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Numerical simulation of shear wave attenuation in borehole inserted by a horizontal fracture

Yan Binpeng, Ou Weiming, Huang Xingguo, da Silva Nuno Vieira

Acta Geophysica

2020

Vol. 68, no. 6

1715--1726

The amplitude of shear waves is attenuated when passing through horizontal fractures crossing a borehole. In this study, we investigate the amplitude attenuation of shear waves throughout simulation of full-wave acoustic logging with the fnite-diference method. As the fracture aperture is very small, it needs to be represented in a very fne gird when carrying out fnite-diference simulation. Therefore, the variable-grids fnite-diference method is adopted to avoid over-sampling in the non-fracture regions, yielding substantial savings in computational cost. We demonstrate the accuracy of waveform modeling with the variable-grid fnite-diference by benchmarking against that obtained with the real-axis integrating method. We investigated the efects of several important parameters including fracture aperture, distance from receiver to fracture, borehole radius and extended distance utilizing that benchmarked variable-grid fnite diference code. We determined a good linear relationship between the attenuation coefcient of shear wave amplitude and the fracture aperture. Then, the efects of distance from receiver to fracture, the borehole radius and the extended distance of fracture on shear wave attenuation are also studied. The attenuation coefcient of shear wave becomes smaller with the increasing borehole radius. While, it increases as the distance from receiver to fracture and the extended distance of fracture increase. These efect characteristics are conducive to the use of shear wave to evaluate fractures.

The influence of the frequency‑dependent spherical‑wave effect on the near‑surface attenuation estimation

Ji Yongzhen, Wang Shangxu, Yuan Sanyi, Yan Binpeng

Acta Geophysica

2019

Vol. 67, no. 2

577--587

The knowledge of Q is desirable for improving seismic resolution, facilitating amplitude analysis and seismic interpretation. The most commonly used methods for Q estimation are the frequency-spectrum-based methods. Generally, these methods are based on the plane wave theory assuming that the transmission/reflection loss is frequency independent. This assumption is reasonable in the far-field situation and makes the transmission/reflection coefficient irrelevant with the Q estimation result. However, in the near-surface context, this assumption is invalid because the seismic wave propagates in the form of spherical wave in the real seismic surveys and the spherical-wave transmission/reflection coefficient is frequency dependent. As a result, deviation will exist. In this paper, the influence of the spherical-wave effect on the Q estimation in the near-surface context was proved in both synthetic data and field data for the first time, and it was found that the deviation due to the sphericalwave effect is of order comparable to the intrinsic attenuation. The compensation method based on the forward modeling is then proposed to correct this deviation, and the effectiveness of the proposed method is proved by the reasonable estimated results of both synthetic data and field data example. These results raise caution for the interpretation of the extracted Q in the near-surface context if they do not account for the spherical-wave effect and point to the necessity of incorporating a frequency-dependent term in the frequency-spectrum-based method when applied to the Q estimation in the near surface.