Response Calculation And Ground-loop Calibration Of Helicopter Transient Electromagnetic System

As China's economy continue developing rapidly, and the living standardimprove continuously, the demand of mineral resources is becoming bigger andbigger in China. At present more than 92% of the one-time energy, 80% of theindustrial raw materials, more than 70% of the agricultural production resources allcome from the mineral resources .But since 1990s, the proportion of the import ofmineral resources of the total consumption has increased year by year, it is thus clearthat mineral resources have played a decisive role on our country's economic securityand development which is the same as many other countries in the world. In view ofthis situation, we must increase the exploration of mineral resources in our owncountry, and must increase the proved reserves to meet the development needs and todeal with special circumstances.However, China has a vast territory, and the workload of the geological survey isvery big; What's more, the terrain structure is complex, many places are not suitablefor investigation personnel enter directly. So aviation geological exploration becomesthe best choice. Our country started the electromagnetic exploration of the 863 plan in2006, as part of the plan: podded time domain helicopter airborne electromagneticexploration system development integration, in order to realize our countrymountainous terrain in complex conditions and geological exploration. Since this isthe first study podded aviation transient electromagnetic system, so a lot of systemlevel problems are needed to solve. In this paper, by taking basic parameters of thehelicopter TEM system, we build a model based on ground loop to research thecharacteristics of the helicopter TEM system. First, we calculate all-time response ofthe system with different transmitter current waveform to determine the best emissioncurrent waveform for different target, also we can study the transmission distortion ofsystem to all-time response for system bandwidth and matching method; Finally wecan also design the system calibration approach according to complete systemparameters calibration. For these problems, we will do a few research based onall-time response of ground loop model:The first, we calculate the all-time response by ground loop for different transmitter current in free space. The characteristic of on-time response and off-timeresponse is studied here and the results show that, for high resistance targets, on-timeresponse and off-time response rate quite, but for low resistance targets, off-timeresponse amplitude decreases quickly with the increase of the target resistivity.On-time response has no much relation with target conductivity. In practice ,we canuse on-time response to detect low resistance target , off-time response to detect highresistance target. Finally we calculated switch off-time response of trapezoidalwaveform current. We find that switch off-time response rarely changes with targetconductivity, and can be used to detect low low resistance target. Off-time responsecan be used to detect high resistance target .The second, the magnetic bucking technology is studied here for airborne transientelectromagnetic method. Single coil is reversely wounded in the center of thetransmitting coil to offset primary field in receiving coil. Through the calculation wefind that if only to offset the magnetic field at the central of the system, we can'tachieve the best effect of offset. Must increase bucking coil diameter properly toachieve the theory of magnetic flux offset. In the actual, we can change the coilposition to get the best result. At last, 1% of the offset effect is achieved.The third, we study the parameters and electrical parameters of sensor coil. Thenwe establish the sensor structure model and give the relationship between sensordiameter, section size, circle number, wire section parameter and sensor inductance,capacitance, resistance. In addition, according to the equivalent circuit of the sensorcoil, we also study the limitations of bandwidth to signal. The results show that to gethigh bandwidth sensor must have a large diameter, more sections. In order to improvethe linearity of sensor amplitude frequency characteristics and reduce the system timedistortion, we suggest system be matched on slightly under damping.The fourth, the system noise can be divided into magnetic noise, electrical noiseand background noise. Electrical shielding method is suggested to reduce the electricnoise due to strong electrical coupling between transmitting and receiving coil. In thispaper, we establish the model of the electrical interference and electrical shielding,theoretically getting the equivalent circuit of interference and the shielding model.Based on the circuit, we analysis electric shielding effectiveness. By analyzing thesensor and preamplifier circuit noise model, we calculated the noise variance in 100kHz bandwidth. We also discuss the Inverse stacking filtering model and giveamplitude frequency characteristics of different filter function for different stackingnumber. Then we get the relation between stacking number and signal-to-noise ratio. At last, we analyzed the ground noise model of the receive system, in transientstacking and channel stacked, the system reached 0.43nv/m2precision.The fifth, we theoretically study the method of using ground loop to calibrate theequivalent parameters of the system. This paper we use four parameters: equivalentreceiving area, migration along the line, height, coil dip Angle to describe the systemparameters. First of all, we establish the system parameters in free space based onground loop response. By using the least squares fitting method, the systemparameters are established accurately through the six profile curves. Then the groundresponses to the influence of system parameters establishing is discussed. Influence ofthe direct ground response, the ground and loop to mutual inductance responses areanalyzed in detail. By increasing the fitting parameters, choosing right fitting channel,we have successfully work to control the errors below 3%.The innovations of this paper are as following: we calculate the all-time responseby ground loop for different transmitter current waveform in free space. We putforward slightly damping matching as the best way for coil. By increasing the fittingparameters, we have successfully work to control the errors below 3%.Based on the theory, this paper studied the transient electromagnetic signaldistortion during the production, transmission and the last process of forming profilecurve; studied the influence of the current waveform on the response; put forward thatwe should choose different emission waveform and different signal acquisitionmethod for detecting different targets; analyzed the influences of sensor parametersand EMC on signal distortion and the resolution of the system; proposed the theoryand method to extract equivalent parameters of the system by using the response ofconductive back line; and theoretically solved the influence of earth resistivity on theparameters extraction.