| As one of the main methods of geophysical exploration, transient electromagnetic method is characterized by sensitization to low resistance object, anti-shielding of high resistance, and high working efficiency. Therefore, it is widely used in various fields such as hydrology, geotherm and gob area of coal mine explorations and it has obtained much better effect. However, the electromagnetic interference caused by atmospheric and human noise in the transient electromagnetic field, especially those resulting from the high voltage transmission line have aroused more and more concerns from the exploration fields. Based on the phenomenon, this thesis makes an analysis and study on transient electromagnetic method influenced by the interference from high voltage transmission line.The thesis has made a conclusion on the main interference factors such as geomagnetic pulsations, thunderstruck, magnetic substance and human noise after referring to a great number of related books. The2-D homogeneous half space forward model is established to make a numerical modeling calculation and the results are as follows:emission current is in linear co-relation with transient electromagnetic primary field, secondary field. The radius of emission loop has a complex relationship with primary field and secondary field. Signal in the superficial part of the small emission loop is strong, but the attenuation is relatively fast and signal in the deep part is weak. The resistivity of geological body influences only on the secondary field with a more complex relationship. The overall signal noise ratio of geologic body with low resistivity is lower than that of geologic body with high resistivity on conditions of the same investigation depth and emission parameters. The110kv and220kv high voltage transmission line models are established in accordance with the characters of flowing current to make a numerical modeling calculation and the results are as follows:magnetic field gradient produced by at any point on the ground presents a sinusoidal variation and the maximum and minimum values with the change of horizontal distance. The interference produced by high voltage transmission line is compared with transient electromagnetic and the results are as follows:the interference from high voltage transmission line as the primary field will produce secondary field. It has only marginal effects on transient electromagnetic. The interference is gathered in the form of secondary field and its relation to investigation depth emission current, size of emission loop and the resistivity of geological body is so close that can not be ignored.The thesis has put forward a method to eliminate the interference produced by high voltage transmission line on the basis of analyzing the interference law and discovering the features of transient electromagnetic in the field work. Furthermore, the method has been verified in the field detection and has gained much better effect. The thesis plays a great role in guiding the transient electromagnetic work operated near high voltage transmission line. |
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