TY  - JOUR
A1  - Esfahani, Reza Dokht Dolatabadi
A1  - Vogel, Kristin
A1  - Cotton, Fabrice
A1  - Ohrnberger, Matthias
A1  - Scherbaum, Frank
A1  - Kriegerowski, Marius
T1  - Exploring the dimensionality of ground-motion data by applying autoencoder techniques
JF  - Bulletin of the Seismological Society of America : BSSA
N2  - In this article, we address the question of how observed ground-motion data can most effectively be modeled for engineering seismological purposes. Toward this goal, we use a data-driven method, based on a deep-learning autoencoder with a variable number of nodes in the bottleneck layer, to determine how many parameters are needed to reconstruct synthetic and observed ground-motion data in terms of their median values and scatter. The reconstruction error as a function of the number of nodes in the bottleneck is used as an indicator of the underlying dimensionality of ground-motion data, that is, the minimum number of predictor variables needed in a ground-motion model. Two synthetic and one observed datasets are studied to prove the performance of the proposed method. We find that mapping ground-motion data to a 2D manifold primarily captures magnitude and distance information and is suited for an approximate data reconstruction. The data reconstruction improves with an increasing number of bottleneck nodes of up to three and four, but it saturates if more nodes are added to the bottleneck.
Y1  - 2021
U6  - https://doi.org/10.1785/0120200285
SN  - 0037-1106
SN  - 1943-3573
VL  - 111
IS  - 3
SP  - 1563
EP  - 1576
PB  - Seismological Society of America
CY  - El Cerito, Calif.
ER  - 
TY  - JOUR
A1  - Esfahani, Reza Dokht Dolatabadi
A1  - Gholami, Ali
A1  - Ohrnberger, Matthias
T1  - An inexact augmented Lagrangian method for nonlinear dispersion-curve inversion using Dix-type global linear approximation
JF  - Geophysics : a journal of general and applied geophysics
N2  - Dispersion-curve inversion of Rayleigh waves to infer subsurface shear-wave velocity is a long-standing problem in seismology. Due to nonlinearity and ill-posedness, sophisticated regularization techniques are required to solve the problem for a stable velocity model. We have formulated the problem as a minimization problem with nonlinear operator constraint and then solve it by using an inexact augmented Lagrangian method, taking advantage of the Haney-Tsai Dix-type relation (a global linear approximation of the nonlinear forward operator). This replaces the original regularized nonlinear problem with iterative minimization of a more tractable regularized linear problem followed by a nonlinear update of the phase velocity (data) in which the update can be performed accurately with any forward modeling engine, for example, the finite-element method. The algorithm allows discretizing the medium with thin layers (for the finite-element method) and thus omitting the layer thicknesses from the unknowns and also allows incorporating arbitrary regularizations to shape the desired velocity model. In this research, we use total variation regularization to retrieve the shear-wave velocity model. We use two synthetic and two real data examples to illustrate the performance of the inversion algorithm with total variation regularization. We find that the method is fast and stable, and it converges to the solution of the original nonlinear problem.
KW  - surface wave
KW  - nonlinear
KW  - inversion
KW  - modeling
KW  - finite element
Y1  - 2020
U6  - https://doi.org/10.1190/geo2019-0717.1
SN  - 0016-8033
SN  - 1942-2156
VL  - 85
IS  - 3
SP  - EN77
EP  - EN85
PB  - GeoScienceWorld
CY  - Tulsa, Okla.
ER  -