| The Transient Electromagnetic Method(TEM)is a Method for Electromagnetic exploration in the time domain.This method is a kind of artificial electromagnetic detection method based on the principle of electromagnetic induction.This is a magnetic field(electric field)pulse sent underground to generate a magnetic pulse,using a grounded or ungrounded loop arranged near the ground or underground,with a pulse current that changes in step.Under this excitation,the induced eddy current in the geological body excited by ohmic loss,(secondary field)an induced electromagnetic field varying with time.The secondary field generated by this excitation contains abundant geoelectric information of underground geological bodies.During the interval of the first pulse magnetic field,the information of the secondary field is measured by ungrounded coil or grounded electrode.The attenuation law of the secondary field is affected by the resistivity,volume scale,buried depth,frequency of transmitting current and waveform of the good conductor.The purpose of detecting the distribution of underground medium and geological body is to measure the secondary electric field and magnetic field generated in the field pulse period of geological body.Although there are many kinds of tem,the eddy current field problem of secondary excitation of conductive dielectric under step pulse is based on mathematics and physics.The section measurement method is mainly used to explore the well-conductive metal ore body,karst,engineering foundation,etc.,and study the transient electromagnetic response of local conductors.The sounding method is mainly used to detect the strata structure,such as coal field,groundwater,geothermal,oil and gas fields,etc.,and to study the transient electromagnetic response of the horizontal layered earth.Transient electromagnetic theory has a strong complexity,and at the present stage of its technical theory understanding is not deep enough,especially in the three-dimensional transient electromagnetic response,still far from the practical level.In the current production practice,the interpretation method and technology of one-dimensional transient electromagnetic theory are mostly used,not only because of the lack of understanding of multi-dimensional characteristics of transient electromagnetic response,but also because of the lack of software to simulate three-dimensional transient electromagnetic field.Three dimensional tem can be solved by finite difference time domain method,finite element method,finite volume method,integral equation method,etc.But in the face of complex and have terrain simulation study still need to use the finite element method(fem)to study the response characteristics of 3D electromagnetic field,so the text using numerical simulation software CST were studied on the transient electromagnetic method in time domain of 3D geological body different response characteristics were analyzed,and different geoelectric model,simulate low resistivity thin layer and different depth,such as high resistance thin layer resistivity,size and depth distribution of the magnetic field around the conductive abnormal body.In this paper,the central loop device commonly used in practical production is adopted,and the transmitting and receiving coils are rectangular.The geological information of the abnormal body is obtained by analyzing the amplitude difference between the internal and external induced electromotive force received by the central loop and the amplitude difference between the induced electromotive force around the wire frame.Therefore,the main purpose of this paper is to study the different responses obtained by fem forward modeling of 3D geological bodies and analyze the difference values of such responses.Therefore,the feasibility of using CST numerical software for transient electromagnetic exploration is verified,and a new research idea is provided for the detection of geological abnormal bodies. |
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