含偏心点声源的随钻测井声场模拟和地层各向异性反演研究

地层横波波速测量以及各向异性评价是目前随钻声波测井应用及方法研究的难题之一.针对这些问题，本文试图从钻铤上的偏心点声源在各向异性地层中的响应出发，寻求和探讨解决方案.我们基于三维有限差分模拟的随钻测井信号，研究了偏心点声源在水平横向各向同性（HTI）慢速地层井孔中激发声波传播规律，研究结果表明：在HTI地层中偏心点声源能够同时激发出快、慢两种波形，并且此两种波形中主要分别包含快、慢弯曲波，由于与钻铤波的耦合作用，其速度总是分别小于地层快、慢横波波速，所以无法直接用于地层横波波速的测量.但根据进一步的灵敏度分析可知当在3 kHz以上时，它们的相速度分别对地层参数水平和垂直剪切模量（c₆₆和c₄₄）最为敏感.基于此特性，本文提出一种基于解析近似解的最小二乘拟合法，能够通过快、慢波频散曲线反演地层横波波速，所测值误差小于3%，并且具有很好的容错率和稳定性；进而由横波速度反演值评价地层各向异性大小.最后，地层快横波偏振方位可以利用垂直于偏心声源方向的偶极采集的信号能量变化曲线来获取.

Simulation of acoustic LWD with an eccentric source and inversion of formation anisotropy

The evaluation of shear wave slowness and anisotropies is a key issue in logging while drilling(LWD). To further address this problem, we attempt to study the response of an eccentric point source in an anisotropic formation. Numerical results by the 3D finite difference method (FDM) show that fast and slow waves will be excited by the eccentric sources in the horizontally transverse isotropic (HTI) formation. But the slowness of them is much larger than those of the real formation because of the influence of the collar, which is why they cannot be directly used to measure the shear slowness or the anisotropy. In order to investigate the characters of the fast and slow waves, we make the sensitivity analysis, and find that the slowness of them is most sensitive to c₆₆ and c₄₄ at high frequencies (f ≥ 3 kHz), respectively. Based on this characteristic, we propose a least squares fitting method based on the analytic method to evaluate the shear slowness and the anisotropy indirectly. Results show the relative errors of the measured slowness is less than 3%, and the fault tolerance is high. Then, the value of the anisotropy is evaluated by the fast and the slow shear slowness. Finally, the polarization direction of transverse shear waves is obtained by the relative energies of waveforms from the dipole acquisition method.