Seismic and Petrophysical Relationships from UK Continental Shelf Rocks at Elevated Pressures

There are few published seismic (P- and S-waves) properties for seafloor bedrocks. At low pressures (1 to 10 MPa), velocities and attenuations are determined mainly by open microcracks. At higher pressures, the microcracks close, and the velocities and attenuations depend primarily on the matrix porosity. We have investigated both the relationships between the acoustic, petrophysical, and geological properties of the rocks at 40 MPa pressure and the effect of microcracks on the acoustic properties at 10 MPa pressure. In this paper we discuss the former; the latter will be discussed separately. P- and S-wave velocity and attenuation measurements were carried out on a suite of seabed sedimentary and igneous rocks at effective pressures from 10 to 40 MPa at ultrasonic frequencies. The porosities and permeabilities of the rocks ranged from 0% to 32% and 0 to 110 mDarcy, respectively. Characterization of the rocks revealed that most of the sandstones have a substantial clay content (kaolinite, illite, and chlorite) and fractures. Most of the igneous rocks are chloritized. The seismic properties of the rocks are markedly lower than those of similar continental rocks because of the microporosity formed by the alteration of feldspars, micas, and mafic minerals to clays (e.g., chloritization of pyroxenes) and the corresponding reduction of the elastic moduli. The results of this study suggest that the values of velocities and quality factors used for ocean acoustic propagation models are lower than normally assumed.     

中国陆架的沉积环境地球化学

中国陆架沉积环境地球化学研究表明,我国近海和远海海域的海洋沉积物质量保持良好,部分近海海域沉积物质量较差。探讨了我国陆架海域沉积物的有机物、重金属元素和生源要素等环境地球化学。     

NWF: Propagation of Tsunami and its Interaction with Continental Shelf and Vertical Wall

In this article, tsunamis represented as solitary waves was simulated using the fully nonlinear free surface waves based on Finite Element method developed by Sriram et al. (2006). The split up of solitary wave while it propagates over the uneven bottom topography is successfully established. Wave transmission and reflection over a vertical step introduced in the bottom topography is in good agreement with the experimental results from Seabra-Santos et al. (1987). The wave transformation over a continental shelf with different smooth slopes reveals that the solitary wave reflection increases while the continental slope varies from flat to steep. The interaction of the solitary wave with a vertical wall for different wave steepness has been analysed. The reflected shape of the profile is in good agreement with the observation made by Fenton and Rienecker (1982) and an increase in wave celerity is observed.     

NWF: Propagation of Tsunami and its Interaction with Continental Shelf and Vertical Wall

This paper describes the use of a transportable photographic zenith camera in a tidal flat area to obtain astrogeodetic deflections of the vertical, and the subsequent calculation of a fine structured geoid. It shows that the newly developed camera system is suited for astronomical observations in areas of this type, and that a control point distance of 20 km is sufficient for the determination of a decimeter‐geoid.     

Temporal Variations of Benthic Bacterial Microflora on the Northwestern Mediterranean Continental Shelf and Slope

Abstract. Bacterial (direct and heterotrophic counts, heterotrophic potentials) and organic matter (org C, org N) parameters were studied over 2 years in the surface sediments of 3 stations located on the continental shelf (station A: 15m; station B: 35m) and slope (station C: 910m) in the northwestern Mediterranean Basin.
Although logistic constraints did not allow a satisfactory sampling periodicity in the offshore survey, some seasonal trends were still observable at the deepest station. Variations were quantitatively more important in the two shallow sediments (from 3×10² heterotrophic bacteria – ml^-1 in December 1984 to 10⁷ heterotrophic bacteria-ml^-1 in March 1986). Epifluorcsccncc direct counts were relatively constant (5×10⁸ cells-ml^-1), while heterotrophic potentials (with ¹⁴C labelled glucose and glutamic acid) showed a succession of very short pulses over the year. Different concentrations of organic matter (from 0.2% orgC at 15m to 0.8% orgC at 35m) appeared to sustain very similar bacterial numbers. Nevertheless, a certain relation between the seasonal evolution of bacterial and organic matter parameters was discernable. Although depth apparently had no measurable effect on maximal spring bacterial densities, it seemed to lessen the summer and winter decreases in the two shallow stations.     

陆架沙丘(波)形成发育的环境条件

陆架大、中、小、巨沙丘(沙波)发育的动力机制与河流沙波有相似之处,也有许多差异,前者含定时多向的潮流、持续定向的洋流和偶发性的暴风浪流,后者为单向持续水流。20～100cm／s的底流速是发育沙丘的动力因素,水流的多向性和浪流的偶发性又常常制约沙丘发育的强度和迁移的速度;陆架浅水区(小于18m)受破浪的影响,沙丘尺度与水深呈微弱负相关,深水区水深的影响甚微。陆架输沙量是沙丘发育的物质条件,特别是细、中砂底质,在此粒度范围内沙波尺度与粒径呈微弱正相关,平缓广阔的地形在沙丘发育中也起一定的作用。     

南海北部陆架区内波的演变与耗散机制

海洋是多尺度强迫-耗散系统,机械能主要在大尺度输入,在小尺度耗散。在大、中尺度运动的能量向小尺度湍流传递过程中,内波扮演着重要角色。内波的生成和破碎可打破海洋动力平衡,而在陆架区,内波（主要是内孤立波）的浅化演变与耗散则是驱动湍流混合的关键过程。通过长期的理论、观测与数值模拟研究,目前已认识到内波浅化过程中主要发生如下演变：波形调制、极性转变、裂变、破碎与耗散。相较于直接发生破碎,浅化演变过程中的裂变及其引发的剪切不稳定和对流不稳定是内孤立波在陆架区的主要耗散机制,显著调制陆架区的跃层混合。从能量串级的角度讲,内孤立波浅化裂变为动力不稳定的高频内波是潮能串级的重要通道。本文简要回顾南海北部陆架区内波的研究历史,并着重总结内波在陆架区演变与耗散机制的研究进展。     

Demersal Continental Shelf and Upper Slope Cephalopod Assemblages from the Balearic Sea (North-Western Mediterranean). Biological Aspects of Some Deep-Sea Species

The analysis of 79 hauls performed by commercial bottom trawlers from 50 to 800 m depth in the Balearic Sea (north-western Mediterranean) from June 1995 to September 1996 yielded a total of 30 cephalopod species belonging to 12 families. Cluster analysis of these data gave as a result two main groups 50–200 m and 200–800 m each subdivided into two other groups (50–100 vs. 100–200 m and 200–600 vs. 600–800 m). These results suggested the existence of two assemblages that could be associated to the continental shelf (50–100 m) and the upper slope (600–800 m) separated by a wide transitional zone (100–600 m) representing a region of overlapping shelf and slope faunas (ecotone). The faunistic bathymetric gradient showed a continuous substitution of species with depth rather than discrete assemblages separated by distinct boundaries. The more coastal species such as Eledone moschata, Loligo vulgarisSepia officinalis and Octopus vulgaris were found on the continental shelf; on the transitional zone, apart from species characteristic of this zone (Illex coindetii, Sepietta oweniana, Rossia macrosoma, Scaeurgus unicirrhus and Pteroctopus tetracirrhus), we also observed species from both the continental shelf and slope. The upper slope was characterized by typical species of deeper waters, such asBathypolypus sponsalis , Histioteuthis reversa, H. bonnellii, Ancistroteuthis lichtensteinii andOnychoteuthis banksii . The octopod O. vulgaris was the dominant species on the continental shelf and upper transitional zone, being substituted by T. sagittatus on the lower transitional zone and upper slope. Mean biomass decreased abruptly from the continental shelf to the transitional zone and from there to the upper slope. Mean species richness and species diversity were higher in the transitional zone than in the continental shelf and upper slope. Finally, some biological aspects of the more abundant deep-sea cephalopod species are studied: Bathypolypus sponsalis, Octopus salutii,Pteroctopus tetracirrhus , Histioteuthis reversa and H. bonnellii.     

Physical Properties of Sediments on the Northern Continental Shelf of the South China Sea

Using physical (acoustical engineering mechanics and scanning electron microscope) and other technical methods, is study makes a comprehensive analysis of the structural characteristics and engineering mechanics properties of sediments, the sediment structure, microstructure and their classification characteristics and in the light of the physical-mechanical characteristic parameters, the relations between the grain contact, grain accumulation, and porosity with the physical-mechanical characteristics of sediment, and reports the relationship between the acoustic physical parameters and the stress-strain properties of sediment. Results show that there are six structural types in the shelf seabed sediment of the northern South China Sea, among which, the mixed contact structural type sediment has a higher compressive strength and a higher sound velocity than others, and that in the shallow seabed there exists a mesoscale structure with high and low velocity layers.     

Physical Properties of Sediments on the Northern Continental Shelf of the South China Sea

ABSTRACT

Using physical (acoustical engineering mechanics and scanning electron microscope) and other technical methods, is study makes a comprehensive analysis of the structural characteristics and engineering mechanics properties of sediments, the sediment structure, microstructure and their classification characteristics and in the light of the physical-mechanical characteristic parameters, the relations between the grain contact, grain accumulation, and porosity with the physical-mechanical characteristics of sediment, and reports the relationship between the acoustic physical parameters and the stress-strain properties of sediment. Results show that there are six structural types in the shelf seabed sediment of the northern South China Sea, among which, the mixed contact structural type sediment has a higher compressive strength and a higher sound velocity than others, and that in the shallow seabed there exists a mesoscale structure with high and low velocity layers.     

海南岛东西陆架秋季悬浮体分布特征对比

2005年8月到10月国家海洋局第二海洋研究所在海南岛周围进行了近海底质和悬浮体的调查。利用悬浮体调查数据对比了海南岛东西两侧陆架悬浮体浓度的分布特征,并探讨影响其分布的因素。研究结果表明,海南岛东西两侧陆架的悬浮体在垂向上的分布形式基本有3种:增加型、C型和稳定型。海南岛东西侧陆架在悬浮体浓度分布方面存在较大差异,自然条件是造成这种差异的主要因素。物质来源对悬浮体体积浓度分布也有较大影响。悬浮体体积浓度在局部的高值、高梯度区与流系(如暖涡和上升流)有较为密切的关系。在东侧陆架,盐度分布与悬浮体的浓度分布有一定的关系。     

末次盛冰期以来东海陆架南部EA05岩心地层划分及其古环境意义

通过综合分析东海陆架南部EA05岩心上部30 m的岩性、有孔虫丰度和一些特征的地球化学参数,探讨了该段岩心的气候地层划分。该段岩心可依次划分为末次盛冰期(30~11.85 m)、冰消期(11.85~5 m)和冰后期(5~0 m)地层。末次盛冰期时气温低,研究区水深浅,沉积物中有孔虫丰度很低,且大多数热带浮游有孔虫种属消失,此时陆源物质输入量较大,沉积物中有机质含量高,黑潮对东海陆架南部影响小。随着冰消期以来气温回升,海面上升,研究区水深增大,岩心中有孔虫丰度急剧增加;陆源物质输入量减少,沉积速率降低。大约在16 ka B.P.左右,黑潮重新开始对东海南部陆架产生影响。在EA05岩心的6~5 m,3.4~3.0 m和1.4~1.2 m处分辨出了3个冷事件,可能分别对应于YD事件、8.2 ka B.P.冷事件和3.8 ka B.P.新冰期Ⅲ,反映出该地区冰消期和冰后期气候波动较大。     

东海北部陆架柱样中底栖有孔虫组合及其古环境研究

东海北部陆架区D-46C和D-74C柱样底栖有孔虫分析结果表明,D-46C柱样自下而上为压扁卷转虫(Am-moniacompressiuscula)组合,并可以进一步划分出压扁卷转虫—花朵虫(A.compressiuscula-Florilusspp.)亚组合、压扁卷转虫—叶状串珠虫(A.compressiuscula-Textularia foliacea)亚组合和压扁卷转虫—强壮箭头虫(A.compressiuscula-Bolivina robusta)亚组合;D-74C柱样自下而上可以划分出三个组合:叶状串珠虫—压扁卷转虫(Textularia foliacea-Ammonia compressiuscula)组合、大西洋花朵虫—异地希望虫(Florilus atlanticus-Elphidium advenum)组合和大西洋花朵虫—结缘寺卷转虫(Florilus atlanticus-Ammonia ketienziensis)组合。两个柱样中底栖有孔虫组合垂向变化明显,结合AMS14C测年结果,初步认为两个柱样均为全新世以来的沉积。其中,D-74C柱样为全新世中期(相当于亚北方期)以来的沉积,在全新世晚期一直受到陆地冲淡水的强烈影响;D-46C柱样为全新世早期(相当于前北方期和北方期)以来的沉积,受到陆地冲淡水流的影响较D-74C柱样小。     

长江口邻近陆架沉积物粒径变化趋势及动力成因

基于长江口外邻近陆架近20年实测数据,分析了邻近陆架区域表层沉积物中值粒径、砂-泥分界线和泥质区变化规律,并探讨了自然机制和人类活动等对其影响。结果表明:1近期(2006-2010年)长江口邻近陆架区域沉积物中值粒径和砂、粉砂和粘土等值线分布格局较2006年以前未发生明显变化;2伴随流域入海泥沙量和海滨区悬沙浓度的减小,陆架区域表层沉积物中值颗粒表现为粗化趋势发展;3陆架区域砂-泥分界线2004-2007年在北侧(31°30′以北)无明显趋势,南侧(31°30′以南)为向口内移动,2008-2010年无论南侧还是北侧均为向口内移动,主要受长江径流枯水期,引起潮流水动力相对增强、悬沙浓度减小及苏北沿岸流等影响;4泥质区域为长江口沉积速率最大区域,由于入海泥沙和海滨区域悬沙浓度降低,三角洲侵蚀等环境下表现为减小趋势,因北槽深水航道整治工程的实施改变了南北槽分流比,南槽下泄径流水动力增加,使得泥质区位置存在南偏趋势。     

Mining Sand on the Continental Shelf of the Atlantic and Gulf Coasts of the U.S.

Essentially all marine mining along the East and Gulf coasts of the U.S. is for sand used in beach nourishment projects. The current minimal commercial production of sand and aggregate may increase as conventional, on-shore sources become exhausted or are lost to competing land use. Studies published in the late 1990s document a history of nearly 900 individual episodes of beach nourishment having a total cost in excess of $2 × 10⁹ with several hundred million cubic meters of sand placed along over 645 km (400 mi) of shoreline. As exemplified by studies in Florida, prospecting for sand for use in beach nourishment can begin before site specific needs are identified. A full prospecting starts with assimilation of pertinent literature, local knowledge, and an understanding of the geologic and geomorphic settings in which suitable deposits of sand or aggregate occur. High-resolution seismic profiling follows to outline the three dimensional extent of the sand bodies. Finally, vibratory cores are collected to verify the interpretation of the seismic data and to provide samples for geotechnical, especially granulometric, analyses. The actual method of production often is determined by the local availability of different dredge types. Because the mining of marine sands disturbs meaningful areas of the sea floor, environmental concerns must be considered. While it generally is assumed that dredged areas will be left barren, it is possible to assess the likely rate of recolonization. The disruption of bottom habitat also can affect feeding and spawning areas for fishes and other organisms. Alteration in local currents and wave transformation processes need to be modeled and their consequences assessed.     

Expulsion of Shallow Gas in the Skagerrak—Evidence from Sub-bottom Profiling, Seismic, Hydroacoustical and Geochemical Data

A combined high resolution seismic, sub-bottom profiling, and multi-beam echo-sounding survey in the Skagerrak (Danish sector of the North Sea) together with gas analyses at a station along the profile exhibit the expulsion of gas (mainly methane) and the presence of gas-charged sediments at shallow depth. The echo-soundings yield detailed insight into the distribution and shape of typical sea-floor features associated with gas seepage, such as pockmarks. The pockmarks reach dimensions of 800 m in length, 300 m in width, and 15 m in depth, with the long axis running parallel to the slope of the Norwegian Trench. Processing of the multi-channel high resolution seismic data and the digitally recorded sub-bottom profiler signals indicate an internal compressional velocity of about 1050 m s^-1 within the gas-charged sediments reaching from the sea-floor to a sub-bottom depth of about 23 m. Using the lateral distribution and thickness of the gas-charged sediments in conjunction with a mean concentration of gas of 3000 ppb, the present amount of trapped gas is estimated to be 6·45 × 10¹¹ g CH₄. The flux of methane through the sea-bed into the water column appears to be 7·2 × 10¹⁰ g CH₄ per year. To explain the small difference in size between the methane pool in near-surface sediments and the annual flux through the sea-bed, a constantly high supply of methane from leaking hydrocarbon reservoirs at greater depths has to be active.     

Seismic and structural characterization of fluid escape pipes using 3D and partial stack seismic from the Loyal Field (Scotland,UK): A multiphase and repeated intrusive mechanism

The potential for fluid leakage from sub-surface reservoirs has important implications for CO2 storage, hydrocarbon reservoirs and water resources. Understanding the genesis, morphology, fluid flow mechanisms and extent of fluid escape from reservoirs allows for better risking of geological resources and storage potential. Here we describe in detail the structures of fluid escape pipes from the Loyal Field, observed from a 3D full and partial stack seismic dataset. The seismic imagery suggests that the fluid escape pipes are rooted at least in the main Paleocene reservoir and by-pass the reservoir seal to cross the post Lista Formation overburden up to the intra-Neogene units. The pipes extend for a few hundred meters to a few kilometres and show varying shape and structure from blow-out structures to incipient mud volcanoes. A detailed analysis of the seismic characteristics observed both from main baseline and partial stack data allows a division of the pipes into two families: (1) seeps and pipes following structural discontinuities and (2) pipes unrelated to the pre-existing structural features. The pipes internal seismic response, the reflector termination of the main conduits and the distribution of stacked bright reflectors suggest an upward migration mechanism (during pipe birth and development), requiring a cyclic switching from non-Darcy hydrofracturing (during overpressure) to Darcy flow lateral migration (during low-pressure stage).