Study On Transient Electromagnetic Method Of Large Loop Source Prospecting Shallow Strata

Transient electromagnetic method is an important method in the field of electrical prospecting.But in actual work,with a larger transmitter coil is seldom used for detecting near surface target for transient electromagnetic method.In view of this situation,this paper focuses on the study of the effect of the large loop source transient electromagnetic method on the shallow anomaly.On the basis of the oblique ramp waveform,this paper discusses some problems about detecting shallow targets,and hopes to provide some references and basis for practical work.The distribution law of the primary field is analyzed,and it is concluded that the energy decreases with the increase of depth.This shows that the stimulated degree of shallow strata is larger than that of deep strata.On this basis,the energy distribution characteristics of the primary field of different transmitter coils are also summarized.The relation between the depth and the maximum line box is derived.The difference between the expressing blind zone and the detecting blind zone is analyzed.The geological model of shallow anomaly is established,and the forward calculation is carried out to study the effect of shallow exploration.In order to meet the actual situation,get observation data more accurately,the transient process of primary induced voltage in receiving coil is studied.The minimum resolvable time is calculated and abandoning the early interference data before this time in subsequent data processing.After giving up the early interference data,the apparent resistivity is calculated by using the later period formula and the global apparent resistivity method,and the data are calculated by inversion method.The buried depth of the anomalous body is Om,10m,20m respectively,and the anomalous body is obvious in the inversion results.The simulation results show that the observation data of late time contain shallow geological information.There is no blind zone in transient electromagnetic method.Simulation results show that:for the same shallow abnormal body is observed with a different size of the transmitting coil,the processing results can reflect the abnormal body;in the same observation system,when the ratio of background resistivity to abnormal body resistivity is 10,the inversion result is better than the ratio is 2.In the case of exiting a cover layer,the covering layer is not reflected,but when the resistivity of covering layer is high,it has larger anomaly area both in the horizontal and vertical directions rather than the low resistivity covering layer.The results show that the low resistivity covering layer has weaken the effect of the low resistivity abnormal body.Finally,the experiment is carried out in a shallow area containing the goaf,and the buried depth of the target layer is 20m-50m.At the end of the experiment,verified by drilling,inferred(water)goaf and the drilling results matches nearly.This shows that detection results are reliable,and detecting shallow anomaly with large transmitter loop can achieve good results.