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Geophysical Prospecting: Multisource least‐squares migration of marine streamer and land data with frequency‐division encoding

Multisource migration of phase‐encoded supergathers has shown great promise in reducing the computational cost of conventional migration. The accompanying crosstalk noise, in addition to the migration footprint, can be reduced by least‐squares inversion. But the application of this approach to marine streamer data is hampered by the mismatch between the limited number of live traces/shot recorded in the field and the pervasive number of traces generated by the finite‐difference modelling method. This leads to a strong mismatch in the misfit function and results in strong artefacts (crosstalk) in the multisource least‐squares migration image. To eliminate this noise, we present a frequency‐division multiplexing (FDM) strategy with iterative least‐squares migration (ILSM) of supergathers. The key idea is, at each ILSM iteration, to assign a unique frequency band to each shot gather. In this case there is no overlap in the crosstalk spectrum of each migrated shot gather m(x, ωi), so the spectral crosstalk product m(x, ωi)m(x, ωj) =δi, j is zero, unless i=j. Our results in applying this method to 2D marine data for a SEG/EAGE salt model show better resolved images than standard migration computed at about 1/10th of the cost. Similar results are achieved after applying this method to synthetic data for a 3D SEG/EAGE salt model, except the acquisition geometry is similar to that of a marine OBS survey. Here, the speedup of this method over conventional migration is more than 10. We conclude that multisource migration for a marine geometry can be successfully achieved by a frequency‐division encoding strategy, as long as crosstalk‐prone sources are segregated in their spectral content. This is both the strength and the potential limitation of this method.

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