WAVE-FIELD SEPARATION BY 3-D FILTERING IN CROSSHOLE SEISMIC-REFLECTION PROCESSING

In processing crosshole seismic reflection data, it is necessary to se parate the upgoing and downgoing primary reflections from each other, from the direct waves, and from other wave types in the data. We have implemented a 3-D f-k-k filter for wavefield separation that is applie d in a single pass. The complete data set is treated as a data volume, with each sample defined by the three coordinates of source depth, re ceiver depth, and time. The filter works well because upgoing primarie s, downgoing primaries, and direct waves lie in different quadrants in f-k-k space. The strongest multiples, including mode-converted multip les, lie in the same quadrants in f-k-k space as the direct waves, so they are readily rejected together. Tube waves and mode-converted prim aries are also suppressed as most of the energy in these wave types li es outside the pass volume for P-wave primaries. Some head wave and S- wave primary energy will be passed by the filter; however, these waves tend to have low amplitudes and late arrival times, respectively, and will be smeared out on imaging with the P-wave velocity field. We hav e processed a real crosshole data set using two different methods of w avefield separation: applying 2-D f-k filtering to common source gathe rs and applying a 3-D f-k-k filter to the whole data set. The migrated image produced after 3-D f-k-k filtering contains less coherent noise and consequently shows improved continuity of reflectors.