The Application Of Ground-airborne Transient Electromagnetic Method With Electric Source In The Gobs Detection

Due to the large-scale exploitation of coal,especially the spontaneous mining of many small coal mines,a large number of unclear underground gobs have appeared.These gobs will not only cause harm to the ground construction and engineering facilities,but also the safety of the mines.Therefore,it is of great significance to detect the underground gobs formed during coal mining.When conducting traditional terrestrial geophysical exploration in areas with complex topography and geomorphology,the work becomes difficultly,inefficiently,and costly.In contrast,the ground-airborne transient electromagnetic method can quickly and extensively conduct the exploration of the gobs area.The efficiency has been greatly improved,especially in mountainous regions with complex terrain condition.Ground-airborne transient electromagnetic method is a new geophysical method.It arranges excitation sources on the surface,and carries flight platforms to collect signals in the air.This operation method takes the advantages of airborne and ground transient electromagnetics.The observation response has high signal-to-noise ratio,high efficiency,and large exploration depth,particularly suitable for the areas with complex terrain conditions.At present,although the ground-airborne transient electromagnetic method has achieved a rapid development,there are few applications in practical engineering.Most of them are based on terrestrial systems during the data process and interpret,and there is no supporting special interpretation method,which restrict its development.The one-dimensional forward problem is the basis for studying ground-airborne transient electromagnetics.In this paper,firstly,one-dimensional forward problem of ground-airborne transient electromagnetic method is discussed.The electric dipole superposition method is used to obtain the one-dimensional frequency domain response of ground-airborne transient electromagnetics of electric source,and then the frequency domain electromagnetic field response is covert to time domian.by sine and cosine transformation,which realize the calculation of forward modeling of ground-airborne transient electromagnetic response in time domain.Secondly,the uniform half-space and layered model are designed,the parameters such as the receiver height,offset,and layers' resistivity are changedin order to analyze the influence of these factors on their response and the response characteristics of the typical layered model.The apparent resistivity can directly and clearly reflect the underground electrical information,and the calculation is simple and fast.It has widely used in practical production.Therefore,the definition of the apparent resistivity of ground-airborne transient electromagnetics is discussed in this paper.The idea of an inverse function establishes an iterative algorithm for resistivity calculations,enabling full field apparent resistivity calculations that can satisfy early and late time paths and offsets.Different layered models were designed to acquire the characteristics of the resistivity curve by changing parameters such as layer thickness and interlayer resistivity.The results show that the full field apparent resistivity calculated by this method can accurately and explicitly reflect the underground electrical information.Aiming at the application of ground-airborne transient electromagnetic method in the gobs detection,a three-dimensional water-filled gobs model was designed and the full field apparent resistivity profile was drawn.The results diaplay that the resistivity profile can clearly reflect the abnormal area of water gobs.At present,the ground-airborne transient electromagnetic method has received more attention because of its own advantages,but it is still in the research stage and rarely used in practical production.In this paper,based on the actual project of ground-airborne transient electromagnetic detection of coal gobs,the full field apparent resistivity method introduced in the paper is applied to the detection of coal gobs,and the ground-airborne TEM data detected in the gob area are processed.According to the definition of resistivity,the resistivity profile and the slice diagram are portraied.The results demonstrate that the anomaly of water-filled gobs can be clearly reflected from the figure.It is proved that the method is validity in the detection of the gobs area.