| The helicopter time domain electromagnetic method（HTEM）has the advantages of high speed and efficiency,high resolution,andstrong terrain adaptability,etc.It is widely used in the non-contact green exploration of metal minerals and groundwater in complex terrain and currently undertakes more than 90%of the global airborne electromagnetic exploration tasks.Concerning the further development of the system,the exploration needs in the areas of groundwater and geotechnical place ever higher demands on the ability of the HTEM system for near-surface detection.Since the shallow earth response mainly occurs in the early off-time and is affected by many factors at the same time,the multi-directional study of early signal correction and interference suppression based on the self-developed HTEM system is of great importance for improving the shallow detection ability of the domestic HTEM system and the expansion of its areas of application.The earliest time channel of the HTEM system can directly reflect the depth of the system’s shallow blind zone.The shorter the earliest time channel,the smaller the shallow blind zone.In shallow detection,several sets of HTEM systems with small shallow blind zones have been commercially used,including the earliest time channel of the Sky TEM system at 4.2μs,which is much better than the self-developed HTEM system.Since the core technology of foreign similar systems is in secrecy,this paper explores correction and suppression methods to improve the surface detection ability of self-developed HTEM systems and break the monopoly of foreign technology on the transfer characteristic of the receiver sensor（TCRS）,primary multiplicative noise（PMN）and attitude effect,which influence the quality of the signal at early off-time.The main research contents of the work on early signal correction and interference suppression are as follows:（1）Research on TCRS correction based on the combination of Wiener filter and calibration device.For the distortion of measurement data caused by the TCRS,especially early data,a correction method is proposed that combines an integrated correction device and Wiener filter.The method obtains a digital filter with the function of TCRS compensation by designing an integrated correction device applied to the HTEM system and training the coefficients of the Wiener filter using ideal and measured wire loop responses.The plane equation method and the wire loop model calculate the ideal wire loop reaction.The measured signal can be passed through the trained Wiener filter to remove the TCRS.Field tests show that the method can compensate the receiver sensors with different transfer functions and advance the earliest time channel of the corrected ground data to 10.42μs.（2）Research the generation mechanism and suppression method of primary multiplicative noise（PMN）in the HTEM system.Responding to the problem of unclear mechanism generation of PMN,this paper combines the time-frequency domain analysis of high-altitude signals based on previous research and finds that the birds perform damped oscillations with a bimodal distribution of the pull period along the flight direction and vertical flight direction.The receiver coil is forced to oscillate during the oscillation of the bird,resulting in a change in the relative position between the transmitter loop and the receiver coil,resulting in the generation of PMN.Since the period of the PMN is much larger than the period of the transmitter current,we designed the elastic network regularized linear regression（ENLR）framework to estimate the PMN in the half-period.In addition,we propose using adaptive moment estimation（Adam）to accelerate the model’s solution.The results of processing the flight data show that the channels that meet the error requirements after processing are advanced from the 12th to the 4th channel,and the earliest time channel is advanced to 20.83μs.（3）Research the correction method of attitude effect in HTEM system based on rotation matrix.The correction factor of the flight attitude effect under the superposed dipole model is theoretically derived using the rotation matrix and the Green tensor to study the changes in the coupling relationship between the bird and the Earth under different flight attitudes.For the influence of the bird’s response to the ground wire loop during an attitude change,the correction factor of the system’s response to the ground wire loop at different flight attitudes is derived and fitted using the rotation matrix and the microelement method.When determining the correction factor,the accurate measurement of the attitude angle determines the accuracy of correcting the attitude effect.This paper proposes a multi-GPS combination attitude meter to measure the attitude angle of the bird,and the static ground test shows that the accuracy of the attitude sensor is better than 0.1°and the output rate can be up to 200Hz.Taking the example correction of the bird’s attitude effect under the layered earth model,the maximum change in the correction factor caused by the measurement error of the attitude angle is±4.53×10-5,confirming the feasibility of the angle measurement scheme.The actual flight test shows that the attitude correction can suppress the amplitude error of all measurement channels by 25.35%.（4）Calculate the wire loop response model based on the microelement method and its application in quantitative system analysis.The mutual inductance coefficient between the bird and the arbitrarily wire loop is derived using the plane equation or the microelement method to calculate the response between the bird and the wire loop.The ideal response of the wire loop is obtained.Regarding the TCRS correction,the results before and after the correction are quantitatively analyzed using the amplitude of each measurement channel of the ideal wire loop response.Regarding PMN suppression,PMN suppression is analyzed based on the air-to-ground noise level.Given that the attitude effect is corrected,the fitting error is quantitatively analyzed using the theoretical wire loop response and correction factor fitting.The quantitative analysis based on this paper’s wire loop response model provides a solid basis for validating the proposed method.This paper proposes corresponding solutions for the correction and interference suppression of the early signal of the HTEM system,which provide strong support for fine detection of large depth ranges in complex terrain.After optimization,the earliest time channel of the HTEM system is advanced to 20.83μs,using the diffusion depth formula,and the shallow blind zone is reduced to 18.21 m for a 10Ω·m homogeneous earth. |
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