4,428 research outputs found
Construction of Artificial Point Sources for a Linear Wave Equation in Unknown Medium
We study the wave equation on a bounded domain of Rm and on a compact Riemannian manifold M with boundary. We assume that the coefficients of the wave equation are unknown but that we are given the hyperbolic Neumann-to-Dirichlet map. that corresponds to the physical measurements on the boundary. Using the knowledge of. we construct a sequence of Neumann boundary values so that at a time T the corresponding waves converge to zero while the time derivative of the waves converge to a delta distribution. The limit of such waves can be considered as a wave produced by an artificial point source. The convergence of the wave takes place in the function spaces naturally related to the energy of the wave. We apply the results for inverse problems and demonstrate the focusing of the waves numerically in the one-dimensional case.Peer reviewe
Micro-seismic Elastic Reflection Full Waveform Inversion with An Equivalent Source
In micro-seismic event measurements, pinpointing the passive source's exact
spatial and temporal location is paramount. This research advocates for the
combined use of both P- and S-wave data, captured by geophone monitoring
systems, to improve source inversion accuracy. Drawing inspiration from the
secondary source concept in Elastic Reflection Full Waveform Inversion (ERFWI),
we introduce an equivalent source term. This term combines source functions and
source images. Our optimization strategy iteratively refines the spatial
locations of the source, its temporal functions, and associated velocities
using a full waveform inversion framework. Under the premise of an isotropic
medium with consistent density, the source is defined by two spatial and three
temporal components. This offers a nuanced source representation in contrast to
the conventional seismic moment tensor. To address gradient computation, we
employ the adjoint-state method. However, we encountered pronounced
non-linearity in waveform inversion of micro-seismic events, primarily due to
the unknown source origin time, resulting in cycle skipping challenges. To
counteract this, we devised an objective function that is decoupled from the
source origin time. This function is formulated by convolving reference traces
with both observed and predicted data. Through the concurrent inversion of the
source image, source time function, and velocity model, our method offers
precise estimations of these parameters, as validated by a synthetic 2D example
based on a modified Marmousi model. This nested inversion approach promises
enhanced accuracy in determining the source image, time function, and velocity
model
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