Research On Wireframe Design And Application Of Shallow Small Loop Transient Electromagnetic Method

With its advantages of simple equipment,convenient operation,and high working efficiency,the transient electromagnetic method has been widely used in many fields,such as hidden underground aquifer structure survey,geological hazard survey,and civil engineering survey.In practical applications,transient electromagnetic systems need to lay a certain size transmitting coil in the field,However,due to the field construction environment,in some construction sites with complicated terrain,the laying of coils is difficult.Not only affect the efficiency of field data collection,but also have a great impact on the quality of the collected data.And in most cases,field surveys such as hydrogeological surveys,geological hazard surveys,and construction surveys for workers and civilians do not require high depth.Under the requirements of shallow exploration depth,the size of the transmitting coil can be effectively reduced by increasing the number of turns and transmitting current of the transmitting coil.Starting from Maxwell's equations,this paper introduces the transient electromagnetic field of uniform half-space and the transient electromagnetic field of layered magnetic dipole sources in detail.The forward simulation simulates the electromagnetic response law under different launch parameters and different geoelectric models.Under the uniform geoelectric model,the transient electromagnetic response amplitude increases linearly with the increase of the transmission current and the number of turns of the transmission coil,and increases exponentially with the increase of the size of the transmission wire frame.In the homogeneous half-space geoelectric model,when the transmitted magnetic moment is constant,the greater the surrounding rock conductivity,the greater the amplitude of the secondary field response.When the surrounding rock conductivity is less than0.001S/m,the response has entered the late stage within 10 ms.Although the induced magnetic field becomes smaller,the diffusion depth according to the transient electric field is more than 200 meters.For the two-layer geoelectric model,the thickness of the overlying layer is greater than 10 m and the difference in conductivity is more than2 times,which will stimulate an effective secondary field.For A-type and Q-type geoelectric models,small emission magnetic moments can stimulate an effective secondary field.In comparison,Q-type geoelectric models are better.For the "H" type geoelectric model and the "K" type three-layer geoelectric model with a thin layer in between,the change rate of the anomalous field increases linearly with the increase in the thickness of the thin layer;it increases exponentially with the increase in the difference in the conductivity of the thin layer.The influence of emission parameters on the turn-off time is studied.If the transmission magnetic moment is constant,the wire frame with a small size should be selected as much as possible,and the number of turns of the coil should be reduced as much as possible to reduce the turn-off time.The off time decreases exponentially.The lower the formation resistivity,the greater the emission magnetic moment required to detect the same depth;the smaller the formation resistivity,the smaller the turn-off time,the greater the minimum detectable depth;and the earliest resolvable time is affected by the emission wireframe,the general line The smaller the frame,the smaller the earliest distinguishable time.The electrical parameters of the receiving coil are analyzed.In general,the resistance and inductance of the receiving coil increase linearly with the increase of the coil size;the distributed capacitance of the receiving coil can be reduced by the method of winding;and the receiving coil The damping resistance needs to be adjusted according to different geological conditions.Based on this,a receiver integrated wireframe suitable for shallow exploration was designed.The application of the small loop transient electromagnetic method to the karst development area and reservoir leakage area has achieved good exploration results?The exploration results were verified by field drilling and high-density resistivity method.Finally,the reliability of the small loop transient electromagnetic method exploration results was verified.