Robert Soubaras

Deghosting by joint deconvolution of a migration and a mirror migration

This paper focuses on marine receiver deghosting. This step of processing has received renewed interest in recent years because it is a key point in increasing the bandwidth of the final images. Morever, any non-conventional marine acquisition method, such as over-under streamers, dual-sensor streamers or variable-depth streamers, requires its own receiver deghosting method. I will adress the deghosting problem from a novel viewpoint, which leads to a deghosting method adapted to any acquisition method. This deghosting method is optimal in terms of signal-to-noise ratio because it is not performed as a preprocessing stage. It is true amplitude, being able to extract the true deghosted reflectivity, that is the reflectivity that would have been obtained should the water surface be not reflecting. The principle of this method is to perform a migration together with a mirror migration, and to perform a joint deconvolution of these two images. The proposed method is illustrated on a synthetic and a real data example with a variable-depth streamer acquisition.

Explicit 3-D migration using equiripple polynomial expansion and Laplacian synthesis

In f-x explicit finite‐difference depth migration schemes, wavefield downward extrapolation is carried out through spatial convolution using finite‐length filters. Existing methods for computing these filters are based on nonlinear least‐squares, with a high computational cost, or on Taylor series expansion, which is suboptimal. In the 3-D case, the physics of wavefield extrapolation requires 2-D extrapolation filters with circular symmetry. Recently, McClellan transformation has been used to design circularly symmetric extrapolation operators. But this approach exhibits artifacts when the data are not spatially oversampled. We describe an alternative method to take advantage of the circular symmetry: the radial response of the filter is expanded as a polynomial in the Laplacian, which is synthesized as the sum of two 1-D second‐derivative filters. Using the Laplacian rather than the McClellan transform yields an artifact‐free impulse response for wavenumbers much closer to the Nyquist wavenumber at the s...

Two-step Explicit Marching Method For Reverse Time Migration

We describe in this paper a new way of solving the twoway wave equation called the two-step Explicit Marching method. Compared to the conventional explicit finitedifference algorithms, which can be second or fourth order but are subject to stability conditions and dispersion problems that limit the magnitude of the time steps used to propagate the wavefieds, the proposed method is based on a high order differential operator and allows arbitrary large time steps with guaranteed numerical stability and minimized dispersion. Synthetic and real data examples show that it allows the reverse time migration to be performed with the Nyquist time step, based on the maximum frequency of the input data, which is the maximum time step that can be used for proper imaging.

Angle Gathers For Shot-record Migration By Local Harmonic Decomposition

It is now recognized that shot-record wave-equation migration is the best method for imaging in complex media. However, it is also recognized that producing one output image is not enough. Velocity analysis can only be done when a gather is produced, as well as AVO analysis or multiple attenuation. This paper starts by describing the local harmonic decomposition: it is an efficient algorithm which can be used to extract angle information from a wavefield. It is then shown that this algorithm can be used to produce reflection angle gathers. A synthetic test proves that these gathers are centered on zero opening angle whatever the dip and the absence of artefacts. Results from the application of this algorithm on real 3D OBC data are shown.

Velocity model building by semblance maximization of modulated-shot gathers

In recent years, wave-equation migration has greatly enhanced imaging in complex velocity models. However, velocity model building is still dependent on ray-theory approximations. We propose a full wave-equation methodology for velocity model building based on the nonlinear inversion of a semblance criterion with respect to the velocity field. A newly described type of migration, called the modulated-shot migration, is used to obtain the necessary gathers, which are indexed in surface angle. The semblance of these gathers, after spatial averaging, is used as the cost function. This methodology is shown to successfully image the Marmousi model and the subsalt part of the Sigsbee model, especially in terms of focusing, which is as good as with the true model, but also in terms of depthing which is enhanced compared with the initial model. Realistic constraints are used in terms of minimum frequency, maximum offset, and crudeness of the starting model. A key point in the success of this methodology is the mu...

Variable Depth Streamer - The New Broadband Acquisition System

The importance of recording the full range of frequencies (low as well as high) is widely accepted. High-fidelity, low-frequency data provides better penetration for the clear imaging of deep targets, as well as providing greater stability in seismic inversion. Broader bandwidths produce sharper wavelets and both low and high frequencies are required for high-resolution imaging of important features such as thin beds and stratigraphic traps. The industry has been facing many issues that have limited the performance of marine seismic surveys with respect to bandwidth. Among them, we find mechanical and acoustic noise, source and receiver ghosts and attenuation with depth. Until recently, conventional de-ghosting was found to be sub-optimal. Thanks to recent advances in technology and also in operational capabilities, we have seen several improvements, in particular with the use of solid streamers, deep towing and notch diversity. We describe a different technique to achieve broadband marine streamer data. The proposed solution is a new combination of streamer equipment, novel streamer towing techniques, and a new de-ghosting and imaging technology. The technique takes full advantage of the low noise and low-frequency response of the new generation of solid streamers (see Figure 1). It then uses receiver notch diversity to yield a broadband spectrum. Conventional acquisition (see Figure 2) with its receiver ghost represents a tuned receiving array which enhances some frequencies and completely cancels others (at the ghost notches). In variable depth streamer acquisition (see Figure 3) a variable depth streamer is used so that the receiving array is detuned and receives all frequencies. As a result, the method creates an exceptionally sharp and clean wavelet for interpretation. It can be optimized for different water depths, target depths and desired output spectra. Figure 4 shows how the variable depth configuration improves low frequency response (it is the average streamer depth that is key parameter here), while at the same time using notch diversity to avoid higher frequency notch problems. This approach to towed streamer broadband seismic is particularly efficient, flexible and customizable for a range of environments and applications. The acquisition parameters such as variable depth streamer profile, maximum streamer depth and source depth can be tuned to provide the maximum possible bandwidth for a given geological setting and water depth. In particular this technique can take full advantage of towing solid streamers at what are currently considered as extreme depths to benefit from the improved low-frequency response of the hydrophones and reduced sea-state noise. To date a variety of test lines have been acquired in different settings with streamer maximum depths as large as 60m. The novel approach to deghosting is fully 3D. It makes no 2D assumptions and has no limitations in the cross-line direction making it suitable for wide-azimuth as well as 3D surveys. This flexibility means that the technique can be used for a range of applications. The increase in penetration from the extension of the bandwidth at the low end will benefit the imaging of deep targets and those below complex overburdens. Shallow targets (such as shallow hazards) will benefit from the fully from the total bandwidth available and recordable. Recent trials have achieved usable bandwidths between 2.5 and 150 Hz.

Multiple attenuation for variable-depth streamer data: from deep to shallow water

Summary Variable-depth streamer acquisition is becoming a key technique for providing wide bandwidth seismic data. Varying the receiver depth creates wide receiver ghost diversity and produces a spectacular increase in the frequency bandwidth. However, compared to conventional data, this variable-depth streamer data implies a major challenge in processing: how to deal with various receiver ghosts. The ghosts have to be preserved up to the deghosting step. Here we present the implication for the following de-multiple methods: Shallow-Water Demultiple, Tau-P deconvolution and Surface-related multiples elimination in deep and shallow water environments.

Challenges in Processing Variable-depth Streamer Data

Summary Variable-depth streamer acquisition is emerging as a key technique for providing wide bandwidth seismic data. With several data sets acquired across the world, it has consistently produced high quality images in terms of seismic resolution, layer stratigraphy and low-frequency penetration. By varying receiver depth, variable-depth streamer acquisition introduces receiver ghost diversity over different offsets. Such diversity enables a joint deconvolution method to fully remove the receiver ghost. Variable-depth streamer data also tends to be less noisy due to the deep tow of cables. These two factors allow variabledepth streamer data to have a spectrum from 2.5 Hz up to the source notch. Challenges in processing include: how to maintain the full bandwidth in the data, how to effectively remove multiples, and how to robustly build a velocity model. This paper will discuss each of these challenges and their solutions.

Multiple attenuation and P-S decomposition of multicomponent ocean-bottom data

τ vx Δz Summary In multicomponent ocean-bottom acquisitions, hydrophone data is acquired together with vertical and horizontal geophone data. The aim of the horizontal geophone is to produce a shear-wave section together with a compressionalwave section. The pressure and velocity measurements are contaminated by ghosts and peg-leg multiples. Also, apart from vertical propagation, P and S waves are not separated between the measurements. In this paper, a method is presented for water-layer multiple attenuation and P and S separation that does not require the subsurface elastic parameters to be known, nor the measurements to be well calibrated.