To make the analysis statistically significant, sixteen wells were selected from the western section of the Gulf of Mexico. The surface location of these wells as well as of a semi-parallel seismic line are marked on the map shown in Figure . The logs were smoothed with a triangle filter, so that the figure represents an averaged version of the subsurface slowness. Figure a shows the sixteen vertically-smoothed sonic logs. This section represents the major lithologic features, with young, poorly-consolidated sediments at the top, a well-delimited high-velocity layer (white region around 8000 ft.), and a low-velocity zone at the bottom. It can be seen as the result of a tomographic inversion or other type of interval velocity estimation using well-to-well, or surface seismic, data.
Because seismic-reflection structures generally represent isochrones rather than iso-velocity lines, sonic log sections like the one in Figure a will generally differ from the seismic sections (Figure ). There are several ways to transform the information contained in the sonic logs into a log that shows a better correlation with the seismic section. The usual way is to generate a synthetic seismogram using the sonic, or sonic and density, logs. Figure b shows the synthetic seismograms generated from the original sonic logs by using a very low-frequency wavelet (central frequency of 3 Hz) to avoid spatial aliasing. Although each log correlates well with its corresponding surface seismic trace, the low-frequency synthetic section shows some lateral continuity, but only in localized regions. Likewise, we would find only localized continuity if we selected sparse traces from a very low-frequency seismic section. The next section presents an alternative chrono-attribute that shows a more extensive lateral continuity than the synthetic seismogram.