Research On Key Techniques Of Broadband Time-frequency Electromagnetic Receiver

The current situation is that geophysical equipment such as electromagnetic instrument heavily depends on imports, and instrument developed has not been given attention for a long time. Development Plan Outline for Medium and Long Term Science and Technology Development （2006—2020） suggest that the demand of time‐frequency electromagnetic Instrument will be a further increase, so it is pressing to develop it with freedom knowledge property right and excellent performance for the healthy development of geophysical exploration in our country. The rapid development of high‐precision analog‐digital conversion technology, GPS timing technology, solid‐state disk storage technology, high performance processor, signal processing technology will help to deal with key technologies such as high‐frequency weak signal detection and extraction,1/f low‐frequency noise removed, synchronization trigger for remote sending and receiving, real‐time storage of large data amounts, so it is feasible to develop broadband electromagnetic receiver.Started with the principle of TEM, CSAMT, MT, collected and compared with technical specifications of the foreign instrument, designed by modular concept, electromagnetic receiver is made of a control board, five high‐frequency board, five low‐frequency board. The control board is based on USB2.0transfer protocol, and control FPGA controller through microcontroller bus, which includes SLAVE controller, large‐capacity FIFO controller, SPI bus converter, GPS information converter, synchronization trigger and calibration signal transmitter. The control board has solved the problem of high‐speed data transmission by combination of slave FIFO and large‐capacity FIFO. Slave FIFO is combined with USB2.0microcontroller and main controller simulated by FPGA. Large‐capacity FIFO is made of SRAM and small scheduled FIFO which is integrated in FPGA. In order to decrease crosstalk, the communicate of each functional board and control board is completed by magnetic coupling isolation, and reasonable layout, shielded, ground handing, power isolation is used too, which can decrease interference and reduce noise.On the basis of analyzing how to decrease1/f noise by chopper technology and Δ‐∑technology, the low‐frequency board achieve targets that1/f noise corner frequency is lower than0.1Hz, and amplitude spectrum of frequency lower than corner frequency is less than10‐8uV. Through the experiment of signal test, short circuit test, sensor matching test, the low‐frequency board can be used in the acquisition of a large dynamic range, which includes MT, LOTEM and CSAMT. It also verify the long‐term reliability and standability of the low‐frequency board. After designed high‐frequency acquisition boards based on AD7763and AK5394, it verify the advantages of the AK5394through noise test and sensor matching test.The final design of the high‐frequency board has achieved these target that the peak‐peak value of noise is less than10^‐7 uV and amplitude spectrum less than10^‐7uV, so the acquisition of AMT, TEM and CSAMT can use high‐frequency board. The successful design of the low and high frequency board laid the foundation of broadband low noise acquisition.Because of non‐consisitency caused by component differences, the calibration souce and synchronous clock board generates1V peak value by GPS PPS trigging benchmark source chopper and analog amplifier operation circuits, in order to providing calibration source for low and high frequency acquisition. It also integrates circuit of OXCO synchronization and GPS synchronization, which is combinationed with FPGA to achieve line trig, gps trig and OXCO trig.Three electromagnetic methods of Data recording and storage strategy has been analyzed in detail. On the basis of thorough analysis of the TEM signal characteristics, TEM data recording is achieved by binary full waveform records and bipolar overlay method of handing. A series of basic logarithmic interval frequencies used for CSAMT is designed by the advantages of the whole cycle of acquisition. CSAMT sweep data is automatically recorded by tbl table completed by own TBLEditor softeware. After analysis of the relevant characteristics of the MT signal,the MT recording method is full‐wave acqusisition by2000Hz, and then PC104generates time series data of125/2Hz,125/64Hz and125/2048Hz by digital filter and extract.The right of time‐frequency electromagnetic design is verified by the experiment in Jilin Changbaishan geothermal resource exploration, Liaoning Dataigou iron mine explortation, Changchun Cultural Square, Changchun Sungengdun and so on. Compared with similar instruments on the market today, it has a great advantage.