Development Of Long Period Magnetotelluric Signal Acquisition

With the rapid development of science & technology and modern economy,the Human is ever-accelerating pace of exploring space,but being know very little about the earth where we are living.Therefore,the global scientists have made unremitting efforts to carry out the deep exploration of the earth.At present in the field of exploration geophysics,long period electromagnetic sounding method(Referred to as LMT)has become an important means to study the deep structure of the earth.By measuring the electromagnetic field signals of different frequencies,it can obtain the electrical information of different deep parts of the earth,which can reflect the geological structure.Want to truly and effectively reflect the characteristics of the earth's internal resistivity,geological structure and other information,high precision,stable and reliable observation instrument is the guarantee of the effective information of the earth electromagnetic,and it is also the foundation of the data processing in the later period.However,our country has not yet been commercialized in the long period of the earth electromagnetic sounding instrument,the research institutions and scholars in China have adopted the LEMI-417 instrument and RC_MTMV data preprocessing software system to study the deep structure of the earth in recent years,which to a certain extent limits the development of deep exploration research in China,and this equipment was developed so long ago,performance indicators and ease of operation can also be improved.In order to satisfy the demand of deep exploration in our country,to carry out the deep exploration of the earth and independent research and development of related detection equipment,which will provide technical support for the exploration of the inner structure of the earth and its movement,the development of the earth's deep prospecting space and the prevention of geological disasters,and has important practical significance and scientific significance.The observation signal of long-period magnetotelluric sounding instrument is low frequency and small amplitude,field observation conditions are complex,all kinds of electromagnetic interference is serious,observation period is long,the instrument has stronger noise suppression capability,low temperature drift and time drift,and low power consumption.This thesis is based on 863 major project topics-long period distributed magnetotelluric observation system(2014AA06A612),the design and development of the signal acquisition system in the observation system are carried out.The main work of this paper is as follows.(1)A design scheme of FPGA signal acquisition system based on ARM and LMT is put forward.Through the research of LMT signal characteristics,basic principles and observation methods,and reference to the functional requirements and performance indicators of the main instruments at home and abroad,the technical requirement and design scheme of signal acquisition system are put forward.(2)A five channel electromagnetic signal acquisition circuit is designed and developed,which is low noise and low drift with potential compensation.Including the design of the five channel signal conditioning circuit based on the chopping(self stabilizing zero)operational amplifier ADA4528;acquisition circuit based on high precision 24 bit analog-digital conversion ADS1281;realization of potential compensation circuit based on 16 digit-analog conversion AD5542 chip and subtraction device;program control amplifier circuit based on the PGA281,according to the input signal adaptive change gain,improve the analog digital conversion circuit of signal acquisition resolution.(3)The digital control circuit based on the combination of FPGA and ARM is completed.Make full use of the I/O device FPGA resource rich,high clock frequency,time delay and less synchronous sequential logic control,modeling and realizing the logic control,data processing and transmission of the input signal of the five channel electromagnetic field,and solving the problem of multi channel data acquisition timing synchronization and sampling frame.Combined with ARM flexible computing control ability and perfect peripheral interface,based on low power ARM processor STM32 chip and peripheral expansion digital module(SD card module,GPS module,TFT LCD module,WiFi module,etc.),programming to completethe data store based on the Fatfs file system and SD card,to achieve timing and positioning base on GPS,to realize of human-computer interaction base on TFT,to realize Wireless Monitoring and Control base on WiFi.(4)System performance test is carried out on the prototype of LMT signal acquisition system.The noise test indicates that the signal acquisition channel of the magnetic field has a peak value of 0.8nT in the 10,000nT(10V)bipolar single ended input,the signal acquisition channel of the electric field has a peak value of 2?V under the 25 mV differential input.In each channel test,the reference error is less than 0.01%,the linearity error is less than 0.07‰,the crosstalk suppression ratio is higher than 110 dB,the channel consistency is more than 99.8%.The results show that the performance indexes of the signal acquisition system basically meet the design requirements.