Semi-Airborne Transient Electromagnetic Apparent Resistivity Imaging And Its Application In Tunnel Survey In Complex Terrain Areas

With the gradual completion of the 13th Five-Year Plan,China's comprehensive well-off society has also entered the stage of decisive success.A series of policies such as "the Belt and Road Initiatives" and "Precision Poverty Alleviation" have been proposed one after another,which means that the focus of infrastructure construction has shifted to the western region.The terrain in the western region is complex and the geological data is lacking.Traditional geological exploration methods such as geological exploration and geological drilling have been unable to meet the engineering needs.New geological exploration methods are needed to improve the geological exploration system,and the Semi-Airborne TEM came into being.Two aspects of Semi-Airborne TEM data interpretation and imaging problems are studied.On the one hand,based on the dipole field response expression,the Semi-Airborne TEM response expression of the grounded wire source is derived and the forward calculation program is written.The typical numerical model is used for forward calculation,and the Semi-Airborne TEM response characteristics of the ground source is summarized.On the other hand,the definition method of full field apparent resistivity based on the attenuation voltage is introduced to calculate the full field apparent resistivity of the typical layered geoelectric model.Combined with engineering application examples,the feasibility and accuracy of the method are analyzed.This paper mainly explores the above two aspects.The superposition thought is used instead of the integral idea to solve the expression of grounded wire source electromagnetic field,and the frequency domain response expression of the dipole field under uniform half-space and isotropic horizontal layered earth conditions is obtained.Combined with the cosine transform,the response expression of grounded long wire source of Semi-Airborne TEM is obtained.We set uniform half-space model and typical layered model for numerical calculation,summarizing the Semi-Airborne TEM response characteristics under different flight heights,different offsets and different geometric positions.The variation of flight height and offset will only affect the early transient response,but they have little effect on the overall response.The smaller the vertical distance between the receiving point and the excitation source,the stronger the early signal and the stronger the data quality.At the same time,compared with the magnetic field signal,the attenuation voltage signal has a stronger ability to describe the Semi-Airborne TEM response characteristics and has a wider range of characterization.Most of the existing Semi-Airborne TEM interpretation methods are based on the traditional apparent resistivity and the limit conditions are difficult to meet.Based on the results of Semi-Airborne TEM forward modeling of grounded wire source,the author introduced the concept of full field apparent resistivity defined on the basis of attenuation voltage.And the author used the inverse function instead of the inversion idea to derive the full field apparent resistivity formula and write the corresponding calculation program.And the typical electric model is used for numerical calculation.The variation law of the full field apparent resistivity curve of the typical three-layer geoelectric model with different offsets,different flight heights and different formation thicknesses is summarized.Finally,the Semi-Airborne TEM of grounded wire source field test was carried out in a targeted manner,and effective data was obtained.Compared with the geological data obtained from the site excavation,the feasibility and accuracy of the interpretation method in practical engineering applications were verified.This paper combines theoretical derivation,numerical calculation and field test to study the Semi-Airborne TEM response characteristics of grounded wire source and the full field apparent resistivity interpretation method,which provides a certain theoretical and practical basis for the formation of the subsequent Semi-Airborne TEM matching interpretation system.It also provides a basis for the popularization and application of this method in practical engineering.