2D FINITE-DIFFERENCE SIMULATIONS: EFFECTS OF PATH, SOURCE AND ANELASTIC ANISOTROPY ON MICROSEISMIC WAVEFORMS Hoda Rashedi* and David Eaton Department of Geoscience, University of Calgary, AB, Canada Work Flow: Theory, Methodology and Inputs In this research, microseismic synthetic data produced in order to investigate the effect of moment tensor, velocity model, anisotropy and attenuation on the waveforms. This data generated by 2D acoustic finite-difference modeling program which originally developed by Boyd[3]. These realistic synthetic data provide an effective way to benchmark processing and interpretation algorithms. An excellent example is provided by the Marmousi dataset created by IFP[2] a shown in Figure1. Therefore, realistic suite of synthetic microseismograms for a scenario in which the source location, mechanism and background model are known and the complete wavefield containing multiples, guided waves etc, is accurately simulated. Figure1: Prestack FD dataset created by IFP in 1988 • Understand the effects of moment tensor on the waveforms • Investigate the wave propagation pattern in different medium with or without anisotropy • Study the impact of various degrees of attenuation during propagation on waveform • Produce a suite of synthetic data to be used for benchmarking and calibration SUMMARY Figure2: Sample realistic velocity model, Central Alberta Velocity model. Vs ρ Vp Equation of motion ( , ) , − = Step 1 : Solve it using FINITE DIFFERENCE Step 2 : Assign properties of medium into Stiffness Tensor ISOTROPIC Shear modulus: K bulk modulus: μ VTI (Vertically Transverse Isotropic) Thomsen Parameters: ε δ ϒ Souce: Moment tensors with various focal mechanism Realistic Velocity Model Attenuation (Anelastic Medium) = ω. N: Damping Factor C: Stiffness Tensor Matrix ω: Angular Frequency Q: Quality Factor Matrix ISOTROPIC 13 = 33 33 − 2 55 33 − 2 55 33 55 VTI 12 = 11 11 − 2 66 11 − 2 66 11 66 • • • ε • ϒ • δ 5 independent elements 2 independent elements • • Double Couple Tensile Crack Opening ISOTROPIC ISOTROPIC VTI ISOTROPIC & ANELASTIC VTI & ANELASTIC VTI ISOTROPIC & ANELASTIC VTI & ANELASTIC = 0 0 1 0 0 0 1 0 0 P-Wave Radiation pattern S-Wave Radiation pattern = 1 0 0 0 1 0 0 0 3 S-Wave Radiation pattern P-Wave Radiation pattern CONCLUSION REFERENCES RESULTS Anelastic attenuation refers to amplitude losses due to conversion of elastic strain energy to other types of energy, such as heat. The effects of attenuation are often approximated using a constant, isotropic Q model. On the other hand, anisotropy refers to the dependence of elastic properties on propagation and polarization directions. Here, these phenomena are combined to evaluate the potential effects of attenuation anisotropy for microseismic recordings. Finite-difference simulations are used to investigate the changes on the waveforms. This will provide a valuable data that model the wave propagation in realistic media due to various rock behavior during hydraulic fracturing with different moment tensors. INTRODUCTION SCIENTIFIC OBJECTIVE Case studies of finite-difference simulations involving various source types and positions incorporating varying degrees of anisotropy establishes a tool to study the behaviour of waves. On the other hand, simulations involving attenuation anisotropy are scarce in microseismic monitoring, , hence, synthetic data can provide valuable insights for an interpretation of microseismic wave propagation and waveforms. This approach enables analysis and identification of wave-field elements such as head waves, post-critical reflections and guided waves. Imperfect absorbing boundaries create artifacts that require model dimensions to be extended to ensure that artifacts do not interfere with desired signals. The effects of strong anisotropy are evident. The main goal of this study is to build a series of synthetic data including attenuation anisotropy for both isotropic and vertically transverse isotropic medium and investigate the wave-field elements and the effect of the attenuation to them. [1] Arts, R. J., and P. N. J. Rasolofosaon, 1992: Approximation of velocity and attenuation in general anisotropic rocks: 62 nd Annual International Meeting: SEG, Expanded abstract, 640- 643. [2] Bourgeois, A. M. Bourget, P. Lailly, M. Poulet, P. Ricarte, and R. Versteeg. Marmousi, model and data. Proceedings of the 1990 EAGE workshop on Practical Aspects of Seismic Data Inversion: Eur. Assoc. Expl. Geophys, 5-16, 1991. 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