Tomograms and seismograms

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.

 

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Figure 2 Surface locations of sixteen adjacent wells in the Gulf of Mexico. The continuous line corresponds to the surface locations of the sections derived from the well-logs (Figures  and ), while the dashed line indicates the surface location of the seismic section shown in Figure .

 

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Figure 3 A seismic section from the Gulf of Mexico. The section is semi-parallel to the sixteen selected wells whose locations are shown in Figure . The left side of the top section connects with the right side of the bottom section.

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.

 

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Figure 4 (a) The sixteen adjacent sonic logs indicated in Figure  after vertical smoothing. (b) A low-frequency (3 Hz) synthetic seismogram corresponding to the sonic logs shown in a.