井孔中多极源在分层介质的声场模拟方法*

水平分层地层是常见的地质模型,为了获得地下水平分层介质中声波测井的响应并了解声波在其中的传播规律,需要对该模型进行声场数值模拟,通常采用基于时域有限差分的数值模拟方法。然而对于地下数百米井段的声场模拟,如何最大化地减小数值模拟的计算区域,提高计算速度和节省内存,是必须考虑的策略问题。本文中,我们提出了动态计算区域的仿真策略,给出了计算区域选取的优化方案,并通过算例验证策略的有效性。假设发射器分布在阵列接收器下方,模拟结果表明,当声源距离下方最近的分层边界超过2米时,位于仪器下方的所有分层介质可以在计算中忽略不计。对于位于阵列接收器上方的分层边界,在选取计算区域时,也同样采用类似的策略。利用这样的策略,我们连续快速模拟了多层介质中单极子和偶极子声波测井响应,其中纵波和横波速度提取结果与输入的连续分层介质参数吻合。应用这种策略,可开发出数百米甚至数千米的井孔声场连续快速仿真的数值模拟软件。

Simulations of multipole acoustic waves in a borehole surrounded by layered media

Horizontal layered formation is a common geological model, to obtain the response of acoustic logging in this media, we need to use the numerical simulation of , a numerical simulation method based on finite difference time domain is usually adopted. However, it is necessary to consider the strategy to solve the simulation of the sound field of the hundreds of meters underground, how to reduce the calculation area of the numerical simulation and improve the calculation speed and save the memory. In this paper, we propose a simulation method for dynamic computing region, and give the optimization scheme of the region selection, and the effectiveness of the method is verified by numerical examples. Assuming the transmitter is located beneath the receiver arrays, it is shown that when the source is more than 2.0 meters above from the nearest bed boundary, all layers beneath the logging tool can usually be ignored in the computation. The layers above the last receiver should be treated similarly. Two numerical examples, for monopole and dipole logging respectively, are presented to validate the developed program. We can study the continuous in-situ responses to a multipole source in a formation containing various layers and hence investigate the detection properties of advanced logging tools using this software.