Imaging of an extinct magmatic system to de-risk geothermal exploration: 3-D joint Magnetotelluric and Gravity inversion on Lānaʻi (Hawaiʻi)

Reducing exploration risk in geothermal projects requires workflows that can robustly connect subsurface characterization to subsequent decisions on well siting and heat/energy extraction. Here, we present an island-scale, multi-physics imaging workflow that integrates magnetotelluric (MT) and gravity data to constrain the geometry of a volcanic magmatic system and its associated hydrothermal system. We invert MT in 3-D on a locally refined, topography-conforming mesh, and jointly invert gravity using a cross-gradient structural coupling to the resistivity model. The resulting coupled resistivity-density models image a dense, resistive intrusive complex beneath the Palawai Basin interpreted as the remnant upper crust magma reservoir, and radially oriented lineaments consistent with rift-zone dike complexes that terminate around 5 km depth. Above the intrusive core, we resolve a moderately conductive layer that may signify hydrothermal alteration products and/or groundwater, while low resistivities near the coastline delineate extensive seawater/brackish intrusion within the basal aquifer system.

Our models identify a region where the intrusive body -- interpreted as a potential heat source -- is shallow, suggesting an elevated thermal gradient. More broadly, this case study shows how multi-parameter geophysical observations and 3-D imaging can inform conceptual models that support geothermal exploration.