3D time-domain forward modeling of airborne transient electromagnetism considering superparamagnetic effect 

 

LI Zizhuo, LIU Yunhe* and REN Xiuyan

Global Geology ›› 2024, Vol. 27 ›› Issue (4) : 216-232.

PDF(753 KB)
PDF(753 KB)
Global Geology ›› 2024, Vol. 27 ›› Issue (4) : 216-232.

 3D time-domain forward modeling of airborne transient electromagnetism considering superparamagnetic effect 

 

  •  LI Zizhuo, LIU Yunhe* and REN Xiuyan 
     
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Abstract

 The superparamagnetic effect arises from the superparamagnetism exhibited by a multitude of nano-sized magnetic mineral particles under an external electric ffeld. This phenomenon manifests in transient electromagnetic data primarily as a deceleration in the attenuation rate of late-stage signals, a characteristic difffcult to discern directly from airborne transient electromagnetic signals, consequently leading to signiffcant misinterpretations of subterranean electrical structures. This study embarks on 3D forward modeling of airborne electromagnetic responses in the frequency domain, accounting for the superparamagnetic effect, utilizing an unstructured finite element method. Superparamagnetic responses in the time domain were obtained through frequency-time conversion. This investigation explores the influence of various parameters—such as magnetic susceptibility, time constants, and ffight altitude—on the superparamagnetic effect by examining the response characteristics of typical targets. Findings indicate that in its late stages, the superparamagnetic effect can induce a relative anomaly of up to 300%. There is a positive correlation between magnetic susceptibility and the strength of the superparamagnetic effect. The inffuence of the time constant's upper and lower limits on the superparamagnetic effect is minimal; however, the range between these limits significantly affects the effect, showing a negative correlation with its intensity. Higher flight altitudes weaken the superparamagnetic signal. The impact is most pronounced when superparamagnetic minerals are shallowly buried, effectively shielding the underlying geology with the characteristics of a good conductivity anomaly, but this effect diminishes with greater depth. The insights from this study provide a theoretical framework for a deeper understanding of the superparamagnetic effect in transient electromagnetic signals and for more accurate interpretations of subterranean geological and electrical structures. 
 

Key words

 electromagnetic exploration / aviation electromagnetism / time domain / superparamagnetic effect / 3D forward modeling / ffnite element method

 

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LI Zizhuo, LIU Yunhe* and REN Xiuyan.  3D time-domain forward modeling of airborne transient electromagnetism considering superparamagnetic effect 
 
[J]. Global Geology. 2024, 27(4): 216-232
PDF(753 KB)

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