Monitoring and application of distributed optical fiber sensors in transient temperature field of frozen soil

A monitoring method for transient temperature field of frozen soil using distributed optical fiber sensors was developed based on Raman optical time-domain reflectometry(ROTDR).Three Xiashu soil specimens with their respective water content of 20%,40% and 60% were monitored under a cycle of freezing and thawing,when the method was employed in real time.A space-time matrix of the monitored data was obtained.Several distribution features of the transient temperature field,including local distribution,negative temperature boundary distribution,specific-position distribution and specific-time distribution,were extracted from the matrix in the respective phases of unfrozen soil,freezing soil,thawing soil and thawed soil.The disturbed feature resulted from an abrupt temperature rising of the environment was identified.It was discovered that the coating of sensing fiber resulted in a lagging effect of temperature response.It was also shown that there was an inverse correlation between the speed of temperature variation and the initial water content for freezing soil,but no monotonic correlation for the thawing soil.