Seismoelectric numerical simulation in 2D vertical transverse isotropic poroelastic medium     

Munirdin Tohti  Yibo Wang  Evert Slob  Yikang Zheng  Xu Chang  Yi Yao 《Geophysical Prospecting》2020,68(6):1927-1943

Seismoelectric coupling in an electric isotropic and elastic anisotropic medium is developed using a primary–secondary formulation. The anisotropy is of vertical transverse isotropic type and concerns only the poroelastic parameters. Based on our finite difference time domain algorithm, we solve the seismoelectric response to an explosive source. The seismic wavefields are computed as the primary field. The electric field is then obtained as a secondary field by solving the Poisson equation for the electric potential. To test our numerical algorithm, we compared our seismoelectric numerical results with analytical results obtained from Pride's equation. The comparison shows that the numerical solution gives a good approximation to the analytical solution. We then simulate the seismoelectric wavefields in different models. Simulated results show that four types of seismic waves are generated in anisotropic poroelastic medium. These are the fast and slow longitudinal waves and two separable transverse waves. All of these seismic waves generate coseismic electric fields in a homogenous anisotropic poroelastic medium. The tortuosity has an effect on the propagation of the slow longitudinal wave. The snapshot of the slow longitudinal wave has an oval shape when the tortuosity is anisotropic, whereas it has a circular shape when the tortuosity is isotropic. In terms of the Thomsen parameters, the radiation anisotropy of the fast longitudinal wave is more sensitive to the value of ε, while the radiation anisotropy of the transverse wave is more sensitive to the value of δ.  相似文献   

Tutorial: unified 1D inversion of the acoustic reflection response     

Evert Slob  Kees Wapenaar  Sven Treitel 《Geophysical Prospecting》2020,68(5):1425-1442

Acoustic inversion in one-dimension gives impedance as a function of travel time. Inverting the reflection response is a linear problem. Recursive methods, from top to bottom or vice versa, are known and use a fundamental wave field that is computed from the reflection response. An integral over the solution to the Marchenko equation, on the other hand, retrieves the impedance at any vertical travel time instant. It is a non-recursive method, but requires the zero-frequency value of the reflection response. These methods use the same fundamental wave field in different ways. Combining the two methods leads to a non-recursive scheme that works with finite-frequency bandwidth. This can be used for target-oriented inversion. When a reflection response is available along a line over a horizontally layered medium, the thickness and wave velocity of any layer can be obtained together with the velocity of an adjacent layer and the density ratio of the two layers. Statistical analysis over 1000 noise realizations shows that the forward recursive method and the Marchenko-type method perform well on computed noisy data.  相似文献   

A classification of dyke-fracture geometry with examples from Precambrian dyke swarms in the Vestfold Hills,Antarctica     

J. D. Hoek 《International Journal of Earth Sciences》1991,80(2):233-248

A classification of dilational fracture geometry is proposed. It applies to structures that can be characterized by the combination of a fracture system and a dilation vector field. The classification is illustrated with examples of igneous dykes, pegmatites, and pseudotachylites. Segmentation in the form of offsets, jogs, or bifurcations is common to most fracture systems. Four basic types of dilational fracture systems are distinguished on the basis of the geometry of segmentation. These are: irregular, braided, en-echelon, and zigzag. Zigzag fracture systems are further differentiated. They consist of newly formed, obliquely dilated fractures or, alternatively, of pre-existing planes in the host-rock that are reactivated by oblique dilation. This paper is the first to actually report an igneous zigzag-dyke involving newly formed fractures.Rotation of the regional stress field in the direction of propagation leads to en-echelon segmentation. Braided fracture systems reflect high local stress-intensities, probably related to propagation rate. Two possibilities exist for the formation of zigzag dykes consisting of newly formed fractures that are obliquely dilated. They may form by extreme interaction of tensile fracture segments in a regional stress field with a low differential stress. Alternatively, they may form in a regional stress field with high differential stress through the propagation of shear fractures.Segmentation of dykes, characterized by offsets, is common. Such segmentation can be the result of protrusions of the fracture termination. Offsets also occur where a dyke cuts an older planar structure. Such offsets are the result of the local inhibition of fracture propagation. The resulting apparent offset can lead to a misinterpretation of relative age.Apophyses form as a result of the dilation of a segmented fracture system.

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Zusammenfassung Eine Klassifizierung der Geometrie von Dilatationsbrüchen wird vorgeschlagen. Sie wird angewendet auf Strukturen die charakterisiert werden können als die Kombination eines Bruchsystems und eines Dilatationsvektorfeldes. Die Klassifizierung wird illustriert durch Beispiele von magmatischen Gängen, Pegmatitien und Pseudotachyliten. Segmentierung in Form von Versätzen, Stoßfugen oder Gabelungen sind häufig in vielen Bruchsystemen. Vier Grundtypen von Dilatationsbruchsystemen können an Hand der Segmentierungsgeometrie unterschieden werden, diese sind: Unregelmäßige -, verzweigte -, kulissenförmige — und zick-zack Typen. Die Zickzack-Bruchsysteme werden weiter untergliedert. Sie bestehen aus neu gebildeten sich schief ausdehenden Brüchen, oder aber aus bereits bestehenden Flächen des Muttergesteins, welche von der schiefen Erweiterung reaktiviert wurden. Dies ist die erste Veröffentlichung, die von magmatischen Zickzack-Gängen berichtet, die neu geformte Brüche entwickeln.Die Rotation des regionalen Spannungsfeldes in Fortplanzungsrichtung führt zu einer kulissenartigen Segmentierung. Verzweigte Bruchsysteme spiegeln hohe lokale Spannungsintensitäten wieder, die in Bezug zur Fortpflanzungsrate stehen. Es bestehen zwei Möglichkeiten für die Bildung von Zickzack-Gängen die aus neugebildeten Brüchen bestehen die sich schief ausgedehnt haben. Sie können sich bilden bei extremen Wechselwirkungen von Dehnungsbruchsegmenten in einem regionalem Spannungsfeld mit einer geringen differentialen Spannung. Andererseits können sie sich auch unter einem regionalen Spannungsfeld mit hohen differentialen Spannungen, durch das Fortschreiten von Scherbrüchen bilden.Häufig findet sich eine Segmentierung von magmatischen Gängen, welche durch Versätze charakterisiert ist. Diese Segmentierung kann das Resultat von Vorsprüngen des Bruchendes sein. Versätze können auch vorkommen wo ein Gang eine ältere ebene Struktur durchschneidet. Diese Versätze treten auf, wenn die Bruchfortpflanzung lokal behindert wird. Der resultierende scheinbare Versatz kann zu Mißdeutungen des relativen Alters führen.Apophysen bilden sich als ein Resultat von Ausdehnungen des segmentierten Bruchsystems.

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Résumé Cette note propose une classification des fractures de dilatation, basée sur leur géométrie. Elle s'applique aux structures que l'on peut caractériser par la combinaison d'un système de fractures et d'un champ de vecteurs de dilatation. Elle est illustrée par des exemples de dykes magmatiques, de pegmatites et de pseudotachylites. La plupart des systèmes de fractures présentent ordinairement une segmentation sous la forme de rejets, de coudes ou de bifurcations. Sur la base de la géométrie de la segmentation, on peut distinguer quatre types fondamentaux de systèmes de fractures de dilatation: les systèmes irrégulier, anastomosé, en échelon et en zig-zag, ce dernier pouvant être à son tour subdivisé. Ces systèmes mettent en jeu soit une fracturation nouvelle à dilatation oblique, soit une réactivation par dilatation oblique de plans préexistants. La présente note signale pour la première fois un dyke magmatique en zig-zag impliquant des fractures nouvelles.La rotation du champ de contraintes régional dans la direction de propagation engendre la segmentation en échelon. Les systèmes de fractures anastomosées traduisent des contraintes locales élevées, probablement en relation avec le taux de propagation. Il existe deux possibilités dans la formation de dyke en zig-zag correspondant à des fractures nouvelles à dilatation oblique. Ils peuvent résulter d'une interaction intense de tronçons de fractures de tension dans un champ de contraintes régional à faible gradient. Ils peuvent aussi se former dans un champ à gradient de contrainte élevé, par la propagation de fractures de cisaillement.La segmentation de dykes avec rejets est une disposition fréquente. Elle peut résulter d'une irrégularité de l'extrémité des fractures. Elle peut aussi se produire lorsqu'un dyke coupe une ancienne structure plane. De tels rejets sont l'expression d'un empêchement local à la propagation de la fracture. Le rejet apparent qui en résulte peut conduire à une interprétation erronée de l'âge relatif. Les apophyses se forment par dilatation d'un système segmenté de fractures.

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Spectroscopic investigation of U(VI) sorption at the calcite-water interface     

Evert J. Elzinga  C. Drew Tait  Kirk D. Rector  David E. Morris 《Geochimica et cosmochimica acta》2004,68(11):2437-2448

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A prototype web-based analysis platform for drought monitoring and early warning     

Mattijn van Hoek  Jie Zhou  Jing Lu  Chaolei Zheng  Guangcheng Hu 《International Journal of Digital Earth》2020,13(7):817-831

ABSTRACT

It has long been recognized that an effective drought monitoring and early warning system, which provides functions for real-time condition monitoring and prediction, risk assessment, information dissemination and response recommendation, is very important for the preparedness for and mitigation of drought impacts. In this article, we review the currently existing drought monitor and early warning systems, discuss applicable remote sensing datasets and drought indicators and present the development of a web-based quasi-real-time Global Drought Monitoring & Analysis Platform (Web-GDMAP). The Web-GDMAP is built upon a series of indicators derived from multi-source satellite remote sensing data and various other sources of data. From a technical perspective, the Web-GDMAP system includes a series of components from data storage, model implementation and distribution, to client-side visualization and user intuitive interaction. From a theoretical perspective, the Web-GDMAP system integrates multi-indicators on different aspects of drought, including anomalies in precipitation, anomalies in land surface thermal and vegetation conditions, water deficit of soil and plants, etc. Several case studies on applying the developed Web-GDMAP in the Asian region are demonstrated. Further improvements and perspectives are discussed.  相似文献   

Fast and accurate three‐dimensional controlled source electromagnetic modelling†     

Ali Moradi Tehrani  Evert Slob 《Geophysical Prospecting》2010,58(6):1133-1146

We present a fast approximate method for three‐dimensional low frequency controlled source electro‐magnetic modeling. We apply the method to a synthetic model in a typical marine controlled source electromagnetic scenario, where conductivity and permittivity are different from the known background medium. For 3D configurations, fast computational methods are relevant for both forward and inverse modelling studies. Since this problem involves a large number of unknowns, it has to be solved efficiently to obtain results in a timely manner, without compromising accuracy. For this reason, the Born approximation, extended Born approximation and iterative extended Born approximation are implemented and compared with the full solution of the conjugate gradient fast Fourier transformation method. These methods are based on an electric field domain integral equation formulation. It is shown here how well the iterative extended Born approximation method performs in terms of both accuracy and speed with different configurations and different source positions. The improved accuracy comes at virtually no additional computational cost. With the help of this method, it is now possible to perform sensitivity analysis using 3D modelling in a timely manner, which is vital for controlled source electromagnetic applications. For forward modeling the solution at the sea‐bottom is of interest, because that is where the receivers are usually located. For inverse modeling, the accuracy of the solution in the target zone is important to obtain reasonably accurate conductivity values from the inversion using this approximate solution method. Our modelling studies show that the iterative extended Born approximation method is fast and accurate for both forward and inverse modelling. Sensitivity analysis as a function of the source position and different reservoir sizes validate the accuracy of the iterative extended Born approximation.  相似文献   

Seismic and electromagnetic controlled-source interferometry in dissipative media   总被引：3，自引：1，他引：2  

Kees Wapenaar  Evert Slob  Roel Snieder 《Geophysical Prospecting》2008,56(3):419-434

Seismic interferometry deals with the generation of new seismic responses by crosscorrelating existing ones. One of the main assumptions underlying most interferometry methods is that the medium is lossless. We develop an ‘interferometry‐by‐deconvolution’ approach which circumvents this assumption. The proposed method applies not only to seismic waves, but to any type of diffusion and/or wave field in a dissipative medium. This opens the way to applying interferometry to controlled‐source electromagnetic (CSEM) data. Interferometry‐by‐deconvolution replaces the overburden by a homogeneous half space, thereby solving the shallow sea problem for CSEM applications. We demonstrate this at the hand of numerically modeled CSEM data.  相似文献   

Using multicomponent GPR to monitor cracks in a historical building     

Luciana Orlando  Evert Slob   《Journal of Applied Geophysics》2009,67(4):327

We tested the usefulness of acquiring multicomponent GPR data to detect cracks in a historical building, and to monitor their dynamics, caused by a slowly and irregularly moving landslide. We used 2 GHz bipolar antennas in a configuration that allows for acquiring the in-line and cross-line electric field components with x- and y-directed antennas. The 2 × 2 data matrix was collected on a floor in the building along transects at four different times over a period of one year. The data were processed with a standard 2D scalar algorithm and with the latest 3D single component vector algorithm that corrects for antenna effects. We have implemented a 3D single component vector migration algorithm in a 2.5D sense to produce 2D slices of a 3D vector migration image by applying the algorithm on line data. This procedure allows for migrating single component line data taking into account all vector effects as well as three-dimensional wave propagation. We show that the 2.5D vector migration images have a much better in-line resolution than the migration images obtained by applying a standard 2D scalar migration algorithm.The GPR profiles agree with the a priori information about the structure of the floor. In particular, we detected two different types of anomalies, only a few of which can be due to utilities and to metallic mesh. Some shallower anomalies agree well with the cracks visible on the tiling, suggesting that some cracks can be directly detected using GPR. Visually there were no changes in the cracks on the floor and no clear changes in the GPR data could be attributed to possible subsurface changes in the cracks. The variations in the GPR images seemed primarily caused by changes in the coupling of the antennas with the investigated structure (floor) depending on the season when the measurements were made. For this reason the monitoring aspect of the survey is not successful.  相似文献   

Equipartitioning is not sufficient for Green's function extraction          下载免费PDF全文

Roel Snieder  Yuanzhong Fan  Evert Slob  Kees Wapenaar 《地震科学（英文版）》2010,23(5):403-415

The extraction of the Earth's Green's function from field fluctuations is a rapidly growing area of research.The principle of Green's function extraction is often related to the requirement of equipartitioning,which stipulates that the energy of field fluctuations is distributed evenly in some sense.We show the meaning of equipartitioning for a variety of different formulations for Green's function retrieval.We show that equipartitioning is not a sufficient condition,and provide several examples that illustrate this point.We discuss the implications of lack of equipartitioning for various schemes for the reconstruction of the Green's function in seismology.The theory for Green's function extraction is usually based on a statistical theory that relies on ensemble averages.Since there is only one Earth,one usually replaces the ensemble average with a time average.We show that such a replacement only makes sense when attenuation is taken into account,and show how the theory for Green's function extraction for oscillating systems can be extended to incorporate attenuation.  相似文献   

The Teaching of Rock Mechanics     

E. Hoek 《Rock Mechanics and Rock Engineering》1972,4(3):135-138

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