随钻声波测井仪器声系的最优化设计理论和方法研究

随钻测量技术近来发展很快，已成为深海钻探的必备技术之一。迄今为止所有随钻声波隔声技术都是采用在发射和接收换能器间刻槽的方法来阻隔沿钻铤传播的波。这种刻槽方式隔声体严重损害钻铤强度，不利于随钻声波测井技术的推广和使用，本项目提出一种简便、实用而有效的，不需要在钻铤上刻槽进行隔声的方法。根据拉伸波在钻铤中传播的固有阻带理论，对长度大于波长的不同横截面积（即内、外径）的钻铤加以组合使得有效阻带比单个钻铤的阻带大大拓宽。针对这一变径钻铤组合模型，理论推导随钻声波测井的多模式耦合理论，计算透射系数与频率的关系曲线，分析钻铤波的隔声阻带特征。在拟设计隔声频率范围内用透射系数构造目标函数，用模拟退火法对多个隔声体参数（钻铤变径次数、内外径组合、变径长度等）进行约束反演，用最优化方法设计一种长度大于波长的变径钻铤组合隔声体结构，在最佳阻带上进行随钻声波测井，就能很大地压制钻铤波，从而有效地测量地层信号。

Logging while drilling tecnology has undergone a rapid development recently, becoming a must have technology in deep-water drilling. A key technology for logging while drilling (LWD) acoustic measurements is the design of an acoustic isolator to suppress tool waves propagating along the drill collar, such that acoustic signals from earth formations can be effectively measured under LWD conditions. Up to now, the LWD acoustic isolation is achieved by periodically cutting grooves along the drill collar between acoustic transmitter and receivers. Such a technique, although it is effective, reduces the mechanical strength of the drill collar and adds cost to the manufacturing and maintenance of the LWD tool, hindering the application of the LWD acoustic technology. We propose to develop an LWD acoustic technology that does not use the groove-cutting design. We utilize the inherent frequency stopband for extensional wave propagation along a cylindrical pipe and effectively broaden the stopband by combining drill pipes of different cross-section areas whose lengths are greater than a wavelength but are shorter than the transmitter-to-receiver distance. For this variable cross-section model, we derive the LWD modes wave couple theory, calculate the relation curve between transmission coefficient and frequency, and analyse the stopband characteristic of extensional wave. Inverting the isolator parameters using simulated annealing method, and design the optimized isolation. After propagation through the combined drill collar system, the stopband in the collar extensional wave is significantly widened and the wave amplitude in the stopband is substantially reduced. Making LWD acoustic measurements in this widened stopband allows for recording acoustic signals from the surrounding formation.