多极声源在裸眼井外的纵横波辐射特性

声波远探测方法已逐渐成为石油测井的一门重要应用技术,声源向井外地层中的辐射特性是远探测声场的主控因数。推导了单极、偶极和四极子声源的远场位移渐近解与辐射指向性公式,模拟分析了低频和常规测井频率下多极子声源在井外地层中的辐射图样,优选出不同声源辐射到地层中的优势波类型,并对比了不同频率下优势波指向性的变化规律。结果表明:单极辐射P波和偶极辐射SH横波在各自的辐射声场中占据了主导地位,它们对应的最佳激发频段分别在10 k Hz和4 k Hz左右,而在四极子声源的辐射声场中,P波和SH横波的指向性相近,均优于SV横波,且两者在7 k Hz左右达到最佳,为多极子声波远探测技术的应用提供了理论支持。

Radiation characteristics of a multipole acoustic source in an open borehole

Single-well imaging technique has become an important application in well logging technology, and the radiation characteristics of a borehole acoustic source are important for imaging reflectors outside the borehole. By using steepest descent method, the asymptotic solution of the elastic wave fields and far field radiation directivities of monopole, dipole and quadrupole sources are derived, from which the multipole source radiation patterns are evaluated under the low and conventional frequencies of well logging. The dominant radiation wave types of various acoustic sources are selected, and their directivity change rules with frequency are analyzed. Our results demonstrate that P and SH-waves are respectively dominant in the radiation fields of the monopole and dipole sources, and their corresponding optimal excitation frequencies are around 10 kHz and 4 kHz. For the quadrupole source, the directivities of P and SH waves are similar, and they are both superior to SV shear wave. The optimal excitation frequency of quadrupole P and SH waves is around 7 kHz. Our theoretical results are useful for the application of borehole acoustic reflection imaging technology.