arXiv subjects

Ayiaz Kaderali

Publications and source records attributed to Ayiaz Kaderali.

6 recordsLinked to original sources

arXiv2020

Selecting velocity models using Bayesian Information Criterion

We present a strategy for selecting the values of elasticity parameters by comparing walk-away vertical seismic profiling data with a multilayered model in the context of Bayesian Information Criterion. We consider $P$-wave traveltimes and assume elliptical velocity dependence. The Bayesian Information Criterion approach requires two steps of optimization. In the first step, we find the signal trajectory and, in the second step, we find media parameters by minimizing the misfit between the model and data.

physics.geo-ph↗

arXiv2020

On effects of inhomogeneity on anisotropy in Backus average

In general, the Backus average of an inhomogeneous stack of isotropic layers is a transversely isotropic medium. Herein, we examine a relation between this inhomogeneity and the strength of resulting anisotropy, and show that, in general, they are proportional to one another. There is an important case, however, in which the Backus average of isotropic layers results in an isotropic -- as opposed to a transversely isotropic -- medium. We show that it is a consequence of the same rigidity of layers, regardless of their compressibility. Thus, in general, the strength of anisotropy of the Backus average increases with the degree of inhomogeneity among layers, except for the case in which all layers exhibit the same rigidity.

physics.geo-ph↗

arXiv2020

On anisotropy and inhomogeneity parameter estimation using traveltimes

We consider an anisotropic inhomogeneous model to simulate measured vertical-seismic-profile traveltimes. In this model, we assume that velocity increases linearly with depth and anisotropy is the result of elliptical velocity dependence. Using a series of sources in a line to a single receiver, we minimize the least-squares residual between measured and modelled traveltimes to estimate the anisotropy and inhomogeneity parameters of the subsurface. To verify the approach, we construct synthetic traveltime data for a one- and two-layer model and perform the traveltime inversion. We justify the convergence-ensuring specifications and model-parameter restrictions that are intrinsic to the parameter estimation. To determine the approach's practicality, we assess the reliability of results under the influence of noise. From this assessment, we discern the noise threshold for both models and determine that the noise restriction is severely restrictive for increasing model complexity. We estimate parameters, using the same methodology, for a real-data case and conclude with a discussion of results.

physics.geo-ph↗

arXiv2020

On possible issues of Backus average

In this paper, we continue the study of Bos et al. (2018) regarding statistical and numerical considerations of the Backus (1962) product approximation. While the approximation is typically quite good for seismological scenarios, Bos et al. (2018) demonstrate a physical scenario that could, in spite of the stability conditions for isotropic media, lead to an issue within the Backus average. Using the Preliminary Reference Earth Model of Dziewoński and Anderson (1981) and a case study in the upper oceanic crust, we investigate whether this issue is likely to occur in the context of seismology.

physics.geo-ph↗

arXiv2020

On inhomogeneity parameters for Backus average

In this paper, we discuss five parameters that indicate the inhomogeneity of a stack of parallel isotropic layers. We show that, in certain situations, they provide further insight into the intrinsic inhomogeneity of a Backus medium, as compared to the Thomsen parameters. Additionally, we show that the Backus average of isotropic layers is isotropic if and only if $γ=0$. This is in contrast to parameters $δ$ and $ε$, whose zero values do not imply isotropy.

physics.geo-ph↗

arXiv2020

On Backus average for oblique incidence

We postulate that validity of the Backus (1962) average, whose weights are layer thicknesses, is limited to waves whose incidence is nearly vertical. The accuracy of this average decreases with the increase of the source-receiver offset. However, if the weighting is adjusted by the distance travelled by a signal in each layer, such a modified average results in accurate predictions of traveltimes through these layers.

physics.geo-ph↗