时移音频大地电磁监测数值模拟研究

音频大地电磁法(Audio Magnetotelluric Method,简称AMT)是一种以天然电磁场作为场源的电磁探测方法,具有频率范围宽、成本低以及适合长时间监测等特点,使其在储层流体时移监测方面相对于其他有源电磁方法有一定的优势.针对沉积地区海水入侵以及CO2地质封存动态监测等问题,通过数值模拟从理论上进行时移音频大地电磁方法响应特征的研究.为此,根据不同案例背景下监测目标体的结构特性建立相应的地电模型,采用TM极化模式下二维音频大地电磁的有限元正演和OCCAM反演开展时移监测模拟,分析两种不同案例中流体介质的响应特征及地下电阻率反演模型.另外以海水入侵为例,通过在响应数据中加入白噪声,对不同信噪比下的响应数据进行反演计算,对比不同信噪比下白噪声对时移监测效果的影响.模拟结果表明,时移音频大地电磁法从理论上可以有效反映出地下介质电性特征随时间的动态变化过程,也能够有效监测地下流体的扩散过程.

Numerical simulation of time-lapse audio magnetotelluric monitoring

Audio frequecy magnetotelluric method is an electro-magnetic detection method with natural electromagnetic field as the field source,which has the characteristics of wide frequency range,low cost and suitable for long time monitoring,making it have certain advantages over other active electromagnetic methods in reservoir fluid time-lapse monitoring.Seawater intrusion and excessive CO2 emissions have brought harm to our living environment,so we must consider how to solve them.To solve the problems of seawater intrusion and dynamic monitoring of CO2 geologic storage in sedimentary areas,the time-lapse audio frequecy magnetotelluric method is proposed to be used to study the response characteristics theoretically through numerical simulation.For this purpose,the corresponding geoelectric model was established based on the structural characteristics of the monitoring target under different case backgrounds,and the time-lapse monitoring simulation was carried out by finite element forward calculation of two-dimensional audio frequecy magnetotelluric and OCCAM inversion under the TM polarization mode.The response characteristics of fluid media in two different cases were analyzed and the underground inveson model was obtained.The strategy of parallel computation was added in the forward calculation,which lays a foundation for the following rapid inversion.Gaussian white noise is a common type of noise and is often used in noise analysis.So taking seawater intrusion as an example,white noise was added to the response data to compare the effect of white noise with different SNR on time-lapse monitoring effect.Meanwhile,inversion calculation of apparent resistivity with different SNR was carried out to obtain a better result.The simulation results show that time-lapse audio frequecy magnetotelluric method can effectively reflect the dynamic change process of the electrical property of underground media with time,and monitor the diffusion process of underground fluid.