SLOPE DATA WATER‐BOTTOM MULTIPLE ATTENUATION¹

Methods for predicting and attenuating water‐bottom multiples by wavefield extrapolation have been discussed by several investigators. Because these prediction methods operate on shot records, boundary conditions must be specified for every shot record.

The approach presented operates in the common‐offset plane; a model of expected water‐bottom multiples is generated from the observed surface wavefield using a finite‐difference wave‐equation migration algorithm with an offset term. An accurate water‐depth profile is required, but there is no restriction on the shape of the water bottom other than a dip limit of approximately 18–20°. In generating a multiple model, the water‐bottom primary and each water‐bottom multiple reflection of the observed surface wavefield are extrapolated to a higher order. Thus, the extrapolated water‐bottom primary of the model is lined up with a water‐bottom multiple in the data and each multiple in the model is lined up with a higher‐order (or later) multiple in the data.

Prestack multiple attenuation is achieved, for one offset at a time, by first adapting the model of expected multiples to the observed data and then subtracting the predicted multiple energy. An error‐constrained adaptation algorithm is proposed in order to control instabilities. No assumptions are made about primary reflections and no subwater‐bottom velocities are required.

Computational efficiency of modelling and adaptation can be improved by applying this method only to near and intermediate offsets as the stacking process usually provides sufficient multiple attenuation at far offsets. A field data example demonstrates the potential of the proposed method for improving the primary‐to‐multiple ratio in prestack and post‐stack data.