Coda wave interferometry and relative source location

A wide range of applications requires relative locations of sources of energy to be known accurately. Most conventional location methods are either subject to errors that depend strongly on inaccuracy in the model of propagation velocity used, or demand a well-distributed network of surrounding seismic stations in order to produce reliable results. A source location method based on coda wave interferometry (CWI) is relatively insensitive to the number of seismic stations and to the source-to-station azimuthal coverage. It therefore opens new avenues for research, for applications in areas with unfavourable recording geometries, and for applications which require a complementary method. CWI uses scattered waves in the coda of seismograms to estimate the small differences between two seismic states, and currently has three types of applications: estimating bulk velocity change of the medium, scatterer displacement, and source location perturbation. When used for source location, CWI is used to estimate the distances between pairs of sources with similar mechanism (equivalent to estimating location perturbation of the same source), which are then used jointly to determine the relative location of a cluster of sources using a probabilistic framework as an optimization problem. However, estimating source separation is a relatively new type of application of CWI. In the first part of this thesis, the performance of CWI is tested in models with varying complexities and types: from point-scatterer media as assumed in the CWI theory, to layered media as in classic Earth models, to media with combinations of point-scatters and layers, and finally to the more realistic Marmousi model. This thesis also presents the first elastic case of testing CWI to estimate source separation in synthetic experiments. The study contributes to better understand and interpret the source separation estimates and therefore relative locations using CWI. The second part of this thesis validates the location algorithm with synthetic data. When applied to real seismic data, the algorithm is found to suffer from the impact of large difference in the dominant wavelength of recordings made on different instruments. This thesis introduces a new formulation for the optimization problem to account for data from multiple station channels. In addition, it proposes a way to standardize the selection of parameters when implementing the method. The algorithm is applied to a micro-seismic dataset of mining induced events recorded in Nottinghamshire, England. The earthquake location results are highly consistent when using different individual seismometer channels, showing that it is possible to locate event clusters with a single-channel seismometer. These microseismic events have shorter distinguishable codas in recorded waveforms, and hence fewer recorded scattered waves than those that have been used to test this method previously. Thus, the potential applications of this cost-effective method are extended to seismic events over a wider range of magnitudes. Given the advantages of this location method, it has been applied only once in literature other than in this thesis. It is likely that one reason that it is not used more widely is the lack of reliable code that implements this multistage method. This thesis develops a well-commented MATLAB code called CWIcluster that does so, accompanied by a clear and thorough user manual. It implements the location method in three stages: classifying events into clusters, computing inter-source separations using CWI, and estimating their relative locations. Each stage can be implemented in an automated sense given criteria chosen by the user. It is shown that the location algorithm is able to correct bias (underestimation) in the CWI separation estimates to some extent. The third part of this thesis returns to the three basic types of applications of CWI. Standard CWI methods require an assumption that a single type of perturbation has taken place in the system (as do most other methods that measure changes in a seismic system). However, in reality more than one type of perturbation can occur simultaneously. This thesis proposes a general treatment to account for multiple types of perturbations, allowing each type to be recognized and estimated with the effects of others being compensated. The appendices include a co-authored submitted paper that examines the influence of velocity change and source location perturbation on one another in the context of a rock-physics laboratory. Overall this thesis intensively tests the relatively new method of coda wave interferometry to estimate inter-source separations in various environments, and explores its potential to detect multiple types of perturbations that have occurred simultaneously, thus extending our understanding of the set of CWI methods in general. In addition, it validates the relative location method based on CWI and provides ways to improve the original method, as well as a way to assess the quality of results when applied to real data. Finally, it presents a new freely-available code package to implement the location method, which the authors hope will introduce this method more widely in both academia and industry

Locating micro-seismic sources with a single seismometer channel using coda wave interferometry

A novel source location method based on coda wave interferometry (CWI) was applied to a microseismic data set of mining-induced events recorded in Nottinghamshire, England. CWI uses scattered waves in the coda of seismograms to estimate the differences between two seismic states. We used CWI to estimate the distances between pairs of earthquake locations, which are then used jointly to determine the relative location of a cluster of events using a probabilistic framework. We evaluated two improvements to this location technique: These account for the impact of a large difference in the dominant wavelength of a recording made on different instruments, and they standardize the selection of parameters to be used when implementing the method. Although the method has been shown to produce reasonable estimates on larger earthquakes, we tested the method for microseismic events with shorter distinguishable codas in recorded waveforms, and hence, fewer recorded scattered waves. The earthquake location results are highly consistent when using different individual seismometer channels, showing that it is possible to locate event clusters with a single-channel seismometer. We thus extend the potential applications of this cost-effective method to seismic events over a wider range of magnitudes

The Terracorrelator: a shared memory HPC facility for real-time seismological cross-correlation analyses

Earthquakes and volcanic eruptions may in some instances be preceded or accompanied by changes in the geophysical properties of the Earth, such as seismic velocities or event rates. The development of reliable probabilistic forecasting methods for these hazards requires real-time analysis of seismic data and truly prospective forecasting and testing to reduce bias. However, potential forecasting techniques, including seismic interferometry and earthquake "repeater" analysis, require a large number of waveform cross-correlations; this is computationally intensive, and is particularly challenging in real-time. Here we describe the "Terracorrelator", a new high performance computing facility at the University of Edinburgh designed for real-time cross-correlational analyses. The machine consists of two 2TB shared memory nodes for cross-correlation and post-processing, and two Intel Xeon Phi nodes for pre-processing. The Terracorrelator has been tested on a seismic interferometry case study using ObsPy for seismic operations and processing, and Dispel4Py for writing and executing the workflow. The workflow is distributed automatically for parallel processing in a shared memory multicore environment. Preliminary results have demonstrated that data from 1000 seismic stations can be pre-processed, and each station cross-correlated with all others (499500 cross-correlations) in hourly or daily intervals sufficiently quickly to keep ahead of new data arriving, on one of the shared memory nodes. The second node is therefore free to perform interpretative analysis on the outputs, for example to look at changes in the resulting correlations. These promising results suggest that it will be possible to undertake real-time interferometric analysis using Sure~1000 stations, and to test the predictive power of current seismic velocity changes for future hazard occurrence

The Shear Strength of Granite Weathered Soil Under Different Hydraulic Paths

At present, there is no clear understanding of the influence of differences in soil mineral composition, particle size grading, and hydraulic paths on the shear strength of unsaturated soil, and the related strength models are not applicable. The shear strength characteristics of different saturation specimens under different hydraulic paths were studied on two granite weathered soils. The experimental results show that the shear strength index of the prepared specimen is “arched” with the increase of saturation, and the dehydration specimen decreases linearly with the saturation. As considering the cementation of free oxides in soils and the interaction among soil particles at different saturations, it is assumed that there are three different contact modes among soil particles: direct contact, meniscus contact, and cement contact. The difference in contact modes will reflect the different laws of shear strength. A shear strength model capable of distinguishing between the capillary effect and the adsorptive effect was established. The model predicted and verified the shear strength data of granite weathered soil under different hydraulic paths well, and then theoretically explained the evolution law of the shear strength of granite weathering soil under the change of saturation

Formation control of Multi-agents Based on Method of Matrices

Formation control is one of the most actively studied topics in multi-agents systems, in which all agents will form a desired formation to complete certain tasks. In this paper, a method of matrices is proposed to describe information about formation. Additionally, a flocking algorithm with virtual agents is proposed to control all agent to complete task of desired formation. Simulation results show that all agents can form a desired formation and their velocities consensus are verified

Table_1_The effect of antioxidant dietary supplements and diet-derived circulating antioxidants on vitiligo outcome: evidence from genetic association and comprehensive Mendelian randomization.xlsx

BackgroundPrevious studies have indicated that antioxidant diets may have a positive impact on vitiligo by interfering with oxidative stress mechanisms. However, there has been a lack of research utilizing the Mendelian randomization (MR) method to analyze the relationship between antioxidant diet intake and vitiligo.MethodsIn this study, we employed both univariate Mendelian randomization (UVMR) and multivariate Mendelian randomization (MVMR) approaches. The specific antioxidant dietary supplements (such as coffee intake, green tea intake, herbal tea intake, standard tea intake, and average weekly red wine intake) as well as diet-derived circulating antioxidants, including Vit. C (ascorbate), Vit. E (α-tocopherol), Vit. E (γ-tocopherol), Carotene, Vit. A (retinol), Zinc, and Selenium (N = 2,603–428,860) were significantly associated with independent single-nucleotide polymorphisms (SNPs). We obtained pooled statistics on vitiligo from a meta-analysis of three genome-wide association studies (GWASs) of European ancestry, including 4,680 cases and 39,586 controls. Inverse variance weighted (IVW) was employed as the primary analytical method, and sensitivity analysis was conducted to assess the robustness of the main findings.ResultsGenetically, coffee intake [odds ratio (OR) = 0.17, 95% confidence interval (CI) 0.07–0.37, p = 1.57 × 10–5], average weekly red wine intake (OR = 0.28, 95% CI 0.08–1.00, p = 0.049), and standard tea intake (OR = 0.99, 95% CI 0.98–0.99, p = 5.66 × 10–7) were identified as protective factors against vitiligo. However, no causal effect between the intake of other antioxidant diets and vitiligo was found. Moreover, no instances of pleiotropy or heterogeneity were observed in this study.ConclusionOur study indicates that coffee, standard tea, and red wine consumption can potentially reduce the risk of vitiligo. However, there is insufficient evidence to support that other antioxidant diets have a significant effect on vitiligo.</p

Data_Sheet_2_The effect of antioxidant dietary supplements and diet-derived circulating antioxidants on vitiligo outcome: evidence from genetic association and comprehensive Mendelian randomization.docx

BackgroundPrevious studies have indicated that antioxidant diets may have a positive impact on vitiligo by interfering with oxidative stress mechanisms. However, there has been a lack of research utilizing the Mendelian randomization (MR) method to analyze the relationship between antioxidant diet intake and vitiligo.MethodsIn this study, we employed both univariate Mendelian randomization (UVMR) and multivariate Mendelian randomization (MVMR) approaches. The specific antioxidant dietary supplements (such as coffee intake, green tea intake, herbal tea intake, standard tea intake, and average weekly red wine intake) as well as diet-derived circulating antioxidants, including Vit. C (ascorbate), Vit. E (α-tocopherol), Vit. E (γ-tocopherol), Carotene, Vit. A (retinol), Zinc, and Selenium (N = 2,603–428,860) were significantly associated with independent single-nucleotide polymorphisms (SNPs). We obtained pooled statistics on vitiligo from a meta-analysis of three genome-wide association studies (GWASs) of European ancestry, including 4,680 cases and 39,586 controls. Inverse variance weighted (IVW) was employed as the primary analytical method, and sensitivity analysis was conducted to assess the robustness of the main findings.ResultsGenetically, coffee intake [odds ratio (OR) = 0.17, 95% confidence interval (CI) 0.07–0.37, p = 1.57 × 10–5], average weekly red wine intake (OR = 0.28, 95% CI 0.08–1.00, p = 0.049), and standard tea intake (OR = 0.99, 95% CI 0.98–0.99, p = 5.66 × 10–7) were identified as protective factors against vitiligo. However, no causal effect between the intake of other antioxidant diets and vitiligo was found. Moreover, no instances of pleiotropy or heterogeneity were observed in this study.ConclusionOur study indicates that coffee, standard tea, and red wine consumption can potentially reduce the risk of vitiligo. However, there is insufficient evidence to support that other antioxidant diets have a significant effect on vitiligo.</p