井间定向震源正演模拟与特性分析

Forward modeling and characteristic analyzing of directional source in crosshole seismic

为减少管波对采集数据的影响,井间地震采用了定向震源,其能量辐射特性影响接收波场的能量分布与后续成像效果.研究定向震源的正演模拟并分析能量辐射特性,为井间地震成像过程中能量补偿提供基础数据.采用圆形震源阵列法,在近源区内按照质点与震源点连线的方向设置振动函数强度,水平方向能量最强,垂直方向能量最弱,模拟的辐射能量具有随方向改变的特点.实际井间地震能量方向性非常强,因而同时采用吸收衰减层法进一步减弱纵向的地震波能量.吸收衰减层法在震源周围设置具有衰减因子的海绵层,通过不同方向设定不同的衰减因子来控制震源的方向特性.衰减因子采用了渐变函数,保证海绵层边界不产生人为反射干扰.将圆形震源阵列法与吸收衰减层法联合进行双层介质模型和实际速度场模拟,得到的炮记录方向性更加明显.两种方法的有机结合,能够更好地模拟井间定向震源.在圆形震源阵列法可行性验证基础上,分析了阵列半径对接收波场能量的影响,初步确定了模拟定向震源的最佳阵列半径.通过分析方向性参数和井间距对接收波场能量分布的影响,获得了定向震源激发下接收波场的能量分布,为在井间地震偏移成像中进行能量补偿建立了基础.

To reduce the impact of tube wave generated in source borehole on acquisition data,directional source is usually used in cross-hole seismic. The energy radiation characteristics of source influence the received energy distribution of the wave field and the subsequent imaging effect. Hence it is necessary to study forward modeling of directional source and analyzing its energy radiation characteristics. This will provide necessary theoretical basis for energy compensation in imaging later. Circle source array is used to simulate source. Here initial vibrating function in the near source region is defined directly according to the direction of source. The energy is strongest in horizontal and weakest in vertical direction,the energy modeling changes with direction change. Considering the actual data of cross-well seismic has very strong directional characteristic,it is necessary to combine attenuation layer method to realize a further weakening of the vertical energy. Attenuation layer method controls the directional characteristic of source by setting different attenuation factor in different direction in sponge layer which is around the source. A gradient function is added to the attenuation factor to ensure that no artificial interference is produced at the sponge layer boundary. The circle array method and attention layer method are combined to simulate practical directional source,and the directional characteristic of the record is more obvious. The combination of two methods can simulate the cross-well directional source better. The circle array method is then verified through numerical experiment. After carefully analyzing the effect of the array radius on the wave field energy,the best array radius in directional source is defined. Through simulation,analysis of the effect of directional parameters and well spacing on the receiving field energy, the energy distribution of wave field excited by directional source is obtained,which establishes the foundation for the energy compensation in cross-well seismic migration.