Tsuneyoshi Sugimoto

Proposal of Non Contact Inspection Method for Concrete Structures Using High-Power Directional Sound Source and Scanning Laser Doppler Vibrometer

Maintenance for concrete structures such as buildings, bridges, and tunnels is necessary, because it is thought that a lot of them show deterioration. As a periodic inspection, a hammering test is the most popular method. However, it has several problems. One of the problems is that it is difficult to inspect the places where people cannot reach. Therefore, non contact inspection methods have been developed. As a non-contact inspection method, we propose a system consisting of a high-power directional sound source and a scanning laser doppler vibrometer (SLDV). In this method, an air-borne sound wave is used for the excitation of a concrete wall, and then the vibration velocities on the concrete wall are measured two-dimensionally by the SLDV. From the vibration velocity, defective parts can be detected. In this paper, we describe two types of experiment on the feasibility of our proposed method. In these experiments, concrete wall test pieces, which have artificial defects, are used. From the experimental results, we confirmed the effectiveness of our proposed method as a non contact inspection method for concrete structures.

Underground Imaging Using Shear Waves: Stacking Method of the Reflected Scattered Waves

Electrical and electromagnetic methods are widely used to detect buried relics. However, in marshes or swamps where the moisture constant is high, these methods are difficult to use, because the electromagnetic wave is strongly attenuated by the presence of water. Although seismic prospecting utilizing elastic waves and the common depth point stacking method (CDP method), which assumes reflection at the stratum plane, are suitable for the investigation of stratigraphic structure ( e.g., at depths of 100 to 1000 m ), commercial equipment and the CDP method are not suitable for detecting relics which exist at shallow depths ( e.g., 0–10 m ). Therefore, we proposed an underground imaging method using shear waves and a method based on stacking of reflected scattered waves in which point reflection is assumed. The experimental results confirm the feasibility of our proposed method. However, the resolution of the underground image is insufficient to detect relics. Therefore, an improvement in the resolution is necessary.

Buried Object Detection Method Using Optimum Frequency Range in Extremely Shallow Underground

We propose a new detection method for buried objects using the optimum frequency response range of the corresponding vibration velocity. Flat speakers and a scanning laser Doppler vibrometer (SLDV) are used for noncontact acoustic imaging in the extremely shallow underground. The exploration depth depends on the sound pressure, but it is usually less than 10 cm. Styrofoam, wood (silver fir), and acrylic boards of the same size, different size styrofoam boards, a hollow toy duck, a hollow plastic container, a plastic container filled with sand, a hollow steel can and an unglazed pot are used as buried objects which are buried in sand to about 2 cm depth. The imaging procedure of buried objects using the optimum frequency range is given below. First, the standardized difference from the average vibration velocity is calculated for all scan points. Next, using this result, underground images are made using a constant frequency width to search for the frequency response range of the buried object. After choosing an approximate frequency response range, the difference between the average vibration velocity for all points and that for several points that showed a clear response is calculated for the final confirmation of the optimum frequency range. Using this optimum frequency range, we can obtain the clearest image of the buried object. From the experimental results, we confirmed the effectiveness of our proposed method. In particular, a clear image of the buried object was obtained when the SLDV image was unclear.

Extremely Shallow Underground Imaging Using Scanning Laser Doppler Vibrometer

Ground penetrating radar (GPR) and elastic wave exploration have been widely used in underground exploration. However, it has not been possible to explore extremely shallow underground areas up to 10 cm below the ground surface, because various waves are received and the reflected wave cannot be separated. Although land mines have become a serious problem all over the world, the existing detection methods are not very effective in the case of soil containing electrolytes, for example, laterite soil in Southeast Asia. Therefore, we propose a method using a sound wave and a scanning laser Doppler vibrometer (SLDV). We conducted an exploration experiment of buried objects both in the laboratory and outdoors. As a result, the frequency responses of the buried objects were obtained and the responses of the buried objects were confirmed by imaging. Therefore, it is possible to use this method at an actual site.

Study on detectable size and depth of defects in noncontact acoustic inspection method

We study a noncontact inspection method for large-scale structures such as tunnels and bridges. This method involves the use of a high-powered sound source and a scanning laser Doppler vibrometer (SLDV). In our previous study, we proposed a tone burst wave method to improve the signal-to-noise ratio (SNR) of the measured result. Using this method, a defect that was difficult to detect using our previous method was detected. In this study, we examined the detectable size and depth of the defect by using a model wall with circular defects. The distance between the sound source and the concrete test piece was 5 m, and the output sound pressure was about 100 dB near the surface of the concrete test piece. As the transmitted wave, tone burst waves with different center frequencies from 500 to 7000 Hz were used. A conventional investigation by the hammer method was also simultaneously carried out for comparison and almost identical performance was confirmed. From the experimental result, we confirmed that the bending resonance frequency detected was proportional to the depth of the circular defect, and was in inverse proportion to the plane size (area) coincident to the analytical result for a circular plate. We also found that the vibration energy of the defect shows a strong dependency on its depth. Therefore, the possibility of defect depth estimation using the resonance frequency and the vibration energy ratio is expected. In the future, a practical investigation system that will replace the hammer method might be developed.

Distinguishing Buried Objects in Extremely Shallow Underground by Frequency Response Using Scanning Laser Doppler Vibrometer

A sound wave vibration using a scanning laser Doppler vibrometer are used as a method of exploring and imaging an extremely shallow underground. Flat speakers are used as a vibration source. We propose a method of distinguishing a buried object using a response range of a frequencies corresponding to a vibration velocities. Buried objects (plastic containers, a hollow steel can, an unglazed pot, and a stone) are distinguished using a response range of frequencies. Standardization and brightness imaging are used as methods of discrimination. As a result, it was found that the buried objects show different response ranges of frequencies. From the experimental results, we confirmed the effectiveness of our proposed method.

Study on the characteristics of sound propagation in shallow underground by air-borne sound

Recently, the importance of type-II dilatational waves generated by air-borne sound was recognized, because it is very useful in shallow underground imaging without making contact with the ground surface. To reconfirm the existence of this wave, the propagation time of air-borne sound in sand is measured using an acryl sand tank, condenser microphones and a loudspeaker. The experimental result shows the existence of this wave and the possibility of a non-contact method of shallow underground imaging.

Underground Imaging Using Stacking Method of Reflected Scattered Waves: Study of the Longitudinal Sound Source Using Super-Magnetostriction Vibrator

Underground imaging using a stacking method of reflected scattered waves has been studied. A sound source made of aluminum has already been employed in the hammer method. However, the amplitude of the reflected scattered waves is not stable. Therefore, we propose a new sound source comprising a super-magnetostriction vibrator whose displacement is 100 times larger than that of a conventional magnetostriction transducer. The experimental results show the potential of this new sound source.

Study on Water Distribution Imaging in the Sand Using Propagation Velocity of Sound with Scanning Laser Doppler Vibrometer

We propose a method for the monitoring and imaging of the water distribution in the rooting zone of plants using sound vibration. In this study, the water distribution measurement in the horizontal and vertical directions in the soil layer was examined to confirm whether a temporal change in the volume water content of the soil could be estimated from a temporal changes in propagation velocity. A scanning laser Doppler vibrometer (SLDV) is used for measurement of the vibration velocity of the soil surface, because the highly precise vibration velocity measurement of several many points can be carried out automatically. Sand with a uniform particle size distribution is used for the soil, as it has high plasticity; that is, the sand can return to a dry state easily even if it is soaked with water. A giant magnetostriction vibrator or a flat speaker is used as a sound source. Also, a soil moisture sensor, which measures the water content of the soil using the electric permittivity, is installed in the sand. From the experimental results of the vibration measurement and soil moisture sensors, we can confirm that the temporal changes of the water distribution in sand using the negative pressure irrigation system in both the horizontal and vertical directions can be estimated using the propagation velocity of sound. Therefore, in the future, we plan to develop an insertion-type sound source and receiver using the acceleration sensors, and we intend to examine whether our method can be applied even in commercial soil with growing plants.

Non contact long distance exploration method for concrete using SLDV and LRAD

The hammering test is a representative method in inspection for cavities and delaminations at shallow area of concrete surface. Although this method is used widely because it is not expensive, efficiency of the defect-judging largely depends on the testers experience and long measurement time is necessary for wide area inspection. Other methods have been developed, however, it is necessary to contact or approach to the inspection object during a measurement. Therefore, we propose a new non-contact acoustic imaging method for nondestructive inspection using scanning laser Doppler vibrometer (SLDV) and long range acoustic device (LRAD). In this method, Surface vibration, which is generated by air borne sound, is measured using SLDV. This time, the styrofoam board was buried at shallow depth in the concrete are used as a substitute of a cavity in the concrete. As an experimental result, a styrofoam board is clearly imaged by the vibration velocity of the concrete surface. Furthermore, we confirmed that our ...