Three-dimensional Forwarding Of Ground Well Transient Electromagnetic And Rapid Localization Of Abnormal Bodies Based On Convolutional Perfectly Matched Layer

Transient Electromagnetic Method(TEM)is an artificial source electromagnetic detection method based on the principle of electromagnetic induction.The transient electromagnetic method transmits a primary pulse field to the underground and collects the secondary field generated intermittently after the turn-off.Based on the electrical characteristics of the secondary field,the distribution of the underground geological structure can be probed.The most common working form of the transient electromagnetic method is to lay out the emission source on the ground and receive signal on the ground,which is called the ground transient electromagnetic method.However,due to the limition of emission magnetic moment and the interference of ground surface noise,the detection resolution of ground transient electromagnetic method is often unable to meet the detection requirements when detecting small-sized and large-buried anomalies.The ground-well transient electromagnetic method is a transient electromagnetic method developed on the basis of the ground transient electromagnetic method,which transmits signal on the ground and receive signal underground.Compared with the ground transient electromagnetic exploration method,the receiving probe of the ground-well transient electromagnetic method is located downhole,which is closer to the detection target,and the response signal obtained is stronger,which can meet the detection requirements of small-size anomalies;at the same time,this device form can also meet the needs of deeper detection.The ground-well transient electromagnetic method has unique advantages in the fine detection of large depths and small targets.However,due to the relative distance between the receiving point and the transmitting source of the ground-well TEM method,it is impossible to directly apply the relatively perfect ground TEM theory to interpret the ground-well TEM data.The relatively complete ground transient electromagnetic theory interprets ground-well transient electromagnetic data.At present,there is no systematic theory and interpretation method for ground-well transient electromagnetic method.In the field of ground-well transient electromagnetic forwarding,qualitative analysis of multi-channel response curves is mainly used to determine the approximate depth range of the underground anomaly;however,for the horizontal orientation of the anomaly and the level between the borehole.The distance cannot be accurately determined,so it is impossible to accurately locate the underground abnormal body.In view of the above problems,this paper adopts the method of combining numerical simulation and field experiment to carry out the research on the location of the abnormal body beside the well.First,in order to meet a large number of 3D forward calculation requirements,the original 3D forward calculation program with dirichlet boundary was improved.CPML boundary was used in place of dirichlet boundary,so that the speed of 3D forwarding calculation was greatly improved.Subsequently,a large number of numerical calculations are carried out by using the improved ground-well transient electromagnetic 3D forwarding program,the response curve of the model is analyzed to summarize the response law,and a rapid localization method of ground-well transient electromagnetic anomalies is proposed which can accurately locate the depth of the abnormal body and delineate it within 45° around the well.Finally,the accuracy and field applicability of the method are verified by numerical model test and field data test.This study can further improve the ability of ground-well transient electromagnetic method to locate underground abnormal bodies,and realize the refined exploration of target bodies,which is of great significance for ensuring construction safety.