DESCRIPTION OF THE VELOCITY ESTIMATION PROCEDURE

The velocity-estimation method that I described in previous reports (Biondi, 1989; Biondi, 1988) is based on the maximization of beam stacks' energy at the traveltimes and surface locations predicted by the velocity function and modeled with raytracing. Therefore in order to estimate the velocity, I computed beam stacks from the prestack data for 12 offset ray parameters p_h, from .05 s/km to .336 s/km, and for three midpoint ray parameters p_y, from -.04 s/km to .04 s/km. I transformed the beam-stacks according to the coordinate transformation introduced in the previous report (Biondi, 1989), assuming a constant velocity of 2.5 km/s. I then smoothed the transformed data along the midpoint axis and the time axis using Gaussian windows. The smoothed and transformed data from the 250 midpoint positions located around the anomaly were used by the velocity estimation.

The starting solution for the iterative estimation algorithm was a velocity profile function of depth, but constant in the lateral direction. This velocity function was derived from conventional stacking velocity analysis applied to a few of the CMP gathers. The estimation procedure started with few conjugate gradient iterations until the velocity model predicted the gross feature of the beam-stacked data, and then continued with some Gauss-Newton iterations for better estimating the finer components of the velocity function.