6th Congress of Balkan Geophysical Society - Budapest, Hungary 3-6 October 2011

A24Deghosting of Seismic Data Based on AdaptiveSubtraction AlgorithmA.A. Bocharova* (DECO Geophysical CK) & M.A. Poluboyarinov. (DECOGeophysical CK)

SUMMARYGhost-waves is a traditional problem for seismic processor, however there is no efficient solution of thisproblem until now. It is considered, that ghosts are less obvious then long-path multiples and are lesseasily removed by seismic processing.This paper propose a new approach for ghost-wave removal. It is based on adaptive subtraction of a ghost-wave model from the original wave field. The method was applied to synthetics seismic data and fieldseismic data recorded in several areas. Results show that the described approach is effective in all cases.

6th Congress of Balkan Geophysical Society - Budapest, Hungary 3-6 October 2011

Introduction

Ghost-wave is a short-path multiple, or a spurious reflection that occurs when seismic energy initially reverberates upward from the shallow subsurface and then is reflected downward, such as the base of weathering or between sources and receivers and the sea surface. (Shlumberger Oiflield Glossary) Ghost-waves often arrive soon after, and sometimes very near, the primary reflections. Moreover, you can get several ghost events following each reflection depending on the towing depths of the source and the streamer. The ghost reflection events are normally considered as noise and need to be removed from the seismic data in order to avoid confusion in the interpretation of the seismic images at a later stage. It is considered, that ghosts are less obvious then long-path multiples and are less easily removed by seismic processing. Ghost-waves is a traditional problem for seismic processor, however there is no efficient solution of this problem until now. This paper proposes a new approach for ghost-wave removal. It is based on adaptive subtraction of a ghost-wave model from the original wave field. Suppression of the ghost model from the initial data by adaptive subtraction allows for the productive result without smoothing or erasing the interfering primary reflections and it is not affecting the original wavelet. The ghost-wave removal method was applied to synthetics seismic data and field seismic data recorded in several areas. Results show that the described approach is effective in all cases.

Basic Principles of Ghost-Wave Removal Approach

The approach based on the adaptive subtraction of ghost-wave model from the original wave field consists of two steps. The first step is creating a ghost-wave model. On the second step, the model is adapted to the input data with matching filters and subsequently subtracted from the initial data. The model of ghost events is obtained from the data by static shifts of the original traces to the ghost-wave arrival time. The main feature of the algorithm is its capability to make adjustments for possible gradual amplitude distortion along the time coordinate. Special filter is calculated for each trace based on both the original data traces and the model traces. This filter, when applied to the trace is trying to minimize the RMS amplitudes of whatever is found similar between the trace and the model. When the filter is calculated, it is accounted for non-stationarity to make it adaptive to the events that are quite similar but not exactly the same. This makes the subtraction to a certain extend efficient even for rather approximate models, when the arrival time or/and the amplitude of the modeled ghost-waves differ from the actual observation. However, the more similar is the model of the ghost-waves to the real ghost events observed, the more efficient is the subtraction. For this reason, the module is less efficient if, for example, marine data is significantly disturbed by sea swelling because the primaries and the ghosts are disturbed differently and the resulting model becomes less similar to the observation. When the filter is calculated for a particular trace of the original data, beside the corresponding trace of the model some adjacent traces of the model can also be used. It might happen that some extra similarity to what is observed on the original data trace can be found there.

6th Congress of Balkan Geophysical Society - Budapest, Hungary 3-6 October 2011

Thus, using adjacent traces could make the subtraction effect stronger. On the other hand, the filter shall not differ too much from trace to trace. For this reason, filters calculated for each of the original traces can afterwards be averaged over the base of several traces. This would make the subtraction milder and help to avoid the effect of erasing a gap around the removed ghost-waves. (Denisov et al., 2006)

Experiment with synthetic data

First, the algorithm was applied to synthetic data with the ghosts. The synthetic data was created based on a real marine seismic record without noticeable ghost events. The initial wave field was multiplied by -1 and then bulk shifted to 10 ms. Then, the initial and shifted wave fields were stacked together. An example of the initial data without ghosts is shown in Fig. 1a, the synthetic wave field with the ghost is shown in Fig. 1b and the result of the ghost-waves suppression is shown in Fig. 1c.

Figure 1 a) Initial data without ghost events b) wave field with ghost events c) result of ghost-waves suppression. Results show that adaptive subtraction method is effective and it does not significantly affect the primary events.

Application to the real data

Next part of the research was dedicated to suppression of ghost events from the real data. It was important to extract some useful information about thin low-amplitude layers which were affected by strong interference with the ghost. The approach was tested on several data examples. Figure 2a shows an example of marine seismic high-resolution data recorded in the White Sea shallow water area. The wave field contains strong ghost events. Figure 2b shows the result of the ghost-waves suppression.

a b c

6th Congress of Balkan Geophysical Society - Budapest, Hungary 3-6 October 2011

Figure 2 a) Initial data with ghost events b) result of ghost-waves suppression.

a

b

6th Congress of Balkan Geophysical Society - Budapest, Hungary 3-6 October 2011

Conclusion

Results of ghost-wave removal from synthetic and real seismic data show that adaptive subtraction method is effective and it does not significantly affect the primary events.

Acknowledgements

All results were created using RadExPro software produced by DECO Geophysical Software Company. References M.S. Denisov, M.A. Polubojarinov, D.B. Finikov, Robust Methods of Adaptive Seismic Multiple Suppression, Extended Abstract, Saint Peterburg 2006, EAGE International Conference&Exhibitors Catalogue Shlumberger Oilflield Glossary (http://www.glossary.oilfield.slb.com/)