Technical Abstracts

Azimuthal AVO models (AVAZ) have been used to characterize fracture distributions in HTI media. The main subject of this paper is a method for the stabilization of AVAZ. We present an extension of Whitcombe et al.'s (2004) technique for gradient stabilization in Shuey's 2-term AVO model to the case of Ruger and Tsvankin?s (1997) azimuthal AVO. We also investigate the estimated errors in the AVAZ model parameters with varying HTI isotropy plane direction for a selection of offset-azimuth distributions, including azimuthal sectors and COV classes. We apply the technique to WAZ land data from Algeria.

Assuming HTI anisotropy due to one dominant vertical fracture set, the relationship between azimuthal Fourier coefficients and fracture parameters is described. These azimuthal Fourier coefficients can be calculated by performing a Fourier transform of azimuthal sectored seismic data over a particular range of angle of incidences. Using only the magnitude and phase of the 2ndFourier coefficient for just one angle of incidence it is possible to calculate the anisotropic gradient and symmetry axis and achieve similar results to that of the near offset Rüger equation. If linear slip theory is assumed, it is then possible using multiple Fourier coefficients and angles of incidence to unambiguously estimate the symmetry axis and the more fundamental normal and tangential fracture weakness parameters. In addition, an unbiased estimate of the anisotropic gradient can be determined. This is demonstrated on synthetic and real seismic data.

Azimuthal velocity models for HTI media are extensively used for land seismic exploration in Canada, North Africa and the Middle East. A surface fitting technique honouring all azimuths can invert for an HTI velocity model. When performing the inversion it is important to estimate the degree of confidence in the estimated velocity model. The main subject of this paper is velocity uncertainty estimation. Furthermore, we investigate the estimated errors in the model parameters with varying acquisition direction for various offset-azimuth distributions including azimuthal sectors and COV classes. We apply this technique to WAZ land data from Algeria.

High-density wide azimuth (WAZ) land surface acquisitions have demonstrated superior imaging capabilities. Apart from the traditional poor signal-to-noise ratio of land data we face a new challenge: the necessity of reconciling the kinematics of the various azimuths. In this paper, we present an imaging case history involving WAZ non-linear slope tomography. Using surface information (kinematic invariants), velocity model updates are performed both in depth and time. We chose to start from an initial pre-stack time migrated (PreSTM) dataset. After applying a structurally consistent filtering to improve the S/N ratio on stacked data, we used a dense automated tool for dip picking. In parallel residual move-out (RMO) was computed on all azimuths simultaneously. Our case study demonstrates that WAZ non-linear slope tomography in the depth domain greatly improves the imaging of the structures when compared to the initial PreSTM result. We observe that even if tomography in the time domain significantly enhances imaging, it cannot successfully honour the kinematics of the various azimuths within the constraints of time imaging assumptions. On the contrary, WAZ non-linear slope tomography in the depth domain offers an efficient way to reconcile these kinematics, thus promoting the use of depth imaging when processing high-density WAZ data, even in the context of mild geological complexity.

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 QPSDM 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.

Seismic amplitudes within target intervals are often affected by localized absorption anomalies in the overburden. In the North-West Australian shelf and in many other regions, the majority of such anomalies are caused by gas trapped in shallow sediments. We apply amplitude tomography to take this effect into account in geological settings typical for the North-West Australian shelf. We propose and apply a mixed absorption model that combines frequency-independent absorption with linear Q-compensation and allows a much better fit with the real seismic data than previously used methods.

In recent years, our ability to image subsalt structure has improved significantly with the availability of wide azimuth data, reverse time migration (RTM) and routine use of anisotropic imaging and tomography. In subsalt exploration, derivation of subsalt velocity, especially velocity directly below the base of salt, i.e., salt exit velocity, remains challenging due to complex salt overburden. Anomalous high BOS amplitudes can often be found, which indicate the relatively strong impedance contrast in the salt-subsalt boundary. Reflectivity inversion for dirty salt is extended to derive salt exit velocity and is further combined with high-resolution tomography with high-fidelity RTM 3D angle gathers to invert the subsalt velocity model for better pressure prediction and imaging in subsalt exploration. We demonstrate the methodology with a synthetic and real WAZ data example.