There are numerous reasons why synthetic seismograms do not tie precisely with the actual seismic data. The most common reasons are problems with the velocity model. In this study, the sonic log and physical properties velocity measurements were used to calculate the two-way traveltime-depth relationship. There are several factors that might affect the accuracy of the sonic log, and thus the time-depth relationship. If the borehole becomes washed out, the sonic log will not read true rock formation velocities. If the formation is anisotropic, the seismic wave energy will not necessarily travel in the same direction as the log measurements. Dispersion effects can cause a poor correlation between the seismic data and the synthetic trace. Ideally, check-shot surveys or vertical seismic profiles would be used to refine the time-depth curve, as well as gain information on formation anisotropy and frequency transmission along the borehole. Unfortunately, this data was not available for this study. As mentioned previously, shipboard velocity measurements may be erroneous because of the removal of overburden pressure (Hamilton, 1979; Fulthorpe et al., 1989; Urmos et al., 1993).
Attempting to tie the synthetic seismograms to single-channel seismic data also causes problems. Single-channel seismic data often have artifacts that do not accurately represent the subsurface geology. Multiples and intramultiples in the seismic data are present in the SCS data and are extremely difficult to remove. SCS data are also highly susceptible to being affected by out-of-plane energy. These artifacts in the seismic data are not recreated in the synthetic seismogram and could affect the correlation. Another problem with SCS data is that the data have no far-offset stacking to help remove random noise from the recorded trace. In such a case, the major reflectors in the seismic data will typically match with the synthetic trace. This noise, however, is impossible to accurately model with synthetic seismograms and can have a significant effect on lowering the correlation coefficient. This is believed to be one of the major reasons for the low correlation coefficients of the synthetic seismograms generated in this study.
There are several other potential reasons that the synthetic might not tie with the real seismic trace. A deviated borehole would cause the drillhole to appear deeper than it actually is, thus causing events in the synthetic to appear "deeper" than those in the seismic data. Despite using the seismic wavelet extraction procedure, the lack of an actual source wavelet could cause differences between the synthetic seismogram and seismic data. Finally, as mentioned earlier, there are numerous factors that could affect the quality of the sonic and density logs as well as the physical properties data, any of which could adversely affect the resultant synthetic seismogram. Unfortunately, these effects are difficult to quantify.