Acoustic Reflectivity From Variously Oriented Orthorhombic Media: Analogies to Seismic Responses From a Fractured Anisotropic Crust

Abstract

The variations in the strength of seismic reflections with both angles of incidence and azimuth are being used to constrain the orientations and degrees of mechanical anisotropy of geological formations. This information is often used in turn to infer the directions of fracturing and stress usually assuming simplified geological structures that in many cases are not realistic. In order to further understand this problem, the reflectivities from a set of variously titled samples cut from a single block of a common anisotropic composite material are measured with both angles of incidence and azimuth in the laboratory. Each sample is analogous to a formation in which the fracture sets increasingly dip. The observed responses differ from plane wave theory due to the finite nature of the experiment. However, the deviations are well accounted for by modeling the acoustic reflectivity using its bounded pulse. As expected, the reflectivities vary with the sample tilt, the angle of incidence, and the azimuth at which the measurements are made. However, some of these responses, such as the precritical reflectivities, do not appear to strongly vary with azimuth for a given sample. Critical angle phenomena, too, must be used cautiously as the observed reflectivity peaks do not lie at the exact critical angle. These results suggest that reflectivities could be an important source of information with regard to the spatial orientation of anisotropic layer.

Sensitivity study of the critical angle in elastic orthorhombic media

“…Furthermore, the shift of the critical angle with azimuth is inverted to derive the relevant anisotropy parameters (Landrø & Tsvankin, 2007). Ultrasonic critical‐angle reflectometry has indeed become the industry standard in a wide range of applications such as the study for oriented ORT media (Melehmir & Schmitt, 2017) and used as a function of azimuth in human bones for some composite materials (Antich & Mehta, 1997).…”

An optimized 2D/3D Finite-difference Seismic Wave Propagator Using Rotated Staggered Grid for Complex Elastic Anisotropic Structures

“…Nowadays, the utilization of Thomsen's parameter is common in industry and this approximation has led better quality seismic imaging (Tsvankin et al, 2010). However, as computational capabilities progress there is a growing realization (Assad, 2005;Cheadle et al, 1991;Malehmir & Schmitt, 2017;Schoenberg & Sayers, 1995;Tsvankin, 1997) that Thomsen's approximations are not sufficient to describe more complicated situations of tilted and fractured formations necessitating that elastic wave solvers capable of handling more general, lower symmetry cases of anisotropy.…”

Crack‐Induced Shear‐Wave Orthorhombic Anisotropy: Modeling and Inversion of Shear‐Wave Borehole Measurements

“…A more complicated anisotropic model, the orthorhombic anisotropy (OA, Figure 1(c)), is used to model this situation. However, the presence of two sets of anisotropic systems in the OA model complicates the analysis of seismic amplitude responses originating from fractured shales (Malehmir & Schmitt, 2017; Schoenberg & Helbig, 1997; Tsvankin, 1997) because the OA of the rock cannot be adequately described using VTI or HTI alone. Therefore, finding appropriate parameters to model the OA anisotropy characteristics is the key for solving this problem.…”