Study On Processing Of Early Transient Electromagnetic Signal

Transient electromagnetic method (TEM) is a kind of advanced geophysicalprospecting technique. Nowadays TEM has been widely used in mineral exploration,drilling, marine and aviation areas and so on. In recent years, the shallow transientelectromagnetic method as an efficient, high-resolution geophysical method, whichshows a good application prospect in the field of shallow detection, is becoming a focusfor researcher. Early transient electromagnetic signal is affected by the receiving coiltransition process. And the emission current nonzero turn-off caused signal aliasing andcurrent turn-off effect. So the early signal especially during the turn off is not fullyutilized, which makes the detection blind area problem has not been fundamentallysolved.The paper has emphasis on the research early TEM signal. The pure secondaryfield can be got by eliminating the impact of the transition process and the primary fieldduring current turn-off. After elimination the nonzero turn-off current effect, thencontaining the turn-off period of apparent resistivity will be got. That can solve theproblem of detection blind area. The following studies are mainly discussed in thisarticle:Firstly, analysis of the existing research on early transient electromagnetic signal islack. By analyzing the whole of transient electromagnetic response signal characteristics,the idea of a shallow transient electromagnetic sounding signal processing. The shallowTEM detection signal processing method is proposed.Secondly, based on TEM basic theory, analysis of the transient electromagneticinduction process and the characteristics of induced eddy current in the early, middleand late; Analysis of the additional effect of the early transient electromagnetic signal;The transient response of a homogeneous half-space excited by a ramp currentwaveform is proposed for zero time selected as the beginning of the current turn-off.The current turn off period is called ‘induction period’, the current completely shut offlater is called ‘decay period’.Thirdly, according to the magnetic field and time derivative of magnetic field(voltage) of the central loop device, the relationship among magnetic field, inducedvoltage and apparent resistivity is discussed. The current turn-off effect correction stillcan’t use during the turn-off signal at present. A method about calculating the all-time apparent resistivity is proposed，which eliminate the primary field in the turn off periodand use ramp step response direct to calculate. Then the effect of current turn-off can beeliminated.Through forward modeling and Ansoft Maxwell simulation homogeneoushalf space data processing to prove the validity and feasibility.Fourthly, based on circuit theory, the equivalent circuit of receiver coil is deeplystudied. Analyze the performance characteristics of the receiving coil and themeasurement of the distribution parameters. Transition process can be simulated byconvolution receiving coil theory response and impulse response. Then analyze theinfluence of the transition process TEM signal。Finally, the noise suppression method about field experimental data is discussed.Using the shallow TEM detection signal processing method and direct inversion methodbased on the apparent resistivity TEM “smoke loop” to an air-raid shelter experimentwhich was carried out in our school with good results. And it applied to a preliminarystudy of the grounding grid fault diagnosis.