Prestack Phase-Screen Migration for VSP Data in Gas Hydrate Zone

Prestack phase-screen migration for surface seismic data was adjusted to apply to VSP data. In addition, it was improved to be able to compensate the absorption, which may occur significantly in gas hydrate formation, if Q-factors of the layers are provided. To verify the algorithm, we applied it to synthetic seismic data obtained from the velocity model simulating gas-hydrate bearing zone, and compared the results of migration without and with compensating for absorption. The compensation for absorption during migration significantly improved the resolution of the migrate image. With spatial spectral analysis, we confirmed that the migration with compensation for absorption recovered the amplitudes of higher frequencies, which are affected more by absorption. Results of the test with real VSP data from gas hydrate zone showed that the base of gas hydrate zone, which is weakly depicted by conventional migration, was clearly identified by the phase-screen migration with compensating for absorption.