Sensitivity of magnetotelluric responses in the exploration of the Oldoinyo Lengai magmatic plumbing system

The Oldoinyo Lengai stratovolcano, located in northern Tanzania, is the only active volcano currently erupting natrocarbonatite lavas. It is situated within the East African Rift, the longest continental rift in the world. While extensive geological research has been conducted on Oldoinyo Lengai, geophysical studies, particularly those using electromagnetic methods, remain limited. A recent seismological investigation in the region revealed a complex, highly interconnected lateral and vertical plumbing system. Additionally, recent Global Navigation Satellite System (GNSS) and Interferometric Synthetic Aperture Radar (InSAR) studies have shown subsidence around the volcano, suggesting the presence of a shallow, deflating magma source connected to deeper reservoirs. This has emphasized the urgent need for a comprehensive study of the inner structure of the volcano to assess potential hazards. Magnetotellurics (MT) has proven effective in imaging magmatic systems by mapping electrical resistivity distributions at depth, which are particularly sensitive to the presence of fluids and melt. Therefore, applying MT to infer the subsurface structure of Oldoinyo Lengai is highly promising, as it could provide valuable insights into the magmatic reservoirs driving volcanic-tectonic events, including their depth, geometry, and distribution.

This study evaluates the feasibility of using MT to detect the magmatic system beneath Oldoinyo Lengai by analyzing MT response distributions (e.g., Phase Tensor). We incorporate previous seismological findings to model the electrical conductivity structures at both shallow and deep levels. Our analysis focuses on: a) evaluating the resolving capacity of MT to identify conductive features that may correspond to magmatic reservoirs, and b) examining the spatial distribution and frequency behavior of MT responses based on equivalent conductivity models. Legacy data from the region has been reprocessed, providing a more accurate reference frame. This allows for a clearer contrast between the modeled structures and background resistivities, suggesting that MT is an ideal method for imaging the internal architecture of the volcano.

The station locations have been selected based on accessibility and logistical considerations. The proposed MT array will have higher resolution around Oldoinyo Lengai, with increasing interstation distances toward the outer array, extending up to ~40 km. This strategy aims to investigate the magmatic system beneath Oldoinyo Lengai and explore potential connections with surrounding volcanoes, opening doors for future regional-scale studies.