Joint 3D traveltime calculation based on fast marching method and wavefront construction

3D traveltime calculation is widely used in seismic exploration technologies such as seismic migration and tomography. The fast marching method （FMM） is useful for calculating 3D traveltime and has proven to be efficient and stable. However, it has low calculation accuracy near the source, which thus gives it low overall accuracy. This paper proposes a joint traveltime calculation method to solve this problem. The method firstly employs the wavefront construction method （WFC）, which has a higher calculation accuracy than FMM in calculating traveltime in the small area near the source, and secondly adopts FMM to calculate traveltime for the remaining grid nodes. Due to the increase in calculation precision of grid nodes near the source, this new algorithm is shown to have good calculation precision while maintaining the high calculation efficiency of FMM, which is employed in most of the computational area. Results are verified using various numerical models.

High signal-to-noise ratio sensing with Shack–Hartmann wavefront sensor based on auto gain control of electron multiplying CCD

High signal-to-noise ratio can be achieved with the electron multiplying charge-coupled-device（EMCCD） applied in the Shack–Hartmann wavefront sensor（S–H WFS） in adaptive optics（AO）.However,when the brightness of the target changes in a large scale,the fixed electron multiplying（EM） gain will not be suited to the sensing limitation.Therefore an auto-gain-control method based on the brightness of light-spots array in S–H WFS is proposed in this paper.The control value is the average of the maximum signals of every light spot in an array,which has been demonstrated to be kept stable even under the influence of some noise and turbulence,and sensitive enough to the change of target brightness.A goal value is needed in the control process and it is predetermined based on the characters of EMCCD.Simulations and experiments have demonstrated that this auto-gain-control method is valid and robust,the sensing SNR reaches the maximum for the corresponding signal level,and especially is greatly improved for those dim targets from 6 to 4 magnitude in the visual band.

Nonlinear analysis of traffic jams in an anisotropic continuum model

This paper presents our study of the nonlinear stability of a new anisotropic continuum traffic flow model in which the dimensionless parameter or anisotropic factor controls the non-isotropic character and diffusive influence. In order to establish traff^c flow stability criterion or to know the critical parameters that lead, on one hand, to a stable response to perturbations or disturbances or, on the other hand, to an unstable response and therefore to a possible congestion, a nonlinear stability criterion is derived by using a wavefront expansion technique. The stability criterion is illustrated by numerical results using the finite difference method for two different values of anisotropic parameter. It is also been observed that the newly derived stability results are consistent with previously reported results obtained using approximate linearisation methods. Moreover, the stability criterion derived in this paper can provide more refined information from the perspective of the capability to reproduce nonlinear traffic flow behaviors observed in real traffic than previously established methodologies.

Application of Gaussian beam pre-stack depth migration in rugged seabed

Strong fluctuation of seabed,abrupt variation in depth and dip of seabed bring seismic imaging problems,such as irregular reflection waves,obvious multiple waves,serious lateral wave development,poor imaging on base surface and depression structure,low signal-to-noise ratio of middle and deep layers.In this paper,Gaussian beam migration imaging method is used to analyze the imaging effect of rugged seabed in deep water area,and the ray tracing method of wavefront construction method is used to analyze the kinematic characteristics of seismic waves.By improving the design of seismic data acquisition and observation system,imaging quality of fine structures is improved.

Simulation of Propagating Acoustic Wavefronts with Random Sound Speed

A method for simulating acoustic wavefronts propagating under random sound speed conditions is presented.The approach applies a level set method to solve the Eikonal equation of high frequency acoustics for surfaces of constant phase,instead of tracing rays.The Lagrangian nature often makes full-field ray solutions difficult to reconstruct.The level set method captures multiple-valued solutions on a fixed grid.It is straightforward to represent other sources of uncertainty in the input data using this model,which has an advantage over Monte Carlo approaches in that it yields an expression for the solution as a function of random variables.