Refraction and scattering into a sandy ocean sediment in the 30-40-kHz band

To observe sound penetration into a sandy sediment, a buried acoustic receiving array was insonified by a wide band sound source carried by a remotely operated vehicle. A slanting array design was used to avoid scattering artifacts. This design overcame possible problems in previous experiments, in which scattering artifacts from the array structure could be mistaken for a propagating wave. The experiments took place in a sandy sediment off the West coast of Florida, as part of the sediment acoustics experiment, which is a multidisciplinary effort to study sediment acoustics. A coherent angle, speed, and height estimation process searched through a four-dimensional search space, of source height and elevation angle, wave speed, and propagation delay to find spherical acoustic wave fronts. Three main categories of waves were found: first refracted, dominant nonrefracted and evanescent. Later acoustic arrivals, a fourth category, remain to be analyzed. Their relative intensities effectively characterize the sediment penetrating acoustic energy. The acoustic sound pressure level of penetrating waves below the critical grazing angle was found to be greater than expected for a flat interface.     

霍尔锚在粉细砂中抛锚深度模型试验

航船应急抛锚时锚板贯入土体可能会影响河床或海床中的结构物甚者造成破坏,因此在通航频繁的航道,结构物埋深的设计需要充分考虑应急抛锚时锚板的贯入深度。本文通过缩尺模型试验模拟了霍尔锚在中等密实度粉细砂中的抛锚贯入过程,研究了不同抛锚速度(1.15～4.4 m/s)及粉细砂相对密实度(0.45～0.65)对抛锚贯入深度的影响;基于太沙基极限承载力理论和能量守恒定律,推导出霍尔锚在粉细砂土中贯入深度的表达式,与模型试验结果对比显示理论计算结果偏于保守。基于试验结果提出修正系数,修正后的理论公式能够较好地快速预测霍尔锚在中等密实度粉细砂中的贯入深度。研究结果为粉细砂土河床或海床中的结构物埋深设计提供了一定的技术参考。     

Acoustic backscattering experiments in a well characterized sand sediment: data/model comparisons using sediment fluid and Biot models

As part of the sediment acoustics experiment 1999 (SAX99), backscattering from a sand sediment was measured in the 20- to 300-kHz range for incident grazing angles from 10/spl deg/ to 40/spl deg/. Measured backscattering strengths are compared to three different scattering models: a fluid model that uses the mass density of the sediment in determining backscattering, a poroelastic model based on Biot theory and an "effective density" fluid model derived from Biot theory. These comparisons rely heavily on the extensive environmental characterization carried out during SAX99. This environmental characterization is most complete at spatial scales relevant to acoustic frequencies from 20 to 50 kHz. Model/data comparisons lead to the conclusions that rough surface scattering is the dominant scattering mechanism in the 20-50-kHz frequency range and that the Biot and effective density fluid models are more accurate than the fluid model in predicting the measured scattering strengths. For 50-150 kHz, rough surface scattering strengths predicted by the Biot and effective density fluid models agree well with the data for grazing angles below the critical angle of the sediment (about 30/spl deg/) but above the critical angle the trends of the models and the data differ. At 300 kHz, data/model comparisons indicate that the dominant scattering mechanism may no longer be rough surface scattering.     

CPT based prediction of foundation penetration in siliceous sand

The load–penetration response of a foundation is one of the fundamental aspects of geotechnical engineering. In sand, the bearing capacity approach requires the operative friction angle to be known, which introduces significant uncertainty to the prediction. The predictive method developed in this paper eliminates the need to determine the friction angle. The central concept is the direct correlation of in situ piezocone penetrometer measurements to the load–penetration response of foundations.The correlation factor is shown to depend primarily on the sand relative density. The footing shape has a minor influence on the correlation factor. This study aims at large diameter foundations used in the offshore industry, where the variation in correlation coefficient is minor. However, context is provided to previous research on smaller diameter foundations, which shows the dependence on the footing diameter (through the well-known stress level effect).The proposed method is shown to perform well against load–penetration data from centrifuge experiments on footings of different diameters and elevation shapes. The performance against field data in particular provides significant confidence in the CPT based prediction method of foundation penetration in sand developed here.     

砂土中吸力式导管架基础沉贯试验研究

吸力式导管架基础具备高承载力、高施工效率、高环境友好度、低造价等特点,使用其作为海上风电底部支撑结构有利于产业的规模化发展,实现上述愿景的前提在于使筒型基础顺利安装以满足设计要求。基于此,在不同初始施加泵压下,对吸力式导管架的沉贯效率特征值、基础内外围渗流变化、筒裙端部土压力特点进行探究。通过抱桩器使吸力式导管架在吸力安装过程中仅具备竖向自由度,从而进行纯沉贯试验。结果表明：初始泵压2 kPa时沉贯效率特征值最高;沉贯过程中基础外围渗流水压小于基础内围渗流水压;筒裙外侧所受土压要大于筒壁内侧所受土压力。     

黄河口粉质土沉积物侵蚀性动态变化试验研究

通过一系列室内试验,研究了黄河口粉质类沉积物在循环动荷载作用后侵蚀性的变化.在试验中,采用真空预压固结装置制备重塑土样,采用液压伺服土动三轴试验机实现了不同幅值及不同振次的动荷载作用条件,并通过动三轴静态试验及贯入强度测试,测定了动荷载作用后土样强度的变化.通过水槽冲刷试验测试了不同条件的动荷载作用后,土样抗冲刷能力的变化,并进行了临界剪切应力的计算.试验结果表明:对于黄河口粉质类沉积物,动荷载作用后其强度及抗侵蚀性均降低;沉积物强度指标(不排水剪切强度、残余强度、贯入阻力)、侵蚀性指标(起动流速、临界剪切应力、冲刷率)与动荷载幅值、动荷载振次均呈现出较好的线性相关性.     

粉沙质海岸港区航道减淤工程措施研究

通过对唐山港京唐港区附近海岸演变进行调查和分析,结合唐山港京唐港区建港近二十年的现场观测资料,对港区附近的泥沙运动和港口航道泥沙淤积机理进行分析,对2003年10月强风暴潮期间,港口航道发生严重的泥沙骤於进行了研究,为解决港口航道特别是外航道的泥沙淤积问题,提出粉沙质海岸港区航道挡沙和固沙的工程措施,并通过物理模型对挡沙固沙措施的工程效果进行了模型试验。试验结果表明挡沙和固沙工程方案对航道泥沙减淤效果很明显,其中,挡沙方案目前已应用到唐山港京唐港区二期挡沙堤改造、三期挡沙堤的设计和建设工程中,已发挥其挡沙减淤的作用。     

底床粉质土液化下波动水体含沙量垂向L型分布特征试验研究

根据现场大风浪条件下的实测资料,粉质土海岸水体中的含沙量沿垂向具有上部均匀、近底突增的分布特点,即呈L型分布特征。利用黄河三角洲粉质土作为试验底床开展波浪水槽试验研究,揭示了底床粉质土在波浪作用下产生液化情况的水体含沙量沿垂向存在L型分布特征。根据试验现象以及悬沙粒度变化,分析认为底部高含沙层的形成主要受粉质土液化后细颗粒析出的影响,上部水体中悬沙由湍流脉动维持。对粉质土海岸大风天气期间水体含沙量剧烈增加采用波致粉质土液化的观点进行了初步解释。     

SCA6—1型声学水位计应用研究

本文介绍ＳＣＡ６－１型声学水位计的测量原理与关于仪器安装方法所引起的水位测量误差的研究，并简述了外保护管井的设计及使用中应注意的问题。     

Phase disturbance of 100-kHz groundwaves by a circular island in the open sea

The hyperbolic position fixing of ships by the Decca navigation system utilizes groundwave propagation. But the hyperbolic pattern will be disturbed by an isolated island. The instantaneous phase velocity and the bearing error are calculated from the equiphase lines near the island. It is found numerically and experimentally that the hyperbolic pattern is bent towards the island that is situated on the baseline between the master station and the slave one.     

Acoustic imaging of cohesive sediment resuspension and re-entrainment in the Changjiang Estuary, East China Sea

 Vertical profiles of instantaneous cohesive suspension concentration, obtained from an acoustic suspended sediment monitor in the Changjiang Estuary, indicate that near-bed high-concentration suspensions consist of upper and lower high-concentration suspensions, separated by a natural breakpoint at ca. 2 g l^-1. Acoustic images revealed near-bed high-frequency resuspension events of a few seconds, which contribute mainly to the formation of the lower near-bed high-concentration suspension. Upper and lower lutoclines are also indicative of re-entrainment of lower and upper high concentration suspension, respectively. Near-bed high-frequency resuspension is caused by turbulence, while re-entrainment is probably attributable to internal wave activity. Received: 26 April 1996 / Revision received: 23 July 1996     

珠江口外伶仃洋海底含气沉积物声波反射及衰减特征

高分辨率地震剖面显示,在珠江口外伶仃洋海底有大面积含气沉积物。根据含气沉积物地震反射特征将其分为声学空白、声学幕、声学扰动、不规则强反射顶部和相位下拉等类型。对含气沉积物声波衰减初步分析表明约在3.3kHz附近有一衰减峰值,将实际资料与理论模型计算进行对比,认为此峰值频率对应于等效半径约6mm的气泡的特征振荡频率,此等效半径气泡的阻尼振荡是引起声波衰减的主要原因。     

Mobile sand deposits and shoreface sediment dynamics in the inner German Bight (North Sea)

This paper presents a conceptual model for the net bedload transport regime on the shoreface of the German Bight. The model is based on the spatial distribution of the surficial sediment cover (North Sea sands) which is identical to the uppermost layer in the seismic recordings. Sediment thickness was measured using very high resolution seismic profiling (chirp sonar) and vibrocoring. The three-dimensional sediment distribution was estimated using geostatistical methods (cokriging). The results demonstrate a longshore sand distribution with three distinct zones. In Zone 1 (0–10 m water depth) the sediments attain their maximum thickness of 10±2.5 m. Between 10 and 15 m water depth a relatively thin sand layer of 0.4–1.5 m is observed within Zone 2. The seaward adjacent Zone 3 (15–20 m water depth) is characterized by an averaged sand thickness of 2–3 m with local maxima of 5–6 m. Further offshore, the sand layer decreases to about 1–2 m thickness. The net bedload transport directions inferred from this sediment zonation comprise a longshore sediment bypassing in Zone 1 which results in a substantial sediment supply to the innermost part of the German Bight due to bedload convergence. Shore-normal bedload transport shifts sand to and fro across the coastal profile although the net directional transport is seawards. This results in sediment depletion between the 10 and 15 m-isobaths (Zone 2) and an adjacent sediment accumulation in deeper waters (Zone 3).     

南黄海辐射沙脊群苦水洋海域的 沉积动力特征及稳定性研究

根据2011 年7 月3 日原4 日在苦水洋海域获取的全潮水文资料和底质样品,计算潮周期水、沙输运率,分析了其沉 积动力特征。基于1979年历史海图资料与最新测量的水道地形进行对比,进一步探讨苦水洋水道的地形演变过程及稳定性。 结果表明,苦水洋潮流作用较强,水道内沉积物主要以悬移质形式输运,在近岸段沿水道向岸净输沙,但在（121.606毅E, 33.029毅N）以东以向海输运为主。近三十多年来,苦水洋西水道深槽与口门北水道逐渐贯通,水道整体趋于顺直。     

南大洋深海嗜冷菌2-5-10-1及其低温脂肪酶的研究

从南大洋普里兹湾的水样中筛选到一株产低温脂肪酶的深海嗜冷菌2-5-10-1,对其生长及产酶情况和酶性质做了初步研究.该菌株的最适生长温度为5℃,此时分泌的胞外脂肪酶最多;添加Tween80,橄榄油可显著促进脂肪酶的产生.该脂肪酶的最适作用温度为35℃,在0～20℃均保持较高的酶活性,在0℃可保持37%的相对酶活性;酶的最适作用的pH值为7.5,在pH6～9的范围内均存在较高酶活性;对热较敏感,在60℃保温15min可丧失50%以上的酶活性.该脂肪酶的催化作用不需要金属离子的参与,Cu2+和Zn2+对酶活有着强烈的抑制作用.     

椒江河口洪季最大浑浊带核部的悬沙分选机制

以实测悬沙粒径、流速、含沙量资料为基础,通过悬沙不同粒级组分、中值粒径以及粒度参数的统计,分析椒江河口最大浑浊带核部悬沙粒径分布变化特性。研究表明,悬沙粒径分选主要受物质来源、潮流动力作用下底部部分物质再悬浮和絮凝沉降3个因素的影响。其中第二个因素起主导作用,大潮期比小潮期显著,第三个因素的作用主要发生在涨落憩前后低流速期,并在表层较近底层的水体明显。     

琼州海峡东口潮流沙脊表层沉积物特征及沉积环境划分

琼州海峡东口潮流沙脊主要受到东西向往复流和地形地貌的控制,是我国重要的海砂资源地。通过对琼州海峡东口海域350个站位的表层样粒度数据的分析,探讨了研究区的沉积物粒度特征,并划分了沉积环境。结果显示,研究区底质类型共17种,以砂、砾质砂、砾质泥质砂、泥质砂质砾、砂质粉砂为主,其中潮流沙脊区,主要分布有砂、砾质砂和砾质泥质砂,以东则分布有砂质粉砂和粉砂质砂,砂质砾和泥质砂质砾多分布于冲刷槽内。研究区沉积物砾、砂、粉砂、粘土的平均占比分别为9.7%、71.0%、15.3%、4.0%,平均粒径为2.15φ,分选系数为1.69,其中潮流沙脊区砂占比多超过95.0%,平均粒径多小于2.00φ,分选较好,随着水动力的减弱,由西向东,沉积物逐渐变细。通过聚类分析,研究区共划分出6类沉积区,Ⅰ类、Ⅱ类沉积区分别代表潮流沙脊区和冲刷海槽区,Ⅲ类代表海湾环境,Ⅳ类代表潮流沙脊前缘,Ⅴ类代表开阔陆架,Ⅵ类代表河口水下三角洲沉积环境。     

The effect of cyclic loading on the undrained shear strength of a silty clay

Abstract

The strength of saturated clays subjected to low‐frequency cyclic loading is of considerable importance in the design of offshore structures. A series of postcyclic undrained triaxial strength tests were carried out on samples of a silty clay (Keuper Marl), which had not failed under cyclic loading, to investigate the effect of this loading on the postcyclic monotonie effective stress path and strength of the soil. Undrained cyclic loading of normally consolidated and lightly overconsolidated samples by generating positive pore pressures caused the apparent degree of overconsolidation to increase. Samples subsequently subjected to monotonie testing followed effective stress paths commensurate with this apparent overconsolidation and failed on the “Hvorslev” surface on the “dry” side of the stable state boundary surface rather than on the critical state as would be expected of samples of this initial stress history. Samples which were initially heavily overconsolidated did not generate large pore pressures under cyclic loading and did not therefore exhibit large changes in postcyclic monotonie strength. The reduction in strength after cyclic loading therefore is greater for normally consolidated and lightly overconsolidated samples. The pore pressures generated under postcyclic monotonie loading are related to the degree of apparent overconsolidation achieved during cyclic loading.     

悬浮体粒度特征所反映的南黄海辐射沙脊群泥沙扩散趋势

基于大量的悬浮体现场粒度和浓度数据,借助细颗粒悬浮体作为示踪物,分析了潮流作用下南黄海辐射沙脊群海底沉积物的再悬浮和扩散特征,并探讨了沙辐射沙脊群以北泥质区的形成机制。研究结果表明：（1）在辐射沙脊群以北,近岸悬沙在离岸方向上的扩散受到限制,向南又被辐射沙脊群中心浅滩所阻挡,只能在辐射沙脊群与废黄河口之间形成沿岸分布的高浓度浑水团。（2）辐射沙脊群以北的往复潮流对海底沉积物有分选作用,倾向于把海底沉积物中的粗颗粒分选搬运至沙脊群,把细颗粒沉积物向北输送,从而在沙脊群以北形成泥质区。（3）在正常海况下,沙脊表层沉积物再悬浮作用十分微弱,沙脊上覆水体中的悬沙主要来自于临近沟槽海底沉积物的水平扩散。