| Applications such as groundwater resource exploration,underground mineral resource exploration and non-destructive detection of cultural relics are constantly increasing.Helicopter-borne electromagnetic(HEM)detection platform has the advantages of high efficiency,economy and adaptability to complex terrain survey.High-resolution exploration of shallow underground targets has become one of the major needs of China now.However,the traditional HEM detection system usually uses a bipolar polygonal transmitting source with a base frequency such as 25 Hz/30 Hz,its energy is mainly concentrated on the base frequency and odd harmonics,and the energy decays rapid.In addition,limited by the current turn-off time,the earliest sampling time,the sampling rate and the noise level,there are blind spots in shallow surface,which is the area of concern for engineering and environmental detection.The high-frequency pseudo-random source expands the frequency band and is more suitable for shallow detection.Its spectral lines are densely distributed at equal intervals and can provide higher resolution.The combination of high-frequency pseudo-random source and HEM provides a new idea for efficient acquisition of shallow geological information.This paper is funded by the national key R&D program "Airborne Underground Mineral and Water Resource Detector Shallow High-resolution Electromagnetic Detection Technology" to carry out research on HEM shallow detection technology based on high-frequency pseudo-random source.A wide-band,low-noise sensor and a high sampling rate receiver are designed,and an analysis method based on the triplecorrelation function(TCF)is proposed for the pseudo-random source error problem.The main research work and results are as follows:(1)A wide-band,low-noise air-core coil sensor is designed.According to the requirement of long-distance transmission noise suppression,the differential signal transmission mode is designed,the influence factors of sensor static noise are analyzed,and the noise level is controlled at 4.86 n T/s.In addition,considering the constraints such as sensitivity,SNR and size of the air-core coil,the resonant frequency of the sensor is increased to above 150 k Hz,which ensures low-noise sensing of signals under high-frequency pseudo-random sources.(2)A high-sampling receiver based on the PXIe bus is designed.The timing and scale of the data flow are analyzed and a reasonable hardware design is carried out.In terms of software,the multi-channel data is integrated by using the composite timestamp,the data is stored in an improved multi-information single file method,and the storage format is optimized to realize the reliable storage of multi-information under large data volume.The receiver can achieve full-wave recording of three-component electromagnetic data,transmitting current and auxiliary information,with a sampling rate of up to 1.25MSa/s/ch and a dynamic range of 112.6 d B.(3)A pseudo-random source error analysis method based on the TCF is proposed.Different from the traditional HEM system,under the excitation of pseudo-random source,the ground response is based on the system identification,and the current data is directly involved in the calculation,so the quality of the recording is particularly important.The transmitting current error analysis method is based on the TCF's ability to distinguish error m-sequences and to suppress Gaussian noise.As a medium,the dual-threshold method is used to extract sequence features,and to determine whether the transmitting current is erroneous.As a functional software component of the receiver,it can indicate the system status and evaluate the data quality.In this paper,a wide-band,low-noise,high-speed sampling receiving system and bit error analysis method designed for high-frequency pseudo-random source HEM detection have been tested in the laboratory and field measurements.The analysis method can achieve the expected goal,and has important theoretical value and reference significance for the design of shallow HEM detection instruments. |
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