Suhas Phadke

Marine synthetic seismograms using elastic wave equation

SUMMARY Computation of synthetic seismograms for marine models is useful for the inversion and interpretation of offshore seismic data sets. In this paper we describe the implementation of an algorithm for solving 2D elastic wave equation for marine models. The resulting hyperbolic system of equations is solved on a Cartesian grid using a MacCormack finite difference scheme. The stability condition and P-wave phase velocity is independent of Poisson’s ratio. Moreover, the Swave phase velocity is insensitive to the Poisson’s ratio. This allows us to use the same algorithm for liquid medium by making the Poisson’s ratio equal to 0.5. The method is first tested for a simple two-layer model. It is shown that as the Poisson’s ratio increases from 0.25 to 0.5 in the top layer, the converted phases are eliminated from the seismograms. Finally, synthetic seismograms and snapshots are shown for a realistic model.

An explicit predictor-corrector solver with application to seismic wave modelling

Abstract Wave-equation-based forward modelling using explicit finite-difference methods is a standard technique for calculating synthetic seismograms. The stability criterion restricts the size of the time step. In this paper a predictor–corrector method for solving the wave equation is described which allows the use of a larger time step. A stability analysis of the method is also carried out. Parallel implementation of the algorithm is described for a distributed computing environment which makes use of MPI and PVM message passing calls for communication between processors.

Absorbing boundaries with Huygens' secondary sources

In this paper we propose a new approach for absorbing boundary reflections during numerical simulation of the seismic wave field. This is the first attempt towards attenuating spurious reflections from the boundary of a model by adding Huygens’ secondary sources in a transition zone surrounding the model. These secondary sources create destructive interference and cancel the wavefield outside the transition zone. Huygens’ sources are derived from an analytical formulation which is based on a symbolic equation and therefore do not depend either on the nature of the wave equation or for instance on the order of the finite difference scheme used. However during numerical simulation, the noise introduced due to the discretization on the problem, needs to be absorbed using other techniques.

On improving performance of migration algorithms using MPI and MPI-IO

SUMMARY High level of I/O performance is necessary in making use of parallel machines for many scientific applications. In this paper we discuss the I/O requirements of one such application: 3D seismic migration. I/O performance is the bottleneck rather than the computational or communication performance in 3D seismic imaging. For large 3D data volume it is not possible to read and keep all the required data and information in computer memory. Therefore the data are partially read and intermediate results are written out at various stages during the execution of the code. In this article we have discussed an approach to handle the massive I/O requirements of seismic migration using MPI-IO. We have also optimised some data communication using MPI calls. The performance of parallel seismic migration code on PARAM 10000, which is a cluster of SUN workstations, is discussed for two data sets.

PVM Implementation of Higher Order Finite Difference Seismic Modelling Algorithms In a Distributed Computing Environment

Seismic wave modelling algorithms used for calculating the seismic response of a given earth model, require large computational resources in terms of speed and memory. In this paper we describe the PVM (Parallel Virtual Machine) implementation of these algorithms in a distributed computing environment. Both the acoustic and elastic wave modelling equations are formulated as a first order hyperbolic system. Numerical solution uses an explicit finite difference scheme, which is fourth order accurate in space and second order accurate in time. A domain decomposition algorithm is used for distributing the workload and the tasks communicate via PVM message passing calls. The efficiency and speed of the algorithms is tested on a cluster . of SUN UltraSparc workstations.

Depth extrapolation of seismic wavefields using cubic spline approximation

Depth extrapolation equation used for seismic migration is often solved by finite-difference technique. The most commonly used migration method is based upon the CrankNicolson implicit scheme. Some modifications to this scheme are in practice which improve the impulse response of the migration operator. In this paper we propose a different approach where the wavefield is approximated by cubic spline function. The approximation to second derivative resulting from this approach is similar to that of the 1/6th trick of Claerbout. There is another spline parameter which controls the nature of impulse response. The scheme is unconditionally stable for 0.5 Il.0 . As the value of increases the dispersed evanescent energy gets more and more attenuated. The method is demonstrated by calculating the impulse response and by applying it to a synthetic data set.

Seismic imaging with a parallel multiple instruction multiple data (MIMD) computer

This presentation is oriented towards the examination and assessment of seismic analysis on the Myrias SPS-2. In particular, we focus on an efficient parallel implementation of a 2-D prestack seismic imaging methods, namely the w-x migration in shot gather and also shot-receiver gather domains, and reverse time finite-difference migration. Also, the implementations of the solution of other seismic methods will be presented. These include synthetic seismograms via ray-tracing and common fault point stacking. Maximum degree of parallelism, efficiency and speed was achieved for most of the methods used.

Imaging steam‐simulated low‐velocity zones during enhanced oil recovery operations

Determination of the size, position and physical parameters of a buried low velocity lens or layer is a difficult problem when using any seismic method. The example presented here is important in showing the effect of travel time triplication, cusping and strong amplitude anomalies in the presence of rays passing through a low velocity lens. A low velocity lens is generated in a heated zone, an example being the area around a steam injection site in a thermal oil recovery operation.