Interpretation of seismic waveforms can be expressed as an optimization problem based on a nonlinear least-squares criterion to find the model which best explains the data. An initial model is corrected iteratively using a gradient method (conjugate gradient). At each iteration, computation of the direction of the model perturbation requires the forward propagation of the actual sources and the reverse-time propagation of the residuals (misfit between the data and the synthetics); the two wave fields thus obtained are then correlated. An extra forwardpropagation is required to compute the amplitude of the perturbation along the conjugate-gradient direction. Thenumber of propagations to be simulated numerically in each iteration equals three times the number of shots. Since a 2-D finite-difference code is employed to solve forwardand backward-propagation problems. the method is general and can handle arbitrary 2-D source-receiver configurations and lateral heterogeneities. Using conventionalvelocity analysis to derive an initial velocity model, the method is implemented on a real marine data set. Thedata set which has been selected corresponds approximately to a horizontally stratified medium. Consequently, a single-shot gather has been used for inversion. In spite of some simplifying assumptions used for wave-field propagation (acoustic approximation, point source), and using synthetics generated by a nearby sonic log to calibrate amplitudes, our final synthetics match the input data very well and the inversion result has clear similarities to the log.

This content is PDF only. Please click on the PDF icon to access.

First Page Preview

First page PDF preview
You do not have access to this content, please speak to your institutional administrator if you feel you should have access.