倾斜地层条件下瞬变电磁一维反演的适应性研究

Study on the adaptability of one-dimensional transient electromagnetic inversion in inclined strata

为了研究倾斜地层条件下瞬变电磁一维反演的适应性,以典型的K型和H型模型为基础,分析了倾斜地层条件下瞬变电磁的响应特征。发现K型倾斜地层的瞬变电磁响应在高阻层埋深较浅时早期响应幅值较小,随着高阻层埋深增大幅值逐渐增大,而H型模型的响应变化规律刚好与K型相反。进一步对4组不同倾斜角度的H型和K型模型进行瞬变电磁一维OCCAM反演,结果表明,在地层倾斜角度较大时,不管是H型还是K型模型的反演电阻率扭曲严重,反演结果均不能代表实际地层的电性特征;在地层倾角较小时,K型模型反演的高阻层电阻率随着埋深增大幅值逐渐减弱,而H型模型在地层倾角较小时,一维反演的结果与实际模型吻合度较高,这也说明一维反演对H型倾斜地层适应性更好。最后通过实测数据证明在小倾角地层条件下,只要能区分倾斜地层与局部异常体的反演电阻率特征,瞬变电磁一维反演仍然是适用的。

In order to study the adaptability of one-dimensional inversion of transient electromagnetic(TEM)in inclined strata,the response characteristics of TEM in inclined strata are analyzed based on typical K-type and H-type models.It is found that the transient electromagnetic response of K-type inclined stratum is smaller in the early stage when the burial depth of high resistivity stratum is shallow,and the amplitude increases gradually with the burial depth of high resistivity stratum increasing,while the response of H-type model is just opposite to that of K-type.Four sets of H-type and K-type models with different inclination angles are further inverted by one-dimensional transient electromagnetic OCCAM.The results show that the inversion resistivity of H-type and K-type models are seriously distorted when the formation inclination angle is large,and the inversion results do not represent the electrical characteristics of the actual formation;when the formation dip angle is small,the inversion of K-type model is serious.The resistivity of high resistivity layer decreases gradually with the increase of burial depth.When the dip angle of H-type model is small,the results of one-dimensional inversion are in good agreement with the actual model.This also shows that one-dimensional inversion is more suitable for H-type dip model.Finally,the measured data prove that the one-dimensional inversion of TEM is still applicable as long as the inversion resistivity characteristics of inclined strata and local anomalous bodies can be distinguished under the condition of small dip strata.