Okwudili C. Orji

Imaging the sea surface using a dual-sensor towed streamer

Sea-surface profile and reflection coefficient estimates are vital input parameters to various seismic data processing applications. The common assumption of a flat sea surface when processing seismic data can lead to misinterpretations and mislocations of events. A new method of imaging the sea surface from decomposed wavefields has been developed. Wavefield separation is applied to the data acquired by a towed dual-sensor streamer containing collocated pressure and vertical particle velocity sensors to obtain upgoing and downgoing wavefields of the related sensors. Time-gated upgoing and downgoing wavefields corresponding to a given sensor are then extrapolated to the sea surface where an imaging condition is applied so that the time-invariant shape of the sea surface can be recovered. By sliding the data time-window, the temporal changes of the sea surface can be correspondingly estimated. Ray tracing and finite-difference methods were used to generate different controlled data sets used in this feasibility study to demonstrate the imaging principle and to test the image accuracy. The method was also tested on a first field data example of a marginal weather line from the North Sea.

Rough sea surface implications on receiver deghosting

Efficient removal of ghost events is the key to achieving broadband seismic data. On the receiver-side, accurate ghost removal can be performed when both pressure and vertical particle velocity information is available. However, deghosting pressure-only measurements by spectral division with a flat sea surface or a statistical ghost function is still commonly applied. In this paper, we evaluate the performance of wavefield separation and deghosting with a flat sea surface (or statistical) ghost function for a rough weather marine seismic data acquisition. The behaviour of the pressure ghost function under rough sea conditions is analysed in comparison to the flat and the statistical pressure ghost functions. The three pressure ghost functions (i.e., true, flat, and statistical) converge to the same amplitude and phase values for the lower frequencies, but they diverge from each other for higher frequencies. The error created by wavefield separation and deghosting by spectral division is quantified, using synthetic and real seismic data. Wavefield separation provides deterministic and full receiver-side deghosted up-going wavefields, while both flat and statistical deghosting methods result in significant errors with implications in pre- and post-stack evaluation.

Imaging time varying sea surface using dual sensor data

The assumption of flat sea surface in seismic processing can lead to misinterpretation and mislocation of events. Sea surface profile and the reflection coefficient estimates along the streamer can be obtained by applying an imaging technique that is based on decomposed wavefields obtained from dual sensor streamers. The accuracy for frozen sea surfaces has been tested with finite-difference data. A further test for time varying sea surfaces has now been performed with synthetic data generated by a KirchhoffHelmholtz integral approach. A field data from the North Sea is also tested.

Imaging 3D Sea Surfaces from 3D Dual-Sensor Towed Streamer Data

3D realistic sea surface imaging from 3D dual-sensor towed streamer data is presented. The technique is based on separating data acquired by collocated dual-sensors into up-going and down-going wavefields. Subsequently, these wavefields are extrapolated upwards in order to image the sea surface. This approach has previously been demonstrated using 2D data examples. Here, the focus is on 3D data. Controlled 3D data based on the Kirchhoff-Helmholtz algorithm is generated, and the 3D sea surface imaging technique is applied. For coarsely spaced streamers from 3D field data, the technique is applied streamerwise (i.e., 2D wavefield separation, extrapolation, and imaging). In the latter case, the resulting sea surface profiles corresponding to each time frame are interpolated to demonstrate that the main sea surface characteristics are preserved, and artefacts due to 2D processing of 3D data are mainly limited to areas corresponding to large angles of incidence. Time-varying sea surfaces from two different 3D field data are imaged. The data examples were acquired under different weather conditions. The imaged sea surfaces show realistic wave heights, and their spectra suggest plausible speeds and directions.