Experimental Examination of Preloading for Strainmeters in Deep Seafloor Boreholes

Geodetic and seismic observation in deep seafloor boreholes can monitor activities around subducting plates with higher precisions relative to other observations taking advantage of shorter distances to plate boundaries and lower noise levels. One of the key techniques to provide precise observation is to couple the sensor to the surrounding media. This study examines the coupling condition of a sensor of a strainmeter in the borehole through a laboratory experiment. The sensor was cemented in a tank that simulates a borehole. Strain and temperature variations were recorded continuously until 30 days after the beginning of the cementing experiment. Since the strain records contained large amplitude of thermal responses, reduction of them by a model fitting was performed. After the reduction, preloading condition was suggested from a strain variation of extension of the sensor with time.