Parallel Seismic Ray Tracing

Seismic ray tracing is a common method for understanding and modeling seismic wave propagation. The wavefront construction (WFC) method handles wavefronts instead of individual rays, thereby providing a mechanism to control ray density on the wavefront. In this thesis we present the design and implementation of a parallel wavefront construction algorithm (pWFC) for seismic ray tracing. The proposed parallel algo- rithm is developed using the stapl library for parallel C++ code.We present the idea of modeling ray tubes with an additional ray in the center to facilitate parallelism. The parallel wavefront construction algorithm is applied to wide range of models such as simple synthetic models that enable us to study various aspects of the method while others are intended to be representative of basic geological features such as salt domes. We also present a theoretical model to understand the performance of the pWFC algorithm. We evaluate the performance of the proposed parallel wavefront construction algorithm on an IBM Power 5 cluster. We study the effect of using different mesh types, varying the position of source and their number etc. The method is shown to provide good scalable performance for different models. Load balancing is also shown to be the major factor hindering the performance of the algorithm. We provide two load balancing algorithms to solve the load imbalance problem. These algorithms will be developed as an extension of the current work

Parallel Adaptive Mesh Coarsening for Seismic Tomography

International audienceSeismic tomography enables to model the internal structure of the Earth. In order to improve the precision of existing models, a huge amount of acquired seismic data must be analyzed. The analysis of such massive data require a considerable computing power which can only be delivered by parallel computational equipments. Yet, parallel computation is not sufficient for the task: we also need algorithms to automatically concentrate the computations on the most relevant data parts. The objective of the paper is to present such an algorithm. From an initial regular mesh in which cells carry data with varying relevance, we present a method to aggregate elementary cells so as to homogenize the relevance of data. The result is an irregular mesh which has the ad- vantage over the initial mesh of having orders of magnitude less cells while preserving the geophysical meaning of data. We present both a sequential and a parallel algorithm to solve this problem under the hypotheses and constraints inherited from the geophysical context

Parallel Seismic Ray Tracing

Seismic ray tracing is a common method for understanding and modeling seismic wave propagation. The wavefront construction (WFC) method handles wavefronts instead of individual rays, thereby providing a mechanism to control ray density on the wavefront. In this thesis we present the design and implementation of a parallel wavefront construction algorithm (pWFC) for seismic ray tracing. The proposed parallel algo- rithm is developed using the stapl library for parallel C++ code.We present the idea of modeling ray tubes with an additional ray in the center to facilitate parallelism. The parallel wavefront construction algorithm is applied to wide range of models such as simple synthetic models that enable us to study various aspects of the method while others are intended to be representative of basic geological features such as salt domes. We also present a theoretical model to understand the performance of the pWFC algorithm. We evaluate the performance of the proposed parallel wavefront construction algorithm on an IBM Power 5 cluster. We study the effect of using different mesh types, varying the position of source and their number etc. The method is shown to provide good scalable performance for different models. Load balancing is also shown to be the major factor hindering the performance of the algorithm. We provide two load balancing algorithms to solve the load imbalance problem. These algorithms will be developed as an extension of the current work

Dynamic Load Balancing in a Geophysics Application Using STAPL

Seismic wavefront simulation is a common method to understand the composition of earth below the surface, especially for hydrocarbon exploration. One of these simulation methods is the wavefront construction algorithm. In this thesis, we reduced the load imbalance in a parallel implementation of the wavefront construction algorithm. We added a generic redistribution framework for data structures in the C++ parallel library STAPL. We present a redistribution algorithm for the parallel wavefront construction application which uses the recursive coordinate bisection method to find a near-optimal data distribution of the data. This algorithm leveraged the added redistribution features in STAPL to improve the running time of our application. We compared the run time of the application with and without redistribution on different geophysics models. We show that the proposed redistribution provides up to 9.45x speedup on a Cray XE6m cluster and 11.85x speedup on an IBM BlueGene/Q cluster

Static Load Balancing using Non-Uniform Mesh Partitioning based on Ray Density Prediction for the Parallel Wavefront Construction Method

The Wavefront Construction (WFC )method, which was developed based on ray theory, is one of the most efficient tools in seismic modeling. The main idea of this method is to propagate a wavefront represented by rays in a computational mesh that is interpolated whenever an accuracy criterion is violated. Recently, a parallel WFC was developed using the Standard Template Adaptive Parallel Library. However, due to wavefront density adaptivity, the parallel implementation exhibits inefficient performance owing to load imbalances between multiple processors.This paper applies a static load balancing approach based on a method for predicting future loads for a synthetic salt dome model, in order to improve the performance.The approach utilizes a preliminary conventional ray simulation to estimate the cost (future load) of each cell in the WFC's initial wavefront mesh.Then it applies a non-uniform mesh decomposition that results in a more efficient parallel WFC. Our implementation shows better and stable scalability in most WFC simulations. Overall, this paper contributes to understanding the behavior of wavefront mesh adaptability and predicting earth model complexities, and it serves as a guide for achieving the ultimate goal, a fully load-balanced parallel WFC

AxiSEM: broadband 3-D seismic wavefields in axisymmetric media

We present a methodology to compute 3-D global seismic wavefields for realistic earthquake sources in visco-elastic anisotropic media, covering applications across the observable seismic frequency band with moderate computational resources. This is accommodated by mandating axisymmetric background models that allow for a multipole expansion such that only a 2-D computational domain is needed, whereas the azimuthal third dimension is computed analytically on the fly. This dimensional collapse opens doors for storing space–time wavefields on disk that can be used to compute Fréchet sensitivity kernels for waveform tomography. We use the corresponding publicly available AxiSEM (<a href="www.axisem.info"target="_blank">www.axisem.info</a>) open-source spectral-element code, demonstrate its excellent scalability on supercomputers, a diverse range of applications ranging from normal modes to small-scale lowermost mantle structures, tomographic models, and comparison with observed data, and discuss further avenues to pursue with this methodology