Second EAGE/SBGf Workshop 2014

The acquisition parameters of a variable-depth streamer (VDS) survey over Martin Linge field offshore Norway were optimized by modelling. Its benefits were estimated by comparison of signal amplitude spectra on synthetic data for both the legacy flat streamer and a new VDS survey. As soon as several sail lines of the VDS survey had been acquired and processed up to 3D migration, new seismic fast-track data were compared to legacy flat streamer data. Real legacy and new data were also compared to the modelling results. The synthetic amplitude spectra explained quite well the improvement in signal-to-noise ratio of the VDS data up to 30 Hz. However, the better signal-to-noise of VDS data in the 30-40 Hz frequency range was not predicted. A new method has thus been developed to predict the final signal-to-noise ratio as a function of the acquisition parameters by adding real ambient noise in the modelling. This new method gave a more accurate prediction of the signal-to-noise ratio of the final migrated sections with a better fit between real and synthetic data.

In this work we present principles and results of the Adaptive Vibroseis Technology (AviSeis) application in standard linear, adaptive sweep and low frequency regimes. Field experiment was performed in complex geology region for deep target reservoir. Standard vibrators and geophones were used in the low frequency sweeping mode. Resulting images show better resolution, reflectivity contrast and deeper imaging due to low and high frequencies enhancement.

Removing ghost energy in marine streamer data is important for both seismic processing and interpretation. Full azimuth data have abundant azimuths, which require more robust 3D deghosting techniques. The coarse and irregular crossline sampling in full azimuth (FAZ) seismic data creates challenges for 3D deghosting. A fully data-driven 3D deghosting technique using a progressive sparse Tau-P inversion has proven to be able to overcome the sparse sampling in the crossline direction and to be effective in attenuating the receiver ghost in a 3D mode. We demonstrate the benefits of 3D deghosting using a staggered FAZ and ultra-long offset data set from Keathley Canyon, Gulf of Mexico. Using the data-driven 3D deghosting method, we observed less residual ghost energy in shot gathers from a side-gun when compared with the 2D pre-migration bootstrap deghosting method. The 3D deghosting method subsequently improved the images of steeply-dipping top of salt (TOS) and provided more coherent common imaging gathers.

We present a new method to remove ghost signal from conventional streamer marine data. The method assumes that each ghost component of the recorded signal is a deformed version of the desired signal, which is free of ghost recorded energy. The free of ghost signal can be transformed into each ghost component by applying a shift in time and a proper amplitude scaling, forming an equation system where it is the unknown vector. The transformations involved in the solution of this system, are carried out by a warping algorithm. The construction of warping operators comprises the computation of traveltime and amplitude for each reflected wave. We applied the method in a deep-water 2D line from the pre-salt area in Brazil. In this case, we use a NMO approximation for time-shift calculation and do not apply any amplitude scaling. The results are compared with those obtained without the deghosting and show considerable gains in the bandwidth, promoting outstanding benefits in the imaging and inversion.

Wavefield separation based on the combination of pressure and particle velocity data is generally used to extract the up- and down-going components from multi-component seabed or towed marine seismic recordings prior to imaging. By carefully combining vector-acoustic (VA) data in the extrapolation of shot gathers in reverse-time migration (RTM) we show that wavefield separation (deghosting) can be performed ‘on-the-fly’ at no extra cost. We call such a strategy VARTM and we successfully apply it to a North Sea OBC field dataset, acquired in the Volve field. We also discuss additional advantages of VARTM over standard RTM of up-going only waves such as improved handling of directivity information contained in the acquired vector-acoustic data for clearer shallow sections and imaging of the down-going component of the recorded field (mirror VARTM) without the need for an additional finite difference modelling.

The seismic bandwidth and resolution of conventional marine streamer data can be easily compromised by the appearance of a well know interference phenomena produced by the towing of sources and receivers below the water. A secondary signal, which bounces off of the sea surface near the sources and receivers, called a ghost signal, is very close to the original primary signals causing an interference pattern that distorts the wavelet. The interference manifests itself as a series of notches in the amplitude spectrum. The appearance of these notches narrows the bandwidth of the usable data and reduces the resolution of the seismic signal. We propose a new processing approach for deghosting the data based on a filtering algorithm applied in the Continuous Wavelet Transform (CWT) domain. The CWT provides a solid mathematical framework for dealing with the non-stationary nature of seismic data, by allowing a time variant analysis for the filter design and application. The application of the procedure on two different data sets demonstrates that our new deghost technique successfully eliminates the ghost notches and enhances the high and low frequencies, hence expanding the bandwidth and increasing the resolution of the data.

Broadband seismic processing has become a key technique to get more out of the data. Deghosting is a key part of broadband techniques, but the location in the processing flow varies significantly. The deghosting operation is performed anywhere from immediately after acquisition to imaging. We will look a the interaction between deghosting and such techniques as demultiple, debubble, residual designature and denoise techniques. We hope to shed some light on the optimal location for deghosting in the processing flow.

On a deep offshore turbiditic field covered by a recent yet conventional 4D baseline, a non-conventional route has been decided for the first monitor, to obtain seismic information related to dynamic heterogeneities but also better characterize reservoirs. The strategy decided for the first monitor (M1), one year after first oil, was to shoot broadband and process twice: - One 4D-dedicated processing mapped to conventional (with a Fast Track and a Full Processing phase) in order to match the baseline characteristics and obtain quality 4D signal to help understanding the early dynamic behaviour of the field. - One 3D broadband processing aiming to improve the existing seismic for reservoir characterization purposes. Despite a complicated workflow involving the generation of a reghosted dataset for 4D purposes, the final 4D data were produced in 4 months. The 3D broadband processing took 10 months overall. Beyond demonstrating that high quality 4D signal can be obtained by reghosting a broadband monitor on a conventional 4D baseline, it is shown that an unprecedented level of detail of dynamic heterogeneities and reservoir description can be reached in this setting using the combination of high resolution 4D fractional velocity change with 3D broadband images.

Increasing the seismic resolution by extending the signal bandwidth has been a technology driver for the seismic industry over recent years. Although some remarkable marine exploration case studies have validated the concept of 3D broadband images, the technology has not shown its full potential in a 4D context. In this paper, we will describe advantages of data acquired using a dual-sensor towed streamer for 4D seismic applications.