Nonlinear Inversion from Partial EIT Data:Computational Experiments

Background: Recently, particle swarm optimization (PSO) has been increasingly used in geophysics due to its simple operation and fast convergence. Objective: However, PSO lacks population diversity and may fall to local optima. Hence, an improved hybrid particle swarm optimizer with sine-cosine acceleration coefficients (IH-PSO-SCAC) is proposed and successfully applied to test functions and in transient electromagnetic (TEM) nonlinear inversion. Method: A reverse learning strategy is applied to optimize population initialization. The sine-cosine acceleration coefficients are utilized for global convergence. Sine mapping is adopted to enhance population diversity during the search process. In addition, the mutation method is used to reduce the probability of premature convergence. Results: The application of IH-PSO-SCAC in the test functions and several simple layered models are demonstrated with satisfactory results in terms of data fit. Two inversions have been carried out to test our algorithm. The first model contains an underground low-resistivity anomaly body and the second model utilized measured data from a profile of the Xishan landslide in Sichuan Province. In both cases, resistivity profiles are obtained, and the inverse problem is solved for verification. Conclusion: The results show that the IH-PSO-SCAC algorithm is practical, can be effectively applied in TEM inversion and is superior to other representative algorithms in terms of stability and accuracy.

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Crustal Velocity Models Retrieved from Surface Wave Dispersion Data for Gujarat Region, Western Peninsular India

Earth Sciences Research Journal ◽

10.15446/esrj.v23n2.58570 ◽

2019 ◽

Vol 23 (2) ◽

pp. 147-155

Author(s):

Vishwa Joshi

Keyword(s):

Wave Dispersion ◽

Data File ◽

Western India ◽

Nonlinear Inversion ◽

S Wave ◽

Surface Wave Dispersion ◽

Peninsular India ◽

Velocity Models ◽

Crustal Velocity ◽

Dispersion Data

The physiographic features of Gujarat state of western India are unique, as they behaved dynamically with several alterations and modifications throughout the geological timescale. It displays a remarkable example of a terrain bestowed with geological, physiographical and climatic diversities. The massive 2001 Bhuj earthquake (M 7.7) over the Kachchh region caused severe damage and devastation to the state of Gujarat and attracted the scientific community of the world to comprehend on its structure and tectonics for future hazard reduction. In the present study, three clusters of wave paths A, B1, and B2 have considered. In each cluster, dispersion data were measured station by station which collectively formed a dispersion data file for a nonlinear inversion through Genetic algorithm. In this way, three crustal velocity models were generated for entire Gujarat. These models are 1) Across Cambay Basin (Path A), 2) Along Saurashtra - Kathiawar Horst (Path B1) and 3) Along Narmada Basin (Path B2), which were formed at different times during the Mesozoic. The average thickness of the crust estimated in the present study for paths A, B1 and B2 are 38.2 km, 36.2 km, and 41.6 km respectively and the estimated S-wave velocity in the lower crust is ~ 3.9 km/s for all the paths. The present study will improve our knowledge about the structure of the seismogenic layer of this active intraplate region

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Sensitivities of surface wave velocities to the medium parameters in a radially anisotropic spherical Earth and inversion strategies

Annals of Geophysics ◽

10.4401/ag-6806 ◽

2015 ◽

Vol 58 (5) ◽

Author(s):

Sankar N. Bhattacharya

Keyword(s):

Surface Wave ◽

Phase Velocity ◽

Group Velocity ◽

Wave Velocity ◽

Love Waves ◽

P Wave ◽

Model Parameters ◽

Nonlinear Inversion ◽

Wave Velocities ◽

Spherical Earth

Sensitivity kernels or partial derivatives of phase velocity (c) and group velocity (U) with respect to medium parameters are useful to interpret a given set of observed surface wave velocity data. In addition to phase velocities, group velocities are also being observed to find the radial anisotropy of the crust and mantle. However, sensitivities of group velocity for a radially anisotropic Earth have rarely been studied. Here we show sensitivities of group velocity along with those of phase velocity to the medium parameters VSV, VSH , VPV, VPH , h and density in a radially anisotropic spherical Earth. The peak sensitivities for U are generally twice of those for c; thus U is more efficient than c to explore anisotropic nature of the medium. Love waves mainly depends on VSH while Rayleigh waves is nearly independent of VSH . The sensitivities show that there are trade-offs among these parameters during inversion and there is a need to reduce the number of parameters to be evaluated independently. It is suggested to use a nonlinear inversion jointly for Rayleigh and Love waves; in such a nonlinear inversion best solutions are obtained among the model parameters within prescribed limits for each parameter. We first choose VSH, VSV and VPH within their corresponding limits; VPV and h can be evaluated from empirical relations among the parameters. The density has small effect on surface wave velocities and it can be considered from other studies or from empirical relation of density to average P-wave velocity.

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Analysis of Stress Drop Variations in Fault and Subduction Zones of Maluku and Halmahera Earthquakes in 2019

Jurnal Penelitian Fisika dan Aplikasinya (JPFA) ◽

10.26740/jpfa.v9n2.p152-162 ◽

2019 ◽

Vol 9 (2) ◽

pp. 152

Author(s):

Rahmat Setyo Yuliatmoko ◽

Telly Kurniawan

Keyword(s):

Subduction Zone ◽

Stress Drop ◽

Subduction Zones ◽

Slip Rate ◽

Disaster Mitigation ◽

Mitigation Measures ◽

Reverse Fault ◽

Nonlinear Inversion ◽

Rock Structure ◽

Geological Conditions

The amount of stress released by an earthquake can be calculated with a stress drop, the stress ratio before and after an earthquake where the stress accumulated in a fault or a subduction zone is immediately released during an earthquake. The purpose of this research is to calculate the amount of stress drop in faults and subduction in Maluku and Halmahera and their variations and relate them to the geological conditions in the area so that the tectonic characteristics in the area can be identified. This research employed mathematical analysis and the Nelder Mead Simplex nonlinear inversion methods. The results show that Maluku and Halmahera are the area with complex tectonic conditions and large earthquake impacts. The Maluku sea earthquake generated a stress drop of 0.81 MPa with a reverse fault mechanism in the zone of subduction, while for the Halmahera earthquake the stress drop value was 52.72 MPa, a typical strike-slip mechanism in the fault zone. It can be concluded that there is a difference in the stress drop between the subduction and fault zones; the stress drop in the fault was greater than that in the subduction zone due to different rock structure and faulting mechanisms as well as differences in the move slip rate that plays a role in the process of holding out the stress on a rock. This information is very important to know the amount of pressure released from the earthquake which has a very large impact as part of disaster mitigation measures.

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