Multiples and multimode events in a 2D marine zero-offset survey: a physical modelling example

Joanna K. Cooper, Don C. Lawton and Gary F. Margrave

ABSTRACT

Several physical seismic models of simple wedges were built to assess amplitude effects commonly associated with the classic 'Widess' wedge. Rather than producing simple tuning associated with thin beds, 2D marine zero-offset seismic surveys over these physical models showed a surprising number of high-amplitude dipping events corresponding to multiples and multimode events associated with the wedge. Finitedifference exploding reflector models, using numerical versions of the same wedge velocity models, were generated to assist in the identification of these events. Amplitudes of the top-wedge and base-wedge reflections show additional complexities not observed in simple acoustic seismic models of the wedges. Migration of the physical model data was accomplished using 2D poststack Kirchhoff time and depth migration. The multiples and multimode events degrade the quality of the migrated image; removal of these events prior to migration was accomplished using f-k filtering. This study suggests that mixed-mode multiples can be significant, high-amplitude events in data recorded in the presence of high-velocity rocks with a wedge-like geometry.

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