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. 2019 Sep 23;19(19):4114. doi: 10.3390/s19194114

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© 2019 by the authors.

Licensee MDPI, Basel, Switzerland. This article is an open access article distributed under the terms and conditions of the Creative Commons Attribution (CC BY) license (http://creativecommons.org/licenses/by/4.0/).

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Spectrogram calculated from C-OTDR signal in single sensor segment within the power cable at a sensing distance of more than 55 km . The excitation frequency $f = 0.6$ Hz is clearly visible.