Sergey Birdus

Compensating attenuation due to shallow gas through Q tomography and Q-PSDM, a case study in Brazil

Summary The presence of gas, both as shallow pockets and as commercial reservoirs, has long been recognized as a significant problem in imaging seismic data. In this paper we describe how we successfully applied Q tomography and Q-PSDM technology to compensate the phase, frequency and amplitude loss due to shallow absorption, thus improving structure imaging and potentially accurate AVO/DHI analysis underneath shallow gas.

3-D Tomographic Amplitude Inversion for Compensating Transmission Losses in the Overburden

A tomographic inversion approach using prestack depth migrated common image gathers is utilized to compensate reflection data for amplitude loss caused by transmission anomalies in the overburden. The approach has the advantage of estimating transmission losses from anywhere within the overburden using the actual seismic raypaths. Examples show that the method can mitigate amplitude attenuation caused by transmission anomalies and should be considered as one of the processes for amplitude preserving processing that is important for AVO analysis when transmission anomalies are present.

Restoring velocity variations below seafloor with complex topography by geomechanical modeling.

Summary Seismic images below a rugose seafloor are always distorted due to (1) lateral velocity contrast between the water and sediments and (2) velocity variations within shallow sediments caused by the variable water depth. Normally, costly iterative tomographic modeling is employed to build the shallow part of velocity model in the same way as it is applied for the deeper intervals. Velocity models for the shallow sediments affected by variable water depth can be more efficiently restored using geomechanical approach. This approach significantly improves velocity models and the corresponding seismic images, thus reduces turnaround time for depth processing projects.

Making anisotropy in PSDM depth-velocity models conformal with geology and velocity. Case study from the NW Australian shelf.

We propose and successfully apply on a real 3D seismic dataset from the North-West Australian shelf a new technique that uses well information to correlate anisotropy with velocity for localized lithology driven anomalies. We assume that localized variations in both velocity and anisotropy are caused by changes in the lithology (shale vs carbonate vs sandstone etc). This should result in some correlation between anisotropy anomalies and velocity anomalies. We use well information to establish such a correlation. Our technique produces geology conformal PSDM anisotropic velocity models and reduces depth misties.

Multi-Azimuth Seismic Data Imaging in the Presence of Orthorhombic Anisotropy

SUMMARY The presence of orthorhombic anisotropy can severely affect the imaging of multi-and wide-azimuth data. Analysis of multi-azimuth (MAZ) data often reveals noticeable fluctuations in moveout between different acquisition directions, preventing constructive summation of MAZ images. Vertical transverse isotropy (VTI) effects can also cause well misties and higher order moveout. We have developed an approach for imaging in the presence of orthorhombic anisotropy. Both synthetic and real data from offshore Australia show that our approach can take into account the co-existing HTI/VTI effects of orthorhombic anisotropic media, reduce the structural discrepancies between seismic images built for different azimuths, producing a constructive summation of MAZ dataset, resolving well misties, and delivering a step-change in the final seismic image quality.

3D tomographic amplitude inversion for compensating amplitude attenuation

Introduction Spatial variations in the transmission properties of the overburden cause seismic amplitude distortions on deeper horizons and hence pose problems to the AVO analysis. One of the common causes of these transmission anomalies is gas within shallow sediment. This induces anomalous amplitude decay in zones beneath the gas anomaly, often making the identification and interpretation of deeper reflectors difficult. This in turn affects the ability to accurately predict reservoir properties. Thus, there is a need to compensate the amplitude loss caused by this kind of transmission anomalies.

3‐D tomographic amplitude inversion for compensating amplitude attenuation due to shallow gas

A tomographic inversion approach using prestack depth migrated common image gathers is utilized to compensate reflection data for amplitude loss caused by transmission anomalies, such as shallow gas, in the overburden. The approach has the advantage of estimating transmission losses from anywhere within the overburden using the actual seismic raypaths. Examples show that the method can mitigate amplitude attenuation caused by transmission anomalies and should be considered as one of the processes for amplitude preserving processing that is important for AVO analysis when transmission anomalies are present.