Bruno Gratacos

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...

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.

Calibration of Horizontal Sensors In the Presence of Azimuthal Anisotropy

Shear wave processing of 3D seismic data involves the analysis of the horizontal particle motion in directions that are different from the acquisition directions. This analysis requires that the X and Y sensors must have equal sensitivity and frequency response to the particle motion. We present corrective methods using only the seismic data are established according to 1D models of wave propagation corresponding to a purely isotropic earth, or more generally for models that take into account the azimuthal anisotropy if present. Since individual traces have a low signal to noise ratio, stacking schemes derived from the propagation model are defined in order to gain robustness. The effect of the calibration process is then demonstrated using synthetic and real data.

Processing the Hod 3D multicomponent OBS survey, comparing parallel and orthogonal acquisition geometries

The Hod 3D multicomponent ocean-bottom seismic (OBS) survey was acquired to image the reservoir through a gas cloud. Data were acquired with shot lines both along the receiver cables (in-line or parallel shooting) and orthogonal to the receiver cables (cross-line shooting).

Amplitude effects associated with shear-wave splitting

Shear-wave anisotropy results in a time delay between two different shear-wave polarizations S1 and S2. If there are associated differences in attenuation of the amplitudes, then simple time shifting is insufficient to match the S1 and S2 images. This ―differential‖ attenuation leads to both an additional time delay and a frequency dependent amplitude effect. Using a dataset acquired over a heavy oil reservoir, we demonstrate that an improvement to the agreement between P-S1 and P-S2 images can be achieved through the use of a match filter approach. For realistic cases, this filter must be adapted to handle spatial and temporal variations of the splitting signature. The resulting attributes suggest that further insights into the reservoir properties can be found.

Reorientation and calibration of non‐gimbaled multicomponents sensors

Summary Shear wave processing of 3D data requires that the particle motion can be accurately reconstructed from the measurements of the seismic instrument. This requirement cannot be assumed honored on the raw field data: the orientation of the geophones are usually not accurately surveyed, and since the geophone response depends on their largely unknown coupling with the earth, within a single multicomponents sensor the frequency response of each geophone can significantly vary. We propose a robust method for the QC of the geophones orientations, and a calibration scheme that takes the hydrophone data as a reference to spectrally calibrate each individual geophone. The methods is illustrated with synthetic example, and applied to real data.