Katsuro Mogi

Ultrahigh Resolution Multiplexed Fiber Bragg Grating Sensor for Crustal Strain Monitoring

We demonstrated a multiplexed fiber Bragg grating (FBG) sensor with static strain resolution of 10 nanostrain (nε) for crustal strain monitoring. Each sensor unit consists of a pair of identical FBGs for strain sensing and reference, respectively. A narrow linewidth tunable laser is used to interrogate the FBGs, and a cross-correlation algorithm is incorporated to demodulate the wavelength difference induced by strain. When no strain is applied, an ultrahigh wavelength precision corresponding to strain resolution of 3.3 nε was obtained, indicating the ultimate resolution of the sensor system. With a variable strain applied by a piezo-stage, strain resolution of 17.6 nε was demonstrated. When the sensor is adopted for the in situ monitoring of crustal deformation, the strain induced by oceanic tide is clearly recorded with a resolution of 10 nε, providing a potential tool for the geophysical measurements.

Field demonstration of 10-nano static strain resolution multiplexed FBG sensor for geophysical applications

We have developed an FBG sensor with a strain resolution better than 10 nano-strain (10-8) for geophysics applications. The sensor consists of a pair of identical FBGs, one for strain sensing and the other for reference. A narrow linewidth tunable laser is used to interrogate the two FBGs simultaneously. Cross-correlation algorithm is utilized to extract the Bragg wavelength difference between the FBGs with high precision. Multiplexed sensing is achieved using WDM technique. With this sensor, the crustal deformation induced by oceanic tide at Aburatsubo Bay in Japan is clearly observed with a strain resolution better than 10 nε. This is the first that 10 nε order static strain resolution is demonstrated with FBG sensors, providing a potential tool for the geophysics applications.

Identifying damaged areas inside a masonry monument using a combined interpretation of resistivity and ground-penetrating radar data

The Bayon Complex in the Angkor heritage site, Cambodia, has been damaged by weathering. To plan its long-term preservation, it is essential to investigate its internal structure and the degree of damage within the masonry monument. This study shows results of ground-penetrating radar (GPR) and electrical exploration surveys, and an interpreted section of the internal structure and moisture distribution in the masonry monument. The GPR can detect boundaries between stone blocks and between stone blocks and compacted soil. Electrical resistivity can indicate moisture distribution with high reliability in combination with GPR sections. The top surface zone of the terrace structure of this monument is composed of three layers of stone blocks, and the zone below a depth of 55–60 cm is composed of compacted soil. Rainwater penetrates into the terrace through gaps between the stone blocks and drains from vertical walls through cavities in the top part of the compacted soil. Damaged areas are limited to a part of the terrace, and a large area has remained in good condition. This study shows that a combination of electrical resistivity and GPR data is useful for investigating the internal structures and classifying the degree of damage to old stone structures. This study shows results of ground-penetrating radar and electrical exploration surveys, and an interpreted section of the internal structure and moisture distribution in the Bayon Complex in the Angkor heritage site, Cambodia. Rainwater penetrates into the monument through gaps between stone blocks and drains from vertical walls through cavities in the top part.

Classifying destruction areas in a stone structure from joint interpretation of resistivity and ground-penetrating radar data

The Bayon Complex in the Angkor Heritage Site, Cambodia has been destructed by weathering effect. Investigating the structure and the progressive degree of destruction inside the masonry monument is needed to preserve it for a long time. This study shows the survey results of electric exploration and groundpenetrating radar (GPR) and the interpreted sections of internal structure and moisture distribution. GPR can detect boundaries between stone blocks and a boundary between a stone block and a compacted soil layer. Electric resistivity can estimate moisture distribution using composed structures estimated from the GPR. The top subsurface area of the terrace structure of the monument is composed of three layers of stone blocks and the area below the depth of 55 cm or 60 cm is composed of compacted soil. Rainwater invades into the terrace through between stone blocks and drains from a vertical wall through cavities in the top zone of the compacted soil. Destruction areas are limited in a part of the terrace and wide areas keep good conditions. This study shows that the joint interpretation of resistivity and GPR data is useful to investigate internal structures and to classify destruction degrees of old stone structures.