Application of AVAZ inversion algorithm based on exact formulas to a wide-azimuth seismic survey data

Results of applying of nonlinear AVAZ inversion optimization algorithm to data from 3D wide-azimuth seismic survey in the Republic of Serbia are presented. The algorithm is based on exact reflection coefficients formulas for PP reflection from anisotropic medium. We compare it with a conventional algorithm based on Ruger linear approximation of P-wave reflection from a boundary between isotropic and anisotropic (HTI) media. Maps of fracture orientation and anisotropy degree are more detailed in the case of using AVAZ inversion based on exact formulas. The results are in general accordance with the FMI well data, which indicates reliable performance of the algorithm.

Study of the influence of anisotropy parameters on reflection coefficients from a boundary between two azimuthally anisotropic media

The paper considers an algorithm for calculating reflection coefficients from boundary between two HTI media. Analysis of the presence of anisotropy above and below the target boundary, as well as variations in the parameters of HTI media, was done. Interpretation of reflection data from the boundary between two HTI media with neglect of anisotropy above or below potentially leads to significant errors in estimation of symmetry axes directions, and hence fracturing orientation. Overestimation/underestimation of an elastic parameter in the overlying HTI medium could lead to a corresponding overestimation/underestimation of similar parameter in the underlying target layer in the result of AVAZ inversion. Furthermore, among the anisotropy parameters Thomsen parameter γ has most significant influence on the reflection coefficients dependences. Thus, the parameter γ could be used foremost as a result of the AVAZ inversion.

Calculation method and characteristic analysis of dispersion curves of Rayleigh channel waves in transversely isotropic media

Coal seams have beddings and fissures and are typically anisotropic media. Current channel wave theories are mainly based on isotropic media, and few studies exist on the dispersion characteristics of Rayleigh channel waves in anisotropic models, such as transversely isotropic (TI) media. We chose the generalized reflection-transmission coefficient method to solve the dispersion curves of Rayleigh channel waves in TI media. However, it is difficult to solve the associated dispersion equations of Rayleigh channel waves using this method directly. Therefore, we have extended the generalized reflection-transmission coefficient method and determined the improved accuracy through numerical simulation. We analyzed the dispersion characteristics of Rayleigh channel waves of several typical coal seam models in TI media. The results showed that in the three-layer model, the difference in fundamental-mode dispersion curves between vertically transversely isotropic (VTI) media and isotropic media was relatively small; however, the differences in the higher order dispersion curves were slightly larger. The difference in the Airy phase velocity between horizontal transversely isotropic (HTI) and isotropic media was relatively large. When the coefficient of variation in the qP waves ([Formula: see text]) was greater than 0, the fundamental-mode and first-order phase velocity curves of HTI media exhibited an evident intersection at the head end. In the dirt-band-containing coal seam model, within the 350 and 550 Hz band, the high-frequency velocity of fundamental-mode phase velocity curve of isotropy and HTI media was slightly higher than the low-frequency velocity, which is a notable phenomenon.

The effect of geologic structure on fracture characterization using azimuthal VSPs

Azimuthal VSP (AzVSP) surveys have been commonly used for fracture characterization by analyzing the P-to-S converted wave response across fractured zones. Fractured media produce characteristic two-cycle patterns on AzVSP gathers that are disrupted in the presence of complex structures. Aiming to characterize the complexity of AzVSP response in the presence of geologic structure, we derive anisotropy parameters for effective horizontal transverse isotropy (HTI) media, generate AzVSP signatures for the flat-interface case as the baseline, and compare the baseline signatures to AzVSP signatures for the dipping-interface cases. Fracture fill and fracture intensity are varied to capture the effect of intrinsic fracture parameters on AzVSP signatures. We find that dry fractures produce a stronger response compared to fluid-filled fractures for the same fracture density. The structural response on AzVSP signatures is isolated by generating synthetic seismograms across structurally equivalent isotropic models. Transverse energy, resulting from structures, could be misinterpreted as evidence for fracturing. AzVSP signatures for a fractured dipping-interface two-layer model show significant distortion of the fracture response for the 10° dip case as compared to the flat-interface signatures. As a possible solution to the structural problem, we use the arrival azimuths of the transmitted P-to-S event in synthetic signatures generated with the isotropic structural model, to orient the radial and transverse components parallel and perpendicular to the isotropic P-to-S event, respectively. Such structurally consistent orientation negates the effect of structure on azimuthal VSP gathers and uncovers the underlying fracture response. This methodology can be extended to complex overburdens that are structurally well constrained.

Compression waves reflection from the low-velocity azimuthally anisotropic medium: a physical model study

SUMMARY Interest in azimuthal anisotropy of rocks is mainly associated with fractured reservoirs, which may contain hydrocarbon deposits. Cracks in such deposits in most cases have a subvertical orientation, which is caused by the predominance of vertical stresses in rocks over horizontal ones. To determine the azimuthal direction of fractures, one can use, in particular, the dependence of the reflection coefficients of elastic waves on the boundaries with such media on the azimuth. This paper presents the results of physical modelling, demonstrating this dependence. To conduct experiments, we developed a technology for manufacturing models of azimuthally anisotropic (HTI) media with a high degree of anisotropy by 3-D printing. The features of the reflection of compression waves from the boundary of water and low-velocity azimuthally anisotropic media were experimentally investigated on the example of the model HTI medium produced by this method. Experiments have shown that at angles of incidence less than 25°, the reflection coefficients are practically independent from the azimuth. However, at larger angles of incidence, an azimuthal dependence of the reflection coefficients is observed, most pronounced at azimuths from 45° to 75°. The results of measurements in the direction of the isotropy plane agree well with the theoretical reflection coefficients for the boundary of isotropic media.

Radiation pattern analyses for seismic multi-parameter inversion of HTI anisotropic media

In seismic waveform inversion, selecting an optimal multi-parameter group is a key step to derive an accurate subsurface model for characterising hydrocarbon reservoirs. There are three parameterizations for the horizontal transverse isotropic (HTI) media, and each parameterization consists of five parameters. The first parameterization (P-I) consists of two velocities and three anisotropy parameters, the second (P-II) consists of five elastic coefficients and the third (P-III) consists of five velocity parameters. The radiation patterns of these three parameterizations indicate a strong interference among five parameters. An effective inversion strategy is a two-stage scheme that first inverts for the velocities or velocity-related parameters and then inverts for all five parameters simultaneously. The inversion results clearly demonstrate that P-I is the best parameterization for seismic waveform inversion in HTI anisotropic media.