A. Pica

Wavelengths of Earth structures that can be resolved from seismic reflection data

The aim of inverting seismic waveforms is to obtain the “best” earth model. The best model is defined as the one producing seismograms that best match (usually under a least‐squares criterion) those recorded. Our approach is nonlinear in the sense that we synthesize seismograms without using any linearization of the elastic wave equation. Since we use rather complete data sets without any spatial aliasing, we do not have the problem of secondary minima (Tarantola, 1986). Nevertheless, our gradient methods fail to converge if the starting earth model is far from the true earth (Mora, 1987; Kolb et al., 1986; Pica et al., 1989).

Simultaneous source separation: A prediction‐subtraction approach

The acquisition of n-shots, more or less simultaneously, increases acquisition efficiency and collects a wider range of information for imaging and reservoir characterisation. Its success relies critically on the ability to separate n-shots from one recording. Stefani et al (2007) showed that while some datasets may be easily separated, others are more difficult. Using the more difficult data example from Stefani et al (loc.cit.), we show that a PEF-based adaptive subtraction (Spitz, 2007) of the estimated wavefield due to a secondary source provides an effective separation of the sources.

Retrieving both the impedance contrast and background velocity; a global strategy for the seismic reflection problem

Recorded seismic reflection waveforms contain information as to the small‐scale variations of impedance and the large‐scale variations of velocity. This information can be retrieved by minimizing the misfit between the recorded waveforms and synthetic seismograms as a function of the model parameters. Because of the different physical characters of the velocity and the impedance, we update these parameters in an alternating fashion, which amounts to a relaxation approach to the minimization of the waveform misfit. As far as the impedance is concerned, this minimization can be performed efficiently using gradient algorithms. For the inversion for seismic velocities, gradient methods do not work nearly as well; therefore, we use different minimization methods for determining impedances and velocities. However, the determination of the impedance and the determination of the velocity are strongly coupled; relaxation is most effective when this coupling is as weak as possible. Weak coupling can be achieved par...

Mirror imaging of OBS data

The world’s demand for energy is accelerating, while its hydrocarbon reserves are diminishing. Producers are compelled to explore and produce oil and gas in more challenging environments and to maximize recovery in existing reservoirs. New technology has always been a key to success. One such new technology is seismic acquisition using ocean bottom station (OBS) nodes (Berg et al., 1994; Ronen et al., 2003; Amal et al., 2005; Docherty et al., 2005; Granger et al., 2005).

Adaptive multiple subtraction with wavelet-based complex unary Wiener filters

ABSTRACTAdaptive subtraction is a key element in predictive multiple-suppression methods. It minimizes misalignments and amplitude differences between modeled and actual multiples, and thus reduces multiple contamination in the data set after subtraction. Due to the high crosscorrelation between their waveforms, the main challenge resides in attenuating multiples without distorting primaries. As they overlap on a wide frequency range, we split this wide-band problem into a set of more tractable narrow-band filter designs, using a 1D complex wavelet frame. This decomposition enables a single-pass adaptive subtraction via complex, single-sample (unary) Wiener filters, consistently estimated on overlapping windows in a complex wavelet transformed domain. Each unary filter compensates for amplitude differences within its frequency support, and can correct small and large misalignment errors through phase and integer delay corrections. This approach greatly simplifies the matching filter estimation and, despit...

3D surface-related multiple modeling

The shape of seismic reflected energy on shot or CMP gathers can be ex-tremely complicated in comparison with the actual geometry of geologic generators. A simple synclinal structure may produce a triplication in the zero-offset domain, and migration processing is needed to resolve this situation. By comparison, multiple generation “squares” (at least for first-order multiples) the degree of complexity of the reverberated reflected energy, and, in general, there is no domain, neither time, depth, nor pre- or postmigrated, where multiples and primaries can be simplified simultaneously.

Wave equation based internal multiple modeling in 3D.

Summary Model-based surface related multiple modeling (3D SRMM) can be achieved by the use of pre-stack demigration algorithms, thus avoiding the constrains on the shot positions distribution required by the data-based methods. In the following, we show a new method for modeling internal multiples in 3D by using a model-based technique. This method follows a parallel flow with respect to those employed by the data-based multiple modeling techniques, and allows for the construction of internal multiple events produced between upper layers reflecting the energy downward, and lower layers reflecting the energy upward. The method has been applied to Wide Azimuth Towed Streamer data from the Gulf of Mexico.

Processing Solutions For Wide Azimuth Data: Outcome From a WATS Field Experiment In Deep Water Gulf of Mexico

Wide Azimuth Towed Streamer (WATS) acquisition has already proved to be a key technique in improving seismic imaging, especially in complex areas such as sub-salt plays. In 2006, a WATS field experiment was conducted in a deep water area of the Gulf of Mexico. The main purpose was to challenge recently developed 3D processing algorithms and find the most suitable processing strategy for a wide azimuth dataset. The results indicate that a 3D shot based processing sequence is an effective solution that accommodates the effects related to the multi-pass acquisition method and realizes the full benefit of the recorded 3D wide azimuth wave field.

Shot-based pre-processing solutions for wide azimuth towed streamer datasets

Wide azimuth towed streamer (WATS) acquisition has been shown to provide improved seismic imaging, especially in areas with complex 3D structures. By making use of additional source vessels shooting into the streamer array from large lateral offsets, a dataset is created with a large cross-line aperture, higher fold, and a broader offset-azimuth distribution than conventional (narrow azimuth) streamer datasets. The improved imaging provided by WATS acquisition geometry has been well illustrated as this method is being more widely adopted. An example which illustrates the superiority of WATS data for imaging was given by Michell et al. (2006). There, a straightforward depth migration of WATS data with limited pre-processing was shown to provide a significantly improved image compared to the results obtained with conventional streamer acquisition. However, to realize the full potential of WATS data, preprocessing (the processing sequence prior to imaging) is essential, especially for applications which depend on the quality of the pre-stack gathers such as: N Velocity model building and update N Pre-stack time and depth imaging N 4D/time lapse processing N AVO analysis and reservoir characterization N Quantitative analysis

3D SRME on OBS Data Using Waveform Multiple Modelling.

Summary Surface-consistent data auto-convolution involved in the modeling of surface related multiple reflections (SRME) does not apply easily to Ocean Bottom Survey data unless surface data is available along the OBS shooting locations, or if the receiver distribution allows for wavefield continuation. For cases where only OBC data are available, and in situations with extremely sparse receiver distributions, 3D SRME can still be achieved by using 3D wavefield multiple modeling (WFM SRME). With this method, surface-related multiples are built by modeling the bounces of primaries and multiples within a migrated section representing any arbitrary 3D reflectivity model. Unfortunately, migration of up-going waves from OBS data suffers from the sparseness of the receiver distribution, thus resulting in blanks along the seafloor reconstruction because of the lack of fold and inapplicability of the imaging condition. Instead, down-going first-order multiples can be easily separated from the data after calibration and P-Z differentiation. This data can then be migrated by using the image of the receiver location with respect to the surface of the sea. This image is unambiguous, and allows for a better illumination and imaging of the seafloor, suitable for WFM SRME. This migration method, followed by WFM SRME, has been successfully applied on real OBS (node) data. Results are shown below..