| With the rapid development of China's economy,the demand for resources such as mineral,oil and natural gas is increasing day by day,and the exploration depth is increasing year by year.However,in the field of deep exploration,less detection means can be used,only strong seismic signals and long period magnetotelluric signals can penetrate deep underground rock,so as to complete the effective acquisition of underground geological structure and electrical information.Long period magnetotelluric sounding has become an important means for deep exploration because of its low cost and deep detection depth.However,the long period instrument has complex construction conditions,weak signal and serious interference.Therefore,the long period magnetotelluric acquisition system has put forward higher requirements.In this paper,the structure and function modules of the long period magnetotelluric acquisition system are optimized,and the system acquisition index is clearly defined to improve the reliability and stability of the system.According to the characteristics of electric and magnetic signals,the following research works are carried out.1.Aiming at the problem of weak long period electric field signal,high common mode level,serious radio interference and low signal to noise ratio of traditional electric field signal acquisition data,an improved method of electric field signal acquisition is proposed.The acquisition module of low noise and low drift electric field signal is realized by using self stabilizing zero,chopping and drift suppression technology.The noise analysis is carried out in hardware design.Experimental results show that the signal to noise ratio and measurement accuracy of electric field acquisition data are improved.2.In view of the large dynamic range of the long period magnetic field signal,and the characteristics of the static component and the alternating component in the magnetic field signal,the traditional magnetic field acquisition method has a low precision on the alternating component acquisition,which is limited by the background noise.On the basis of the traditional magnetic field signal acquisition method,the dual core synchronous acquisition technology is proposed as the core.A new acquisition method of magnetic field signal is applied to realize the high-precision acquisition of the magnetic field signal's alternating components in the large dynamic range.3.In order to ensure the consistency of the electric field and magnetic field data in the long period system and the reliability and stability of the data transmission between plates,a system MCU acquisition program based on GPS second pulse,data FIFO and CRC check technology is designed,which can be divided into three parts: electric field,magnetic field and control program.In order to facilitate instrumentation system debugging and data processing,a LabVIEW based operation software for data storage,reading and display of host computer is developed.4.The system test and comparison experiment were carried out.The test results of the electric field acquisition module show that the DC bias noise is about 6.71 uV in the time of up to 20000 s,and the peak and peak noise is about 1.4 u V without obvious drift.The noise spectrum is better than-156 dB.Combined with high-precision acquisition technology,when the polar distance is 100 m,the electric field signal can be collected and resolved to 0.16 V/km.The magnetic field dynamic range test results show that the acquisition accuracy of magnetic field can reach 0.069 nT.And compared with the foreign LEMI-417,the experimental results show that the trend of the electric field and magnetic field signal acquisition curve is basically consistent.The data processing can get the smooth apparent resistivity and impedance phase,which verifies the accuracy of the system and meets the basic requirement of the long period magnetotelluric signal acquisition. |
PDF Full Text Request