Seismic reflection and transmission coefficients at an air-water interface of saturated porous soil

Based on the modified Biot＇s theory of two-phase porous media, a study was presented on seismic reflection and transmission coefficients at an air-water interface of saturated porous soil media. The major differences between air-saturated soils and water-saturated soils were theoretically discussed, and the theoretical formulas of reflection and transmission coefficients at an air-water interface were derived. The characteristics of propagation and attenuation of elastic waves in air-saturated soils were given and the relations among the frequency, the angle of incidence and the reflection, transmission coefficients were analyzed by using numerical methods. Numerical results show that the propagation characteristic of the wave in air-saturated soils is great different from that in water-saturated soils. The frequency and the angle of incidence can have great influences on the reflection and transmission coefficients at interface. Some new cognition about the wave propagation is obtained and the study suggests that we may carefully pay attention to the influence of air on the dynamic analysis of seismic wave.     

Ionospheric reflection coefficients at vlf from sferics measurements

Summary A method for calculating the complex ionospheric reflection coefficients at vlf using sferic waveforms is presented. The mathematical analysis was carried out for a number of different waveforms to illustrate the method. Reflection coefficients determined from sferics observations were compared with those calculated using an ionospheric model. In most cases, the agreement with theory is fairly good although in some cases, reflection coefficients exceeding unity were obtained. The discrepancies are believed to result from horizontally polarized flashes rather than limitations in the ionospheric model.     

Transmission and reflection of Rayleigh waves at a thin low velocity vertical layer: A laboratory and theoretical study

A three-dimensional ultrasonic modelling was used to study the Rayleigh waves reflected from a thin low velocity vertical layer and those transmitted through the same zone. The reflection and transmission coefficients of the Rayleigh waves have been calculated by an approximate method based on the Green function technique. Nonrigid contact boundary conditions were used for the low velocity layer modelling. A rather good correspondence between the theoretical and experimental absolute values of the reflection and the transmission coefficients was obtained. Some discrepancies between experimental and theoretical results, in particular with regard to phase shifts, may be explained by a certain inadequacy of the theoretical model to the low velocity layer investigated by the ultrasonic modelling.     

流体-孔隙介质界面震电反射和透射系数的简化表达式

震电测井和海洋地震勘探中均遇到流体-孔隙介质界面.对于平面声波入射平面界面模型, 本文利用前人给出的震电反射和透射电磁波的电场式, 导出了震电反射和透射系数的表达式, 其中声波与电磁波的物理参数是解耦的.在给定介质参数和入射角且已知三类弹性体波的透射系数时, 就能通过该表达式计算震电反射和透射系数.该表达式对于所有入射角都是成立的, 当入射角为复数时对应非均匀平面波入射的情况.利用电磁波数远小于界面切向波数的前提条件, 将震电反射和透射系数的表达式化简为了简化表达式.简化式中与快纵波、慢纵波和横波有关的量均以透射系数与渗-固位移比的乘积形式出现, 而动电耦合系数、电导率和渗透率以乘积形式作为一个整体出现.简化式表明, 孔隙介质中的快纵波、慢纵波和横波通过渗流位移势影响震电反射和透射系数.数值算例表明, 当平面声波入射角不是极其小时, 震电反射和透射系数的简化式是适用的; 快纵波对震电反射和透射系数的影响可以忽略.震电反射和透射系数的简化表达式不仅简化了震电波场的计算, 而且明确表达了三类弹性体波引起反射和透射电磁信号的物理因素, 为地下和海底储层的参数反演提供了思路.

     

Generalized reflection and transmission coefficients for seismic sources in a multi-layered spherical earth model

Summary A method of propagating the components of the displacement across interfaces in a multi-layered spherical earth model is presented. It is useful both for explosion sources and tangential dislocation sources, buried in an arbitrary layer of the model. Combined with known techniques, our method yields a useful tool for generating synthetic seismograms for earthquake sources in realistic earth models.     

Numerical modelling of complex resistivity effects on a homogenous half-space at low frequencies

The many different existing models describing the spectral behaviour of the resistivity of geological materials at low frequency, combined with the lack of available field data, render the interpretation of complex resistivity (CR) data very difficult. With a recent interest in CR‐measurements for environmental applications and thanks to technological progress, the use of wide‐band frequency equipment seems promising, and it is expected to shed light on the different results among the published solutions to the electromagnetic (EM) coupling problem. We review the theory of EM‐coupling over a homogeneous half‐space with CR‐effects and study some aspects of the complex coupling function. We advocate the use of the CR‐based coupling function in the interpretation process, in order to obtain a better understanding of the physical processes involved in CR‐effects. Application of the model to real field data shows systematic good agreement in two simple cases, even over wide ranges of frequencies. Interpretation with a double Cole–Cole model is applied for comparison, and in spite of good fits to the data, large differences are observed in the interpreted low‐frequency dispersion. We conclude that the use of a second Cole–Cole model to describe EM‐coupling may corrupt the interpretation of the low‐frequency dispersion, even when only the normal range of frequencies (<100 Hz) is considered, and that the use of the actual EM‐coupling expression is essential when the goal is a better understanding of interaction between CR‐effects and EM‐coupling.     

含黏性流体孔隙介质分界面上反透射系数特征研究

地下岩石由岩石骨架和孔隙流体组成,通常流体含黏性.地震波在地下介质中传播时受岩石骨架和黏性流体的影响会呈现出复杂的变化.本文将流、固体位移和应力连续作为边界条件,推导出含黏性流体孔隙介质分界面上反透射系数方程;通过建立上层为饱油、下层为饱盐水的砂岩孔隙介质模型,开展反透射系数特征研究,分别分析不同频率、不同黏滞系数条件下,含黏性流体孔隙介质分界面上反透射系数随入射角的变化.研究表明,孔隙介质分界面上和等效介质分界面上的反透射系数分别随入射角的变化趋势基本一致,说明方程推导和数值计算的正确性;快纵波反透射系数受频率、流体黏性的影响较小,而快横波反透射系数在一定入射角范围内受频率、流体黏性的影响比较大;由于黏性孔隙流体的作用,慢纵波和慢横波的反透射系数受入射角、频率及流体黏性的影响都很大.

     

Mechanical hysteresis in rocks at low strain amplitudes and seismic frequencies

A sensitive capacitance displacement transducer has been used to record hysteresis loops in the stress-strain diagrams of laboratory samples of granite, basalt, sandstone and concrete subjected to cyclic axial strains with amplitudes of order 10^?5 and periods of 10–300 sec. The ends of the loops are always cusped, whether the load cycle is sinusoidal or not, and at low strain amplitudes the loop shape becomes symmetrical and appears to be independent of amplitude. Thermal relaxation influences the observed loop shapes, so that the strain cycles represent a compromise between adiabatic and isothermal compressions. However, this does not affect the conclusion that stress-strain loops are always cusped. This observation does not appear to be consistent with linear theories of damping of acoustic and seismic waves, which indicate elliptical loops.     

Effects of anelasticity on reflection and transmission coefficients

It is known that the reflection and transmission coefficients used in the zeroth order approximation of asymptotic ray theory (ART) are identical to those obtained for the plane wave impinging on a plane interface separating two perfectly elastic half-spaces. We have used ART to compute reflection and transmission coefficients for two viscoelastic media separated by a plane interface. Our method is different from the plane-wave approach because the ART approach requires only a local application of the boundary conditions both for the eikonal and the ray amplitudes. Several types of viscoelastic media were studied. For a given model, the elastic case was emulated by setting all the quality factors Q equal to each other. Several anelastic cases were computed by keeping the same velocities and densities while changing the Qs. The quality factor is a relatively difficult parameter to measure exactly. Hence elastic coefficients are used in most synthetic seismogram computations, and the quality factors are chosen from experimental measurements or simply estimated. From these computations, amplitude and phase differences between elastic coefficients and coefficients for dissipative media are observed in some cases. These differences show the importance of knowing the exact values of Q. Incorrect Q values can lead to unrealistic moduli and to noticeable phase differences of these viscoelastic coefficients.     

Some evidence of ground power enhancements at frequencies of global magnetospheric modes at low latitude

A statistical analysis of the power spectra of the geomagnetic field components H and D for periods ranging between 3 min and 1 h was conducted at a lowlatitude observatory (Aquila, L = 1.6) at the minimum and maximum of the solar cycle. For both components, during daytime intervals, we found evidence of power enhancements at frequencies predicted for global modes of the Earths magnetosphere and occasionally observed at auroral latitudes in the F-region drift velocities (approximately at 1.3, 1.9, 2.6, and 3.4 mHz). Nighttime observations reveal a relative low frequency H enhancement associated with the bay occurrence together with a peak in the H/D power ratio which sharply emerges at 1.2 mHz in the premidnight sector. The strong similarity between solar minimum and maximum suggests that these modes can be considered permanent magnetospheric features. A separate analysis on a two-month interval shows that the observed spectral characteristics are amplified by conditions of high-velocity solar wind.     

Seismic imaging of mid-Pleistocene tunnel-valleys in the North Sea Basin—high resolution from low frequencies

Seismic reflection methods provide continuous access both to stratigraphy (vertical) and to subsurface morphology (horizontal), for which the scales of interest may differ by orders of magnitude. Seismic surveys of Quaternary successions have generally sought to optimise vertical resolution, through the use of higher source frequency content. Here, I show that low-frequency bandwidth is not necessarily a limiting factor for the seismic resolution of glacigenic morpho-sedimentary features. Observations are presented from a buried network of large mid-Pleistocene (Elsterian) tunnel-valleys in the southern North Sea Basin, across a 100×130 km study area with water depths less than 30 m. Low-frequency 2D and 3D seismic multi-channel data, acquired for deeper hydrocarbon exploration, are compared with previously available high-frequency single- and multi-channel profiles (5–15 km grid spacing). The low-frequency data contribute to a new understanding of the basal morphology and fill stratigraphy of the tunnel-valleys, in part due to higher data densities (≥1 km grid spacing), but also to improved imaging of reflectors at depth. The tunnel-valleys are seen to be overdeepened troughs, shallow (≤0.5 km) relative to their widths (≤6 km). The basal unconformity defines a series of arborescent elements, convergent to the south; erosional overlap by younger elements to the north has resulted in anastomosing patterns in places. The fill is dominated by axially downlapping clinoforms, descending to the north, onlapped and overlain by subhorizontal reflectors. Well data show that sand-dominated glaciofluvial sediments are overlain by glaciolacustrine to marine muds. Better definition of the clinoforms on low- versus high-frequency multi-channel data is suggested to reflect the coarse spatial scale of the backset glaciofluvial strata. The results support a simple interpretation of time-transgressive tunnel-valley formation by coeval glaciofluvial erosion and backfill beneath the outer tens of kilometres of the northward receding Elsterian ice sheet margin. Comparable submarginal interpretations have been proposed for drainage features (tunnel-valleys and eskers) of the last deglaciation of both northern Europe and North America using integrated geomorphologic and stratigraphic methods. Commercial 2D and 3D seismic data are widely available from exploration areas such as the North Sea and are argued to constitute an underexploited resource for Quaternary research.     

塔里木盆地结晶基底的反射地震调查

大型克拉通内部沉积盆地基底组构与盆地起源和油气聚集有密切关系.除了钻井岩心及周缘露头研究可提供盆地结晶基底类型证据外,反射地震信号记录了沉积盆地起源时期的有关大地构造作用信息,盆地基底组构可通过记录长度大的反射地震剖面研究.2007年中石化在塔里木盆地将1400 km的地震剖面接收记录从6 s加长到12 s,为研究克拉通盆地结晶基底的组构和类型提供了难得的第一手资料.这篇文章主要介绍这次调查的反射地震剖面,讨论深反射地震数据处理的关键技术,展示塔里木盆地巴楚—塔中地区的四条12 s反射地震剖面,并对反射剖面的散射模式作初步分析.本次调查表明,将地震剖面接收记录从6 s加长到12 s,采集处理的成本只增加了3％左右,但是可为研究克拉通盆地结晶基底和上地壳不均匀性提供很有价值的第一手资料.     

塔里木盆地结晶基底的反射地震调查

大型克拉通内部沉积盆地基底组构与盆地起源和油气聚集有密切关系.除了钻井岩心及周缘露头研究可提供盆地结晶基底类型证据外,反射地震信号记录了沉积盆地起源时期的有关大地构造作用信息,盆地基底组构可通过记录长度大的反射地震剖面研究.2007年中石化在塔里木盆地将1400 km的地震剖面接收记录从6 s加长到12 s,为研究克拉通盆地结晶基底的组构和类型提供了难得的第一手资料.这篇文章主要介绍这次调查的反射地震剖面,讨论深反射地震数据处理的关键技术,展示塔里木盆地巴楚—塔中地区的四条12 s反射地震剖面,并对反射剖面的散射模式作初步分析.本次调查表明,将地震剖面接收记录从6 s加长到12 s,采集处理的成本只增加了3％左右,但是可为研究克拉通盆地结晶基底和上地壳不均匀性提供很有价值的第一手资料.     

Reactivation of basement faults beneath volcanoes: a new model of flank collapse

In order to explain the presence of voluminous volcanic debris avalanche deposits around a stratovolcano, reactivation of vertical faults beneath a volcanic cone has been tested using analogue models. Reactivation of a single vertical fault beneath a cone generates a normal fault and an upturning of the layers creating a bulge on the flank. The upturning induces a flank collapse characterized by a typical horseshoe-shaped scar called an avalanche caldera. Reactivation of two vertical faults beneath a cone also generates a normal fault and a summit bulge. This bulge may result from the movement along a reverse fault. A large collapse is generated within the angle created by the two vertical faults. The angle of the collapse can be up to 140° whereas this angle is typically 120° for a dome intrusion. Collapse is instantaneous and is favoured by the presence of ductile layers (ash-and-pumice formations in the example considered) in a stratovolcano complex. The model may be applicable to volcanoes in a state of dormancy (or extinction) in regions with active regional tectonism. We suggest this mechanism of collapse in the case of the Cantal stratovolcano (Massif Central, France) to explain the presence of voluminous volcanic debris avalanche deposits around this volcano.     

呼和浩特基准地震台S波分裂研究

选取呼和浩特基准地震台2008—2015年产出的全球MS 6.0以上远震地震事件资料,使用Splitlab软件,对远震震相(SKS、SKKS、SS)综合分析,运用旋转相关法和最小能量法,进行S波分裂研究。在计算过程中,使用图形特征和结果对比的方法,对计算结果的质量进行判定,最终得到快波偏振方向主要集中在-12°—6°,延迟时间主要集中在0.9 s左右,与其他文献研究结果吻合。通过本次单台S波分裂研究,为今后更大范围的S波分裂研究提供了有效方法,也为今后单台测震学科研究工作提出新的思路。     

Seismic regime of a single fault model

A special experimental technique enabled us to study in detail seismic events on a fault model in a uniaxial stress field. The recording system used made it possible to investigate the radiation pattern for all the events observed including the precise determination of the dislocation origin.The aim of the present paper is (1) to find the relation between shear and tensile displacements, (2) to check a possible influence of the process of tensile crack generation on the seismic energy release and (3) to compare the seismic regime of a single fault before and after the tensile crack generation, respectively.Results prove the dominating importance of shear mechanism for the seismic energy release. The tensile displacement can be seismoactive only under special contact conditions on the fault plane.The existence of tensile cracks at the fault tips changed the pattern of seismic energy radiation. This feature is probably caused by subsequent changes in contact conditions on the fault plane and in the stress field around the fault.A comparison of some results of the present model experiments with the already published results of geological and seismological measurements and investigations shows the analogous character in laboratory and in nature of the process of tectonic earthquake preparation, the displacement course on the fault during the earthquake and the manner of seismic energy release on faults.     

Seismic reflection along the path of the Mediterranean Undercurrent

Seismic reflection profiling is applied to the study of large scale physical oceanographic processes in the Gulf of Cádiz and western Iberian coast, coinciding with the path of the Mediterranean Undercurrent. The multi-channel seismic reflection method provides clear images of thermohaline fine structure with a horizontal resolution approximately two orders of magnitude higher than CTD casting. The seismic data are compared with co-located historical oceanographic data. Three seismic reflectivity zones are identified: North Atlantic Central Water, Mediterranean Water and North Atlantic Deep Water. Seismic evidence for the path of the Mediterranean Undercurrent is found in the near-slope reflectivity patterns, with rising reflectors between about 500 and 1500 m. However, the core of the undercurrent is largely transparent. Seismic images show that central and, particularly, intermediate Mediterranean Waters have fine structure coherent over horizontal distances of several tens of kilometers. However, the intensity of the reflectors, and their horizontal coherence, decreases downstream. This change in seismic reflectivity is probably the result of diminished vertical thermohaline contrasts between adjacent water masses, so that double-diffusion processes become unable to sustain temperature and salinity staircases. Comparison of root-mean-square seismic amplitudes with temperature and salinity differences between the Mediterranean Undercurrent and the overlying central waters suggests a causal relationship between observed thermohaline fine structure and true seismic amplitudes. We estimate that, within this intermediate water stratum, impedance contrasts are mainly controlled by sound speed contrasts (a factor between 3.5 and 10 times larger than density contrasts), which are mainly controlled by temperature contrasts (a factor between 1.5 and 5 times larger than salinity contrasts).     

Effects of a rigid core on the reflection and transmission coefficients from a multi-layered core-mantle boundary

Summary The aim of this paper is to present the formulations which can be used in calculating reflection and transmission coefficients when the rigidity in the core is taken into consideration. The theoretical curves presented can be used as a guide for studies of the physical parameters of the core-mantle boundary. It is hoped that these curves may lead to a clarification of the great differences between observed data and theoretical calculations, when the geometrical spreading and attenuation are taken into account.The Thomson-Haskell matrix formulations are used to calculate the reflection and transmission coefficients for a multi-layered medium imbedded between two half-spaces representing the solid mantle and a rigid core. A rigid core is defined here as having a rigidity in the range 10¹⁰<<10¹¹ cgs units. For five proposed models of the core-mantle boundary the rigidity in the core is varied and the results are compared with those for a liquid core.     

Use of low frequencies for sub-basalt imaging

Many prospective passive ocean margins are covered by large areas of basalts. These basalts are often extremely heterogeneous and scatter the seismic energy of the conventional seismic reflection system so that it becomes difficult to obtain information on deeper reflectors. Since high frequencies are scattered more than low frequencies, we argue that the acquisition system for sub-basalt targets should be modified to emphasize the low frequencies, using much larger airguns, and towing the source and receivers at about 20 m depth. In the summer of 2001 we obtained seismic reflection data over basalt in the northeast Atlantic using a system modified to enhance the low-frequency energy. These new data show deep reflections that are not visible on lines shot in the same places with a conventional system.