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Migration velocity analysis on synthetic seismic data from South Crete

Tsiolis Alexandros

Πλήρης Εγγραφή

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Περίληψη

In the present thesis, migration velocity analysis (MVA) method was applied on synthetic seismic data from South Crete. The synthetic data were obtained from a previous thesis conducted in the Applied Geophysics laboratory of Technical University of Crete.At first, the initial model of RMS velocities, resulting from the interval velocities used for the synthetic data creation, was produced. Secondly, prestack Kirchhoff migration was conducted for each and every common shot gather and the migrated section was produced in order to examine Kirchhoff migration. It should be mentioned that the number of sources is limited, so it is recommended to increase this number. The examination of other prestack migration methods is recommended, which may perform better at areas with dipping reflectors.Eventually, common image gathers (CIGs), were created using the CMP_Synthetics algorithm. CIG Supergathers contribute to increasing the number of traces for the velocity analysis, but the required analysis was not achieved because of the large distance between the CIGs. Furthermore, inverse NMO correction was applied on the CIGs and three inverse NMO-corrected CIG Supergathers were created. Subsequently, velocity analysis was conducted for each and every inverse NMO-corrected CIG Supergather and interval velocities were calculated.Two different picking methods were conducted. The first picking was based on the MVA data and the second was based on the initial data with their results being compared with each other. For large offsets, move-out is possible to be large and as a consequence, reflections at distant traces are not visible. Move-out at distant traces could be visible if the records were not cut at two-way traveltime 5,5 s. This might be the source of the migration velocity analysis complications. The migration velocity analysis seems to perform better for the first reflector at 3,5 s and the third at 4,5 s because they are flat in contrast to the rest. For the CIG Supergathers, the reflectors should be flat at the involved image point locations. It is recommended to reduce the spacing between the CIGs used for the creation of the three CIG Supergathers, which could be achieved by increasing the fold of the initial data. Hence, the process could be more successful utilizing these CIG Supergathers.

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