Research On Key Techniques Of Networked Receiver For Controlled Source Audio-frequency Magnetotelluric Method

China is in the rapid development of new industrialization and urbanization,so the shortage of resource and environmental pollution issues are increasinglyprominent. Economic construction and social development requires so adequateresources and so powerful environmental carrying capacity that the application ofgeophysical methods put forward the urgent demand. Mainly reflected in thefollowing several aspects:(1)As shallow mineral resources runs out, the developmentof second prospecting space is in an urgent needs;(2)Engineering geophysicalprospecting demand is huger and huger;(3)Environmental geological problem isincreasingly prominent, and environmental geophysical is of significance.Looking formineral and resolving engineering and environmental problems request for deeperinvestigation depth, more powerful resolving ability and higher operating efficiency.Using artificial source, CSAMT is widely used in deep prospecting, Oil and gasreservoirs, coal field detecting, groundwater resources investigating, engineeringenvironmental geology surveying and many other aspect.which has deeperinvestigation depth, more powerful resolving ability, smaller topographic influence,higher operating efficiency. CSAMT is based on observation, so the receiving systemis the key. Seeking resources on the edge of the critical mine requests the receivingsystem to have stronger capacity of resisting disturbance and higher accuracy ofmeasurement, etc.According to the characteristic of the CSAMT, the article manufactures a newreceiver system used in CSAMT with independent intellectual property rights andnarrows the gap with foreign similar instruments to provide equipment forgeophysical exploration applied in resolving the shortage of succeeding resources inChina’s industrialization and urbanization and overcoming the environment geologicaland engineering geological problems. The article has been competed under the fundof ‘National production potential of oil and gas resources(Development andutilization of oil shale) in cooperative innovation project’,‘Study of key techniquesof oil shale’s electromagnetic instruments(Item no.OSR-02-01)’ and Nationalgeological survey projects ‘Oil and gas resources in concealed basin of easternHeilongjiang using controlled source electromagnetic detection research’. In thispaper, the main work is as follows: 1. A high-precision synchronous data acquisition circuitry is developed. Theanalog conditioning circuit which comprises a low noise tunable narrowband filter isdesigned, SNR and noise immunity of the receiver are improved. A high performanceanalog to digital converter is used. A digital filter based on the programmable filter isdesigned to achieve "oversampling", ability to distinguish is improved. GPS-basedsynchronization scheme is designed to achieve a high-precision phase measurements,the synchronization sweep CSAMT program improves measurement efficiency,reducing the construction strength. The digital control circuit is designed by usingUSB controller and FPGA, the circuit is connected to the IPC PC104via USBinterface. PC software was programmed with the multi-threading technology, whichintegrates acquisition control, data storage, data pre-processing and extraction,communications and other functions. Measured results of main technology parametersshow that receiver circuit noise is less than10nV/Hz, the signal to noise anddistortion ratio is120dB, and the effective resolution is19.7bits, relative error ofamplititude is0.0122%, relative error of amplititude is0.6°, synchronization accuracyis better than200ns.2. A calibration method based on correlated identification is proposed. Thegeneration method of invert-repeated m-sequence is illustrated. Spectral analyticexpression of invert-repeated m-sequence is derived from the self-correlation function.The principle for instrument calibration based on correlated identification is analyzed;the calibration equations of the instrument and the magnetic field sensor are givenwhen the source is invert-repeated m-sequence. Selecting principle of invert-repeatedm-sequence parameters is discussed. Finally, calibration tests of instrument and themagnetic field sensor based on correlated identification are deduced respectively. Theresults show that amplitude calibration error is less than0.2%, the phase calibrationerror is less than0.5°; the frequency resolution is0.244Hz, calibration efficiency is75times higher than the traditional square wave. The calibration method is accurate,fast and high frequency resolution.The measurement accuracy is improved bycalibrating the receiver system.3. Receiving system is designed based on GPRS network communication.Network technology based on GPRS was introduced from the following four aspects:wireless communication media, communication hardware interface, communicationprotocol and the monitoring center. First, contrast on the characteristics of a variety ofwireless communication mode is made, the advantages of GPRS wirelesscommunication mode and its role in realizing network is analysised. Next, theindustrial PC serial port is programed by using MScomm control, including serial portinitialization, reading and writing of serial port, response functions for serial portevents, the connection between GPRS module and acquisition system hardware is realized. Then, the features of xml file and its advantages in data transmission areintroduced, the examples using xml file format for transmission of the networkedreceiver system for CSAMT are given. Finally, in order to realize the connection intothe Internet, the Scoket for network programming is used, basic functions and steps ofSocket programming are given, function of application software in the monitoringcenter server are described. Experimental results show that, the GPRS networkcommunication can be applied to the field remote monitoring for CSAMT.4. Contrast tests between the system developed in this paper and a foreignadvanced instrument V8were made in concealed basin geology resources survey inShengli farm, RaoHe County, Heilongjiang Province and geothermal exploration inZacang temple, Guide Country, Qinhai Provice. The accuracy of measurement systemis verified through contrast experiments with the V8for many times. The receiversystem was applied in state deep probing in Yangjiazhangzi, Huludao City, LiaoningProvice; oil shale exploration in,Yijiadian Fuyu County, Jilin Provice; hydrologygeophysical in Pinggou coal mine, Inner Mongolia autonomous region. Several fieldexploration application results show that the receiver system is reliable and meets theneeds of high-precision CSAMT exploration.