共查询到20条相似文献，搜索用时 69 毫秒
1.
High-resolution bathymetric data obtained in July 1996 during a survey of the Kick-'em-Jenny submarine volcano north of Grenada in the Lesser Antilles revealed changes in the structure of the volcanic edifice compared to previously available surveys. The volcano's summit, at 178 m below sea level, was found to be approximately 18 m farther from the surface than was reported by Bouysse et al. (1988) and others. No dome was observed. Instead, an open crater, surrounded by walls that dropped significantly in elevation from one side to the opposite, suggest that eruptions, earthquakes, rockfalls or explosions may have altered the structure since the last detailed survey. The deepest contour of the volcano's crater was found 106 m below the summit. 相似文献
2.
The paper presents the results of a study on the geomorphic structure, tectonic setting, and volcanism of the volcanoes and volcanic ridges in the deep Central Basin of the Sea of Japan. The ridges rise 500–600 m above the acoustic basement of the basin. These ridges were formed on fragments of thinned continental crust along deep faults submeridionally crossing the Central Basin and the adjacent continental part of the Primorye. The morphostructures of the basin began to submerge below sea level in the Middle Miocene and reached their contemporary positions in the Pliocene. Volcanism in the Central Basin occurred mostly in the Middle Miocene–Pliocene and formed marginal-sea basaltoids with OIB (ocean island basalt) geochemical signatures indicating the lower-mantle plume origin of these rocks. The OIB signatures of basaltoids tend to be expressed better in the eastern part of the Central Basin, where juvenile oceanic crust has developed. The genesis of this crust is probably related to rising and melting of the Pacific superplume apophyse. 相似文献
3.
J. Frisbee Campbell 《Geo-Marine Letters》1984,4(1):31-36
Evidence collected on the Kohala Terrace off northwest Hawaii shows that the volcanic edifice has subsided rapidly. This rapid subsidence, combined with Pleistocene glacio-eustactic sea level changes, twice produced relative sea level changes sufficient to drown coral reefs. Such activity can be an explanation for the paradox of reef drowning, i.e., reefs should be able to grow fast enough to avoid drowning by normal subsidence. This explanation can only accommodate the paradox under restrictive circumstances where rapid island subsidence is combined with rapid sea level changes. Because of these restrictive conditions, the general paradox of reef drowning remains. 相似文献
4.
Eleven seismic reflection profiles across Shirshov Ridge and the adjacent deep-water sedimentary basins (Komandorsky and Aleutian Basins) are presented to illustrate the sediment distribution in the western Bering Sea. A prominent seismic reflecting horizon, Reflector P (Middle—Late Miocene in age), is observed throughout both the Aleutian and Komandorsky Basins at an approximate subbottom depth of 1 km. This reflector is also present, in places, on the flanks and along the crest of Shirshov Ridge. The thickness of sediments beneath Reflector P is significantly different within the two abyssal basins. In the Aleutian Basin, the total subbottom depth to acoustic basement (basalt?) is about 4 km, while in the Komandorsky Basin the depth is about 2 km.Shirshov Ridge, a Cenozoic volcanic feature that separates the Aleutian and Komandorsky Basins, is an asymmetric bathymetric ridge characterized by thick sediments along its eastern flank and steep scarps on its western side. The southern portion of the ridge has more structural relief that includes several deep, sediment-filled basins along its summit.Velocity data from sonobuoy measurements indicate that acoustic basement in the Komandorsky Basin has an average compressional wave velocity of 5.90 km/sec. This value is considerably larger than the velocities measured for acoustic basement in the northwestern Aleutian Basin (about 5.00 km/sec) and in the central Aleutian Basin (5.40–5.57 km/sec). In the northwestern Aleutian Basin, the low-velocity acoustic basement may be volcaniclastic sediments or other indurated sediments that are overlying true basaltic basement. A refracting horizon with similar velocities (4.6–5.0 km/sec) as acoustic basement dips steeply beneath the Siberian continental margin, reaching a maximum subbottom depth of about 8 km. The thick welt of sediment at the base of the Siberian margin may be the result of sediment loading or tectonic depression prior to Late Cenozoic time. 相似文献
5.
The outer slope of the eastern reefs of Mauritius shows: from 0 to 15 m depth, a spurs-and-grooves zone of coral morphogenesis, sometimes replaced by a flagstone of coral morphogenesis (down to 20 m) or a spurs-and-grooves zone of volcanic morphogenesis; from 20 to 50 m, a volcanic flagstone with a thin coral-built layer and a buttresses-and-valleys zone of volcanic morphogenesis. The living cover of the upper part (0–15 m) is also quite similar to the cover of the leeward side. On the contrary, the volcanic basement in the lower part is poorly covered by reef-building organisms, while soft or slightly calcified algae are of widespread occurrence. 相似文献
6.
Analysis of multi-channel seismic data from the northern East China Sea Shelf Basin (ECSSB) reveals three sub-basins (Socotra, Domi, and Jeju basins), separated by structural highs (Hupijiao Rise) and faulted basement blocks. These sub-basins show a typical rift-basin development: faulted basement and syn-rift and post-rift sedimentation separated by unconformities. Four regional unconformities, including the top of acoustic basement, have been identified and mapped from multi-channel seismic data. Faults in the acoustic basement are generally trending NE, parallel to the regional structural trend of the area. The depths of the acoustic basement range from less than 1000 m in the northwestern part of the Domi Basin to more than 4500 m in the Socotra Basin and 5500 m in the Jeju Basin. The total sediment thicknesses range from less than 500 m to about 1500 m in the northwest where the acoustic basement is shallow and reach about more than 5500 m in the south.Interpretation of seismic reflection data and reconstruction of three depth-converted seismic profiles reveal that the northern ECSSB experienced two phases of rifting, followed by regional subsidence. The initial rifting in the Late Cretaceous was driven by the NW-SE crustal stretching of the Eurasian Plate, caused by the subduction of the Pacific Plate beneath the Eurasian Plate. Extension was the greatest during the early phase of basin formation; estimated rates of extension during the initial rifting are 2%, 6.5%, and 3.5% in the Domi, Jeju, and Socotra basins, respectively. A regional uplift terminated the rifting in the Late Eocene-Early Oligocene. Rifting and extension, although mild, resumed in the Early Oligocene; while fluvio-lacustrine deposition continued to prevail. The estimated rates of extension during the second phase of rifting are 0.7%, 0.8%, and 0.5% in the Domi, Jeju, and Socotra basins, respectively. A second phase of uplift in the Early Miocene terminated the rifting, marking the transition to the post-rift phase of regional subsidence. Regional subsidence dominated the study area between the Early Miocene and the Late Miocene. An inversion in the Late Miocene interrupted the post-rift subsidence, resulting in an extensive thrust-fold belt in the eastern part of the area. Uplift and subsequent erosion were followed by regional subsidence. 相似文献
7.
Marie Tharp 《Marine Geodesy》2013,36(1):63-65
Mixed layer depth (MLD) is an important parameter in the study of air‐sea interaction, acoustic propagation, and fisheries. With the onset of the southwest monsoon, a jetlike surface water flow develops from west to east along the equatorial Indian Ocean (EIO) creating an upward west to east sea surface slope. This in turn creates a slope in MLD in the opposing direction. In this paper, emphasis is placed on obtaining monthly coefficients between MLD and sea level for three regions across the EIO. Using these coefficients, MLD has been estimated from Geosat altimeter data. MLD in this region has been computed with an RMS error of 16 m, from altimeter data, as verified by TOGA in situ profiles. 相似文献
8.
一种分层海底反向散射模型 总被引：1，自引：1，他引：0
In order to predict the bottom backscattering strength more accurately, the stratified structure of the seafloor is considered. The seafloor is viewed as an elastic half-space basement covered by a fluid sediment layer with finite thickness. On the basis of calculating acoustic field in the water, the sediment layer, and the basement, four kinds of scattering mechanisms are taken into account, including roughness scattering from the water-sediment interface, volume scattering from the sediment layer, roughness scattering from the sediment-basement interface,and volume scattering from the basement. Then a backscattering model for a stratified seafloor applying to low frequency(0.1–10 kHz) is established. The simulation results show that the roughness scattering from the sediment-basement interface and the volume scattering from the basement are more prominent at relative low frequency(below 1.0 kHz). While with the increase of the frequency, the contribution of them to total bottom scattering gradually becomes weak. And the results ultimately approach to the predictions of the high-frequency(10–100 kHz) bottom scattering model. When the sound speed and attenuation of the shear wave in the basement gradually decrease, the prediction of the model tends to that of the full fluid model, which validates the backscattering model for the stratified seafloor in another aspect. 相似文献
9.
在确定温跃层三要素 (深度 (上界深度 )、厚度和强度 )及测站温度垂直最大梯度的基础上 ,分别计算了南沙深水测站 (水深大于 1 0 0 0m)在温跃层上界深度层范围内的平均温度、在温跃层下界深度以下自 3 0 0m层至 80 0m层之间的平均温度。分析表明 ,在温跃层上界深度范围内 ,海水平均温度的水平分布明显显示出低温海水自南沙的西北部向东南部缓慢推进之势 ,似是东北季风驱动的结果。温度垂直梯度越大 ,它在垂直方向上阻碍上层海水的热量往深层扩散的能力就越强。 相似文献
10.
Gary Hoffmann Eli Silver Simon Day Neal Driscoll Bruce Appelgate 《Marine Geophysical Researches》2009,30(4):229-236
Extinct volcanic islands in the Bismarck volcanic arc are fringed by well-developed coral reefs. Drowned platforms offshore
from these islands provide evidence for subsidence in the central section of the arc, north of the Finisterre Terrane–Australia
collision. Bathymetric and backscatter data collected onboard the R/V Kilo Moana in 2004 reveal regularly spaced (~200 m interval)
drowned platforms at depths as much as 1,100 m below sea level. However, the adjacent mainland coast has well documented raised
terraces indicating long-term uplift. Local subsidence may be due to cessation of magmatic activity and cooling, flexural
loading by the uplifting Finisterre Range, loading by nearby active volcanic islands, and/or sediment loading on the seafloor
north of the Finisterre Range. We present some simple models in order to test whether flexural loading can account for local
subsidence. We find that volcanic and sedimentary loading can explain the inferred relative subsidence. 相似文献
11.
In the equatorial Atlantic the Ceará and Sierra Leone rises lie on opposing sides of the mid-ocean ridge and are equidistant from its axis. The northern and southern boundaries respectively, of the two rises are formed by the same fracture zones. The area of shallowest acoustic basement under the Ceará Rise coincides with the presence of a 1–2 km thick seismic layer (velocity: 3.5 km/sec) lying over the oceanic layer 2. This 3.5 km/sec layer is interpreted as a sequence of volcanics which began erupting about 80 m.y. ago when the sites of the two rises lay at the ridge axis. As the “abnormal” volcanic activity ceased, the breakup of this volcanic pile into two pieces has formed the Ceará and Sierra Leone rises.
In the South Atlantic, the northern and southern boundaries of the Rio Grande Rise are also formed by fracture zones and an approximately 1 km thick layer with a velocity of 3.5 km/sec exists also under this rise. The same fracture zones appear to bound the Walvis Ridge. Drilling data suggests that both the Rio Grande Rise and Walvis Ridge have subsided continuously since their creation. The igneous rocks recovered from both rises consist of alkalic basaltic suites typical of oceanic volcanic islands. The existing data favor a model in which “excessive” volcanism along the same segment of the Mid-Atlantic Ridge created both the South Atlantic aseismic rises between 100 and 80 m.y. ago. In both the examples, the northern and southern boundaries of the rises are formed by the same fracture zones which originally bounded the abnormally active segment of the ridge axis. 相似文献
12.
High-frequency bubble layer scattering investigations require the measurement of the intensity of backscattered sound and the corresponding depth of the scatterers below the moving surface. Especially at high sea state conditions and high acoustic frequencies, bubbles acoustically mask the surface, i.e., the surface return cannot be detected. However, this environmental condition is the most interesting one in bubble scattering investigations and a reliable method is required to determine the range of the scatterers to the surface displacement. A method for the determination of the vertical profiling of acoustic scattering in the presence of bubbles at high sea state conditions is presented. It is based on the transmission of a low-frequency signal alternately to the high-frequency signal at which the scattering investigations are performed. The only information that is extracted from the low-frequency echo is the onset of the surface return. It is used to compute the true depth of scatterers at the high frequency. Experiments were conducted to determine the optimum low frequency at which the detection of the surface onset in the presence of a high bubble concentration is ensured. A screening ratio is defined to give a measure of the acoustic masking of the sea surface. It is depicted for an extreme wind condition (20 m/s) for the frequency range of 5-25 kHz and as a function of wind speed for 50 kHz measurements. Selected results of subsurface bubble scattering at 50 kHz from experiments under open sea conditions are presented for the wind speed regime from 9 to 22 m/s. Additionally, the two-frequency scatterometer is used to measure sea state characteristics simultaneously to the scattering investigations by remote sensing techniques 相似文献
13.
The May 2001 Geoacoustic Inversion Techniques Workshop provided synthetic transmission loss (TL) data for four cases with range-dependent shallow-water all-liquid environments. In two of these cases ("0" and "1"), the sea floor has constant slope and the geoacoustic model (GAM) is range independent. Cost functions have been computed using a new adiabatic-mode TL algorithm (which uses an exact velocity boundary condition at the sloping sea floor), as one parameter in the GAM is varied. Two frequencies (80 and 220 Hz) were selected. In case 0, the sea-floor slope is 0.0183 and the GAM comprises an inhomogeneous layer over a basement. The sea-floor sound-speed was selected as the variable parameter. The resulting cost minima at 80 and 220 Hz are displaced from the actual sound speed by 2.3 and 3.4 m/s, respectively. In case 1, the sea-floor slope is 0.012 and the GAM comprises one homogenous layer, five inhomogeneous layers, and a basement. The selected parameter was the sound-speed in the homogeneous layer. The corresponding cost minima are displaced by -1.2 and +1.1 m/s. The relative values of these four errors indicate that mode coupling increases with sea-floor slope and that there may be a dependence on frequency at the greater slope. 相似文献
14.
声散射是重要的声学现象,海洋水体产生的高频声散射信号既可用于开展多种目的的声学海洋学研究,也可能对水下声学设备产生干扰,而海洋水体背景声散射具有显著的时空变异特征,因此针对特定海区开展声散射时变观测具有重要意义。本文利用在南海北部布放的锚系系统所搭载的声学多普勒流速剖面仪,获取了覆盖4个季节的累计约80 d的声散射数据,数据包括75 kHz和300 kHz两个频段,观测水深几乎覆盖了从海面到约600 m水深的整个水体。结果表明,水体在垂向上分布着上散射层和深散射层2个主要散射层。上散射层分布深度在冬夏较浅,位于约100 m以浅,在春秋较深,位于约200 m以浅;深散射层分布深度同样为冬季最浅,位于约300 m以深,但夏季则最深,位于约400 m以深。因此,两散射层的距离在夏季最远,在春秋最近。2个散射层的声散射强度（Sv）同样具有明显的季节变化,上散射层散射强度夏秋较强而春冬较弱,深散射层则正好相反。 相似文献
15.
Aeromagnetic data collected over the Offshore Mahanadi Basin along the Eastern margin of India display high amplitude magnetic anomalies. The presence of a Cretaceous volcanic sequence masks the seismic response from the underlying basement and results in poor quality seismic data. In this study spectral analysis of the aeromagnetic data collected over this part of the Offshore Mahanadi Basin was carried out. Results of this analysis indicate the presence of a high density, highvelocity (6.45 km/s) mafic layer within the crystalline basement varying from 4–6 km depth. This intra-basement layer seems to have been affected by a number of lineaments, which have played a role in the evolution of the Mahanadi Offshore Basin. The western part of the offshore basin is affected by the volcanism related to the 85°E Ridge, whereas the intense anomaly band (900 nT) offshore Puri, Konark and Paradip is interpreted as a combined effect of crystalline Precambrian basement overlain (i) by Cretaceous volcanic rocks of variable thickness (25–860 m) and (ii) by a mafic layer within the basement. 相似文献
16.
将Argo浮标资料与卫星遥感再分析数据相结合,调用基于抛物方程算法的RAM(Range-dependent Acoustic Model)模型,研究了2012年第14号台风“天秤”对不完整深海声道(3 000 m)和完整深海声道(5 500 m)两种水深条件下声传播特性的影响。结果显示:台风对海水的影响局限于表层水体,具体为混合层加厚,混合层内温度梯度接近于零,声速在混合层内正梯度分布;混合层下方一定深度的水体增温,相应的声速也增大。声源在混合层内时,主要对海表层的声传播产生影响,两种水深条件下均出现表面波导声传播模式以及泄漏模式。声源在混合层以下时,不完整深海声道条件下台风使得会聚区向着声源方向靠近;完整深海声道条件下台风对会聚区的位置影响不明显,但声波的翻转深度增加近500 m。 相似文献
17.
In this study, the dB difference and characteristics of krill swarms inhabiting Subarea 48.1, which includes the west and south of the South Shetland Island and the Elephant Island peripheries, were estimated to distinguish Antarctic krill, using acoustics. From April 13 to 24, 2016, acoustic data were collected along 24 survey lines using the frequencies 38 and 120 kHz, and middle trawling was performed at 7 stations. Using the difference between the dB values of two volume backscattering strength (Sv) frequencies (38 and 120 kHz), a clear acoustic distinction could be made between Antarctic krill (4.9 to 12.0 dB) and fish (?4.0 to ?0.2 dB). The distributions and mean Sv of krill swarms in the Elephant Island peripheries and south of South Shetland Island were higher than those in the west of South Shetland Island. The mean length/ height ratio of krill swarms in the west of the South Shetland Island (64.5) was higher than that in the south (35.9) and the Elephant Island peripheries (33.8), with the length of the aggregations exceeding their height. Most krill swarms were distributed between the surface layer (less than 10 m below sea level) and within 200 m of water depth. These results are expected to serve as baseline data for evaluating krill density and biomass by distinguishing them from fish, using acoustics. 相似文献
18.
A number of samples have been dredged from the upper parts of Amanay and El Banquete Seamounts, yet volcanic materials have been collected only on Amanay Seamount. Based on textural features and the presence or absence of kaersutite, two main types of olivine pyroxene basaltic rocks have been identified. The rocks are basanites with high enrichment in the most incompatible elements, similar to that displayed by Ocean Island Basalts. Samples from Amanay Seamount formed due to a low degree of melting of an enriched mantle, very similar to that which probably caused the Miocene volcanic activity of Fuerteventura. The age of Amanay volcanic rocks, 15.3 ± 0.4 and 13.1 ± 0.3 Ma, is similar to those of the older volcanic units exposed in the nearby islands (Gran Canaria, Fuerteventura and Lanzarote). This proves the formation of a separate submarine volcanic edifice coeval with the other edifices of the Eastern Canarian Volcanic Ridge. Volcanic activity on the submarine edifice is thought to have ceased at about 13 Ma, simultaneous with the adjacent main volcanic construction. 相似文献
19.
Detailed morphological data collected from the submarine flanks of the Canary Islands have revealed numerous submarine canyons down to water depths of >3,000 m. These canyons are interpreted to have formed by submarine erosion. We postulate formation of proto-canyons by downslope-eroding mass flows which originate on land, enter the sea, and continue below sea level for several tens of kilometers. Once proto-canyons have been formed, they become deepened by further erosion and failures of the canyon walls and/or floor. Large amounts of sediments, funnelled through the canyons from the islands into the adjacent deep-ocean sedimentary basins, play an important role in the evolution of volcanic aprons surrounding ocean islands. Some major canyon systems appear to have persisted for at least 14 million years. 相似文献
20.
Jinhae Bay, a semi-enclosed, tide-dominated coastal embayment on the southeastern coast of Korea, receives large amounts of sediment derived from the Nakdong River. The irregular surface of the acoustic basement is overlain by a modern sedimentary sequence up to 25 m thick, characterized by an acoustically semitransparent subbottom. Sediments, consisting mainly of terrigenous and bioturbated mud, accumulate at a rate of 2–5 mm/yr. About 21% of the suspended sediments discharged from the Nakdong River, that is approximately 1.0 × 106 tons per year, accumulate in Jinhae Bay. Modern sedimentation began probably at about 5000 yr BP, when sea level approached its present level. 相似文献