13,985 research outputs found
2D approach for modelling self-potential anomalies. Application to synthetic and real data
The aim of this work is to present a 2-D Matlab code based on the finite element method
for providing numerical modelling of both groundwater flow and self-potential signals.
The distribution of the self-potential is obtained by starting with the solution of the
groundwater flow, then computing the source current density, and finally calculating
the electrical potential. The reliability of the algorithm is tested with synthetic case
studies in order to simulate both the electric field resulting from the existence of a leak
in the dam and SP signals associated with a pumping test in an unconfined aquifer. In
addition, the algorithm was applied to field data for the localization of piping sinkholes.
The results show that the outputs of the algorithm yielded satisfactory solutions, which
are in good agreement with those of previous studies and field investigations. In details,
the synthetic data and SP anomalies calculated by using the code are very close in
terms of sign and magnitude, while real data tests clearly indicated that the computed
SP signals were found to be consistent with the measured values
Estimation of the hydraulic parameters of unsaturated samples by electrical resistivity tomography
In situ and laboratory experiments have shown that electrical resistivity tomography (ERT) is an effective tool to image transient phenomena in soils. However, its application in quantifying soil hydraulic parameters has been limited. In this study, experiments of water inflow in unsaturated soil samples were conducted in an oedometer equipped to perform three-dimensional electrical measurements. Reconstructions of the electrical conductivity at different times confirmed the usefulness of ERT for monitoring the evolution of water content. The tomographic reconstructions were subsequently used in conjunction with a finite-element simulation to infer the water retention curve and the unsaturated hydraulic conductivity. The parameters estimated with ERT agree satisfactorily with those determined using established techniques, hence the proposed approach shows good potential for relatively fast characterisations. Similar experiments could be carried out on site to study the hydraulic behaviour of the entire soil deposi
Imaging of a fluid injection process using geophysical data - A didactic example
In many subsurface industrial applications, fluids are injected into or withdrawn from a geologic formation. It is of practical interest to quantify precisely where, when, and by how much the injected fluid alters the state of the subsurface. Routine geophysical monitoring of such processes attempts to image the way that geophysical properties, such as seismic velocities or electrical conductivity, change through time and space and to then make qualitative inferences as to where the injected fluid has migrated. The more rigorous formulation of the time-lapse geophysical inverse problem forecasts how the subsurface evolves during the course of a fluid-injection application. Using time-lapse geophysical signals as the data to be matched, the model unknowns to be estimated are the multiphysics forward-modeling parameters controlling the fluid-injection process. Properly reproducing the geophysical signature of the flow process, subsequent simulations can predict the fluid migration and alteration in the subsurface. The dynamic nature of fluid-injection processes renders imaging problems more complex than conventional geophysical imaging for static targets. This work intents to clarify the related hydrogeophysical parameter estimation concepts
EIT Reconstruction Algorithms: Pitfalls, Challenges and Recent Developments
We review developments, issues and challenges in Electrical Impedance
Tomography (EIT), for the 4th Workshop on Biomedical Applications of EIT,
Manchester 2003. We focus on the necessity for three dimensional data
collection and reconstruction, efficient solution of the forward problem and
present and future reconstruction algorithms. We also suggest common pitfalls
or ``inverse crimes'' to avoid.Comment: A review paper for the 4th Workshop on Biomedical Applications of
EIT, Manchester, UK, 200
Iterative Updating of Model Error for Bayesian Inversion
In computational inverse problems, it is common that a detailed and accurate
forward model is approximated by a computationally less challenging substitute.
The model reduction may be necessary to meet constraints in computing time when
optimization algorithms are used to find a single estimate, or to speed up
Markov chain Monte Carlo (MCMC) calculations in the Bayesian framework. The use
of an approximate model introduces a discrepancy, or modeling error, that may
have a detrimental effect on the solution of the ill-posed inverse problem, or
it may severely distort the estimate of the posterior distribution. In the
Bayesian paradigm, the modeling error can be considered as a random variable,
and by using an estimate of the probability distribution of the unknown, one
may estimate the probability distribution of the modeling error and incorporate
it into the inversion. We introduce an algorithm which iterates this idea to
update the distribution of the model error, leading to a sequence of posterior
distributions that are demonstrated empirically to capture the underlying truth
with increasing accuracy. Since the algorithm is not based on rejections, it
requires only limited full model evaluations.
We show analytically that, in the linear Gaussian case, the algorithm
converges geometrically fast with respect to the number of iterations. For more
general models, we introduce particle approximations of the iteratively
generated sequence of distributions; we also prove that each element of the
sequence converges in the large particle limit. We show numerically that, as in
the linear case, rapid convergence occurs with respect to the number of
iterations. Additionally, we show through computed examples that point
estimates obtained from this iterative algorithm are superior to those obtained
by neglecting the model error.Comment: 39 pages, 9 figure
Nonlinear Inversion from Partial EIT Data: Computational Experiments
Electrical impedance tomography (EIT) is a non-invasive imaging method in
which an unknown physical body is probed with electric currents applied on the
boundary, and the internal conductivity distribution is recovered from the
measured boundary voltage data. The reconstruction task is a nonlinear and
ill-posed inverse problem, whose solution calls for special regularized
algorithms, such as D-bar methods which are based on complex geometrical optics
solutions (CGOs). In many applications of EIT, such as monitoring the heart and
lungs of unconscious intensive care patients or locating the focus of an
epileptic seizure, data acquisition on the entire boundary of the body is
impractical, restricting the boundary area available for EIT measurements. An
extension of the D-bar method to the case when data is collected only on a
subset of the boundary is studied by computational simulation. The approach is
based on solving a boundary integral equation for the traces of the CGOs using
localized basis functions (Haar wavelets). The numerical evidence suggests that
the D-bar method can be applied to partial-boundary data in dimension two and
that the traces of the partial data CGOs approximate the full data CGO
solutions on the available portion of the boundary, for the necessary small
frequencies.Comment: 24 pages, 12 figure
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