Research On Underground Very Low Frequency Coupling Telecommunication System And Key Technologies

Along with the progress of the resources development and utilization of underground spaces,applications such as underground environment perception,instrument monitoring,mine emergency rescue,are putting forward higher request to communication system in the underground environments.It is difficult to lay communication cables underground,and in certain circumstances wired connection become unreliable.Wireless communication is the only alternative to establish reliable underground communication.Formation is a conductive medium,and radio frequency wireless signal attenuation is extremely high.Traditional radio communication system of communication distance is limited within a few meters,which cannot satisfy the actual needs.Carrier will have to be reduced to very low frequency(VLF)band,in order to get effective signal transmission.Aiming at the application of mine emergency rescue communication,oil measurement while drilling(MWD),geological monitoring and underground personnel positioning applications,VLF coupling communication system is researched based on loop antennae,the concrete research content is as follows:(1)Research on the underground coupling communication channel model and system structureThe earth is made up of soil and rock,a mixture of sand,water,etc.,and the composition and geological structure are extremely complex.The electromagnetic signal propagating in stratum suffers greatly attenuation.Based on the theoretical analysis of vertical magnetic dipole radiation field in stratified medium and mathematical simulation model,the propagation of electromagnetic signals with different frequencies in the formation are simulated by numerical calculation method,and the VLF coupling signal propagation model is proposed.The noise sources are identified and analyzed,and the relevant parameters of the very low frequency noise model are determined.On the basis of channel modeling,the system structure and models of VLF coupling communication are proposed,which is verified by experiment system.(2)The active noise reduction method based on dual loop antennae is proposed to fight the heavy non-Gaussian noises of VLF bandNon-Gaussian noise is the main noise components of very low frequency band,appearing as a large number of random amplitude spikes in the time domain with extremely high amplitude.Even clipped by nonlinear limiter,the residual part of the non-Gaussian pulses still present as strong transient signals in the narrow band receiver,which is difficult to be eliminated through the linear filter.A large number of statistical measurements are carried out in different environments,and based on the analysis of non-Gaussian noise characteristics the receiving system and antenna is discussed,and the active noise reduction method based on double loop antennae is put forward.The non-Gaussian noises received by the wide band loop antenna receiver are extracted by timing filter,and narrow band receiving system model is set up to process the extracted non Gaussian pulses.The narrow band non-Gaussian noise samples are then subtracted from the useful signals to improve the signal to noise ratio.The method can effectively improve the VLF coupling communication channel capacity with limited transmission power and signal bandwidth.(3)The long-distance two-way coupling communication system for the mine rescue is researched and realizedBased on the VLF coupling communication system model,system designing principles and rules are discussed considering the limited transmission power and underground spaces.The coupling communication system with the communication distance of hundreds of meters is designed and realized,including the transmitters,receivers,transmitting antennae,receiving antennae,high power drivers,and high efficient modulation and channel coding scheme are proposed and realized.In mine site the communication experiments with the distance of 323 meters are carried out to verify the system performance and indicators.(4)The communication network of VLF band is researched based on the coupling waveguideAiming at the applications of petroleum MWD,intelligent monitoring of underground pipelines,distributed sensing of underground environments,etc.,coupling waveguide theory is researched.The coupling waves are propagated through the existing drilling pipes and transmission pipelines,or the buried periodic conductor structure to establish the VLF communication network.A new type of one dimensional capacitor coupling waveguide is put forward based on the one dimensional magneto-inductive waveguide to realize high speed oil drilling data communication,and a new approach based on the plane wave expansion to model the 2-dimensional coupling waves excited by a point source is proposed also.A two-dimensional cellular waveguide model is proposed to achieve a more uniform bandwidth and group velocity in all propagation directions.Simplified experiment platform and time domain simulation models are established to verify the feasibility and effectiveness of VLF one-dimensional and two-dimensional coupling waves.The research will establish the theoretical foundation for long distance pipeline communication and VLF wireless underground communication networks.Research of this paper will establish theoretical basis for long distance underground coupling communication and large-scale underground monitoring networks while providing system designing basis.The research of the electromagnetic signal amplitude distribution and active noise reduction can also be extended to the underground personnel and facilities coordinate positioning,wireless power transmission,and underwater communications,etc.,with high theoretical and application value.