Andreas Rüger

Variation of P-wave reflectivity with offset and azimuth in anisotropic media

P-wave amplitudes may be sensitive even to relatively weak anisotropy of rock mass. Recent results on symmetry‐plane P-wave reflection coefficients in azimuthally anisotropic media are extended to observations at arbitrary azimuth, large incidence angles, and lower symmetry systems. The approximate P-wave reflection coefficient in transversely isotropic media with a horizontal axis of symmetry (HTI) (typical for a system of parallel vertical cracks embedded in an isotropic matrix) shows that the amplitude versus offset (AVO) gradient varies as a function of the squared cosine of the azimuthal angle. This change can be inverted for the symmetry‐plane directions and a combination of the shear‐wave splitting parameter γ and the anisotropy coefficient δ(V). The reflection coefficient study is also extended to media of orthorhombic symmetry that are believed to be more realistic models of fractured reservoirs. The study shows the orthorhombic and HTI reflection coefficients are very similar and the azimuthal v...

P‐wave reflection coefficients for transversely isotropic models with vertical and horizontal axis of symmetry

The study of P‐wave reflection coefficients in anisotropic media is important for amplitude variation with offset (AVO) analysis. While numerical evaluation of the reflection coefficient is straightforward, numerical solutions do not provide analytic insight into the influence of anisotropy on the AVO signature. To overcome this difficulty, I present an improved approximation for P‐wave reflection coefficients at a horizontal boundary in transversely isotropic media with vertical axis of symmetry (VTI media). This solution has the same AVO‐gradient term describing the low‐order angular variation of the reflection coefficient as the equations published previously, but is more accurate for large incidence angles. The refined approximation is then extended to transverse isotropy with a horizontal axis of symmetry (HTI), which is caused typically by a system of vertical cracks. Comparison of the approximate reflection coefficients for P‐waves incident in the two vertical symmetry planes of HTI media indicates...

Influence of structural dip angles on interval velocity analysis

Common scattering-angle and traditional common-offset gathers can be of limited use for interval velocity analysis in regions with complex geologic structures. In the summation process, which occurs when generating each trace in the common-image gather, vital information about structural dip is lost during prestack depth migration. This inadvertently lost data can provide important input to moveout-based velocity-updating algorithms. Maintaining this crucial dip information can improve the quality of the velocity analysis and imaging processes.

Meshing for velocity modeling and ray tracing in complex velocity fields

In seismic processing, velocity fields are commonly represented on finely sampled Cartesian grids. Attractive alternatives are unstructured grids such as meshes composed of triangles or tetrahedra. Meshes provide a space-filling framework that enables editing of velocity models while facilitating numerical tasks such as seismic modeling and inversion. In this paper, we introduce an automated process to generate meshes of subsurface velocity structures for highly resolved velocity fields without providing additional external constraints such as horizons and faults. Our analysis shows that these new meshes can represent both smooth and discontinuous velocity profiles accurately and with less computer memory than grids.

Analytic Insight Into Shear-wave AVO For Fractured Reservoirs

Approximate reflection coefficients in the two vertical symmetry planes of azimuthally anisotropic media explicitly describe the influence of medium parameters on the AVO gradients and the higher-angle terms of the shear-wave reflection response. The anisotropy in the subsurface is hereby characterized by the shear-wave splitting parameter and anisotropy parameters similar to Thornsen’s (1986) coefficients. The important result of this study is that the AVO gradients of shear waves propagating in the vertical symmetry planes of fractured media are sensitive to the shear-wave splitting parameter and to a parameter combination important for time processing in anisotropic media. Analytic insight developed in this study naturally leads to an inversion algorithm for the anisotropy parameters of the fractured medium.

Geo-literature for the young mind (because inquiring minds want to know)

When youthful exuberance discovers the fascinations of nature and science, positive outcomes are assured. Career decisions are often triggered by inspiring events and engaging experiences early in life, long before the study of Maxwells equations and plate tectonics.