GNGTS 2015 - Atti del 34° Convegno Nazionale

GNGTS 2015 S essione 3.3 125 water layer reverberations only (i.e., travelpaths a, c and f for receiver-side reverberations, and d, e, f for source-side reverberations in Fig. 1). However, the water layer reverberations only can be separately estimated by a model based approach (Jin et al. , 2012). Then, the different estimations can be jointly subtracted from OBS data by cooperative matched filtering (Costagliola et al. , 2015), allowing to cope with the potential weaknesses of the different algorithms. Coarse receiver spacing are often encountered in OBS acquisition geometries. A pre- processing interpolation step can be required in order to improve the result of the multiple estimation approach. For example, compressing sensing based interpolation schemes (Fioretti et al. , 2015) can be pursued to obtain a well-sampled wavefield. Synthetic data example. The proposed approach has been tested with a synthetic OBS dataset to prove the feasibility of the technique. An OBS acquisition has been simulated, using a finite difference kernel and a free surface constraint to allow the correct recording of multiples. The average water-layer depth is 600 m, the shot interval (at surface) is 50 m and the hydrophones spacing (at sea-floor) is 20 m. A receiver line for a selected Common Shot Gather is shown in Fig. 3. Input data is shown in Fig. 3a, the multiple estimation result obtained with the proposed approach is shown in Fig. 3b and it is compared to the result obtained applying the OBS adapted MWD technique (Jin et al. , 2012) that is shown if Fig. 3c. The comparison of estimated multiples (Fig. 3b), obtained by the proposed approach, and the reference input data shown in Fig. 3a, proves that all OBS multiples are correctly predicted (water layer reverberations – red arrows, and deeper reflections – green arrows), at least from a kinematic point of view. In fact, the proposed algorithm is not strictly amplitude consistent, and the seismic wavelet is distorted in respect to input data (a squared source wavelet is superimposed to seismic reflectivity, due to the data convolution): an adaptive subtraction step is required as post-processing to obtain the up-going demultipled wavefield only. The comparison of the results in Figs. 3b and 3c proves how the MWD method cannot predict all the multiples except for water-layer reverberations (red arrows). The two different algorithms can however complement each other (especially when the direct wavefield is not correctly recorded in seismic data): joint adaptive subtraction of both multiple estimations can improve the overall quality of the resulting seismic data. The processing of real OBS acquisitions (not shown) leads to the same conclusions. Fig. 3 – Simulated data example: a) input (reference) data; b) estimated multiples by OBS SRME: all multiples events are correcly predicted c) estimated multiples by OBS MWD: only water layer reverberation are correctly predicted.