Coulomb stress evolution in Northeastern Caribbean over the past 250 years due to coseismic, postseismic and interseismic deformation   总被引：5，自引：0，他引：5  

Syed Tabrez Ali  rew M. Freed  Eric Calais  David M. Manaker  William R. McCann 《Geophysical Journal International》2008,174(3):904-918

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Style and evolution of salt pillows and related structures in the northern part of the Northeast German Basin     

Dirk Kossow  Charlotte Krawczyk  Tommy McCann  Manfred Strecker  Jörg F.W. Negendank 《International Journal of Earth Sciences》2000,89(3):652-664

The northern part of the Northeast German Basin contains a large number of Late Permian (Zechstein) salt pillows, whereas diapiric structures are almost completely absent. This lack of diapirs facilitated the study of early stages of salt movement in the basin. Salt pillows and related structures were investigated in terms of distribution, geometry and time of initiation of salt flow within the regional geological context. The primary Zechstein thickness in the study area was reconstructed to gain more insight into the relationship between the geometry of the salt layer and the style of the salt-related structures. In this study, no clear spatial relationship between the salt structures and basement faults has been found and the location of the salt structures in this area appears to be highly independent of the underlying structural grain. The overburden is affected by minor faulting. We propose that buckling of the overburden due to regional compression significantly contributed to the initiation of the Late Jurassic to Early Cretaceous salt structures in the basin. Reverse faulting of the Gardelegen and Haldensleben Faults is related to inversion tectonics and exerted a compression on the basin fill. During the deformation, the Late Permian salt layer acted as an efficient detachment and led to a marked decoupling of the Mesozoic overburden from the underlying pre-Zechstein rocks.  相似文献   

An attenuation-based sediment classification technique using Chirp sub-bottom profiler data and laboratory acoustic analysis     

Stevenson  I.R.  McCann  C.  Runciman  P.B. 《Marine Geophysical Researches》2002,23(4):277-298

A remote sediment classification technique based on attenuation measurements from Chirp sub-bottom profiler data is described. This differs from previously published work in that attenuation measurements are obtained for each stratigraphic unit within a complex, thinly interbedded sedimentary sequence. Compressional wave attenuation measurements are obtained for a wide variety of lithologies, including muds, silts, sands, clayey sands, silty clays and gravel lags, with grain sizes ranging from 8 Phi to -4 Phi. In addition, attenuation measurements from sub-bottom profiler data were calibrated against laboratory acoustic measurements of vibracores and seabed samples from corresponding geographic locations, under simulated in-situ conditions using a Pulse Tube method. We adapt an instantaneous frequency matching method using a causal attenuation filter to model the decay of the Chirp transmitted waveform. From this modelling, a relationship between t* (a causal attenuation operator) and change in instantaneous frequency is established. The Hilbert transform is used to extract instantaneous frequency information from Chirp seismic, which is used to derive attenuation information for selected individual stratigraphic layers imaged by the sub-bottom profiler. This paper draws attention to the limitations in comparing attenuation measurements derived from Chirp sub-bottom profiler data against previously published literature on experimental attenuation measurements, which are limited by the wide variance of these data, and the difficulty in finding a meaningful best fit to these data. This demonstrates the importance of calibrating remote sediment classification observations using complimentary acoustic analysis of seabed samples to generate a site-specific geoacoustic database. A positive correlation between laboratory and sub-bottom profiler attenuation measurements was obtained, with a correlation coefficient of 0.885. Poorly sorted gravels with a mixed lithic and biogenic pebble component are characterised by very high attenuation with values of Q from 4 to 19. These sediments are considerably coarser-grained than those typically described in previously published experimental studies.  相似文献   

The relationships between the velocities,attenuations and petrophysical properties of reservoir sedimentary rocks1     

A.I. Best  C. McCann  J. Sothcott 《Geophysical Prospecting》1994,42(2):151-178

We have measured the velocities and attenuations of compressional and shear waves in 29 water-saturated samples of sandstones and shales at a confining pressure of 60 MPa and at frequencies of about 0.85 MHz. The measurements were made using a pulse echo method in which the samples (diameter 5 cm, length 1.5 cm to 2.5 cm) were placed between perspex buffer rods inside a high-pressure cell. The velocity of each seismic wave was determined from the traveltime difference of equivalent phase points (corrected for diffraction effects) of the signals reflected from the top and from the base of each sample. Attenuation was determined in a similar way by comparison of the diffraction corrected amplitudes of the signals. The attenuation data are presented as ‘quality factors’: Q_p and Q_s for compressional and shear waves respectively. The results show that Q_s is strongly correlated with V_s, that Q_p is weakly correlated with V_p, and that Q_p is strongly correlated with Q_s. Q_p is strongly dependent on the volume percentage of the assemblage of intra-pore minerals, whether they are clays or carbonates. It is concluded that the attenuation mechanism is due to the local fluid flow arising from the differential dilation of the solid rock frame and the intra-pore mineral assemblage, which is a result of their very different elastic moduli.  相似文献   

Velocities of compressional and shear waves in limestones   总被引：2，自引：1，他引：2  

Solomon Assefa  Clive McCann  Jeremy Sothcott 《Geophysical Prospecting》2003,51(1):1-13

Carbonate rocks are important hydrocarbon reservoir rocks with complex textures and petrophysical properties (porosity and permeability) mainly resulting from various diagenetic processes (compaction, dissolution, precipitation, cementation, etc.). These complexities make prediction of reservoir characteristics (e.g. porosity and permeability) from their seismic properties very difficult. To explore the relationship between the seismic, petrophysical and geological properties, ultrasonic compressional‐ and shear‐wave velocity measurements were made under a simulated in situ condition of pressure (50 MPa hydrostatic effective pressure) at frequencies of approximately 0.85 MHz and 0.7 MHz, respectively, using a pulse‐echo method. The measurements were made both in vacuum‐dry and fully saturated conditions in oolitic limestones of the Great Oolite Formation of southern England. Some of the rocks were fully saturated with oil. The acoustic measurements were supplemented by porosity and permeability measurements, petrological and pore geometry studies of resin‐impregnated polished thin sections, X‐ray diffraction analyses and scanning electron microscope studies to investigate submicroscopic textures and micropores. It is shown that the compressional‐ and shear‐wave velocities (V_p and V_s, respectively) decrease with increasing porosity and that V_p decreases approximately twice as fast as V_s. The systematic differences in pore structures (e.g. the aspect ratio) of the limestones produce large residuals in the velocity versus porosity relationship. It is demonstrated that the velocity versus porosity relationship can be improved by removing the pore‐structure‐dependent variations from the residuals. The introduction of water into the pore space decreases the shear moduli of the rocks by about 2 GPa, suggesting that there exists a fluid/matrix interaction at grain contacts, which reduces the rigidity. The predicted Biot–Gassmann velocity values are greater than the measured velocity values due to the rock–fluid interaction. This is not accounted for in the Biot–Gassmann velocity models and velocity dispersion due to a local flow mechanism. The velocities predicted by the Raymer and time‐average relationships overestimated the measured velocities even more than the Biot model.  相似文献   

The acoustic signature of fluid flow in complex porous media   总被引：2，自引：0，他引：2  

Morten Jakobsen  Tor Arne Johansen  Clive McCann   《Journal of Applied Geophysics》2003,54(3-4):219

Effective medium approximations for the frequency-dependent and complex-valued effective stiffness tensors of cracked/porous rocks with multiple solid constituents are developed on the basis of the T-matrix approach (based on integral equation methods for quasi-static composites), the elastic–viscoelastic correspondence principle, and a unified treatment of the local and global flow mechanisms, which is consistent with the principle of fluid mass conservation. The main advantage of using the T-matrix approach, rather than the first-order approach of Eshelby or the second-order approach of Hudson, is that it produces physically plausible results even when the volume concentrations of inclusions or cavities are no longer small. The new formulae, which operates with an arbitrary homogeneous (anisotropic) reference medium and contains terms of all order in the volume concentrations of solid particles and communicating cavities, take explicitly account of inclusion shape and spatial distribution independently. We show analytically that an expansion of the T-matrix formulae to first order in the volume concentration of cavities (in agreement with the dilute estimate of Eshelby) has the correct dependence on the properties of the saturating fluid, in the sense that it is consistent with the Brown–Korringa relation, when the frequency is sufficiently low. We present numerical results for the (anisotropic) effective viscoelastic properties of a cracked permeable medium with finite storage porosity, indicating that the complete T-matrix formulae (including the higher-order terms) are generally consistent with the Brown–Korringa relation, at least if we assume the spatial distribution of cavities to be the same for all cavity pairs. We have found an efficient way to treat statistical correlations in the shapes and orientations of the communicating cavities, and also obtained a reasonable match between theoretical predictions (based on a dual porosity model for quartz–clay mixtures, involving relatively flat clay-related pores and more rounded quartz-related pores) and laboratory results for the ultrasonic velocity and attenuation spectra of a suite of typical reservoir rocks.  相似文献   

Space, citizenship, and the right to the city: A brief overview     

Eugene J. McCann 《GeoJournal》2002,58(2-3):77-79

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Modelling the seismic attributes of overpressured siliciclastic rocks, with a genetic annealing (GAN) algorithm     

Patricia Domnesteanu  Robert Domnesteanu  Clive McCann 《Geophysical Prospecting》2002,50(2):169-182

ABSTRACT A genetic annealing (GAN) algorithm is used to derive an empirical model which predicts compressional-wave velocity values for overpressured siliciclastic rocks. The algorithm involves non-linear random searching and mutation techniques and its annealing component imposes a very strict control over the rate of convergence of the search. This technique provides an alternative to the standard calculations involving the effective stress coefficient ( n ). The pore pressure is introduced into the model as an explicit variable and as part of an overpressure coefficient, ( P _p/ P _c) − the ratio of pore to confining pressure. Empirical model-derived data and known laboratory data are compared and their differences are shown to be within statistically acceptable error limits. The empirical equation fits all under- and overpressured data simultaneously, irrespective of pore fluid pressure level, with the same parameters. It is used to predict seismic velocities very accurately for extreme levels of overpressure, starting from normally pressured experimental data. The model highlights the effect of pore pressure on the compressional-wave velocity of fully saturated samples with different clay contents. It can be used when the experimental data available are sparse and particularly when a prediction of material behaviour is necessary at specific pore fluid pressure and depth conditions.  相似文献   

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

Solomon Assefa  Clive McCann  Jeremy Sothcott 《Marine Georesources & Geotechnology》2000,18(2):119-140

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.  相似文献   

Geochemistry and provenance of the Miocene sandstones of the Surma group from the Sitapahar anticline,Southeastern Bengal Basin,Bangladesh     

M. Julleh Jalalur Rahman  A. S. M. Sayem  Tom McCann 《Journal of the Geological Society of India》2014,83(4):447-456

Geochemical composition (major and trace elements) of Miocene sandstones of the Surma Group exposed in Sitapahar anticline, Southeastern Bengal Basin was determined to reveal their provenance, tectonic setting and source area weathering conditions. The sandstones are sub-arkosic, sub-lithic and greywacke in composition with abundant low-grade metamorphic, sedimentary lithics (mainly chert with some shale fragments), low feldspars and little volcanic detritus. Compared to the average sandstone value, the Surma Group sandstones are depleted in CaO and enriched in Al₂O₃, Fe₂O₃ and Na₂O. The Chemical Index of Alteration (CIA) values for the Miocene Surma Group sandstones vary from 57 to 73 with an average of 65, indicating low to moderate weathering of the source areas. The geochemical characteristics suggest an active continental margin to passive margin setting for the Surma Group sandstones; preserve the signatures of a recycled provenance that is agreement with sandstone petrography and derivation of these sandstones from felsic source rocks.  相似文献