Research On The Digital Key Technology For Shallow Surface Frequency Domain Electromagnetic Detection System

Human society,economy and life can not be separated from shallow surface geophysics.With the deepening of urbanization and the continuous progress of science and technology,we continue to carry out a variety of economic activities on the shallow surface.For example,laying underground communication cables,rail transportation construction,geological exploration of real estate construction and so on.In addition,due to the fact that people are not familiar with the underground structure,the damage to the cultural relics also occurs.In military detection,unexploded bombs and other weapons of destruction leaved by military exercises and war are covered by the sand to be difficult to find.There is a big security risk,and these are potential problems caused by insufficient cognition of shallow surface.So we need to develop shallow surface detection equipment to meet the needs of people and boost the development of society.In this paper,several shallow surface detection methods are compared.Among them,the frequency domain electromagnetic detection method is one of the main research fields because of its wide frequency range,controllable time,sweeping frequency detection,strong anti-interference ability and high efficiency.The frequency domain electromagnetic detection instruments at home and abroad are enumerated,and their advantages and disadvantages are compared.And we use the disadvantages of the electromagnetic detector of the Jilin University as starting point to design a new digital system and build a new detection system prototype.This paper analyzes and studies the principle of using the shallow surface frequency domain electromagnetic method to detect metallic materials.The detection system is divided into transmitter system,signal conditioning system and digital system,and the digital system is the core and key of the detection system.The overall design and key technologies of the improved digital system are given.It realizes the collection of multi-channel and high-precision data,accelerates the speed of data transmission and shortens the time of each acquisition point.The digital key technologies include: 1.Data acquisition and synchronization control technology: It realizes the fast acquisition of multi-channel 32 bits ADC datas by FPGA.Because of the strict requirement for the accuracy of the phase information in the frequency domain electromagnetic method,the synchronization of the transmitted signal and the data acquisition is realized by the internal module to ensure the accuracy of the phase information.2.Data fast transmission technology: It achieves the transmission of a large number of raw data through the construction of FIFO+UPP structure to ensure the smooth of information and rapid detection.3.Embedded data processing technology: It uses embedded DSP to realize data processing.To achieve accurate and complete received data by using the whole cycle interception and integration technology.The extraction of the in-phase component and quadrature component is achieved by using orthogonal vector lock-in amplifier of weak signal detection.The interference of channel phase delay is removed by the channel phase correction.The soil mineralization reaction interference caused by soil mineralization is removed by the phase correction technology.In this paper,the control and display interface is designed,and the final result of DSP is received and displayed in real time.In this paper,a new digital system,combined with the host computer and the transmitting and receiving antenna,is used to test the induction and sensitivity of the metal in the laboratory and field.The experimental results show that the detection system can detect the metals with different quality within a certain range quickly and accurately.The curves of different metals are obviously different.Compared with the original system,the data transmission speed is increased by 9.5 times,and the acquisition time is reduced by 68.75% in the fast acquisition mode.The real-time and efficiency of the detection system are improved to achieve the purpose of improvement.