{"status":"ok","message-type":"work","message-version":"1.0.0","message":{"indexed":{"date-parts":[[2026,7,30]],"date-time":"2026-07-30T09:29:49Z","timestamp":1785403789274,"version":"3.56.0"},"reference-count":36,"publisher":"MDPI AG","issue":"12","license":[{"start":{"date-parts":[[2026,6,9]],"date-time":"2026-06-09T00:00:00Z","timestamp":1780963200000},"content-version":"vor","delay-in-days":0,"URL":"https:\/\/creativecommons.org\/licenses\/by\/4.0\/"}],"funder":[{"DOI":"10.13039\/501100003725","name":"National Research Foundation of Korea","doi-asserted-by":"crossref","award":["RS-2024-00412141"],"award-info":[{"award-number":["RS-2024-00412141"]}],"id":[{"id":"10.13039\/501100003725","id-type":"DOI","asserted-by":"crossref"}]},{"award":["RS-2024-00412141"],"award-info":[{"award-number":["RS-2024-00412141"]}],"id":[{"id":"https:\/\/ror.org\/013aysd81","id-type":"ROR","asserted-by":"publisher"}]},{"DOI":"10.13039\/501100002641","name":"Konkuk University","doi-asserted-by":"crossref","id":[{"id":"10.13039\/501100002641","id-type":"DOI","asserted-by":"crossref"}]},{"id":[{"id":"https:\/\/ror.org\/025h1m602","id-type":"ROR","asserted-by":"publisher"}]}],"content-domain":{"domain":[],"crossmark-restriction":false},"short-container-title":["Sensors"],"abstract":"<jats:p>Accurate and reliable groundwater-level monitoring in deep observation wells remains difficult for conventional non-contact ultrasonic systems because narrow tubular geometries intensify multipath reflections, signal attenuation, and echo ambiguity. This study proposes a dual-signal direct time-of-flight (ToF) method that combines radiofrequency (RF) synchronization with one-way airborne ultrasonic propagation to a floating receiver located at the groundwater surface. In the proposed architecture, the RF signal provides a near-instantaneous time reference, whereas the ultrasonic signal defines the propagation delay, thereby eliminating dependence on echo-based ranging. The system integrates a wellhead surface unit for synchronized transmission and control, a floating unit for ToF acquisition and embedded processing, and an optional reference channel for in situ estimation of the effective sound speed. A duty-cycled power architecture is used to support low-power long-term deployment, while a multi-shot acquisition strategy with a median-like estimator improves robustness against startup transients, timing jitters, and false detections. Field validation was conducted over a 12-month period under actual groundwater-monitoring conditions, during which the groundwater depth varied between 14 m and 30 m below the wellhead datum. Within this field-validation interval, the proposed system achieved a mean absolute error of 0.048 m, a maximum absolute error of 0.050 m, and an overall valid detection rate of 99.4% over 358 valid cycles out of 360 scheduled cycles. In addition, a separate range-dependent confined-tubular propagation test was conducted to evaluate the extended detection capability of the RF-synchronized one-way ultrasonic ToF architecture. This test demonstrated stable acoustic-link ToF detection up to 300 m inside the tested 170 mm confined plastic pipeline. Therefore, the 300 m result should be interpreted as a range-dependent valid-detection result rather than as a 12-month groundwater-depth validation over the full 300 m interval. These results demonstrate that the proposed direct-ToF method provides an RF-synchronized one-way ultrasonic ToF framework with a floating receiver for groundwater-level monitoring in deep observation wells, while remaining compatible with low-power and IoT-based environmental monitoring systems.<\/jats:p>","DOI":"10.3390\/s26123672","type":"journal-article","created":{"date-parts":[[2026,6,9]],"date-time":"2026-06-09T09:46:54Z","timestamp":1780998414000},"page":"3672","update-policy":"https:\/\/doi.org\/10.3390\/mdpi_crossmark_policy","source":"Crossref","is-referenced-by-count":0,"title":["Dual-Signal Direct Time-of-Flight Method for Long-Range Groundwater Level Monitoring in Observation Wells"],"prefix":"10.3390","volume":"26","author":[{"ORCID":"https:\/\/orcid.org\/0000-0001-8024-6200","authenticated-orcid":false,"given":"Abror Shavkatovich","family":"Buriboev","sequence":"first","affiliation":[{"name":"Department of Artificial Intelligence, Gachon University, Seongnam 13120, Republic of Korea"}],"role":[{"vocabulary":"crossref","role":"author"}]},{"given":"Farkhat","family":"Rajabov","sequence":"additional","affiliation":[{"name":"Department of Computer Systems, Tashkent University of Information Technologies Named After Muhammad Al-Khwarizmi, Tashkent 100200, Uzbekistan"}],"role":[{"vocabulary":"crossref","role":"author"}]},{"given":"Jamoljon","family":"Djumanov","sequence":"additional","affiliation":[{"name":"Department of Computer Systems, Tashkent University of Information Technologies Named After Muhammad Al-Khwarizmi, Tashkent 100200, Uzbekistan"}],"role":[{"vocabulary":"crossref","role":"author"}]},{"ORCID":"https:\/\/orcid.org\/0009-0001-8668-1032","authenticated-orcid":false,"given":"Khudoyorkhon","family":"Jamolov","sequence":"additional","affiliation":[{"name":"Department of Computer Systems, Tashkent University of Information Technologies Named After Muhammad Al-Khwarizmi, Tashkent 100200, Uzbekistan"}],"role":[{"vocabulary":"crossref","role":"author"}]},{"given":"Akmal","family":"Abduvaitov","sequence":"additional","affiliation":[{"name":"Department of Information Technologies, Samarkand Branch of Tashkent University of Information Technologies Named after Muhammad Al-Khwarizmi, Samarkand 140100, Uzbekistan"},{"name":"Department of Artificial Intelligence, Samarkand State University Named After Sharof Rashidov, Samarkand 140100, Uzbekistan"}],"role":[{"vocabulary":"crossref","role":"author"}]},{"given":"Temur","family":"Azamov","sequence":"additional","affiliation":[{"name":"Agency Innovative Development, Tashkent 100174, Uzbekistan"}],"role":[{"vocabulary":"crossref","role":"author"}]},{"ORCID":"https:\/\/orcid.org\/0000-0002-4872-4265","authenticated-orcid":false,"given":"Ilhom","family":"Rahmatullayev","sequence":"additional","affiliation":[{"name":"Department of Exact Sciences, Kimyo International University in Tashkent, Tashkent 100121, Uzbekistan"}],"role":[{"vocabulary":"crossref","role":"author"}]},{"ORCID":"https:\/\/orcid.org\/0000-0002-2778-2562","authenticated-orcid":false,"given":"Cheolwon","family":"Lee","sequence":"additional","affiliation":[{"name":"Department of Computer Engineering, Konkuk University, Chungju 27478, Republic of Korea"}],"role":[{"vocabulary":"crossref","role":"author"}]}],"member":"1968","published-online":{"date-parts":[[2026,6,9]]},"reference":[{"key":"ref_1","doi-asserted-by":"crossref","first-page":"1648","DOI":"10.22214\/ijraset.2019.5278","article-title":"Novel Water Monitoring System: A Python Based Aqua Monitoring System using Raspberry PI","volume":"7","author":"Mohan","year":"2019","journal-title":"Int. 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