The Influence of Different Overburden Conditions to the Detectability of Controlled-Source Audio-Frequency Magnetotellurics at a Deep Depth

Based on the theory of three-dimensional forward and inverse modeling of controlled source audio frequency magnetotellurics (CSAMT), we devised a working apparatus model according to current real working conditions in the field. This model carried out several 3D forward and inverse calculations using different values of the thickness-to-depth ratio of anomaly, the difference between anomaly resistivity and wall rock resistivity, and conductive characteristics of low-resistive overburden. The influence of different overburden conditions to the detectability of CSAMT at a deep depth is analyzed and summarized. The calculation results show that: (1) Under the existence of low-resistive overburden, by using similar model parameters, an anomaly is resolvable at the burial depth of about 500m. An anomaly buried 700m deep can be poorly resolved. (2) After the overburden layer is removed, the inversion effect improves significantly for anomalies of the same size. At a shallow depth, the anomaly's inversed value approaches the actual one in relation to size, location and resistivity. The depth displayed by inverse modeling of an anomaly buried relatively deeply is generally lower than it actually is. (3) In uniform half-space, a general conclusion is that a low-resistive anomaly will be resolved if its thickness-to-depth ratio exceeds 0.22 times the square ischolar_main of the ratio of anomaly resistivity to background resistivity. The inversion effect is no longer good to resolve an anomaly buried over 1600m beneath the surface.