套管-水泥界面微间隙厚度的低频超声反演方法

研究了应用低频超声垂直入射反射波实现套管-水泥界面流体微间隙薄层厚度的反演方法。应用灵敏度函数比较分析了分层系统反射谱各阶谐振频率处的相位跳变量较之各阶谐振频率作为反演特征量的优势。通过目标函数分析,选取分层系统3~10阶谐振频率处相位跳变量作为特征量,利用BP神经网络实验获得0.1~1.0 mm内不同间隙层厚度的反演结果。反演结果与真实值的平均相对误差为4.6%,最小反演厚度仅为波长的18%。所选取的相位特征量也可用于其它层状高声阻抗介质下的薄层厚度反演。 

A thickness determination method of thin layer between casing and cement using low frequency ultrasound

A thickness determination method of fluid thin layer between casing and cement is put forward by using pulse-echo low frequency ultrasonic waves emitting into a cased well vertically. Sensitivity function shows that comparing with the resonant frequencies of the reflection model the phase jump values at these frequencies are more suitable for inversion. The 3rd~10th phase jump values are selected to be the inversion characteristics by using error function analysis. A back propagation neural network is applied to inverse different thin layer thicknesses which are between 0.1 mm and 1 mm. The average relative error between the inversion value and the actual value is 4.6% and the minimum inversion thickness is 18% of the wavelength. The inversion characteristics mentioned can also be used in thickness determination of thin layer beneath layered media with high acoustic impedance.