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Frequency domain elastic wave modeling for polygonal topography using rotated average derivative diference operators

Li Zheng, Chen Jing-Bo, Cao Jian

Acta Geophysica

2020

Vol. 68, no. 5

1387--1409

Modeling of seismic wave propagation in areas with irregular topography is an important topic in the feld of seismic exploration. As a popular numerical method for seismic modeling, the fnite diference method is nontrivial to consider the irregular free surface. There have been extensive studies on the time-domain fnite diference simulations with irregular topography; however, the frequency-domain fnite diference simulation considering irregular topography is relatively less studied. The average-derivative approach is an optimal numerical simulation scheme in the frequency domain, which can produce accurate modeling results at a relatively low computational cost. Nevertheless, this approach can only deal with the modeling problems with a fat free surface. To address this issue, we design a new frequency-domain fnite diference scheme by introducing the polygonal representation of topography into the average-derivative method. The irregular topog raphy is represented by line segments with various slopes. An extension of the conventional average-derivative diference operator in the local rotated coordinate system is used for formulating the spatial derivatives aligned with the topographic line segments. As a result, new average-derivative diference schemes are obtained for irregular topography. In this way, the average-derivative optimal method is generalized to the model with irregular topography. Numerical examples show the efectiveness of the presented method.

Motion control and collision avoidance algorithms for unmanned surface vehicle swarm in practical maritime environment

Zhuang Jiayuan, Zhang Lei, Qin Zihe, Sun Hanbing, Wang Bo, Cao Jian

Polish Maritime Research

2019

nr 1

107--116

The issue of controlling a swarm of autonomous unmanned surface vehicles (USVs) in a practical maritime environment is studied in this paper. A hierarchical control framework associated with control algorithms for the USV swarm is proposed. In order to implement the distributed control of the autonomous swarm, the control framework is divided into three task layers. The first layer is the tele-operated task layer, which delivers the human operator’s command to the remote USV swarm. The second layer deals with autonomous tasks (i.e. swarm dispersion, or avoidance of obstacles and/or inner-USV collisions), which are defined by specific mathematical functions. The third layer is the control allocation layer, in which the control inputs are designed by applying the sliding mode control method. The motion controller is proved asymptotically stable by using the Lyapunov method. Numerical simulation of USV swarm motion is used to verify the effectiveness of the control framework.