The last image of Figure is displayed at full scale in Figure . It is generated by downward continuing the data to a depth of 1500 m in one datuming step. The downward continued data are then migrated and combined with the previous image of the upper 2000 m. This overlap is used to preserve image quality because the portion of the image directly below the 1500 m datum suffers from the effects of limited offset and near-field Kirchhoff distortion. The overlap simply avoids the near-surface distortion suffered by most Kirchhoff migration algorithms.
Figure is a clear improvement over the standard migration displayed in Figure . The anticlinal structure below the salt and the target are now clearly imaged.
An even greater imaging improvement is attained by making the datuming step shorter since this insures that the first-arrival traveltimes are a better approximation to the most energetic arrivals. Continuation of the data to 1500 m in three steps of 500 m each, results in an even better-focused image of the anticline and the target (Figure ). In both Figure and , the events which unconformably define the top of the anticline, the anticline events themselves, and the target events are clearly imaged. The lateral continuity and event coherency in the target zone are substantially improved in Figure .
In Figure , I compare the images in the vicinity of the target zone to the velocity model and a synthetic reflectivity model which represents the desired image. The synthetic reflectivity (Figure b) is obtained by combining the velocity and density models and convolving with a wavelet. This type of resolution cannot be expected from migration, but ideally, the migration results should provide a comparable structural image.
Both of the layer-stripping images in Figures c and d compare favorably with the desired reflectivity. The image obtained by downward continuing the data in three steps of 500 m is superior since the events display better lateral continuity and the image is clearer. This is because the traveltimes calculated for each of the 500 m steps are better behaved than the traveltimes calculated for one step of 1500 m. The resolution is so good that the flat spot in the reservoir and the strongly reflective cap stand out clearly. The phase of the events is a good match to that of the synthetic reflectivity section.