| Meta-instable stress state is hopefully identified by monitoring the dynamic changes of underground stress in seismic observation,which is the key stage for the faults turning into instability.The possibility of earthquake can be determined by recognizing the meta-instable stress state.The change of strass can be generally observed about 0.01MPa before and after strong earthquake.Therefore,observation devices with which resolution is at least up to 0.01MPa are needed to obtain enough observation signal to noise ratio.In the study of Academician Ma jin,it showed that there is a clearly linear connection between the change of volume strass and the change of temperature under the situation of elastic deformation.In addition,temperature is a scalar,so it's easier to handle temperature data.With the function of?T=?T??,??7.5×10-4K/MPa?diorite granite,for example?,and the thermodynamic temperature T=300K in the normal temperature,so?T?2.25×10-4K.In other word elative resolution of temperature measuring is at least 1×10-4?.The research of Ma Jin etc.shows that the temperature of the station near fault in the stage of instability are heating and cooling,but was small?around 0.1mK magnitude?.In order to get enough temperature observation SNR,it is required that the resolution of temperature sensor can reach 1×10-5?order of magnitude.Current temperature with which highest resolution is 1×10-4°C,can not meet the demand of temperature monitoring and recognizing the meta-instable stress state in deep well.As a result,a special thermometer with higher resolution is needed.In order to monitor the changes of deep well temperature with high resolution,a distributed measurement method is adopted,and the N groups of temperature sensors are evenly distributed in the temperature measurement environment.So that it is possible to simultaneously monitor the changes of the ambient temperature at multiple points,and the obtained N groups of data are averaged to improve the resolution of temperature measurement.The temperature sensor adopts the patch typed Ptl000precision resistor as a temperature-sensitive unit.And a four-wire connection method is used.The temperature measurement circuit is designed based on the balanced bridge of a plurality of Pt1000 sensors connected in series.The distributed temperature measurement system utilizes Li-ion rechargeable battery as a driving power source to supply power for the multiple temperature measurement units.The output voltage signal of the temperature measurement circuit is amplified by a differential amplification circuit.And then the analog signal is converted into a digital signal by the data acquisition unit of an EDAS-24GN.Finally,the signal is transmitted to the PC and temperature data processing is performed.At the same time,in order to reduce the self-heating effect of the temperature measurement system,just the temperature sensor is placed in the deep well and other devices are placed on the well.Due to the limitation of the dynamic range,increasing the resolution will result in the smaller range of the temperature measurement.The measurement range in this paper is only 22.6°C31.8°C.It can adapt to different requirements of the temperature measurement range by changing the equilibrium point of the temperature measurement bridge.In this paper,adopting a number of measures such as the distributed temperature measurement method,a plurality of platinum resistance connected in series,improving the bridge power supply voltage and good thermal conductivity of aluminium base PCB improves the sensitivity of the temperature measurement circuit and the heat transfer coupling between temperature measurement circuit and measured medium,which reduces the effect of spontaneous heat for platinum resistance on temperature measurement.In the temperature resolution and noise test,the contrast observation and the related analysis method was introduced.Tests result shows that the temperature monitoring range of the temperature sensor constructed in this paper is adjustable,and the monitoring resolution is in the order of 1×10-5°C,and the effective value of noise of the system is better than the initial proposed resolution indicator of 1×10-5°C,which can meet the needs of seismic precursor temperature observations. |
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