east of ~122o30’E.
Side-scan sonar is designed and used for a variety of survey work, in both military and civilian fields. These systems provide acoustic imageries that play a significant role in a variety of marine ...and inland applications. For this reason, it is extremely important that the recorded sonar image is characterized by high resolution, detail and sharpness. This article is mainly aimed at the demonstration of the impact of side-scan sonar resolution on the imaging quality. The article also presents the importance of acoustic shadow in the process of analyzing sonar data and identifying underwater objects. The real measurements were carried out using two independent survey systems: hull-mounted sonar and towed side-scan sonar. Six different shipwrecks lying in the Baltic Sea were selected as the objects of research. The results presented in the article also constitute evidence of how the sonar technology has changed over time. The survey findings show that by maintaining the appropriate operational conditions and meeting several requirements, it is possible to obtain photographic-quality sonar images, which may be crucial in the process of data interpretation and shipwreck identification.
The paper presents results of bathymetric surveys in the remote nearshore zone of the south Baltic (c.a. 1–2 Nm off the shoreline at depths of 16–20 m). Measurements were collected twice in the ...vicinity of the Coastal Research Station (CRS) in Lubiatowo (Poland), first on November 2017 and then on December 2018. In addition the wave and wind parameters were collected during the “in-between” period of bathymetric measurements. The wind velocities and directions were measured at CRS Lubiatowo while the wave data were collected near CRS Lubiatowo, 1.5 Nm from the shoreline, at the depth of 18 m. The analysis is based on a differential map determined from the bathymetric data, as well as on hydrodynamic conditions between the measurements. The results show changes in the seabed elevation ranging from a few to 70 cm in a year. Sonar measurements were also made in 2017. The images revealed bottom ripples with an approximate height of 5–20 cm and length of 100–200 cm. Theoretical studies focus on the dimensionless shear stresses (Shields parameter) computed from wave and wind data. The uniqueness of this research lies in the fact that at such depths sediment movement has recently been proven and it brings more light to this yet not well-known topic. This paper proves the validity and shows the evidence of recent assumptions of the appearance and migration of rippled forms at the bottom of non-tidal or micro-tidal basins.
•The differential map shows bed changes up to 70 cm/year at relatively deep waters.•Discovery of moving seabed forms with 5–20 cm height and 100–200 cm length in a year.•The evidence of bedforms formation under wave-current induced hydrodynamics.
Recent sonar surveys of Okanagan Lake, in the central interior of British Columbia, revealed the presence of linear, contour-parallel furrows at depths of 50-130 m. Underwater video imagery shows ...these furrows to be approximately 0.5-1 m wide by 0.5-1 m deep with variable lengths extending 1-100 m. Their cross-sectional topology varies from semi-rectangular features with steep-walled sides that expose weak laminations in the sediments to shallow U-shaped troughs that appear to be infilled by lacustrine mud. The most likely origin of the furrows is extension deformation (i.e., tension fracturing) caused by periodic failure along the steeply inclined marginal slopes of this deep, glaciated lake basin. A positive feedback mechanism leading to arrested downslope movement, based on the recent findings reported in Carey et al. (2019), is suggested to apply. Less likely alternative explanations are considered and dismissed. The furrows highlight the nature of long-term geomorphic processes in lacustrine environments that contribute to sediment transport and deposition on the lake bed but are not directly linked to the action of surface waves and currents.
Underwater archaeological work at Soli-Pompeiopolis in 2017 has challenged previously published descriptions of the harbour works. Excavation, sonar survey, and aerial drone survey have provided an ...asymmetric plan of the harbour moles with an entrance to the east, which reflects local knowledge of the wind and wave conditions. The presence of beachrock blocking the harbour is explained by Beaufort's depiction in 1811-1812 of a defunct sluice, no longer visible on the site.
Following the successful utilisation of lake preserved sub-fossil woody material to extend living Scots pine chronologies in Scandinavia, ongoing research in the Scottish Highlands aims to build a ...similar multi-millennial long climatically sensitive pine chronology. This paper details explorative research testing the use of sonar methods to facilitate the search for sub-fossil material in lake environments. Although the method clearly identifies elongate anomalies that are consistent with submerged tree stems in water depths >1.5m, it does not allow the identification of sub-fossil wood remnants in shallow water (<1.0m) or heavily vegetated bays. Therefore, for the successful survey of lakes, a combination of traditional and sonar methods must be applied. Ongoing research will now explore the utilisation of these methods to more remote locations where boat access is not possible.
Assessment of the fish stock in the Theodorushaven was conducted with a dual‐beam hydroacoustic system. Species composition determined by a seine and trawl fishery showed large concentrations of fish ...in the harbour during winter. The hydroacoustic survey showed that the distribution of fish in the harbour area was stratified with fish near the bottom. The overall density in the harbour area was 0.67 fish m–3. The density in the associated channel area was estimated at 0.1 fish m–3. The coefficient of variation for the estimated total fish abundance was >25%.Fish >25 cm were not caught in the Theodorushaven fishery, although analysis of the sonar data indicated their presence. Fishery data from the adjacent Volkerak‐Zoommeer showed that bream was the dominant species. Therefore, the biomass estimate of the stock >25 cm in the Theodorushaven was based on the standard length–weight relationship of bream. This resulted in maximum biomass estimates of 5800 kg ha–1 for the harbour area and 1100 kg ha–1 in the channel area. The minimum biomass estimate, based on the assumption that the more slender zander was the dominant species >25 cm, gave values of 4100 kg ha–1 in the harbour area and 780 kg ha–1 in the channel area.
Tang, Y., Nishimori, Y., and Furusawa, M. 2009. The average three-dimensional target strength of fish by spheroid model for sonar surveys. – ICES Journal of Marine Science, 66: 1176–1183. When ...surveying fish schools by sonar, the fish are insonified from various directions. Because the fish target strength (TS) has three-dimensional directivity, according to its orientation relative to the sonar beam, the TS must be appropriately averaged. By connecting the geometries of the sonar beam and the fish body, the relationship between the apparent orientation of fish, as viewed by the sonar, and the actual orientation in space is derived. Using this relationship, equations for calculating the three-dimensional-averaged TS (3D) are presented. A prolate-spheroid, modal-series, scattering model is then used to determine the characteristics of 3D against various parameters, such as fish attitudes, fish length, sonar frequency, and method of beam scanning. The model is evaluated with two boundary conditions at the spheroid surface: a gas-fluid boundary relevant to fish with a swimbladder, and a fluid-fluid boundary relevant to fish without a swimbladder. The results reveal that 3D varies greatly with the horizontal aspect (yaw angle), but only slightly with the vertical aspect (pitch angle) of the fish. The difference in 3D between the side-on aspect and the end-on (head or tail) aspect increases with the ratio of the fish length to the acoustic wavelength.
The submarine flanks of Anatahan volcano were surveyed comprehensively for the first time in 2003 and 2004 with multibeam and sidescan sonar systems. A geologic map based on the new bathymetry and ...backscatter data shows that 67% of the volcano's submarine flanks are covered with volcaniclastic debris and 26% is lava flows, cones, and bedrock outcrops. The island of Anatahan is only 1% of the volume of the entire volcano, which has a height from its submarine base of 3700 m and an average diameter of ∼35 km. NE Anatahan is a prominent satellite volcano located 10 km NE of the island, but it is only 6% of Anatahan's volume (40 km
3 vs. 620 km
3). Seventy-eight submarine eruptive vents are mapped associated with lava flows and cones between depths of 350 and 2950 m, and 80% of these vents are located in a cluster on the east flank of the volcano. The distribution of cones and lava flows vs. depth suggests a possible change in eruptive style from explosive to effusive between 1500 and 2000 m. Eruptive vents below 2000 m have produced mostly lava flows. There is no evidence of major landslides on the submarine flanks of Anatahan volcano, in contrast to many basaltic islands and seamounts, suggesting that mass wasting at felsic oceanic arc volcanoes may be characterized by sediment flows of unconsolidated volcaniclastic debris instead of mass movements of relatively large intact blocks.
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