Transient Electromagnetic Method Shallow Prospecting Technology With Small Loop Device

Transient electromagnetic method is one of the mainly geophysical explorationtechniques. Nowadays TEM has been widely used in mineral exploration, drilling,marine and aviation areas and so on. With the development of social economic and theexpansion of city construction, more and more attention is paid to engineeringgeological environment. Therefore, near-surface and high-resolution shallow transientelectromagnetic detection technology has become a hot issue of this area. However,because of some shortcomings of transient electromagnetic prospecting system itself,such as the switch-off time of transmitting current、low sampling rate、 transientprocess of receiver coil and so on, a blind zone exits during TEM shallow application.On account of transient electromagnetic central loop manner shallow prospectingtechnology with small loop device, the following studies are mainly discussed in thisarticle:①Based on circuit theory, the equivalent circuit of receiver coil is deeply studied.The relationship between damping coefficient of equivalent circuit and distributionparameters of receiver coil is given. And the calculation method of critical dampingresistance is also offered in this essay. The negative step response characteristic ofreceiver coil under different damping coefficients is discussed.②All time transient electromagnetic response signal characteristic ofnonmagnetic electrical conductor under negative tilting step excitation is analyzed.And primary induced electromotive force and its transient process and secondaryinduced electromotive force and its transient process are studied. At the same time, therelationship between all time transient electromagnetic response and primary fieldresponse、 secondary field response is analyzed under the influence of transientprocess, which provides a theoretical basis for the extraction of all time transientsecondary field response signal.③Based on the negative step response signal of homogeneous half-space, theinfluence of transient process on data processing is discussed. And the impact law ofdamping coefficient to effective sampling time is given. Changing trend of apparentresistivity error with radius of receiver coil is studied under different effective receiverareas to put forward the concept of effective radius, which can guide parametersselection of small loop device during engineering applications. ④The relationship between all time transient electromagnetic field and primaryfield、all time transient secondary field is studied when transmitting current begins toshut off. And the method of removing primary field to extract all time transientsecondary field is also offered with a validation that removing primary field andeliminating the influence of receiver coil transient process can improve the resolutionand accuracy during shallow detection.⑤Based on the theory of “smoke loop”, TEM1-D fast inversion method isstudied. With the use of transient electromagnetic detection system and GDP-32multi-function receiver, an air-raid shelter experiment was carried out in our schoolwith good results. The experiment shows that, during TEM shallow detection, theinfluence of transient process and primary field signal on earlier secondary field signalis very large. After data processing, imaging results becomes clearer. And shallowdetection resolution is also improved. The results gives a demonstration that, the TEMprospecting system designed in this article can detect the location of the air-raid shelter,having the basic functions of TEM shallow detection. And the goal of shallowdetection is reached preliminarily.