Development Of Prototype Of Probe And Receiver Of NMR Logging Tool For Shallow Surface Hydrogeological Parameters

With the continuous progress of technology and the rapid development of economy,the shortage of water resources is becoming more and more serious.The rational utilization of water resources,the monitoring of the groundwater level change,the influence of the dark current on the construction of the project,and the investigation and management of the hydrogeology of the polluted land have become an important task in the new age.Through the detection of shallow surface hydrogeology,the situation of groundwater can be obtained intuitively,especially in the aspects of urban construction,farmland irrigation and other industry and agriculture sectors,which all cannot be separated from the measurement of shallow surface hydrogeological parameters.As a hydrogeological detection technique,NMR logging can measure the porosity of groundwater directly.By Comparing with other logging techniques,it has the characteristics of fast speed,high resolution and unique inversion interpretation.It also avoids the formation pollution,environmental interference and risk to workers,which are caused by radioactive element detection.At present,the nuclear magnetic resonance logging tool is mainly used in oil and gas detection,but the instrument itself is large,inconvenient to move,and has high service cost.Furthermore,it cannot be applied to the survey of shallow geology,such as water level monitoring,building road foundation.Therefore,this paper has developed a prototype of the probe and receiver of NMR logging tool,which is miniaturized,easy to carry and low cost,and is applied to shallow surface hydrogeological survey.In the first place,according to the logging demand,the design requirements of the probe are clarified.The probe is the core of the whole nuclear magnetic resonance logging tool,its design determines the logging performance index and the parameters of the launch system and receiving system and can be achieved by comparing with other probes and combined with the characteristics of logging tool,which is miniaturization,easy assembly and low cost.On the basis of the method of solving the positive problem of electromagnetic field,using the finite element simulation software COMSOL to simulate the static magnetic field of permanent magnet.By adjusting the parameters of the shape and distance of the permanent magnet repeatedly,the design of the probe structure is completed on the premise of ensuring the performance of the instrument.Moreover,the depth and working frequency of the instrument are given through actual measurement.In order to reduce the influence of skeleton on static magnetic field and RF magnetic field,this paper chooses skeleton material with magnetic permeability close to air.At the same time,considering the problem of antenna heating in the instrument experiment,the material of "plastic king"-polytetrafluoroethylene is chosen to be the skeleton of the probe,and protracting the model with the drawing software AutoCAD,then the actual skeleton of the probe is processed,and completes the assembly of the probe.Secondly,the receiver adopts the design principle of low noise.Using the the steps of scheme comparison,theoretical analysis and actual noise test to adopte for the design of power circuit.Through the theoretical analysis of the noise of the amplifier circuit,the preamplifier circuit is selected by multi-stage amplification cascade structure,and the low noise chip is selected as the preamplifier.Aiming at the characteristics of low signal to noise ratio and high frequency,paper designed an eight-order bandpass filter amplifying circuit with Sallen-Key structure and simulated the circuit with Multisim.A zero-drift amplifying circuit is designed,which avoids signal distortion in principle.Finally,this paper tested the probe,the receiver and the system in the shielding room.The test of probe mainly includes the probe radial magnetic field test and the magnetic field test in the detection area ring.According to the measured data,the detection depth of the instrument is 6 to 7cm from the center of the probe,and the working frequency is 255 kHz;The equivalent input noise is 7.99nV?Hz^1/2,and the equivalent input noise density is 2.95?V with the test of background noise of the receiver;The joint test of the probe and the receiver use the simulated NMR water signals and the water signals added to the noise respectively,and the observation is carried out through the probe antenna induction and signal receiving processing.The results show that the received water signal has an ideal water signal attenuation trend,the waveform is stable and complete,the system has a certain noise filtering ability,and the design of the probe and receiver for the shallow surface NMR logging tool is realized.