关于频率电磁测深几个问题的探讨(二)——频率电磁测深探测深度的几个问题分析

Analysis of some problems on the detecting depth of frequency electromagnetic sounding

分析了天然源频率电磁测深(MT/AMT)及人工源频率电磁测深(FEM/CSAMT)的探测深度及评估方法。人工源频率电磁测深在观测点侧面有发射源,分析其探测深度的有关问题比较复杂,除受地电断面和发射频率的影响外,收发距(偏移距)的作用很大。考虑实际应用,在探测对象已定和仪器频率范围已知情况下,通过模拟计算研究了偏移距对探测深度的影响。将偏移距和频率、地电断面参数同时考虑,综合反映出场区对探测深度的影响,通过模拟计算出典型频率电磁测深曲线。曲线特征说明:近区场仅能做几何测深,只有满足远区或中区(过渡区)的条件才能实现频率电磁测深。人工源频率电磁测深探测深度的影响因素较天然源频率电磁测深多了偏移距的作用,其深度评估式应是经验的。所阐述的频率电磁测深的探测深度及评估方法是偏重理论的,并以此指导施工设计或作半定量解释;要更准确得出频率电磁测深的探测深度应通过正确的反演解释。

The detecting depth and depth estimation of natural-source frequency electromagnetic sounding are firstly analyzed in this paper. Then the detecting depth and depth estimation of artificial-source frequency electro- magnetic sounding is discussed. Artificial-source frequency electromagnetic sounding has emission source at the side of the observation point, analysis of the detecting depth is more complicated. In addition to geoelectric section and emission frequency effect, the size effect of transmitter-receiver distance （offset distance） is a significant role. Considering the practical application, in case of defined objects to detect and known frequency range of an instru- ment, the simulation calculation of offset distance effect on the detecting depth has been studied. The offset dis- tance, frequency and geoelectric parameters were considered, the field-zone effect on the detecting depth was syn- thetically reflected. Through the simulation calculation of typical frequency electromagnetic sounding curves, the characteristics of curve shows that for near-field, geometric sounding can only be carried out, only in the far-field or central-field （transition-field） can be realized the frequency electromagnetic sounding. Compared with the natu- ral source frequency electromagnetic sounding, the artificial-source frequency electromagnetic sounding has addi- tional offset distance effect, its depth estimation should be experiential. The detecting depth and evaluation method of the frequency electromagnetic sounding is theoretically focused and can be used to guide the operation design or semi-quantitative interpretation. More accurate detecting depth of frequency electromagnetic sounding was ob- tained through the correct inversion and interpretation.