Development Of Array Transient Electromagnetic Detection System For Urban Underground Space

Along with China's urbanization process,a large number of people have poured into cities.Some problems began to emerge in the process of urbanization.The first problem was the large shortage of urban space.To this end,the city began to expand to surrounding areas and underground.In order to conduct efficient and safe underground space development,accurate preliminary exploration is the key to ensuring safe and efficient construction.At present,the main methods used in the detection of medium and shallow underground structures include shallow seismic methods,ground penetrating radar methods,and transient electromagnetic methods.Due to the inherent limitations of the shallow seismic method and the ground penetrating radar method,this paper uses the transient electromagnetic method to solve the problem of urban underground space detection.The transient electromagnetic method is a method of radiating a transient electromagnetic field to the ground by using an active source coil,and using the observed induced electric field data to determine the underground structure.When using an ungrounded loop for transmission,due to the inductive characteristics of the transmitting coil.The falling edge has the problems of long off-time and insufficient off-line linearity.The turn-off time is closely related to the minimum effective detection depth.Although there are theoretical studies on the full waveform inversion of the transient electromagnetic method at home and abroad,there is still no reliable and effective method to invert the entire process of transient electromagnetic emission.Therefore,reducing the system's off-time is the main effective method to reduce the detection of shallow blind zones.In addition,due to the special nature of urban detection,the size of the transmitting coil is also limited,and the method of laying large-sized coils cannot be used to increase the magnetic moment.Therefore,in the detection of urban underground space,the problem of how to use a small-sized coil to achieve a large transmission magnetic moment is also needed.Another feature of urban transient electromagnetic applications that is different from traditional transient electromagnetics is that the resolution is more demanding.Because the main detection targets of urban detection are mostly pipelines,smaller water bodies,abnormally buried steel bars and other anomalous bodies left over from historical construction,the transient electromagnetic system should be optimized for such detection requirements.Aiming at the above problems,this paper makes a series of improvements to the existing transient electromagnetic system.Firstly,an array transmitting antenna solution is proposed for the problems of insufficient resolution and insufficient transmitting magnetic moment.By setting multiple array type transmitting coils,the transmitting magnetic moment is effectively improved;the off-time is long for the existence of traditional transient electromagnetics.In order to solve the problem of insufficient linearity during shutdown,an active constant voltage clamping structure based on self-resonant oscillation technology is proposed to achieve fast linear shutdown under high power transmission,effectively reducing the detection blind zone of the transient electromagnetic method;The problem of higher lateral resolution required by urban transient electromagnetics is to propose an array receiving solution.By setting an array coil,the information of multiple dimensions obtained from the underground by a single detection can be effectively increased.