A multitasking implementation of a 3D high order finite difference forward modeling program on the CRAY X-MP/416

We present a multitasked implementation of a 3D out-of-core seismic forward modeling on the CRAY XMP/416. The algorithm is based on the forward explicit high order finite difference method. We give a brief theoretical overview of 3D forward modeling and the resolution of the 3D wave equation. We discuss the coefficient matrix generated by the above mentioned approach. An in-core and an out-of-core version of a reentrant subroutine were designed to perform any fraction of the matrix vector multiplication independently. The rest of the program takes advantage of the microtasking feature which enables the system to treat independent iterations of Do Loops as subtasks to be performed by any available processor. The comparison of the measured speed-ups obtained of the multitasked programs (two, three, and four processors) versus the unitasked programs shows that the combination of the macrotasking and microtasking features enabled us to reach approximately 80 percent of the ideal speed-up. The modeling results are significantly improved by using the absorbing boundaries. We tested the program to determine the number of points per wavelength. We found that the number of grid points per wavelength is inversely proportional to the order of the finite difference; and this led us to conclude that there is a significant reduction in memory requirements and in CPU time for using higher order. We discuss the effect of the elimination of a point from the high order configuration on the modeling result and the execution time. We show why the only acceptable result is when the outer point is eliminated. Finally the time sections of the SALFRH model collected at the plane Z = 3 are presented when we have a point source and a plane wave source in the model and for the exploding reflector model. Due to the availability of the SSD (Solid State Storage Device) and its 1250 Mbps dual channel, the I/O wait time was virtually close to zero.