Imaging, characterizing, and monitoring volcanic plumbing systems at Montagne Pelée and Mayotte using experimental petrophysics and land–marine magnetotellurics

How can experimental magnetotellurics and petrophysics provide critical constraints on the structure of magmatic plumbing systems? How can magnetotellurics contribute to volcanic monitoring? We present a workflow applied to two French volcanoes: Montagne Pelée (Martinique, West Indies) and Mayotte.Montagne Pelée volcano has experienced renewed seismic activity since 2019, with earthquakes occurring below 10 km depth and more superficial activity within the first few kilometers. In 2023, a broadband magnetotelluric (MT) survey was conducted, allowing the construction of both finite-difference and finite-element 3D electrical conductivity models down to 20 km depth. These models reveal key features of the magmatic plumbing system, constrained by the joint interpretation of MT data and experimental petrophysics, using high-pressure laboratory measurements of electrical conductivity as a function of temperature on lava samples. In June 2025, an experimental array of three continuous MT monitoring stations was installed in strategically selected locations to track fluid migration and the progressive development of partial melt within the plumbing system. We describe the complete workflow, from 3D imaging to characterization and monitoring.Since 10 May 2018, Mayotte has been experiencing one of the largest offshore seismovolcanic crises of the past three centuries. The MAYOBS1 scientific mission (2–19 May 2019, Marion Dufresne vessel) led to the discovery of a new 820-m-high volcanic edifice, named Fani Maore, with an estimated volume exceeding 6.55 km³. Between 2018 and 2021, geophysical observations revealed an eastward displacement of the island of 21–25 cm, combined with 10–19 cm of subsidence, as well as more than 100,000 earthquakes occurring at unusually large depths (22–45 km). A combination of broadband land and marine MT surveys enabled the construction of a 3D resistivity model down to 30 km depth, revealing two major conductive bodies at approximately 12 km and 22 km depth. The deeper conductor is interpreted as a magmatic mush zone with an estimated melt fraction of 22–42%. As part of the REVOSIMA (Réseau de Surveillance Volcanologique et Sismologique de Mayotte), a network of permanent MT stations is currently monitoring the magmatic plumbing system. The latest imaging results and monitoring developments will be presented.