中文摘要：

针对随钻地震技术在声波长距离传播导致的衰减和信号混叠等问题，本文提出采用井下发射和接收声波的方式，利用偶极声波实现前方地质情况探测。结合数值模拟结果，分析了偶极辐射声场和反射声场特征，重点讨论了反射波信号与钻头前方反射面之间的关系。结果表明，偶极横波在低倾角反射面的反射强度较大，尤其是SH波具有振幅强、倾角覆盖范围大的特点，可以用于前方地质情况探测。偶极横波还表现出了较高的方位灵敏度，有助于识别前方反射体的方位。

英文摘要：

Seismic while drilling （SWD） is a major method of providing reliable geophysical information ahead of the drill-bit. However, the complicated wave-field caused by the long distance between the seismic source and the sensors makes it difficult to process data. To overcome this difficulty, a new approach for structural investigation ahead of the drill-bit is proposed by introducing a downhole dipole system to emit and receive elastic waves. A 3D finite-difference method is used to simulate the radiated and reflected wave-field and the characteristics of the waves received from the reflectors of different angles are analyzed. The analyses show that the dipole shear wave has high reflection sensitivity especially for reflectors of small dip angles. The SH wave has wider radiation coverage and stronger amplitude than SV wave and therefore, can be utilized for structural investigation ahead of the drill-bit. The shear wave also proves to be useful for azimuth determination due to its sensitivity to reflector azimuth