半航空瞬变电磁探测敏感区域研究与影响因素分析

On sensitivity detection area and analysis of influence factors for semi-airborne transient electromagnetic method

接地线源半航空瞬变电磁具有探测深度大、工作效率高、适用复杂地形的优点,得到广泛关注.然而,该方法的理论研究仍处于初步阶段,研究接收点信号与下方地质体关联性对半航空瞬变电磁数据正确解释至关重要.借鉴航空电磁中的footprint方法对上述问题展开研究.首先通过张量格林函数积分推导了地下离散电流单元对空中接收点二次磁场响应的贡献,分析了半航空瞬变电磁footprint时间-空间变化规律;之后,研究了飞行高度、偏移距对半航空瞬变电磁探测敏感区域的影响.研究结果表明有效探测区域的时变特征与地下感应电流场的扩散特性有关,并受接收点位置的影响;早期探测敏感区域主要集中在发射源正下方,随着感应电流场的扩散逐渐向接收点方向偏移,同时接收点下方区域的贡献占比逐渐增加;研究发现晚期贡献最大的部分集中在接收点两侧近地表处,该发现对基于电阻率成像的半航空瞬变电磁具有重要价值.

Grounded wire source semi-airborne transient electromagnetic has received much attention for its advantages of large detection depth and high efficiency. However, the theoretical research on this method is still in its preliminary stage. Studying this correlation is crucial for accurately interpreting the data obtained from ground-based transient electromagnetic surveys. This study employs the footprint technique, commonly used in airborne electromagnetics, to investigate the aforementioned issue. Initially, by employing tensor Green's integral, the contribution of discrete current elements beneath the surface to the secondary magnetic field response at the airborne receiving point is derived. Subsequently, an analysis is performed to investigate the distribution of footprint, along with the temporal and spatial evolution patterns of the semi-airborne transient electromagnetic method. Furthermore, the impact of flight altitude and offset distance on the footprint area in the context of semi-airborne transient electromagnetic surveys is examined. The research findings indicate that the temporal characteristics of the footprint are closely related to the diffusion properties of the induction current field in the subsurface, and they are influenced by the relative position of the receiving point. In the early stages, the footprint is primarily concentrated directly beneath the emission source. As the induction current field gradually diffuses towards the direction of the receiving point, the contribution from the subsurface region beneath the receiving point increases gradually. In the later stages, the highest contribution is mainly concentrated on either side of the receiving point, near the near-surface region. The discovery is valuable for semi-airborne transient electromagnetic method based on resistivity imaging.