Definition Of Apparent Resistivity Of Ground To Air Transient Electromagnetic Method Based On Differential Pulse

The ground to air transient electromagnetic method has become a promising electromagnetic exploration method in recent years because of its advantages such as high construction efficiency and large detection depth.In the data processing of ground to air transient electromagnetic method,the apparent resistivity of underground media has always been a common means to explain the electrical structure of underground geological bodies,the main reason is that the apparent resistivity is the most intuitive reflection of the electrical characteristics of underground media.However,due to the limitation of the emission waveform in forward modeling theory and practical application,in the previous studies on the definition of apparent resistivity,a single pulse such as step wave and square wave is basically used as the main emission waveform.Due to the lack of effective harmonic components,this traditional emission waveform has limited detection resolution and is difficult to conduct high-resolution detection of complex geologic bodies of different depths and scales.The differential pulse,a new transmitting waveform,has good spectral characteristics: its spectrum is mainly concentrated near a specific main frequency,so it can accurately detect the geological body at a specific depth.Therefore,on the basis of previous studies on the definition of global apparent resistivity,this paper introduces differential pulse into the iterative algorithm of apparent resistivity,trying to complete the visual and high resolution apparent resistivity mapping of underground objects with different depths and scales from the emission waveform.The calculation of forward transient response is the basis of the definition of apparent resistivity.In this paper,the 1D forward response is calculated by linear superposition of the analytic solutions of the transient response under three groups of step wave emission to obtain the transient response under differential pulse,while in the 3D response calculation,the time domain vector finite element numerical algorithm is used to calculate.In the definition of apparent resistivity,the iterative algorithm of global apparent resistivity based on the inverse function relationship between half space response and resistivity is chosen as the basic method.Through the study of the variation law of each component of the transient response with half space resistivity under differential pulse,it is found that in the conventional resistivity range,each component of the magnetic field response is not a single value function of resistivity.Therefore,the component Bz with only one extreme value and relatively good monotonicity is selected for this study on the definition of apparent resistivity.In the definition,the solution is independently solved on both sides of the extreme value in the way of single valued function iterative calculation,and then the more reasonable solution on both sides of the extreme value is selected as the final apparent resistivity results.By calculating the apparent resistivity of a simple layered medium in half space,the correctness of the proposed algorithm is preliminarily verified.By changing the offset and flight altitude of the receiving point,it is found that the apparent resistivity definition results are almost not affected by the flight altitude and flight offset within the allowable error range.It is proved that the differential pulse based apparent resistivity definition method still extends the spatial "global" advantage of the apparent resistivity definition method when the step wave is transmitted.One-dimensional theoretical models containing one,two and three low-resistivity layers were set respectively,and the apparent resistivity of the step wave,square wave with different pulse widths and differential pulse with different pulse widths were calculated,and the defined results of differential pulse with different pulse widths were superimposed.By comparing the definition results of different emission waveforms,it is found that the three waveforms can be distinguished effectively when the number of low resistance layers is small,but the low resistance of differential pulse is the most obvious.When the number of low resistance layers is large,step wave and square wave can not achieve accurate resolution of each low resistance layer,but the result of differential pulse superposition can achieve accurate resolution of multiple low resistance layers.In addition,a three-dimensional theoretical model with one or two low resistance plates was set up.It was found through calculation that,in terms of the resolution of the plate anomalous body,both the step wave and the differential pulse could distinguish the underground anomalous body to different degrees,but the apparent resistivity of the anomalous body under the differential pulse was closer to the real resistivity,that is,the difference with the background resistivity was greater,and the abnormal effect was more obvious.Thus greatly improve the ability to distinguish anomalies.