Research On Multi-frequency Transmitting Technology For Electromagnetic Exploration Of Shallow Surface

With the rapid development of science and technology,the rapid development of modern urban construction,shallow geological space exploration plays an increasingly important role in it,and shallow surface frequency domain electromagnetic detection technology because of its high stability,detection bandwidth,detection targets Without damage and other advantages,it stands out in many shallow surface detection technologies and is already a hotspot in such field.The shallow geological frequency domain electromagnetic detection equipment is mainly divided into transmitting system and receiving system.The high-quality transmitted waveform can enhance the secondary field strength and SNR when acquiring signals and the resolution of the entire detection system,and improve detection efficiency.The main work accomplished by this article includes:The traditional equipment of shallow surface electromagnetic exploration generally adopts the single-frequency transmittion method,and the working efficiency is not high.To address this issue,the technology for multi-frequency transmitting electromagnetic of shallow surface has been studied in the paper.The application of multi-frequency transmitting will greatly improve the exploration efficiency and help to collect a more comprehensive target frequency characteristic curve,which has a high practical value.The commonly used five multi-frequency driving signal generation methods are analyzed and compared,in combination with the electromagnetic exploration of shallow surface requirements,the paper determines SPWM multi-frequency modulation method and a~n pseudo-random sequence method as the research object.Based on these two methods,the corresponding circuits are designed,then they are compared and analyzed through theoretical simulation and actual measurement.Based on a~n pseudo-random sequence principle,the pseudo-random sequence function expression and coding of multi-frequency is derivated,and its feasibility is proved by calculating and analyzing the energy distribution of each frequency.Because the multi-frequency driving control signal it generates only two states,so the H-bridge inverter is used as the transmitting bridge.In the SPWM multi-frequency modulation method,a carrier-wave inversion laminated modulation method is adopted,and the modulation signal is generated by superposition of a plurality of sine waves with different frequencies.The main circuit uses a diode-clamped three-level transmitting bridge circuit.This circuit can reduce harmonics,and the output waveform more closely approaches the signal wave.Two transmitting bridge circuits corresponding to the two launch modes are designed respectively,and the corresponding driving circuits are designed for these two transmitting bridges.Their working status and the conducting timing orders of the power switch tubes are analyzed,and digital control logic is implemented in the FPGA.By analyzing the working process of the power switch tube,the resulting losses and circuit parameters are calculated and the main device type is determined.In order to improve the quality of the transmitting waveform,an analogy to a common-mode filter was designed to suppress common-mode interference and to filter out the high-frequency harmonics in a certain extent.And absorption loops designed at both ends of the transister on the bridge can effectively suppress the peak interference of the transmitting waveform.Through system simulation and actual testing,the effectiveness of the two design schemes is verified;Through comparing the advantages of two kinds of multi-frequency transmitting methods and make suggestions on the using situition of the two methods.