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1.
We discuss the results of a field experiment in the Malaya Botuobiya area (West Yakutia) at a site where earlier surveys revealed slowly decaying transient responses. That time-dependent voltage decay indicated magnetic viscosity effects associated with magnetic relaxation of superparamagnetic grains in rocks. In this study, we have applied a high-resolution array TEM survey to contour the anomaly and parametric soundings with systems of different configurations to explore the vertical pattern of magnetic viscosity. The parametric data have been inverted, by means of manual and automated fitting, with a reference model of a layered magnetically viscous earth, using, respectively, analytical formulas and simulation based on a forward solution by separation of variables. According to both automated and manual inversion, the section at the center of the anomalous site fits a three-layer earth model with an intermediate magnetically viscous layer between two nonmagnetic layers. This model is consistent with a priori evidence of local geology and may provide more details of the latter. The inversion results have been further used to estimate the volumetric percentage of superparamagnetic grains in the magnetically viscous layer, assuming magnetite to be the main ferrimagnetic phase.  相似文献   

2.
Transient electromagnetic responses measured in the field or in the laboratory may bear effects of viscous remanent magnetization (VRM) associated with magnetic relaxation of ultrafine grains of ferrimagnetic minerals or superparamagnetism. The behavior of VRM can be studied in time or frequency domain, TDEM measurements being advantageous because they are done in the absence of primary field and owing to broad time range providing high accuracy of VRM parameters. Another advantage is that the rate of viscous decay measured as voltage decay does not need to be corrected for stable and/or slowly decaying viscous component of total remanence. Time-dependent transient responses of viscous decay follow the power law a ⋅ t–b, where a is the initial emf signal (varying in a broad range) and b is the exponent approaching 1. Laboratory tests with a pulse induction coil system reveal a strong linear correlation of the parameter a with frequency-dependent magnetic susceptibility Δκ used commonly for constraining the relative abundances of superparamagnetic particles. Such systems are thus suitable for quick measurements of the large number of samples for detection of superparamagnetic (SP) particles and quantifying their contribution. The difference of b from 1, though being minor, exceeds markedly its error in estimates from measured data. Simulated TDEM responses of a superparamagnetic ground show both parameters (a and b) to depend on particle volume distribution, which is prerequisite for inversion of time-domain transients to magnetic properties of rocks and soils.  相似文献   

3.
A numerical experiment has been applied to explore the potentialities and the limitations of the inversion of IP-affected TEM responses of a two-layer earth with a polarizable layer above (Model 1) or below (Model 2) a nonpolarizable layer. The IP effects have been incorporated into the models via a Cole–Cole complex frequency-dependent conductivity. One of us computed synthetic in-loop and coincident-loop transient responses with added Gaussian noise, and the other performed single and joint inversion of the two sets of pseudo-experimental data. Model 1 turns out to be advantageous over Model 2 in TEM applications and gives a good fit of the Cole–Cole parameters (chargeability, IP time constant, and exponent) even in the absence of a priori information. In the case of Model 2, the lack of a priori information causes problems as to recognize which layer is polarizable, and the fit of the Cole–Cole parameters is generally worse. The layer thicknesses and resistivities are rather accurate in both groups of models, irrespective of whether a priori information is available. As the upper layer increases in thickness (H1), the fit of its parameters ever improves in both models while the parameters of the lower layer, on the contrary, contain a greater error. Joint inversion of in-loop and coincident-loop transients improves the fit in most cases.Relative rms error (σrel) does not depend on the upper layer thickness for Model 1 but decreases as H1 increases in the case of Model 2. The error in joint inversion is times that in single inversion, which means that additional criteria other than σrel may be useful to estimate the inversion quality.  相似文献   

4.
Structures are commonly founded on layered soil deposits. In this study, settlement due to a circular load on a two-layer soil system overlying a rock stratum or a stiff soil deposit is estimated and compared with the values obtained using Steinbrenner’s and Finite element approaches. Design tables are proposed for settlements due to uniform circular loading (of radius equal to a) applied on two-layer soil system for top-layer thickness H 1 = 0.2a–6.0a, bottom-layer thickness H 2 = 1.0a–6.0a, moduli ratio of top and bottom layers E 1 /E 2 = 0.01–100 and Poisson’s ratio v = 0.2–0.5. The settlement factors from the study were also compared with the Ueshita and Meyerhof’s values for a finite top layer overlying a semi-infinite bottom layer. Results were compared with the observed settlement of a structure resting on a layered soil profile overlying a rock stratum. Proposed settlement of the two-layer system shows good agreement with these studies.  相似文献   

5.
In this paper, we have developed a least-squares minimization method to estimate the depth of a buried conducting sphere using electromagnetic induction (EMI) data. This approach is basically based on the solving a set of algebraic linear equations to estimate the depth of sphere embedded in an insulating media. In electromagnetic induction method, the transmitter coil produces the incident magnetic and electric fields that obey the Maxwell’s equations. In the receiver coil, the received response is created in two modes—eddy-current mode (V ec) derived from the perfectly conductor placed in the shallow depth and another mode called current-channeling response (V cc) which depends on the conductivity of the medium. As expected, these responses differ depending on the direction of the incident field related to the receiver coil’s axis. In our case, the transmitter coil’s axis is parallel to the ground surface, and only the eddy-current response is measured in the receiver coil. The validity of this new method is demonstrated through studying and analyzing synthetic EMI anomalies, using simulated data generated from a known model with different random error components and a known statistical distribution.  相似文献   

6.
To the extent that rock deformation can be approximated by a two-dimensional Newtonian model, a powerful stream-function simulation method is applicable. The significance of stream functions is that velocity, strain, stress and energy derived from the same stream function satisfy automatically three basic conditions of dynamics:
1. (1) the condition of continuity.
2. (2) the Navier-Stokes equations.
3. (3) conservation of energy.
Hence we state with Jaeger: “If a stream function can be found which satisfies the boundary conditions of a dynamic model the complete solution follows.” All pertinent bits of dynamic information are implied in the stream function from which they can be directly derived, guaranteed—so to speak—not to violate the basic conditions of dynamics. Stream functions useful in structural geology are solutions of: A double-polynomial solution of max. degree 14 is developed, in which the coefficients are related controlled by the 4ψ = 0 constraint, and their absolute values are determined by the boundary conditions of specific models and by the condition of maximum rate of energy dissipation or maximum rate of decline of potential energy. The polynomial stream function is applied to a collapsing viscous “nappe” consisting of a thin basal layer with low viscosity on which a thicker layer with high viscosity slides due to gravitational spreading. The velocity of forward movement depends upon absolute and relative values of the following parameters: viscosity, thickness, the aspect ratio and density. The velocity of a variety of nappes with different thicknesses, aspect ratios, viscosities and densities is determined.  相似文献   

7.
A nearly 500-km-long seismic profile with reflective and refractive wide-angle Ocean Bottom Seismometer (OBS) data and Multi-Channel Seismic (MCS) data was acquired across the northeastern continental margin of the South China Sea (SCS). The S-wave crustal structure and Vp/Vs ratios have been obtained based on a previously published P-wave model using the software RayInvr. Modeling of vertical- and horizontal-component OBS data yields information on the seismic crustal velocities, lithology, and geophysical properties along the OBS-2001 seismic profile. S-wave velocities in the model increase generally with depth but exhibit high spatial variability, particularly from the shelf to the upper slope of the northeastern SCS margin. Vp/Vs ratios also reveal significant lithological heterogeneity. Dongsha–Penghu Uplift (DPU) is a tectonic zone with a thicker crust than adjacent areas and a high magnetic anomaly. With a Vp/Vs of 1.74 and a P-wave velocity of 5.0–5.5 km/s, the DPU primarily consists of felsic volcanic rocks in the upper crust and is similar to the petrology of Zhejiang–Fujian volcanic provinces, which perhaps is associated with a Mesozoic volcanic arc. The ocean–continent transition (OCT) in the northeastern SCS is characterized by a thinning continental crust, volcanoes in the upper crust, and a high velocity layer (HVL) in the lower crust. The S-wave velocity and Vp/Vs ratio suggest that the HVL has a mafic composition that may originate from underplating of the igneous rocks beneath the passive rifted crust after the cessation of seafloor spreading.  相似文献   

8.
Susceptibility, magnetisation and Mössbauer measurements are reported for a representative selection of 2:1 layer phyllosilicates. Eight samples from the mica, vermiculite and smectite groups include examples diluted in iron which are paramagnetic at all temperatures, as well as iron-rich silicates which order magnetically below 10 K. Anisotropic susceptibility of crystals of muscovite, biotite and vermiculite is quantitatively explained with a model where the Fe2+ ions lie in sites of effective trigonal symmetry, the trigonal axis lying normal to the sheets. The ferrous ground state is an orbital singlet. Ferric iron gives an isotropic contribution to the susceptibility. Fe2+-Fe2+ exchange interactions are ferromagnetic with y ～ 2 K, whereas Fe3+-Fe3+ coupling is antiferromagnetic in the purely ferric minerals. A positive paramagnetic Curie temperature for glauconite may be attributable to Fe2+ → Fe3+ charge transfer. Magnetic order was found to set in inhomogeneously for glauconite at 1–7 K. One biotite sample showed an antiferromagnetic transition at T N =7 K marked by a well-defined susceptibility maximum. Its magnetic structure, consisting of ferromagnetic sheets with moments in their planes coupled antiferromagnetically by other, weak interactions, resembles that found earlier for the 1:1 mineral greenalite.  相似文献   

9.
By carrying out the hydraulic experiments in a one-dimensional open channel and two-dimensional basin, we clarified the process of how a landslide on a uniform slope causes the generation of a tsunami. The effect of the interactive force that occurs between the debris flow layer and the tsunami is significant in the generation of a tsunami. The continuous flow of the debris into the water makes the wave period of the tsunami short. The present experiments apply numerical simulation using the two-layer model with shear stress models on the bottom and interface, and the results are compared. The simulated debris flow shows good agreement with the measured results and ensures the rushing process into the water. We propose that the model use a Manning coefficient of 0.01 for the smooth slope and 0.015 for the rough slope, and a horizontal viscosity of 0.01 m2/s for the landslide; an interactive force of 0.2 for each layer is recommended. The dispersion effect should be included in the numerical model for the propagation from the shore.  相似文献   

10.
Magnetization, susceptibility and Mössbauer spectra are reported for representative chlorite samples with differing iron content. The anisotropy of the susceptibility and magnetization of a clinochlore crystal is explained using the trigonal effective crystal-field model developed earlier for 1:1 and 2:1 layer silicates, with a splitting of theT 2g triplet of 1,120K. Predominant exchange interactions in the iron-rich samples are ferromagnetic withJ=1.2 K, as for other trioctahedral ferrous minerals. A peak in the susceptibility of thuringite occurs atT m=5.5 K, and magnetic hyperfine splitting appears at lower temperatures in the Mössbauer spectrum. However neutron diffraction reveals no long-range magnetic order in thuringite (or biotite, which behaves similarly). The only magnetic contribution to the diffraction pattern at 1.6 K is increased small angle scattering (q<0.4 Å?1). A factor favouring this random ferromagnetic ground state over the planar antiferromagnetic state of greenalite and minnesotaite is the presence of pairs of ferric ions on adjacent sites, in conjunction with magnetic vacancies in the octahedral sheets. Monte Carlo simulations of the magnetic ground state of the sheets illustrate how long range ferromagnetic order may be destroyed by vortices forming around the Fe3+-Fe3+ pairs.  相似文献   

11.
A new model for the evolution of substellar objects is proposed, which enables computation of their internal structure and evolution in an adiabatic approximation without allowance for axial rotation, magnetic fields, and nuclear reactions. The basic set of differential equations in the model contains a new equation for the gradient of the electron degeneracy parameter and adiabatic coefficients of the new type. The results of numerical simulations of evolutionary sequences of hydrogen (X = 1), helium (Y = 1), and hydrogen-helium (X = 0.75, Y = 0.25) substellar objects with masses M < 0.08 M ⊙ during the evolutionary stage 106 yr < t < 1010 yr are reported. Luminosity-effective temperature, mass-radius, radius-time, central temperature-central density, and central temperature-age relations are analyzed for the indicated chemical composition.  相似文献   

12.
To quantify the effect of salinity stratification on phytoplankton density (denoted as P) patterns, experiments were conducted with an idealised model that couples physical and biological processes. Results show that the idealised model is capable of capturing the main features of observed P patterns in the Columbia River estuary during the spring season: during weak stratification, P is almost vertically uniform with values decreasing towards the estuary mouth, whereas during strong stratification, high values of P extend further seawards but are confined to the upper layer. Sensitivity studies reveal that the strong vertical gradients of P can only occur if the intensity of turbulence (measured by depth-averaged values of vertical eddy viscosity and eddy diffusivity) is weak. The advection of P by subtidal currents is important in obtaining a smaller along-estuary gradient of P during weak stratification and in obtaining a smaller horizontal gradient and a larger vertical gradient of P during strong stratification. Accounting for stratification controlled vertical distribution of vertical eddy viscosity and eddy diffusivity is necessary for obtaining realistic P patterns if stratification is strong, but not if stratification is weak. A higher osmotic stress, which leads to faster loss of phytoplankton in salt water, results in a larger along-estuary gradient of P if stratification is weak and in a larger vertical gradient of P if stratification is strong.  相似文献   

13.
Sheath folds are highly non-cylindrical folds occurring in a variety of geological settings, and have been studied using different approaches. With the present work, we provide a quantitative analysis of the generation and development of sheath folds in a viscously layered system in simple shear conditions. The sheath folds develop from an initial non-cylindrical deflection located on the highly viscous layer. The analogue experiments investigated the influence of (1) variations in the viscosity ratio between the high viscosity layer and the matrix (ηhvl/ηm), (2) variations in the ratio between the amplitude of the initial deflection and the thickness of the high viscosity layer (Af/Thvl), and (3) progressive simple shear (γ). The results show that increases in ηhvl/ηm will produce progressively less elongated sheath folds, while increases in Af/Thvl will result in more elongated sheath folds. We present regime diagrams with ηhvl/ηm and Af/Thvl for different shear strains illustrating under which conditions sheath folds form. In case the original deflection amplitude and layer thickness as well as γ can be retrieved for sheath folds in nature, then their geometry can be used to quantify the effective ηhvl/ηm.  相似文献   

14.
《地学前缘(英文版)》2019,10(6):2135-2145
In fractured reservoirs characterized by low matrix permeability,fracture networks control the main fluid flow paths.However,in layered reservoirs,the vertical extension of fractures is often restricted to single layers.In this study,we explored the effect of changing marl/shale thickness on fracture extension using comprehensive field data and numerical modeling.The field data were sampled from coastal exposures of Liassic limestone-marl/shale alternations in Wales and Somerset(Bristol Channel Basin,UK).The vertical fracture traces of more than 4000 fractures were mapped in detail.Six sections were selected to represent a variety of layer thicknesses.Besides the field data also thin sections were analyzed.Numerical models of fracture extension in a two-layer limestone-marl system were based on field data and laboratory measurements of Young's moduli.The modeled principal stress magnitude σ_3 along the lithological contact was used as an indication for fracture extension through marls.Field data exhibit good correlation(R~2=0.76) between fracture extension and marl thickness,the thicker the marl layer the fewer fractures propagate through.The model results show that almost no tensile stress reaches the top of the marl layer when the marls are thicker than 30 cm.For marls that are less than 20 cm,the propagation of stress is more dependent on the stiffness of the marls.The higher the contrast between limestone and marl stiffness the lower the stress that is transmitted into the marl layer.In both model experiments and field data the critical marl thickness for fracture extension is ca.15-20 cm.This quantification of critical marl thicknesses can be used to improve predictions of fracture networks and permeability in layered rocks.Up-or downsampling methods often ignore spatially continuous impermeable layers with thicknesses that are under the detection limit of seismic data.However,ignoring these layers can lead to overestimates of the overall permeability.Therefore,the understanding of how fractures propagate and terminate through impermeable layers will help to improve the characterization of conventional reservoirs.  相似文献   

15.
Special features of linear marine magnetic anomalies and the magnetic measurements made on samples of the ocean crust require that the thickness of the marine magnetic source layer be thicker than the 0.5 km thick pillow lava layer (seismic layer 2A) alone. It is proposed here that the magnetic properties of the samples studied to date indicate a two-layered source to be the most likely: an upper 0.5 km thick pillow lava layer with a natural remanent magnetization of 5 A/m and a lower 3.5 km thick dike and upper gabbro layer with a magnetization of 0.5 A/m.  相似文献   

16.
The idea that a direct hydraulic connection between a man-made reservoir and the foci of post-impoundment earthquakes may not exist at all sites is eminently credible on geological grounds. Our aim is to provide a simple earth model and related theory for use during investigations of earthquakes near new man-made reservoirs. We consider a uniform circular reservoir which rests on the top surface of a no-hydraulic-connection earth model (NHCEM). The model comprises a top elastic (E) layer, an intermediate poroelastic (P) layer, and a bottom elastic half space. The focus of a potential earthquake in the P layer is located directly under the reservoir. The E layer disrupts the hydraulic connection between the reservoir and the focus. Depth of water in the reservoir varies as H ′ + hcos(ω t). Expressions for reservoir-induced stresses and pore pressure in different layers of the NHCEM are obtained by solving the boundary-value problem invoking full coupling between mean normal stress and pore pressure in the P layer. As an application of the derived mathematical results, we have examined and found that earthquakes on 60° normal faults may occur in the P-layer of a selected NHCEM at epochs of low reservoir level if the reservoir lies mostly in the footwall of the fault. The exercise was motivated by observations of such earthquakes under the man-made Lake Mead after it was impounded.  相似文献   

17.
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18.
In this paper, we present and discuss the results of modeling of the transient induction processes observed at different distances from the center of a transmitter loop in studies of a two-layer earth in which the magnetic susceptibility of the top layer or the base depends on frequency. Profiling graphs (the offset is plotted on the abscissa, and the EMF induced in the receiver coil at a fixed time is plotted on the ordinate) show that the polarity of the EMF changes as the offset increases. For the model with a magnetic layer of any thickness, the polarity of the EMF reverses immediately after the receiver crosses the loop wire. For the model with a magnetic base, the offset at which the EMF polarity reversal occurs is the larger the greater the thickness of the layer. For both models, the EMF at a fixed time depends on the thickness of the layer and the offset. Particularly strong dependence of the EMF on layer thickness or offset is observed near the loop side. Therefore, measurements near the wire make it possible to determine the thickness of the layer in the case where it is much less than the length of the loop side. The overall effect of magnetic relaxation and transient eddy currents leads to the fact that small changes in the layer thickness or offset can lead to a dramatic change in the transient response.  相似文献   

19.
Studies of photosynthesis and respiration in the sheet-like green macroalgaUlva lactuca L. have typically made use of cut discs or individual thalli. Because this species accumulates in layered mats in the field, its structure has the potential to significantly alter metabolic rates compared to those measured for a single layer. The effect of biomass layering on photosynthesis and respiration inU. lactuca was assessed by incubating stacks of 1, 2, 4, and 8 thalli across a range of surface irradiance and in the dark. The photosynthesis-irradiance curve for a single thallus was hyperbolic in shape with maximum photosynthetic rates of 16–24.5 mg O2 g?1 dry weight h?1, a photosynthetic efficiency of 0.18 mg O2 g?1 dry weight h?1 (μE m?2 s?1)?1, and dark respiration rates of 0.95–1.2 mg O2 g?1 dry weight h?1. Weight-specific photosynthesis and respiration rates decreased as the amount of layering increased, with rates approximately twice as high in a single layer as for 8 layers. A simple model revealed that the reduced photosynthetic rates were due to attenuation of light through the stack. Each layer ofUlva reduced the light available to the next layer by approximately 55%, and variability in this absorption was directly related to thallus chlorophyll content. Model runs suggested that acclimation of the thalli by changes in chlorophyll content has the potential to reduce some, but not all of the effect of layering. Reduced respiration rates were atributed to the depletion of oxygen from the interstices between the layers. Results were incorporated into a model of anUlva mat, which predicted decreasing rates of weight-specific total mat production with increasing mat thickness as additional layers are added to the bottom of the mat that neither produce nor consume significant amounts of oxygen. The model predicted area-weighted mat production to increase up to a thickness of 10 layers, after which the rate was constant with increasing mat thickness for the same reason. SinceUlva has a tendency to accumulate in layers under natural conditions, these results improve upon metabolic measurements made with single thalli and will be useful for calculations of macroalgal production as well as in ecosystem models in whichUlva is an important primary producer.  相似文献   

20.
In this paper, a probabilistic study of a one-dimensional soil consolidation problem has been carried out. The Collocation-based Stochastic Response Surface Method (CSRSM) was employed for the probabilistic analysis. The Young modulus E, the Poisson ratio ν, the hydraulic conductivity k h and the uniform surcharge loading q applied at the ground surface were considered as random variables. The probabilistic system responses considered in the analysis were the surface settlement and the consolidation time. Numerical simulations that make use of Biot theory were used for the computation of these system responses. A global sensitivity analysis based on Sobol indices was performed to identify the random variables that have the most contribution in the variability of the system responses. Also, a parametric study was undertaken to investigate the effect of the input geotechnical parameters and the statistical parameters of the random variables on the probability distribution functions of the system responses.  相似文献   

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