偶极随钻声波测井声压和径向位移响应的差异*

声波测井仪接收到的电信号通常是多个压电片响应的叠加,它主要是由声压还是径向位移响应转化而来,或是两种响应兼有目前未有定论。该文通过实轴积分法和复变函数法计算并对比分析随钻声波测井的声压和径向位移场,发现这两种响应特性有着显著的差异。首先,软地层的偶极随钻测井时,声压信号包含钻铤波和舒尔特波两个波群,而径向位移信号仅有钻铤波波群;其次,单极声源情况下,声压和径向位移信号的钻铤波能量分别集中在钻铤内、外壁,而偶极情况恰好相反,可见,钻铤按照单极情况的分析结果进行刻槽后,高频时的拖尾现象会影响偶极信号中舒尔特波对横波速度的反演。因此,阐明两类信号的差异对横波速度的反演和钻铤波的压制都具有重要意义。

Difference in pressure and radial displacement response of dipole acoustic logging while drilling

The transmitting and receiving piezoelectric transducers of acoustic logging instruments are respectively used to convert electrical signals into sound source signals required for excitation and acoustic responses with formation information into electrical signals. Whether the electrical signals are converted from acoustic pressure signals or displacement signals has not been determined yet. In this paper, the acoustic pressure and radial displacement fields of LWD acoustic logging are calculated and analyzed by means of real axis integral method and complex variable function method. The results shows that when in soft formation dipole logging while drilling the acoustic pressure signal consists of two wave groups, drill collar wave and Scholte wave, while the displacement signal consists of only collar wave group without apparent Scholte wave . On the other hand, when the monopole acoustic source case, in the acoustic pressure signal, the collar wave energy is concentrated in the outer wall of collar and in the displacement signal the collar wave energy is concentrated in the inner wall. Interestingly, in the case of dipole sound source, drill collar wave energy distribution is just opposite to that of monopole. Besides, when grooving according to the results of the monopole case, the phenomenon of tail dragging at high frequency will affect the inversion of shear wave by Scholte wave in the dipole source. So it is of great significance to clarify the properties of the two kinds of signals for shear wave inversion and drill collar wave suppression.