Small Fixed Loop Transient Electromagnetic Method For Advanced Forecast Of Geology And Experimental Study

Transient electromagnetic method(TEM)is a time domain electromagnetic method of artificial source.It has the advantages of high depth of detection,high transverse resolution,low resistance sensitivity and high efficiency.It has been widely used in mineral exploration,hydrogeological investigation and engineering investigation.At present,the application of transient electromagnetic method in the advanced geological prediction of the tunnel surface mainly uses the moving source device of center return line.This detection method has a smaller transmission line,so the depth of exploration is limited.And only arrange a line,we get less data of detection as well as low precision of detection accuracy.It only reflect a single line on the electrical structure and electrical distribution and can only get the front geological apparent resistivity sectional drawing in the data processing imaging time.It cannot be three-dimensional fine prediction in front of the face of the poor geological.In order to increase the area of transmission coil as much as possible and increase the detection depth and to forecast the three-dimensional scale and the spatial form of the poor geological body in front of the face of the palm,it is very important to use the fixed source emission coil to increase the area of the transmitting coil and make full use of each receiving point inside the transmitting coil to interpret the data.However,in the excitation of small fixed source loop,the internal or even 1/3 within the middle of the loop will not be as same as the large loop,in contrast,dramatic changes will be performed,which makes the collected internal data not be approximately calculated as the resistivity formula at the center point.Therefore,it is imperative to study the data interpretation method of the vertical component of the non-center point in the transient electromagnetic detection of micro-source return line.Currently,at theoretical aspects,the study on the interpretation of non-central data is still not mature enough,so it is difficult to directly give the resistivity formula of non-central point.In this paper,through the numerical simulation and experiment,we study the law of the change of the received signal due to the change of the relative position of the receiving point and the transmitting coil in the uniform half-space geo-electric model,and obtain the proportion relationship of the non-center point data and the center point data.The proportional function of each point and the center point data is obtained,and the resistivity formula of the center point is used for calculating the data of the non-center point.Then a transient electromagnetic advance geological prediction method with small fixed source return line is achieved.