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Perfectly matched layers simulation in 2-D VTI media

Implementing computational boundary conditions, such as perfectly matched layers PML does have advantages for forwarding modeling of the earth's crust. The mathematical modeling of many physical problems encountered in industrial applications often leads to a system of linear partial differential equations-PDEs. It considerably improves the visualization of seismic events relevant to oil and gas exploration. In this work, we present an efficient numerical scheme for hyperbolic partial differential equations where a computational technique takes care of reflections at the borders domain using a linear 2-D elastic-wave system of decoupled equations with PML-type boundary conditions. The key idea is to introduce a layer that absorbs the reflections from the borders improving images visualization. Anisotropy has been reported to occur in the Earth's three main layers. The crust, the mantle, and the core; but this implementation refers to the case of a vertical transversal anisotropic medium VTI in the crust-layer. Images and screen-shots of the longitudinal P impulse-response and the SV transverse impulse-response are obtained at different times. This computational method enables us to achieve images for the P and SV response-impulses, and to obtain high quality synthetic seismograms for the PP and PS reflection events in a 2-D VTI two-layer model.


Introduction
The wave equation plays an important role in seismic oil and gas exploration since it allows modeling the earth structure. In the past decades, seismic anisotropy has been gaining attention from academic and industry, in part thanks to advances in anisotropy parameter estimation 1,2 A material is said to be anisotropic if the value of one or more of its properties varies with direction, consequently anisotropy consideration improves the subsurface imaging. Seismic Anisotropy can be defined as the dependence of seismic velocity on a direction or upon an angle. Anisotropy is described by a 4 th order elasticity tensor with 21 independent components for the lowest-symmetry case 3-6 . However in practice, observational studies are unable to distinguish all 21 elements and anisotropy considerations are usually simplified. For seismic exploration, the most complicated case occurs in fractured monoclinic media, with 9 elastic constants 7 . In general, two or more sets of vertical non-corrugated and not perpendicular fractures produce an effective monoclinic medium with a horizontal symmetry plane. The second most important application occurs for the orthorhombic model. The orthorhombic model describes a layered medium fracture in two orthogonal directions 9,10 , However in the simplest form that seismic exploration uses it, there are two main kinds of transverse anisotropy TI. One is called horizontal transverse isotropy-HTI which is a common model in shear-wave studies of fractured reservoirs that describes a system of parallel vertical penny-shaped cracks embedded in an isotropic host rock 8 , henceforth this kind of anisotropy is associated with cracks and fractures. The other one is called vertical transversal anisotropy-VTI 11 and it is associated with layering and shales. Sometimes people call it vertical polar anisotropy.
In the beginning, forward modeling was done by simulating the scalar P wave field obtained from the acoustic wave equation. However, the earth-crust is elastic and anisotropic 4,7,10 and all propagation modes should be considered in order to observe anisotropy effects. Forward modeling and parameter estimation are almost the most fundamental to all other anisotropy applications in oil exploration 2 . In part, this can be done by numerically solving the hyperbolic elastic wave equation WE. The vector and tensor fields which represent the WE improves the information about the 3D earth-crust geology. This translates into a better subsurface imaging and it is of extreme importance for the oil and gas seismic exploration.
The most common type of anisotropy that occurs in the earth's crust is the vertical transverse anisotropy VTI which is observed in sedimentary rocks 12 . To achieve 3D seismic modeling with VTI anisotropy requires the knowledge of five elastic constants. However in two dimensions only four constants are involved, namely C 11 , C 33 , C 13 and C 44 (in this work we use Voigt notation for the elastic constants) 11,13 . Henceforth shear wave splitting is not considered in a 2-D modeling because of the lack of the elastic constant C 66 . Abundant geological evidence of shales shows the importance of VTI models in seismic reservoir characterization 1,2 .
In this work, we implement an algorithm using a linear finite difference decoupled PDEs in terms of the velocities arXiv:1901.00255v1 [physics.geo-ph] 2 Jan 2019 and stress tensor components. To implement such a model we use central-difference second order approximation for space variables and time partial derivatives. Henceforth we use the finite-difference time-domain FDTD method 17 . In addition, we use staggered stencils for computational storage 14 . Staggered cells grant the calculation of different physical quantities at different mesh-grid points (see Figure 2), reducing the computational cost storage. Additionally, with this algorithm we can introduce different source-receiver configurations, making it ideal for simulation in oil and gas seismic modeling.
To implement a FDTD solution of the WE, a computational frame must first be established. The computational domain is the physical region over which the simulation is performed. To achieve the condition of non-reflective borders, we use the artificial technique of perfectly coupled layers PML 15,16 . Consequently, the effect due to the reflection of the waves at the borders of the domain is automatically reduced.
PML implementation in seismic exploration requires a reformulation of the linear WE to eliminate the unwanted reflections. Thus, this work is structured in the following way: Section I introduces the topic of seismic anisotropy and its relevant application in R&D Oil industry. Section II briefly describes the PDEs for a 2-D medium with VTI shale anisotropy, and the theoretical implementation of the PML obtaining a decoupled EWs (elastic equation system) including the new border computational conditions. Section III describes the numerical implementation that will allow CPU time reduction. Subsequently, we find the correct decoupled finite-difference scheme in terms of the staggered-stencil mesh taking into consideration the PML (see Figure 2). Finally, in section IV the outcome shows the applicability of this technique. We were able to achieve very neat and sharp subsurface images without reflection events at the edges of the 2-D earth-crust two dip layer shale model.

2-D linear VTI media
As we stated before, the formulation of a linear WEs in terms of temporal derivatives of the velocity and the stress fields can be used to propagate waves. The FDTD technique reproduces the fields forward in time-domain. This is called forward modeling. This procedure is designed through a 3D staggered finite-difference grid 13 . This formulation is widely used in the literature for seismology purposes 14 . In 2-D VTI anisotropy, it involves the transformation of five coupled first-order PDEs, namely the two general equations of motion for the vector velocity (v x , v z ) field which are: where ρ is a constant density, f x and f z are external forces driven by the source. Since we use the FDTD time-domain technique, a seismic pulse is used as the source, then the response of the system over a wide range of frequencies can be obtained with a single simulation. And the three equations for the stress σ ij second order tensor field where only four elastic constants are used as the input variables for the WEs: the longitudinal P and the transverse SV modes given by the previous five PDEs are coupled 13 .

2-D VTI decoupled PDEs with PML
In this subsection, the PML technique is briefly explained and applied to the 2-D modeling. In order to make the finite difference method more stable and convergent, we follow the arguments presented previously 15,16 for the stability and the dispersion conditions in a 2-D EWs (we should mention that a 3D anisotropy implementation has not been achieved with this technique, due to failures in the stability and dispersion conditions when considering anisotropy 13 ). Even in works dedicated to seismology, the use of this technique helps to improve 2-D imaging resolution at larger scales. The seismological domains have borders as it happens in seismic exploration. PML in this case, reduces the reflections at the borders in a significant amount compared with the use of traditional mathematical boundary conditions .
The PML mathematical implementation consist in the introduction of a modified coordinate system, where the expansion coefficient is a complex number with an evanescent imaginary part. This generalization is achieved through the following substitution where p(x) and p(z) are the coefficients of the PML. They are given by the following expressions 16 p(x) = p 0 (x/L) N , and p(z) = p 0 (z/L) N , where L is the thickness of the perfectly coupled layer, N the size of the problem and p 0 has an approximate value of 341.9 When the above coordinate system is replaced in the equations for the VTI medium of the previous section, the system becomes a new linear decoupled PDE-EWs. Because of that replacement appears new equations for v 1 x and v 2 x new decoupled velocities: For the decoupled v 1 z and v 2 z velocities the equations are: These decoupled velocities 16 are related to the coupled ones by the relation v The stress tensor field is mathematically treated in the same way, obtaining a decoupled system for the new stress equations: The new decoupled σ 1 xx and σ 2 xx stress equations are: and the new equations for the σ 1 zz and σ 2 zz decoupled stress tensors components equal to: The decoupled shear σ 1 xz and σ 2 xz stress tensor components follow the equations: where now σ xx = σ 1 xx + σ 2 xx , σ zz = σ 1 zz + σ 2 zz , and σ xz = σ 1 xz + σ 2 xz . These 5 linear PDE equations will be used for a forward modeling in the following section.

Computational spacetime implementation
The computational model that we propose in this simulation is divided into a mesh of N x × N z points. Fig. (1) the finite-difference scheme shows the computational edge domain where the PML are applied according to the two-layer dip model. ∆x and ∆z are defined as the distances between the points in such a way that x = n x ∆ x y z = n z ∆z with n x = 1...N x y n z = 1...N z . For the step in time ∆t we have that t = n∆t, being n the step time (in Fig. 2) the stencil time is not showed 13 .
The five variables v 1 x , v 1 z , σ 1 xx , σ 1 zz , and σ 1 xz are discretize into a two dimensional staggered grid mesh, see Fig. (2) for a better explanation, where the velocity components are stored in both stencils, the normal components of the stress tensors are stored in one of the stencils, while the shear stress components are located at another stencil. However, the major difference of a staggered-grid scheme is that the velocity and stress components are not known at the same grid point, as it can be seen from Fig. (2), henceforth we use a previous scheme proposal with little modifications 13 where different stencils are used for normal and shear stress field computations. Following Fig. (2) in the space domain v x is calculated at the points (i ± 1/2, k), v z is calculated at the points (i, k ± 1/2), the normal stress tensor components σ xx and σ zz are calculated at the points (i, k) and finally the shear stress σ xz is calculated at the points (i ± 1/2, k ± 1/2). The density ρ(x, y, z) is considered a constant density ρ 0 (see the last line of Table I for its correspondent values and units) Physically it means that the VTI model is homogeneous and that is does not take into consideration earth-crust heterogeneities.
An explosive source is simulated using the amplitude of a Ricker wavelet with a peak frequency f 0 ∼ 30 Hertz. Amplitudes are added to the velocities v 1 x and v 1 z at each n time-step. The stability limit of the discrete system is given by the following condition: where V p account s for the horizontal longitudinal speed 13 . The dispersion condition for the discrete system is given by the inequality: where V s is the transverse wave speed. However some authors 18 showed that the stability of the classical PML model depends upon the physical properties of the anisotropic medium and that it can be intrinsically unstable. However the elastic constants in table where previously proven 13 to meet Becache stability criteria. This algorithm could be extended to 3-D situations by further studying the stability and dispersion conditions. Henceforward we proceed to obtain the discretization of the velocity fields as follows for the decoupled finitedifference system: the v 1 x and the v 2 x components are computed at time-steps points t n+1 and t n (the stencil-time is not shown here 13 ) with the following finite-difference equations: The v 1 z and v 2 z components are calculated at the times-step t n+1 and t n points with the help of equations: For the stress fields the discretization is as follows: First, the decoupled equations for the normal stress tensor components σ 1 xx and σ 2 xx are computed at the mesh step-time points t n±1/2 using the equations: and Second, the decoupled equation for the normal stress components σ 1 zz and σ 2 zz are computed at the time points t n±1/2 by means of the following expressions: and Third, the decoupled equations for the shear σ 1 xz and σ 2 xz components are computed at time points t n±1/2 by means of: and The elastic constants used for the two-layer model are given according to table . These values correspond to different types of VTI media and they are able to accomplish the stability condition using a PML computer domain 13,16,18 .

Analysis and Results
The present paper describes a new space-time 2-D methodology with time-step and space-step control to solve 2-D VTI linear decoupled EWs with PML conditions using staggered-grids Fig. 1 and Fig. 2. Henceforth, we develop a computational model for an elastic wave propagation simulation for 2-D VTI anisotropic media, by combining a FDTD method on staggered grids with a PML boundary condition. For that, a High-Performance Linux application in C (over 1,000 lines of code) using Make, GDB and Valgrinds memcheck, to generate and visualize 2-D responseimpulses and synthetic seismograms was developed.
We establish that visualization of the impulse-response of the P and SV modes are improved by the PML. As a consequence, unwanted reflections from the borders in Fig. 3 are totally eliminated according to Virieux 14 .
In Fig. 3 simulations were performed with an explosive source (Ricket-wavelength) centered in the middle of the model to eliminate reflections from the edges. In Fig. 3 time screen-shots for t = 0.2, 0.3, and 0.5 sec. are presented. From those snapshots, we conclude that the PML boundary conditions absorb the reflections from the border, implying an exceptional numerical performance of the computational PML technique. Another conclusion is that the P 2-D response-impulse behave as a simpler quasi-elliptical way in Fig. 3, while the SV response-impulse presents triplications. Moreover, the effects of the VTI elastic constants on SV response-impulses affects the direction of propagation. We observe from the snapshots in Fig. 3 how the value of the elastic constant C 13 affects the direction of the SV. Triplications in the SV wavefront are modeled. However, one question is not solve yet in this work, the asymmetric behavior of the SV impulse-response in Fig. 3 with respect to the vertical axis.
Subsequently, the explosive source is relocated to the upper central part of the 2-D model sketched in Fig. 4 A. On the surface, fifty receivers are uniformly distributed on both sides of the source. We see how the velocity fields cancel out at the edges of the model in Fig. 4 A. Then we obtain the synthetic seismograms for the two layers model, each of them has different elastic constants values. The synthetic seismograms of the vertical component V z (Fig. 3  B) and the horizontal component V x (Fig. 3 C) are shown with the primary reflections events due to the dip layer.
Henceforth, it remarkably shows the effectiveness of the PML using the decoupled linear system of equations (1)-(10) for 2-D VTI media simulation in Oil and Gas R&D. In 4B and 4C screen-shops for the PP and the PSV reflection events due to the dip are presented.