Geophysical Imaging of the Shallow Geyser and Hydrothermal Reservoir Structures of Spouter Geyser, Yellowstone National Park: Geyser Dynamics I.

Yellowstone National Park (YNP) is home to roughly 500 geysers, making it the most concentrated geyser field in the world. Recent studies exploring the mechanics of geyser eruptions utilizing laboratory models were limited by a lack of knowledge of the subsurface geometry of the geyser features. This study presents results from active hydrogeophysical surveys, including ground penetrating radar, nuclear magnetic resonance (NMR), seismic refraction, electrical resistivity tomography, and transient electromagnetics to image subsurface geyser structure and constrain the geophysical response of the reservoir structure of Spouter Geyser, Yellowstone National Park. Previous geophysical studies on similar geyser systems in Yellowstone characterize the hydrothermal reservoir that supplies the geyser with hydrothermal fluids and vapors as high porosity, low density structures. Whereas our imaging highlights the hydrothermal conduits and reservoir as high resistivity and high velocity structures, interpreted as silica precipitate decreasing the porosity and increasing the bulk modulus (i.e., from unconsolidated media to semi/fully consolidated). Ground penetrating radar identifies sinter thickness around the geyser, NMR provides porosity measurements of the geyser reservoir, and transient electromagnetics maps the depth to the Biscuit Basin Rhyolite bedrock throughout the field site. Electrical resistivity tomography and seismic refraction image the shallow geyser conduit structures to ∼15 m depth and establish the extent of the geyser hydrothermal reservoir structure. Furthermore, a spatial correlation of the electrical resistivity and seismic velocity results establishes the interpreted hydrothermal geyser reservoir as a high resistivity, high velocity structure located to the northeast of Spouter Geyser at depths greater than 15 m. Plain Language Summary: The dynamics of geyser eruptions are uncertain. Much of this uncertainty stems from a lack of knowledge of their subsurface structure. Therefore, to better understand geyser eruption dynamics, we must first establish the subsurface structure of geysers and their associated hydrothermal geyser reservoirs that supply the erupted hydrothermal fluids and vapors. Geophysical imaging provides an opportunity to image these structures, but active geophysical methods have not been extensively explored for this application. In this study, we use various active geophysical methods to (a) constrain the characteristics of the structure and (b) better understand the subsurface geometry of Spouter Geyser (YNP) and its hydrothermal reservoir. We find that Spouter Geyser's structure and its reservoir have high electrical resistivity and seismic velocity responses, indicative of low porosity and highly cohesive characteristics due to silica precipitate from hydrothermal fluids. We also find that Spouter Geyser's subsurface geometry consists of silica‐lined conduits that extend 15 m to a broad, laterally offset geyser hydrothermal reservoir structure. With the knowledge of the characteristics and geometry of Spouter Geyser and its hydrothermal reservoir, an additional study (Geyser Dynamics II) is better positioned to explore the dynamics of geyser eruptions with active geophysical methods. Key Points: Geophysical results reveal a laterally offset, low porosity and highly consolidated geyser reservoir structure at Spouter GeyserDistinct vertical conduit structures extending to the hydrothermal geyser reservoir at 15 m depth are interpreted at Spouter GeyserVelocity and resistivity distribution characterize Spouter Geyser's reservoir structure as high velocity‐high resistivity [ABSTRACT FROM AUTHOR]