Month: June 2014

Fast elastic mode separation in anisotropic media

June 24, 2014 Documentation No comments

A new paper is added to the collection of reproducible documents:
Fast algorithms for elastic-wave-mode separation and vector decomposition using low-rank approximation for anisotropic media

Wave mode separation and vector decomposition are significantly more expensive than wavefield extrapolation and are the computational bottleneck for elastic reverse-time migration (ERTM) in heterogeneous anisotropic media. We express elastic wave mode separation and vector decomposition for anisotropic media as space-wavenumber-domain operations in the form of Fourier integral operators, and develop fast algorithms for their implementation using their low-rank approximations. Synthetic data generated from 2D and 3D models demonstrate that these methods are accurate and efficient.

How do I do interactive picking?

June 12, 2014 FAQ No comments

While interactive picking is generally discouraged because of its non-reproducibility, occasionally it might be useful. Using interact= option with xtpen outputs mouse-click coordinates in a text file. However, they are Vplot coordinates, not easily related to physical coordinates of the image. Joe Dellinger has a more comprehensive plan for adding interactivity to Vplot graphics.

sfipick is a simple Tkinter script which allows for interactive picking. The interface is straightforward. Use left-button mouse clicks to add picks, right-button mouse clicks to remove wrong picks, middle-button to drag picks. The picks are written in a plain text file and can be processed later.

See also:

Program of the month: sfeikonal

June 11, 2014 Programs No comments

sfeikonal solves the eikonal equation using the Fast Marching Method. This computation produces first-arrival traveltimes on a fixed grid.

The following example from sep/fmeiko/fmarch shows traveltime contours for a point source inside the SEG/EAGE salt model.

The point source can be specified by its coordinates xshot=, yzhot=, and zshot= (note that the depth coordinate zshot corresponds to the first axis). In a small box around the source, the solution is computed analytically to avoid errors from the point-source singularity. The size of the box can be specified in samples (b1=, b2=, and b3=) or in physical dimensions (br1=, br2=, and br3=). For a plane-wave source instead of a point source, use plane1=y, plane2=y, or plane3=y. The plane is assumed to be aligned with the grid. For computing a traveltime table with multiple sources, the source coordinates can be specified in a file given by shotfile=. The order of accuracy in the finite-difference scheme is specified by order= parameter.

The following plot from sep/fmsec/cvel shows the error difference between the first- and second-order computations in a constant-velocity medium.

sfeikonal computes isotropic traveltimes. For an extension to VTI anisotropy, see sfeikonalvti.

10 previous programs of the month:

Lowrank on a staggered grid

June 2, 2014 Documentation No comments

A new paper is added to the collection of reproducible documents:
Lowrank seismic wave extrapolation on a staggered grid

We propose a new spectral method and a new finite-difference method for seismic wave extrapolation in time. Using staggered temporal and spatial grids, we derive a wave extrapolation operator using a lowrank decomposition for a first-order system of wave equations and design the corresponding finite-difference scheme. The proposed methods extend previously proposed lowrank and lowrank finite-difference wave extrapolation methods from the cases of constant density to those of variable density. Dispersion analysis demonstrates that the proposed methods have high accuracy for a wide wavenumber range and significantly reduce the numerical dispersion. The method of manufactured solutions coupled with mesh refinement is used to verify each method and to compare numerical errors. 2-D synthetic examples demonstrate that the proposed method is highly accurate and stable. The proposed methods can be used for seismic modeling or reverse time migration.

Second working workshop

June 1, 2014 Celebration 2 comments

Registration is open for Madagascar’s Second Working Workshop.

As a reminder, Working Workshops as opposed to “talking workshops” are meetings where the participants work together (possibly divided into pairs or small teams) to develop new software code or to conduct computational experiments addressing a particular problem. The First Working Workshop took place last summer in Austin. This year’s workshop will take place on at Rice University in Houston, Texas, on July 31 – August 2, 2014. The topic of this year’s workshop is parallel and high-performance computing. The objective is

  1. To develop convenient tools for high-performance and parallel computing.
  2. To create new examples of distributed-memory and shared-memory parallel computing.
  3. To explore hardware-accelerated parallel computing (NVIDIA GPU and Intel® Xeon Phi™).

Registration is free by an application is required. If you are interested in participating in this workshop, please fill an application form.