海底土的电阻率特征及其腐蚀性评价

海底土的电阻率对于海底土的腐蚀性评价具有重要意义．综述了海底土电阻率的特征及洲定方法,介绍和分析了国内外利用土壤电阻率进行土壤腐蚀性评价的方法,并结合我国海底土特征提出了根据海底土电阻率进行腐蚀性评价的分级标准。     

In Situ Time-Series Measurements of Subseafloor Sediment Properties

The capabilities and diversity of subsurface sediment sensors lags significantly from what is available for the water column, thereby limiting progress in understanding time-dependent seabed exchange and high-frequency acoustics. To help redress this imbalance, a new instrument, the autonomous sediment profiler (ASP), is described herein. ASP consists of a four-electrode, Wenner-type resistivity probe and a thermistor that log data at 0.1-cm vertical intervals over a 58-cm vertical profile. To avoid resampling the same spot on the seafloor, the probes are moved horizontally within a 20 times 100-cm^-2 area in one of three preselected patterns. Memory and power capacities permit sampling at hourly intervals for up to 3-mo duration. The system was tested in a laboratory tank and shown to be able to resolve high-frequency sediment consolidation, as well as changes in sediment roughness. In a field test off the southern coast of France, the system collected resistivity and temperature data at hourly intervals for 16 d. Coupled with environmental data collected on waves, currents, and suspended sediment, the ASP is shown to be useful for understanding temporal evolution of subsurface sediment porosity, although no large depositional or erosional events occurred during the deployment. Following a rapid decrease in bottom-water temperature, the evolution of the subsurface temperature field was consistent with the 1-D thermal diffusion equation coupled with advection in the upper 3-4 cm. Collectively, the laboratory and field tests yielded promising results on time-dependent seabed change.     

舟山海洋沉积物及沿海沉积物流动相石油污染的电阻率特性及其影响因素研究

海洋沉积物作为难以降解的石油类污染物的最终归宿,其污染程度的监测至关重要。本文通过现场采集舟山海洋沉积物及沿海沉积物(包括中砂、粉砂质砂和粉砂),以流动相原油为代表,采用单因素分析法室内配制标准污染沉积物样品测定电阻率,探讨沉积物类型、含水率、含油率、时间因素对石油污染海洋及沿海沉积物的电阻率影响及变化规律。结果显示,未污染和原油污染后海洋沉积物中砂、粉砂质砂、粉砂的电阻率随含水率的升高均呈幂函数降低趋势,并符合Archie公式。在含水率15%时,电阻率随含油率总体上升;而在含水率5%时,中砂电阻率呈现一个小幅上升的趋势;粉砂电阻率先小幅下降后略上升再小幅下降,粉砂质砂电阻率则是先大幅下降后略上升再小幅下降。15%含水率污染沉积物的电阻率随时间增加持续上升;5%含水率污染沉积物电阻率先上升后略下降再大幅上升。研究成果可为海洋及沿海沉积物石油污染电阻率法快速监测提供理论支持,以期为海洋环境管理部门提供相应的决策依据。     

Physical and Acoustic Properties of Gas-bearing Sediments in Jinhae Bay,the South Sea of Korea

High-resolution seismic survey and sediment core sampling were conducted to investigate acoustic characteristics of gas-bearing sediments in Jinhae Bay, the southeast of Korea. The sediment in Jinhae Bay is mostly homogenous mud deposited after the Holocene transgression. Along with the 410 km of chirp seismic profiling, five piston core samples were collected on the track lines.

Gassy sediments are common and occur widely in the bay. Core samples were analyzed for sediment texture, physical properties (porosity, water content, bulk density, and grain density), acoustic properties (compressional wave velocity and attenuation), and electrical resistivity. X-radiograph image analysis was also performed to observe the shape of degassing cracks. There is no significant downcore variation on physical and sediment textures regardless of existence of gas bubbles. However, compressional wave velocity dramatically decreases from average 1480 to 1380～739 m/s for the cores that penetrate the gas-bearing zones. This is probably due to degassying cracks that developed by escaping gases and free gas bubbles that are still trapped in the cores. Electrical resistivity is the only geotechnical property that increases in the gas-bearing zone where compressional wave velocity abruptly decreases. This indicates the possibility of using both electrical resistivity as an index variable as well as to compressional wave velocity to identify gassy sediment microstructure because there are little changes in texture and composition of sediment.     

Electric potential due to a ring electrode

Electrical sensors utilizing ring electrodes mounted on cylindrical rods have been used by several workers to measure the resistivity of marine sediments. As the resistivity of sediment is dependent upon the water content of the sediment, such measurements are extremely valuable for determining the density of sediments. Recently, ring electrodes have also been used in instrumentation to determine fluctuations in the marine sediment-seawater interface due to erosion or deposition. In the past, approximate formulas have been used to evaluate the response of the sensors. In this work, an exact formula is derived, and it is shown that the previous formulas are in error by up to 5%. Although this is not serious for most borehole logging applications, it is essential to use the exact formulas to interpret data from electromagnetic sediment level sensors     

Influence of diagenesis on the electrical resistivity and the formation factor of deep-sea carbonate sediments

Laboratory measurements of electrical resistivity on two DSDP pelagic carbonate sequences permitted the study of the effect of diagenesis on the electrical and other physical properties such as velocity and porosity. Electrical resistivity and formation factor increase with sediment depth. Changes in porosity with progressive diagenesis (that is, ooze-chalk-limestone) are observed, and changes in pore geometry are inferred. These changes are interpreted in terms of systematic variations in electrical and physical properties. Influence of silica on carbonate diagenesis may result in significant change in pore geometry. This effect inhibits electrical conduction leading to complicated but generally high resistivity values for siliceous and cherty limestones. Compressional velocity correlates well with resistivity for both sequences.     

The ICE-MOSES experiment: Mapping permafrost zones electrically beneath the Beaufort Sea

A novel variation of the geophysical technique known as MOSES, for Magnetometric Off-Shore Electrical Sounding, has been developed to map the electrical properties of the sea floor in Aretic regions. The particular target is the permafrost layer under the Beaufort Sea, a layer containing frozen or partially frozen sediment from 100 to 600 m thick underlying shallow sea water, typically 10 to 100 m deep, and several tens of metres of soft sediment. A detailed knowledge of the location and physical properties of the permafrost layer is essential for accurate interpretation of reflection seismic data. The permafrost can contain pockets, regions or layers of gas hydrate. The latter is both a possible resource and a hazard to drilling operations or hydrocarbon production. A local map of the permafrost zone is essential geotechnical information required prior to the construction of an offshore structure or pipeline.The MOSES method is particularly suitable for offshore electrical mapping as it can be made relatively insensitive to the shielding effects of the highly conductive sea water, in sharp contrast to many other electrical techniques. The transmitter is a vertical, long-wire bipole, extending from the sea surface to the sea floor. A commutated current is fed to two large electrodes: one near the sea surface and the other on the sea floor. The return current is through the sea water and the subjacent sediment. The receiver consists of two horizontal orthogonal coils located on the sea floor, and the data are measurements of two components of the magnetic field as a function of frequency and transmitter-receiver horizontal separation.The electrical conductivity of a sample of frozen material is much smaller than that of unfrozen or partially frozen sediment of the same type. Frozen and unfrozen thin layers are often observed sequentially throughout the geological section. The resistivity measured as a function of depth by an electrical logging tool is consequently highly variable. The resulting depth-averaged resistivity, the resistivity resolved by a surface electrical method, is macro-an-isotropic. An experimental design study reveals that both the vertical and horizontal averaged resistivities could be determined in a MOSES sounding without vertical scale distortion.A test of the methodology in very shallow water was conducted in the spring of 1986 at a site, approximate coordinates (70° N, 134.5° W), 85 km north-west of the town of Tuktoyaktuk. The instrumentation was lowered and subsequently recovered through holes in the ice which covers the Beaufort Sea at that time of the year. The transmitter power was obtained from a single lead-acid battery. Transmitter-receiver separations ranged from 10 to 300 m. A rapid increase in sediment resistivity with depth was observed. The higher resistivity values are consistent with those expected for a partially frozen zone.     

Electrical Resistivity Features of Compacted Expansive Soils

An electrical resistance apparatus with a highly sensitive alternate current bridge was developed in the paper for the purpose of measuring the electrical resistivity of soils and eliminating the low frequency polarization. The electrical resistivity of Guangxi expansive soils was measured in the laboratory using the developed apparatus. The investigation result demonstrates that the electrical resistivity of the Guangxi expansive soil is a function of the electrical resistivity of soil skeleton and pore water, water content, degree of saturation and void ratio. Along with electrical resistivity, the matrix suction was measured, and the relationship between the electrical resistivity and matrix suction was analyzed. The electrical resistivity-volumetric water content curves are compared with the matrix suction-volumetric water content curves for these soils. The relationship of matrix suction-volumetric water content is observed to be the similar feature to that of the electrical resistivity-volumetric water content curve.     

A community level sediment bioassay applied to an estuarine heavy metal gradient

A simple laboratory microcosm system, previously used as a community level sediment bioassay for laboratory contaminated sediments, has been used for testing contaminated field sediments. Meiobenthic communities from the Lynher Estuary were incubated with defaunated sediment collected from the Fal Estuary system where a gradient of heavy metal concentrations is reflected in meiofaunal community structure. In the microcosms changes in nematode community structure were related to sediment metal concentrations. These changes in community structure were similar to those previously observed between creeks in the Fal Estuary system indicating that an experimental microcosm approach can be useful both as a sediment bioassay and in the validation of conclusions drawn from field surveys.     

冷泉气体渗漏过程海洋多电极电阻率法探测效果模拟分析

为分析评价一种新型海洋多电极电阻率法对海底沉积物中冷泉气体渗漏过程的探测能力,根据前人理论研究,结合具体实例构建沉积物中快速及慢速冷泉气体汇聚、渗漏、喷发阶段地电模型,模拟采用海洋多电极电阻率法进行探测,利用数值计算方法得到理论剖面图像,并和室内实验实测剖面图像进行对比分析。研究结果表明,在快速冷泉探测剖面图像中,含气层和渗漏通道会因冷泉气体扩散状态不同表现为不同的电阻率异常特征,但易于识别。在慢速冷泉探测剖面图像中,浅层沉积物中气体富集区呈明显的高阻异常区,随着气体渗漏异常区逐步消失。两种喷发过程形成的微地貌特征也可在探测图像中得到反映。海洋多电极电阻率法是一种可以图化描述沉积物中含气层、渗漏通道及气液界面空间分布位置的有效方法。     

A two-phase numerical model for sediment transport prediction under oscillatory sheet flows

To predict sediment transport under oscillatory sheet flow condition, especially for fine sand, is still a challenging research subject in coastal engineering. This paper describes a newly-developed numerical model based on two-phase theory with the use of a one-equation turbulence closure, and its applications in predicting fine sediment suspension in near-prototype oscillatory sheet flow conditions. Model results were compared with comprehensive laboratory measurements of flow velocity and sediment concentration under both symmetrical and asymmetrical oscillatory sheet flows from a large-scale water tunnel. Good agreements between the model results and measurements were achieved and the results demonstrated that the model is capable of reproducing detailed characteristics of sediment entrainment process in the sheet flow regime. The comparisons also revealed the fact that the concentration peaks at flow reversal is associated with the strong vertical sediment transport flux in the pickup layer, which has been widely observed in many laboratory experiments. The effects of flow reversal events on total sediment transport were also discussed.     

Swash overtopping and sediment overwash on a truncated beach

New experimental laboratory data are presented on swash overtopping and sediment overwash on a truncated beach, approximating the conditions at the crest of a beach berm or inter-tidal ridge-runnel. The experiments provide a measure of the uprush sediment transport rate in the swash zone that is unaffected by the difficulties inherent in deploying instrumentation or sediment trapping techniques at laboratory scale. Overtopping flow volumes are compared with an analytical solution for swash flows as well as a simple numerical model, both of which are restricted to individual swash events. The analytical solution underestimates the overtopping volume by an order of magnitude while the model provides good overall agreement with the data and the reason for this difference is discussed. Modelled flow velocities are input to simple sediment transport formulae appropriate to the swash zone in order to predict the overwash sediment transport rates. Calculations performed with traditional expressions for the wave friction factor tend to underestimate the measured transport. Additional sediment transport calculations using standard total load equations are used to derive an optimum constant wave friction factor of f_w = 0.024. This is in good agreement with a broad range of published field and laboratory data. However, the influence of long waves and irregular wave run-up on the overtopping and overwash remains to be assessed. The good agreement between modelled and measured sediment transport rates suggests that the model provides accurate predictions of the uprush sediment transport rates in the swash zone, which has application in predicting the growth and height of beach berms.     

FICUS,a new in-situ probe for resistivity measurements in unconsolidated marine sediments

Physical properties of shallow sediments measured at a particular site can not easily be extrapolated over a given profile. The number of samples required to define sediment properties adequately can then become time-consuming and expensive. Laboratory and in-situ experimentations have shown that electrical probing provides a useful complementary technique to extrapolate results from cores. These experiments have pointed out the need for quantitative, easily-transportable and fast resistivity measurements combining high vertical resolution with azimuthal resolution and full coverage, in the shallow subsurface. A new prototype probe called FICUS (Formation Imaging and Coring for Unconsolidated Sediments) has been developed to bridge this gap. FICUS is designed to provide in-situ high resolution electrical resistivity images of the upper few meters of shallow unconsolidated sediments.Laboratory experiments have been completed to test the feasibility of this technique to provide resistivity images of unconsolidated sediments. Laboratory images agree with theoretical predictions from numerical modelling. The obtained cm-scale resolution could be used for petrophysical and sedimentary purposes. The probe may offer additional information about changes in porosity and pore morphology caused by climatic cycles, since electrical resistivity is known to be especially sensitive to these changes. The probe could also allow to detect and map organic pollutants in the future.     

南海某气田A平台腐蚀环境探讨

根据某气田海区海水及沉积物腐蚀性因子调查的资料,概述了该区阻率、溶解氧、Ｆｅ＾３＋／Ｆｅ＾２＋、Ｅｈ有机质等腐蚀因子的特征。对腐蚀环境进行了综合分析,指出调查区海水腐蚀情况与临近海区海水接近,各站表层海土腐蚀性偏向于较强级,从而提出了防腐蚀对策。     

南海某气田B平台腐蚀环境探讨

根据1997年2月国家海洋局第一海洋研究所和广州二海海洋勘察公司对北部湾南部某气田海区海水及沉积物腐蚀性因子调查的资料，概述了该海区电阻率、溶解氧、Fe^2 /Fe^2 、Eh、有机质等腐蚀因子的特征。对腐蚀环境进行了综合分析，指出调查区海水腐蚀情况与邻近海区海水接近，各站表层海土腐蚀性偏向于较强级，从而提出了防腐蚀对策。     

Geophysical ground-truthing experiments in Eckernförde Bay

During the 1994 Coastal Benthic Boundary Layer Project research cruise in Eckernförde Bay, multichannel digital seismic and electrical resistivity data were collected using surface- and bottom-towed arrays. Profiling with a bottom-towed sled yielded shear wave velocity and electrical resistivity data indicative of the structural strength of the sediment and of the properties of the pore space. Shear wave velocities for the gassy mud were, as expected, extremely low, ranging from < 10 m s^–1 at the surface to around 16 m s^–1 at 2 m. Variations in electrical properties were correlatable with lithological change. It is anticipated that analysis of reflection responses will provide significant additional geotechnical ground-truthing.     

Determination of In Situ Sediment Shear Strength from Advanced Piston Corer Pullout Forces

The advanced piston corer (APC) has been used by the Ocean Drilling Program since 1985 for recovering soft sediments from the ocean floor. The pullout force measured on extracting the core barrel from the sediment is shown to correlate with the average shear strength of the sediment core measured in the ship's laboratory. A simple rule of thumb is derived relating the shear strength of the sediment to the pullout force. Multiple APC holes at individual sites allow the consistency of the pullout measurements to be assessed. The effects of different operational procedures during APC coring are also explored. Although generally applicable, the correlation between pullout force and laboratory measurements of shear strength breaks down for some APC holes, possibly because of the disturbance of some sediment types during the APC coring process. A better understanding of the physical process of APC coring, and its effect on the properties of the sediment both inside and immediately outside the core barrel, would indicate what confidence can be put on the measurement of pullout force as a way of evaluating the in situ shear strength of deep sea sediments.     

渤海锦州油田某平台及管线路由区腐蚀环境探讨

分析了渤海锦州20-2北和21—1油田海区海水及沉积物腐蚀性因子调查结果,阐述了该海区电阻率、溶解氧、Fe^3 ／Fe^2 、Eh、有机质、Es等腐蚀因子特征。在对腐蚀环境进行综合分析基础上指出,调查区海水腐蚀因子的数值与临近海区大体一致,但本海区海水腐蚀性的一个显著特点是：全浸区的海水腐蚀性远大于潮间区与浪溅区,因而该海区海水腐蚀性将大于渤海南部一些海区。调查区内表层海土腐蚀性不强,但管道在经过海土时有可能发生电偶腐蚀,故提出了防腐蚀对策和建议。     

东方1-1气田外输管线(路由区)腐蚀环境探讨

本文根据1997年2月国家海洋局第一海洋研究所和广州二海海洋勘察公司对北部湾南部东方1-1气田海底管线路由区海水及沉积物腐蚀性因子调查的资料，概述了该海区电阻率、溶解氧、Fe^3 /Fe^2 、Eh、有机质等腐蚀因子的特征。对腐蚀环境进行了综合分析，指出调查区海水腐蚀情况与临近海区海水接近，各站表层海土腐蚀性偏向于较强级，从而提出了防腐蚀对策。