平面电磁波法探测浅层局部良导体的响应特征

    Response characteristics of shallow good local conductors using the plane electromagnetic wave method

    • 摘要: 针对均匀半空间背景下的三维良导体模型进行正演计算,研究其在平面波电磁探测条件下的响应特征。三维良导体的电磁异常主要由分界面静态积累电荷产生的二次场所引起,因此目标体电阻率与围岩电阻率相对差异越大,响应相对异常也越大;观测电极距离目标体越近,响应相对异常越大。三维体在水平方向上和垂直方向上的尺寸变化对异常的影响程度不同:垂向厚度变化对异常影响较小;水平方向两个尺寸相差不大时,视电阻率响应曲线形态呈现类似于二层D型测深曲线形态,响应相对异常随水平尺寸变大而变大;当水平方向两个异常体尺寸相差较大时(大尺寸达到小尺寸的8倍以上),沿着大尺寸方向观测与沿着小尺寸方向观测得到的曲线形态不同。沿着大尺寸方向观测时,视电阻率响应曲线形态呈现类似于高低高变化的三层H型测深曲线。此外,视电阻率相对异常一般为相位相对异常值的2倍以上,而且视电阻率异常纵向上遵循静态效应规律,在高频出现异常后将一直延伸至低频更有利于发现异常,因此利用视电阻异常更有利于识别异常。

       

      Abstract: This study conducted the forward modeling of a 3D good conductor model under a uniform half-space background to investigate its response characteristics in a plane electromagnetic wave survey. The electromagnetic anomalies of a three-dimensional good conductor are primarily caused by the secondary field generated by static charge accumulated at interfaces. Consequently, higher relative resistivity differences between the target conductor and the surrounding rock correspond to greater response anomalies. Additionally, a smaller distance between observation electrodes and the target conductor is associated with greater relative response anomalies. Changes in the horizontal and vertical dimensions of a conductor pose different impacts of anomalies. Specifically, variations in vertical thickness have minor impacts on the anomalies. When a conductor has similar dimensions in the horizontal direction, its apparent resistivity response curve resembles a two-layer D-type sounding curve, with the relative anomalies intensifying as the horizontal sizes increase. However, in the case of significant differences between the two dimensions in the horizontal direction (with the larger dimension being at least eight times the smaller), the response curves observed in the directions of the larger and smaller dimensions differ. Notably, the apparent resistivity response curve observed in the direction of the larger dimension resembles a three-layer H-shaped sounding pattern characterized by high, low, and high values sequentially. In addition, the relative anomalies of apparent resistivity are generally more than two times those of phase, with apparent resistivity anomalies following the static effect law in the vertical direction. Specifically, apparent resistivity anomalies in high frequencies tend to extend to low frequencies, creating favorable conditions for anomaly identification. Therefore, apparent resistivity anomalies are more conducive to anomaly identification for good conductors.

       

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