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

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 electromagnetic 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 instrument, the simulation calculation of offset distance effect on the detecting depth has been studied. The offset distance, frequency and geoelectric parameters were considered, the field-zone effect on the detecting depth was synthetically 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 natural source frequency electromagnetic sounding, the artificial-source frequency electromagnetic sounding has additional 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 obtained through the correct inversion and interpretation.