Using IWAVE

Introduction

Domain-specific simulation such as seismic modeling begs for software re-use via modular design. All applications of this type have the same structure: static fields are initialized, dynamic fields updated, output extracted. A modular approach to code architecture is implicit in this structure, and further specialization leads to even more opportunity for code re-use via modular design.

The software package described in these pages, IWAVE, takes advantage of the aforementioned intrinsic modularity. IWAVE is open source software for finite difference or finite element time-domain simulation on regular rectangular grids, written exclusively in the C99 dialect of ISO C. IWAVE is built around a core framework: that is, a collection of separate software packages which together provide essential services upon which applications may be built. These service components completely define the interfaces to which additional code must be written to formulate a complete application.

Along with the core framework, the current release contains a complete finite difference time-domain acoustic modeling application, featuring

• simple parameter-driven job control;
• modeling in 1D, 2D, and 3D space;
• staggered grid schemes of orders 2 in time and 2k, k = 1, 2,..., in space
• serial, loop-parallel, and task-parallel execution models, scaling to thousands of threads;
• flexible specification of PML absorbing or pressure-release boundary conditions on all faces of the simulation cube;
• arbitrary source and receiver locations, and flexible point and array source specification including simultaneous source modeling (random, plane-wave,...)
• standard input and output data formats (SEGY, RSF)

An isotropic elastic modeling application with similar features, and built around the same core framework, has been developed and will be included in a forthcoming release.

The primary purpose of this short paper is to illustrate the use of IWAVE to calculate synthetic acoustic seismograms. To that end, the paper describes a simple application - 2D synthetic seismogram generation over a simple structural model of the sedimentary column - and provides a set of demonstration examples (``demos'') which the reader may reproduce, along with complete annotation of the files needed for job specification and sample graphics derived from the results (as well as commands to produce these graphics).

A secondary purpose is to supply the user with the means to independently verify some of the claims in the paper by Symes and Vdovina (2009), namely the existance of an error component in synthetic data derived from strongly heterogeneous models, in addition to the well-known grid dispersion error. The examples presented here are essentially the same as those presented in that paper. By installing IWAVE and running the demonstrations described here, the reader may reproduce the computational content of (Symes and Vdovina, 2009).

IWAVE is both a standalone application, and a component of the Madagascar software suite (Fomel, 2009) The application package and the examples discussed here (and indeed this paper itself) may be built either independently, or within Madagascar, as explained in detail below.

IWAVE was used in a quality control role in the SEAM Phase I project - see Fehler and Keliher (2011) for an account, including discussion of the many difficulties of large scale numerical simulation of seismograms.

The internal details of IWAVE are not discussed here, except insofar as is necessary to explain the use of the main commands. Symes et al. (2011) briefly describe the structure of the IWAVE framework, with emphasis on its object-oriented design and the resulting mechanisms for coupling modeling with optimization packages to produce inversion applications. The IWAVE project web page (Terentyev et al., 2012) provides extensive reference material, and further information about the design.

The paper begins with a brief review of the system of partial differential equations solved (approximately) by IWAVE's acoustic application, and the choice of finite difference method. The following section presents the examples of Symes and Vdovina (2009), along with some additional examples based on the same distribution of mechanical parameters which shed light on the impact of finite difference order on solution accuracy. Instructions for recreating these examples follow. The paper ends with a brief discussion of the prospects for improvements in performance and accuracy in FD technology, and the evolutionary advantages flowing from the modular, or object, orientation of IWAVE. Two appendices describe the job parameters used in the examples, and download and install instructions.

Using IWAVE