RESOLUTION

Conventional stacking velocity analysis is able to estimate the RMS velocity with sufficient accuracy (2%-3%) if the layers are nearly horizontal and if the incidence angles are small. In this case, we can simply apply Dix's formula to get the interval velocities, which is the information we need to identify anomalous variations of the wave velocity.

Although the accuracy of stacking velocity is sufficient, the space and time resolution of the velocity spectra may be quite limited. In stacking velocity analysis, a spatial average is carried out over the acquisition spread, whose length in our case is 750 m (half of the streamer length). Consequently, we are not able to measure directly sharp lateral velocity variations. Along the time dimension, the coherency values are usually averaged within time windows whose length is comparable to that of the seismic wavelets. Seismic wavelets are band-limited signals; therefore, their direct comparison by semblance or cross-correlation carried out in stacking velocity analysis necessarily provides a band-limited estimate.

Tomographic inversion of traveltimes is able to improve the resolution significantly for two main reasons:

1. The picking procedure transforms seismic reflections into punctual events; although the picking is a spiking process, which concentrates an event into a single point, the picking accuracy is limited by noise presence, interference with other events, phase rotations at supercritical incidence angles, and so on. Therefore, we have to consider this point in a statistical sense, which means that averaging is still necessary for our velocity estimate.
2. There is no spatial average because each event preserves its individual contribution to the velocity estimation during the whole inversion procedure. We can say that the nominal spatial resolution is on the order of the trace spacing, which is 12.5 m in our case. The actual spatial resolution, however, is affected by the size of the Fresnel zone, whose radius is on the order of 60 m at the seafloor with the dominant frequency of 100 Hz.