MDPI Acta Medica Okayama 1424-8220 23 1 2022 Thickness Measurement at High Lift-Off for Underwater Corroded Iron-Steel Structures Using a Magnetic Sensor Probe 380 EN Shoya Adachi Graduate School of Interdisciplinary Science and Engineering in Health Systems, Okayama University Minoru Hayashi Graduate School of Interdisciplinary Science and Engineering in Health Systems, Okayama University Taisei Kawakami Graduate School of Interdisciplinary Science and Engineering in Health Systems, Okayama University Yuto Ando Graduate School of Interdisciplinary Science and Engineering in Health Systems, Okayama University Jin Wang Graduate School of Interdisciplinary Science and Engineering in Health Systems, Okayama University Kenji Sakai Graduate School of Interdisciplinary Science and Engineering in Health Systems, Okayama University Toshihiko Kiwa Graduate School of Interdisciplinary Science and Engineering in Health Systems, Okayama University Toshiyuki Ishikawa Department of Civil, Environmental and Applied System Engineering, Faculty of Environmental and Urban Engineering, Kansai University Keiji Tsukada Graduate School of Interdisciplinary Science and Engineering in Health Systems, Okayama University Infrastructure facilities that were built approximately half a century ago have rapidly aged. Steel sheet piles, the inspection object in this study, are severely corroded, resulting in cave-in damages at wharfs. To solve such a problem, non-destructive inspection techniques are required. We previously demonstrated plate thickness measurement using extremely low-frequency eddy current testing. However, when the steel sheet piles are located in water, shellfish adhere to their surface, causing a lift-off of several tens of millimeters. Therefore, this large lift-off hinders the thickness measurement owing to fluctuations of magnetic signals. In this study, sensor probes with different coil diameters were prototyped and the optimum size for measuring steel sheet piles at high lift-off was investigated. Using the probes, the magnetic field was applied with a lift-off range from 0 to 80 mm, and the intensity and phase of the detected magnetic field were analyzed. Subsequently, by increasing the probe diameter, a good sensitivity was obtained for the thickness estimation with a lift-off of up to 60 mm. Moreover, these probes were used to measure the thickness of actual steel sheet piles, and measurements were successfully obtained at a high lift-off. No potential conflict of interest relevant to this article was reported.