Research Of Resolving Capacity And Development Of Full-Waveform Electromagnetic Receiver In Time Domain

Electromagnetic detection is an effective method of detecting useful information underground in time domain in time domain. It is widely used not only in deep exploration of mineral resources,but also promoted in engineering investigation in recent years.The detection scale gradually range from shallow to deep areas of earth.Full waveform electromagnetic detection method in time domain explore earth during transitmitting current Switch-Off and fully closed.It collect and interpret the early signals which contain shallow information and late signals which contain deep information.Because above,it needs develop high speed,high resolution,low noise time-domain full-waveform electromagnetic receiver,and testing and calibrating its performance,analysising the detectability of targets in order to enable it detect target effectively.For above problem,Supported of national 863 major scientific and technological research project"podded airborne electromagnetic helicopter survey system in time domain",based on ATTEM-2 receiver and three-dimensional algorithm of step wave,this paper develop full-waveform,low noise,high resolution,high speed time-domain ground receiver.And it research one-,two-and three-dimensiona full-waveform electromagnetic response algorithm,refer to calibration method of airborn time-domain electromagnetic system, test and calibrate the performation of receiver,combine those to analysis the detectability of targets in order to enable it detect target effectively.Main research content and the results achieved in this paper are as follows:(1). We developed 24-bit,high resoulution and low noise receiver .Analysised of noise source which interfered the analog channels of receiving system, according vertical layout of the plates to achieve low noise transmission of signals in analog channels. To improve the dynamic range of the system, we take use of the high repetitiveness of system, used two - stage amplifiers and filter, time-sharing collection and superposition to achieve the wide dynamic acquisition for system.To achieve the complete collection of signal, articles used theΣ-Δ24 - bit sampling unit with noise pressuring capacity to achieve low noise,high speed and high resolution acquisition of time domain electromagnetic signal.Combine with CPLD and IPC,it use VHDL and VC++ to write sample program and control sequence. It use absolute value estimated solution for superposition with equal number of bipolar signal.(2). Use ground calibration system and other laboratory equipments to test the performance of receiver. At first,we take use standard signal source, wide band oscilloscope,ATEM-2 transmitter and other equipments to test frenquency band, repeatability,lowest equivalent background noise and minimum resolution of receiver.After that,we use ground calibration system to measure distortion rate and valid time of signal collected by receiver according to contrast actual second field and theoretical second field of calibration coil. Test results show, that the frequency band of receiver is 30KHz, average error of repetitiveness is less than 0.02%,the lowest equivalent background noise is 2μV, the minimum resolution of receiver is 2μV,the valid time are full time excluding shaking period,the average distortion rate is 1%.(3). We researched full waveform electromagnetic response of layered ground. At first, according to the relation between second iuduced voltage excited by trapezoid wave and second iuduced magnetic excited by impact wave , we take use F.N.Kong241 point filters and Guptasarma filters to calculate second iuduced voltage in central loop and overlap loop,and initial field of three-dimension finite difference method.At last,we make"double rings"theory in equivalent to second iuduced voltage.(4). Studied the three-dimension finite difference method for any source in time domain. the paper disperse emission current into small trapezoidal pulses with segmented superposition method,and transform second iuduced voltage excited by emission current into second iuduced voltage excited by several trapezoidal pulses in no source space after they switch off completed and several trapezoidal pulses in no source space.Then take them into source-free Maxwell equations as initial field,and transform source-free Maxwell equations into active Maxwell equations at Off-time at last.After these,we disperse electric field,magnetic field and their derivative by second-order central difference method in time domain and spatial domain.And convert continuous diffusion equations in to explicit discrete difference equations by introduce artificial item.We studied the stability conditions and boundary conditions of difference equations,and use Upward continuation method to calculate electromagnetic field in the air on ground-air interface.(5). Analyse the resolving capacity to target underground of receiver.Utilizding the performance of receiver tested above and combine with full-waveform response of earth,according to absolute error of decay curve and inducd field of background,we analyse the detectability of receiver by center loop and overlapping loop method for homogeneous ground, two-dimensional layered earth,K-earth,H-earth, three-dimensional high and low resistance target in uniform half space, three-dimensional high and low resistance target in uniform half space covered with low resistance layer.At last,we take low resistance target in uniform half space as example,study the influence of radius of transmitter loop ,Switch-Off time and electromagnet interference in the wild for resolving capacity of receiver.This innovations are reflected in the following areas:(1). Develop 24-bit time-domain full-waveform ground electromagnetic receiver at first, Use ground calibration system to calibrate ground receiver ,study the degree of distortion of instrument in signal acquisition,test valid time of signal acquired by receiver,and provide reference for the actual detection of the receiver.(2). Calculating the full-waveform second field of loop source within and outside the loop by combine Bessel function spread method and small argument asymptotes at first .(3). Study the full-waveform time domain three-dimensional finite difference method, accomplish the calculation of full-waveform response in time domain for any transmitted waveform and arbitrary complex earth at first, lay the foundation for analysis of full-waveform time-domain electromagnetic response of the Earth .(4). Combine with the performance of receiver,analyse the detectability of receiver for one-,two- and three-dimensional target by integral equation method and full-waveform time domain three-dimensional finite difference method, providing theoretical basis for Actual detection the wild.