MIGRATION OF THE 3-D DEEP-SEISMIC REFLECTION SURVEY AT THE KTB LOCATION, OBERPFALZ, GERMANY

A near-vertical deep 3-D seismic survey covering an area of approximat ely 17.85 X 19.1 km has been carried out across the German continental deep borehole (KTB) site. This survey is the first of this kind to in vestigate complicated crystalline crust. Two major problems were encou ntered during the post-stack 3-D migration of these data. First, the s urvey region includes areas with strong lateral velocity gradients, wh ich are difficult to treat with many migration algorithms. Velocities in the sedimentary rocks are similar to 3000 m/s and in the crystallin e rocks similar to 5500 m/s. Secondly, the data are sparsely sampled ( 50 x 50 m binning), but include steeply dipping events with true dips up to 60 degrees. To determine the optimum 3-D migration approach thre e different algorithms have been tested: summation (modified Kirchhoff ), finite-difference and phase-shift. Results of the various tests sug gest that one-pass 3-D migration algorithms produce noticeably superio r results to the much faster two-pass methods. The combination of stee p dips and sparsely sampled data was a problem for the finite-differen ce migration, whereas shallow dipping features were imaged well by thi s technique. Although the adapted phase-shift migration algorithm coul d not handle lateral velocity gradients, it produced excellent results for both shallow and steeply dipping structures. Both of these migrat ion algorithms are relatively fast. By comparison, the summation migra tion algorithm produced overall good results for regions containing st eep dips and strong lateral velocity variations, but it requires a lot of computing time.