The Development Of A Multi-Channel Transient Electromagnetic Receiver And Its Acquisition Tests

Based on the electromagnetic induction theory,transient electromagnetic method(TEM)adopts the method of time-domain electromagnetic detection excited by artificial field source.With the conductivity and permeability of rocks and ore in the earth regarded as the physical preconditions,pulse currents are transmitted to the underground by means of ungrounded magnetic source or grounded source,and the earth is excited to produce transient electromagnetic fields.Conductive geologic bodies are induced to generate eddy currents,and produce secondary fields.Underground anomalous bodies can be detected through observing and analyzing the transitional process of secondary fields by TEM.In recent years,there are stricter requirements about the development of instruments of transient electromagnetic method both at home and abroad.In order to meet the needs of engineering geophysical exploration and oil-gas geophysical exploration,new multi-component and multi-parameter transient electromagnetic instruments for great depth detection have entered into electromagnetic detection field.The original single vertical magnetic component dectection has been replaced by time and frequency electromagnetic method(TFEM),which measures electric fields and magnetic fields simultaneously,long offset transient electromagnetic method(LOTEM)and multi-channel transient electromagnetic method(MTEM).In addition,the transmitting system has transitioned from the magnetic source transmission towards high power,high current grounded source transmission,and the signals have developed from bipolar signals to pseudo-random ones.There is large room for further development of transient electromagnetic application in various depth detection and the exploration in different demand areas.In the light of demands for the development of time domain transient electromagnetic method and function expansions of detection instruments,the dissertation designs a multi-channel transient electromagnetic receiving system.The grounded source transmission,the precise clock synchronized communication and high precision GPS positioning are adopted in the system.Six-channel synchronized data acquisition is researched to collect five ground electromagnetic components(Ex,Ey,Hx,Hy,Hz)and magnetic flux variation rate(dB/dt).The design scheme and the tests on the prototype conform to the future development direction of multi-component,great depth detection and multi-parameter interpretation of transient electromagnetic observing system.Therefore,multi-channel and multi-component detection,and the mode of quasi-seismic observation and interpretation have been realized.The major research achievements are as follows:Firstly,guided by electromagnetic induction method in electrical prospecting,and supported by electronic information technology and computer technology,the design of the new multi-channel transient electromagnetic receiver has been accomplished.Through further exploring the extraction method and processing technology of related characteristics of transient electromagnetism,a low-noise multi-channel power supply board,an acquisition and storage control board,and an analog signal acquisition board with a number of channels to be assembled flexibly have been developed to realize the design of a new transient electromagnetic receiver with a minimum of two channels,and up to six channels.According to its design scheme,the receiver can be used as a current waveform acquisition instrument for recording transient electromagnetic transmission system,and also as a collection system for earth responses of the information of electric fields and magnetic fields.And the flexible,diversified and efficient receiving system has been achieved.Secondly,during the development of the receiver,the technology of cross clock domain is adopted to realize multi-channel synchronized data acquisition and storage at a high sampling rate up to 128 k,and information synchronized transmission and data exchange among each module of the receiver.In the process of its development,a 28-digit high precision timer designed on the basis of FPGA,to cooperate with GPS second pulse signals has been adopted to achieve the storage of time information and the received data,which realizes high precision synchronization of data acquired by each receiving site.Meanwhile,external unit circuit is designed specifically to calibrate the acquisition system.In addition,a small-sized monitoring module which utilizes the ultra-low power technology,is devised to observe the running state of the receiver system from far distance.Thirdly,multi-channel transient electromagnetic method can receive magnetic inductance intensity variation rate(dB/dt),and observe the electric field components in two directions and the magnetic field components in three directions at the same time.Therefore,this method can provide more information and larger space coverage.With the dynamic range expansion of the receiving instrument and the increase of the sampling rate,it can collect more data types in a short time.In addition,multi-channel transient electromagnetic method can be compatible with various transmitters,such as Digital PEM system of Canadian CRONE Geophysical Company,T-4,TXU-30 transmitter of Phoenix Company and GDP-32 electrical instrument of American ZONGE Company.Many kinds of sensors such as hollow coils,non-polarizing electrodes and magnetic probes,and so on can be used as the receiving devices.Fourthly,DC resistance network performance tests of each module of the multi-channel transient electromagnetic receiver in the laboratory,and AC signal acquisition tests have been finished.Signal precision measurement,and calculation and analysis of signal to noise ratio have been conducted.The tests of the stability and consistency between channels of the receiver have been carried out to verify that the receiver can realize real-time acquisition and storage of data at a high sampling rate.After numerous tests,the author has continuously improved the design of the system circuit and software,and finished the development of the multi-channel transient electromagnetic receiver.In order to further test the receiver performance,the researcher has utilized T-4 transmitter of Phoenix Company to transmit bipolar currents by means of grounded electric dipole source,and conducted field experiments of the receiver's function and performance.After multi-channel receiving and acquision tests of two measuring lines were completed,two profiles have been formed.The author has compared single-point data of the secondary fields collected by the receiver and V8 system,and found that the attenuation curve patterns formed by both systems are consistent.Fifthly,in order to verify the practical exploration application of multi-channel transient electromagnetic receiver,the author has conducted its application tests in a copper mine area located in Leshan,sichuan province,and the acquisition of transient electromagnetic information of a measuring line across the mineral vein.After the extraction of the secondary field,and denoising by deeplearning method,the corresponding high anomaly measuring points displayed are consistent with the actual field terrain and the mineral vein trend.By contrast with the data curve of chemical prospecting corresponding to the measuring line,the anomaly points of copper element distribution are similar in the two curves.Meanwhile,the receiver and the acquisition station of Electronic Research Institute of Chinese Academy of Sciences were both used to collect electric field information of the same measuring point.By comparing both waveforms,their graphic display results are identical.In conclusion,the receiver can be configured flexibly to receive information such as electric field intensity E,magnetic flux density variation rate dB/dt and magnetic field strength H by flexible configuration by the program,so it can realize the observation system with transmission by a single transmitter,multi-channel synchronized acquisition and transmitted current synchronized recording.The advantages of its multi-channel lie in the information synchronized collection and storage of multiple measuring points on the same offset.Signals are transmitted by ground line source,and the information of electric field and magnetic induction intensity can be received.The information processing of the same offset is close to the data processing method of seismic exploration.The geoelectric information of a certain depth target can be estimated by the common offset profile.In a word,the receiver is the receiving system featuring multiple acquisition channels,wide dynamic range,large storage capacity and high precision synchronization.Deep learning methods are also added in data processing.The research on the design of the receiver plays a vital role in the precise exploration of depth ore body,which will increase the exploration efficiency and lower the production cost significantly.