裸眼井中软地层多极子源激发的弹性波的传播

利用全波场和分波场算法对裸眼井软地层中由单极子源和多极子源（偶极子源和四极子源）激发的弹性波进行了数值研究,给出了声波全波列理论波形、纯纵波和纯横渡理论波形及纵横波的频率传输特性曲线.结果表明,纯纵波和纯横波是无频散几何衰减波;除了在v_s>（1-ρ₁/ρ₂）^1/2v₁的井眼中由单极子源激发的横波无传播特征频率外,纵横波都具有各自的传播特征频率,纵波的传播特征频率有一系列离散值,横波的传播特征频率只有一个且等于非对称模式波的截止频率;多极子源激发的纵波传播特征频率的最小值大于横波的传播特征频率,随着极子数增多,二者的特征频率值皆增高,差别也越大,因此,当声源发射主频较低,接近横波传播特征频率而远离纵波传播特征频率时,低频多极子源将压制纵波并激发出较强的横波,分析结果还表明,通常所说的低频多极子源激发出的横波波群实际上是纯横波与截止频率附近的非对称模式波叠加的结果,在其它条件不变时,二老对横波波群相对贡献的大小取决于声源激发主频的高低及其带宽的大小.

PROPAGATION OF ELASTIC WAVES ALONG A BOREHOLE EXCITED BY ACOUSTIC MULTIPOLE SOURCES IN SOFT FORMATIONS

Propagation of elastic waves along a borehole excited by acoustic multipole sources in soft formation was studied by using "real axis integration" and "branch cut integration". The full wave trains, the pure P and S waves and their excitation spactra have been given. Results obtained with the two numerical methods were compared with each other. It has been shown that: Pure P and S waves are undi-spersed and attenuated, and possess characteristic propagating frequencies respectively, except for the 5 wave under the conditions of the P waves have a series of discrete characteristic frequencies, while the S waves have only one; the minimum of such frequencies of P waves is larger than that of S waves; when the pole number n increases, the two kinds of characteristic frequencies will become higher, and the difference between them will also become larger. Therefore, if the center frequency of acoustic sources is in the vicinity of S wave characteristic frequencies and far away from the P wave characteristic frequencies, the multipole sources with lower excitation frequencies can efficiently suppress P waves and excite strong S waves. The numerical results have also shown that the so called shear wave trains in general shear wave logs are composed of pure S waves and asymmetric modes at cut off frequencies, and their contributions to the full wave trains depond on the center frequencies of multipole sources and their frequency bands.