Effect analysis on frequency-time conversion for transient electromagnetic method of electrical source
Short offset transient electromagnetic method with electrical source is one of the newly proposed deep electromagnetic detection methods for solid mineral resources in recent years.Compared with traditional loop source transient electromagnetic method,it has the advantages of large detection depth,lightweight instrument,and high resolution.However,due to its short offset distance and close distance to the emission source,its efficiency and accuracy need to be carefully considered during frequency time conversion,especially in areas with high resistivity.This article compared the factors that affect frequency time conversion and four commonly used frequency time conversion methods(Guptasarma linear filtering method,line approximation method,cosine transform numerical filtering method,and sine transform numerical filtering method)in the forward calculation of short offset transient electromagnetic waves in electrical sources.The conversion effects of each method under different conditions were analyzed from the perspectives of calculation time and conversion accuracy,so as to provide a reference for frequency time conversion methods for one-dimensional and high-dimensional transient electromagnetic forward modeling calculations.The comparison results show that:1.The accuracy of forward calculation of transient electromagnetic fields in the time domain(i.e.frequency time conversion effect)is not only related to the conversion method itself,but also to parameters such as offset and underground medium resistivity.The smaller the offset distance,the worse the frequency time conversion effect.The higher the resistivity of the underground medium,the worse the frequency time conversion effect.Therefore,in high resistance detection areas,the use of short offset electrical source transient electromagnetic method requires frequency time conversion;2.In the low resistance detection area,the numerical filtering method using line approximation and sine transform has higher conversion accuracy and overall smaller errors compared to the Guptasemar linear filtering method and cosine transform numerical filtering method.However,the Guptasemar linear filtering method has the fastest calculation speed,with higher accuracy in the early stage and gradually increasing accuracy errors in the later stage;In the detection environment with high resistance characteristics,the accuracy and speed of using the line approximation method are relatively good,while the Guptasemar linear filtering method,sine transform numerical filtering method,and cosine transform numerical filtering method have poor calculation results,especially in the late stage.Overall,for unknown detection areas,if high-precision transient electromagnetic forward calculation of electrical sources is required,the use of the line approximation method for frequency time conversion is the option.
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