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Wavepath traveltime tomography 总被引：1，自引：1，他引：1
The elastic-wave equation is used to construct sensitivity kernels relating perturbations in elastic parameters to traveltime deviations. Computation of the functions requires a correlation of the forward-propagating seismic wavefield with a backward propagation of the residual wavefield. The computation of the wavefields is accomplished using a finite difference algorithm and is efficiently executed on a CM-2 parallel processor. The source and receiver locations have maximum sensitivity to velocity structure. The sensitivity kernels or wavepaths are well suited for transmission traveltime inversion such as cross-borehole tomography and vertical seismic profiling. Conventional ray tomography and wavepath tomography are applied to a set of P -wave arrival times, from a cross-borehole experiment at Kesterson, California. Because the wavepaths have increased sensitivity near the source and receiver there are differences in resolution of the velocity structure. Both techniques recover the same relative variations in velocity where the coverage is adequate. The wavepath solution is more laterally continuous and the dominant variation is vertical, as is expected for the layered sediments in this region. 相似文献
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Summary. The inverse problem of using static displacements observed at the surface to infer volume changes within the Earth is considered. This problem can be put in a form such that the method of ideal bodies and the method of positivity constraints may both be applied. Thus all of the techniques previously developed for the gravity inverse problem can be extended to the static displacement problem. Given bounds on the depth, the greatest lower bound on the fractional volume change can be estimated, or, given bounds on the fractional volume change, the least upper bound on the depth can be estimated. Methods of placing bounds on generalized moments of the perturbing body are also developed, and techniques of handling errors in the data are discussed.
Examples are given for both two- and three-dimensional problems. The ideal body method is suited for both 2- and 3-D problems when only two data points are considered, but is unwieldy for more data points. The method of positivity constraints is more versatile and can be used when there are many data points in the case of 2-D problems, but it may lead to an excessive amount of computation in 3-D problems. 相似文献
Examples are given for both two- and three-dimensional problems. The ideal body method is suited for both 2- and 3-D problems when only two data points are considered, but is unwieldy for more data points. The method of positivity constraints is more versatile and can be used when there are many data points in the case of 2-D problems, but it may lead to an excessive amount of computation in 3-D problems. 相似文献
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Singularity and Branching: A Path-Following Formalism For Geophysical Inverse Problems 总被引：5，自引：0，他引：5
D. W. Vasco 《Geophysical Journal International》1994,119(3):809-830
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D. W. Vasco 《Geophysical Journal International》2000,142(3):970-990
Many geophysical inverse problems derive from governing partial differential equations with unknown coefficients. Alternatively, inverse problems often arise from integral equations associated with a Green's function solution to a governing differential equation. In their discrete form such equations reduce to systems of polynomial equations, known as algebraic equations. Using techniques from computational algebra one can address questions of the existence of solutions to such equations as well as the uniqueness of the solutions. The techniques are enumerative and exhaustive, requiring a finite number of computer operations. For example, calculating a bound to the total number of solutions reduces to computing the dimension of a linear vector space. The solution set itself may be constructed through the solution of an eigenvalue problem. The techniques are applied to a set of synthetic magnetotelluric values generated by conductivity variations within a layer. We find that the estimation of the conductivity and the electric field in the subsurface, based upon single-frequency magnetotelluric field values, is equivalent to a linear inverse problem. The techniques are also illustrated by an application to a magnetotelluric data set gathered at Battle Mountain, Nevada. Surface observations of the electric ( E y ) and magnetic ( H x ) fields are used to construct a model of subsurface electrical structure. Using techniques for algebraic equations it is shown that solutions exist, and that the set of solutions is finite. The total number of solutions is bounded above at 134 217 728. A numerical solution of the algebraic equations generates a conductivity structure in accordance with the current geological model for the area. 相似文献
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Changes in geophysical properties caused by fluid injection into porous rocks: analytical models 
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下载免费PDF全文 Steven R. Pride James G. Berryman Michael Commer Seiji Nakagawa Gregory A. Newman Donald W. Vasco 《Geophysical Prospecting》2017,65(3):766-790
Analytical models are provided that describe how the elastic compliance, electrical conductivity, and fluid‐flow permeability of rocks depend on stress and fluid pressure. In order to explain published laboratory data on how seismic velocities and electrical conductivity vary in sandstones and granites, the models require a population of cracks to be present in a possibly porous host phase. The central objective is to obtain a consistent mean‐field analytical model that shows how each modeled rock property depends on the nature of the crack population. The crack populations are described by a crack density, a probability distribution for the crack apertures and radii, and the averaged orientation of the cracks. The possibly anisotropic nature of the elasticity, conductivity, and permeability tensors is allowed for; however, only the isotropic limit is used when comparing to laboratory data. For the transport properties of conductivity and permeability, the percolation effect of the crack population linking up to form a connected path across a sample is modeled. However, this effect is important only in crystalline rock where the host phase has very small conductivity and permeability. In general, the importance of the crack population to the transport properties increases as the host phase becomes less conductive and less permeable. 相似文献
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This paper analyzes the socioeconomic determinants of consumption of wild fish among the Kichwa and Shuar indigenous peoples in the Ecuadorian Amazon. The results of a random-effect linear model show that the consumption of wild fish is higher for households with younger heads that do not have off-farm work and reside far from urban centers, in communities with low population densities. Although various actors promoting aquaculture in the region often claim that it helps to relieve the pressure on wild fish stocks, no statistically significant effect of the consumption of cultivated fish on the consumption of wild fish could be shown. Thus, our analysis suggests that public policies and development interventions which increase access to off-farm employment can both improve local livelihoods and conserve biodiversity, but that the same affirmation cannot be made for the promotion of aquaculture. 相似文献
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D. W. Vasco 《Geophysical Journal International》1997,131(1):9-23
Mathematical methods from the theory of continuous groups are used to determine whether a non-linear inverse problem, in the form of a functional, can be transformed into a linear inverse problem. If such transformations exist they can be constructed from the solutions of a linear system of differential equations. An illustration of the methodology is given by the linearization of the functional relating basement topography to observed surface gravity. The linearized inversion of gravity data for basement topography is applied to observations from Yucca Mountain, Nevada. A 2.0 km step in the basement to the west of Yucca Mountain, corresponding to the Bare Mountain fault, matches the Bouguer gravity anomaly. The resolution and uncertainty associated with the estimates of basement topography indicate that the structure directly beneath the gravity line is well constrained. 相似文献