# Multi-Scale Geophysical Imaging of a Hydrothermal System in Yellowstone National Park, USA

Sylvain Pasquet, W. Steven Holbrook, Bradley J Carr, Neil Terry, Martin A Briggs, Carol A Finn, Paul A Bedrosian, Esben Auken, Jesper Pedersen, Pradip Maurya, et al.

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To cite this version:

Sylvain Pasquet, W. Steven Holbrook, Bradley J Carr, Neil Terry, Martin A Briggs, et al. Multi-Scale Geophysical Imaging of a Hydrothermal System in Yellowstone National Park, USA. Journal of Geophysical Research: Solid Earth, 2025, 130 (4), 10.1029/2024JB029839. hal-05059966

HAL Id: hal-05059966

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Submitted on 7 May 2025

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Distributed under a Creative Commons Attribution 4.0 International License
# JGR Solid Earth

## RESEARCH ARTICLE

10.1029/2024JB029839

**Key Points:**

* We use multi-scale geophysical imaging to delineate the lateral extent of a hydrothermal system in Yellowstone

* We highlight topographic and subsurface structures that favor mixing of hydrothermal and meteoric waters

* We delineate the main hydrothermal upflow zone and its controlling geological structures

**Supporting Information:**

Supporting Information may be found in the online version of this article.

**Correspondence to:**

S. Pasquet,
sylvain.pasquet@sorbonne-universite.fr

**Citation:**

Pasquet, S., Holbrook, W. S., Carr, B. J., Terry, N., Briggs, M. A., Finn, C. A., et al. (2025). Multi-scale geophysical imaging of a hydrothermal system in Yellowstone National Park, USA. *Journal of Geophysical Research: Solid Earth*, 130, e2024JB029839. https://doi.org/10.1029/2024JB029839

Received 2 JUL 2024
Accepted 19 FEB 2025

**Author Contributions:**

**Conceptualization:** Sylvain Pasquet

**Data curation:** Sylvain Pasquet

**Formal analysis:** Sylvain Pasquet

**Funding acquisition:**

W. Steven Holbrook, Carol A. Finn, Paul A. Bedrosian, Kenneth W. W. Sims

**Investigation:** Sylvain Pasquet,

W. Steven Holbrook, Bradley J. Carr

**Methodology:** Sylvain Pasquet,

Neil Terry, Martin A. Briggs, Paul A. Bedrosian, Esben Auken, Jesper Pedersen, Pradip Maurya

**Project administration:**

W. Steven Holbrook, Carol A. Finn, Paul A. Bedrosian, Kenneth W. W. Sims

**Resources:** Bradley J. Carr

© 2025. The Author(s). This article has been contributed to by U.S. Government employees and their work is in the public domain in the USA.

This is an open access article under the terms of the Creative Commons Attribution License, which permits use, distribution and reproduction in any medium, provided the original work is properly cited.

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## Multi-Scale Geophysical Imaging of a Hydrothermal System in Yellowstone National Park, USA

Sylvain Pasquet¹ ID, W. Steven Holbrook² ID, Bradley J. Carr³ ID, Neil Terry⁴ ID,

Martin A. Briggs⁴ ID, Carol A. Finn⁵ ID, Paul A. Bedrosian⁵ ID, Esben Auken⁶, Jesper Pedersen⁷ ID,
Pradip Maurya⁶, and Kenneth W. W. Sims³ ID

¹Observatoire des Sciences de l'Université Ecce Terra, CNRS, Sorbonne Université, Paris, France, ²Department of Geosciences, Virginia Polytechnic Institute and State University, Blacksburg, VA, USA, ³Department of Geology and Geophysics, University of Wyoming, Laramie, WY, USA, ⁴U. S. Geological Survey, Hydrologic Remote Sensing Branch, Storrs, CT, USA, ⁵U. S. Geological Survey, Denver, CO, USA, ⁶TEMcompany Aps, Aarhus, Denmark, ⁷Department of Geoscience, HydroGeophysics Group, Aarhus University, Aarhus, Denmark

**Abstract** Little is known about the local plumbing systems that fuel Yellowstone's famous hot springs, geysers and mud pots. A multi-method, multi-scale geophysical investigation was carried out in the Obsidian Pool Thermal Area (OPTA) to: (a) delineate the lateral extent of the hydrothermal area and associated surface features; (b) estimate the dimensions of the upflow zone and identify its main controlling structures; (c) assess fluids circulation pathways from depth to surface. Ground and airborne geophysical data were acquired to connect local and regional scales, from shallow to large depths. Maps of surface electrical resistivity show a strong correlation with hydrothermal features. At intermediate depths, electrical resistivity permits delineating the upper limit of the upflow zone, while Poisson's ratio highlights differences in subsurface fluid content. Combining these results with surface observations and topographic information, we speculate that differential mixing of hydrothermal and fresh water could explain the wide diversity of features observed at OPTA. Low electrical resistivity observed at large depths also suggest that a vast upflow zone, controlled by rhyolite flows and conjugate faults, underlies the OPTA. We speculate that hydrothermal fluids rise along fractures and reach the surface in topographic lows to form hydrothermal features. Our results show that synoptic, multi-scale geophysical measurements provide a roadmap for understanding where and how geologic heterogeneity, topography, fluid-gas separation, and the mixing of thermal and meteoric waters conspire to produce the wide variety of Yellowstone's renowned hydrothermal features.

**Plain Language Summary** Yellowstone National Park is famous for its geysers, hot springs, and mud pots. These features are fueled by a hidden network of pathways underground. Scientists do not fully understand how this “plumbing system” works. This study investigated the underground network feeding Yellowstone's Obsidian Pool Thermal Area, near Mud Volcano. Researchers used various geophysical techniques to map geological structures and water flow paths, identify hot water upwelling zones, and understand how hot and cold water mix to create different features. The study revealed that hot water rises along fractures in the rock and reaches the surface in low-lying areas. Deeper electrical readings suggest a vast network of hot water pathways controlled by faults and lava flows. Hot and cold water mixing likely explains the variety of geothermal features observed at the surface. Understanding Yellowstone's plumbing system helps us learn how these geothermal wonders work and what influences their diversity. This knowledge can guide future research on Yellowstone's unique hydrothermal features.

## 1. Introduction

The Yellowstone Plateau Volcanic Field (YPVF) is the latest manifestation of a mantle hot spot that has produced several large calderas along the Snake River Plain in the last 17 million years (Pierce & Morgan, 2009; Smith et al., 2009). Over the past 2.1 Ma, it has led to three cataclysmic volcanic eruptions, the most recent (~0.631 Ma) forming the 2,900-km²-large Yellowstone caldera (R. L. Christiansen, 2001; Matthews et al., 2015). Although this caldera has been dormant for tens of thousands of years since the last eruption, a large silicic magma reservoir is still present in the shallow crust (Farrell et al., 2014; H.-H. Huang et al., 2015). This reservoir is the source of the intense activity observed in the YPVF (Smith et al., 2009), expressed by widespread seismicity, significant ground deformation and extensive hydrothermal activity, and leading to a wide variety of hydrothermal features at
**Supervision:** W. Steven Holbrook, Carol A. Finn

**Writing – original draft:** Sylvain Pasquet

**Writing – review & editing:**
W. Steven Holbrook, Bradley J. Carr, Neil Terry, Martin A. Briggs, Carol A. Finn, Paul A. Bedrosian, Esben Auken, Jesper Pedersen, Pradip Maurya, Kenneth W. W. Sims

the surface (e.g., hot springs, fumaroles, geysers, mud pots...). These features also support unique microbial ecosystems that thrive in these extreme environments (Boyd et al., 2012; Meyer-Dombard et al., 2005). Research in Yellowstone has thus been primarily driven by three key objectives: hazard assessment (i.e., hydrothermal explosions), the preservation of unique natural features, and the elucidation of life's origin and evolution in extreme environments (Fournier, 1989; Hurwitz & Lowenstern, 2014; Morgan et al., 2009). Although Yellowstone National Park is protected from geothermal energy exploitation and mineral exploration, research conducted there also shares common ground with investigations in other large hydrothermal systems worldwide, such as the Taupo Volcanic Zone in New Zealand (Rowland & Sibson, 2004; Rowland & Simmons, 2012; Wilson et al., 1995), the Long Valley Caldera in California (USA) (Hildreth, 2017; Hill et al., 1985), and the Campi Flegrei in Italy (Chiodini et al., 2022; Troise et al., 2019).

Despite extensive ground investigations targeting the temporal and spatial variability of Yellowstone's hydrothermal features (Fournier, 1989; Hurwitz et al., 2007, 2012; Werner & Brantley, 2003; Werner et al., 2000; White et al., 1971), relatively little is known about the plumbing systems that link deeper hydrothermal fluids to these charismatic surface features. In this context, near-surface, non-invasive geophysical methods have been increasingly proposed to image the inner structures of volcanic and hydrothermal areas whilst meeting National Park safety and environmental regulations (Hurwitz & Lowenstern, 2014). Because it is sensitive to fluid saturation and ion concentration, clay content and temperature, bulk electrical resistivity ($ρ$) is the main physical parameter used to study hydrothermal systems. Large scale structures (~100–500 m) have traditionally been investigated with ground based (Aizawa et al., 2005, 2009; Bruno et al., 2007; Srigutomo et al., 2008; Troiano et al., 2014) or airborne (Dumont et al., 2019, 2021; Finn, Bedrosian, Holbrook, et al., 2022; Finn et al., 2018; Mogi et al., 2009; Pryet et al., 2012) electromagnetic methods. Two-dimensional (2D) direct current electrical resistivity measurements are also commonly implemented to image shallower structures (< 50 m) at higher spatial resolutions (Barde-Cabusson et al., 2013; Fikos et al., 2012; Finizola et al., 2006; Miller et al., 2020). More recently, seismic methods have been applied to infer shallow mechanical properties of hydrothermal systems from seismic wave velocities, with either passive (Letort et al., 2012; Vandemeulebrouck et al., 2013, 2014; Wu et al., 2017) or active (Amoroso et al., 2017; De Landro et al., 2017; Gammaldi et al., 2018; Gebhardt et al., 2011) implementations. More particularly, Pasquet et al. (2016) recently showed the benefits of estimating Poisson's ratio from the combined analysis of pressure and shear wave velocities to constrain fluid distribution in the upper 10–15 m of the subsurface and image degassing areas.

Finn, Bedrosian, Holbrook, et al. (2022) proposed the first high-resolution synoptic view of pathways of the Yellowstone hydrothermal system derived from electrical resistivity and magnetic susceptibility models of airborne transient electromagnetic (ATEM) and magnetic data. They show that most thermal features are located above low resistivity and low susceptibility structures, which they interpret as upflow zones. These upflow zones are characterized by high vertical fluxes of hydrothermal fluids, leading to the destruction of magnetic minerals and the formation of large amounts of clays through the intense alteration of volcanic rocks. Fluids rise along faults beneath clays, then spread laterally within basins and under adjacent lava flows as subsurface outflow zones. Yet, the large footprint (~100 m) of the ATEM system used in their study precludes detailed investigation of local hydrothermal features. In the meantime, an increasing number of studies have been targeting local features in Yellowstone with site-scale geophysical surveys. Although most of them have focused on geyser structure and dynamics (Ciraula et al., 2023a, 2023b; Vandemeulebrouck et al., 2013, 2014; Wu et al., 2017, 2019, 2021), a handful have attempted to study the subsurface structure and mixing processes that lead to the formation of hot springs, fumaroles and mud pots (Bouligand et al., 2019; Sims et al., 2023; Smeltz et al., 2022; Zohdy et al., 1973).

Here we present the results of extended, multi-scale geophysical surveys carried out in the vapor-dominated, acid-sulfate Obsidian Pool Thermal Area (OPTA). This area, previously studied by Hurwitz et al. (2012) and Pasquet et al. (2016), hosts several close-by active hydrothermal features with different behaviors (mud pots, fumaroles and alkaline-chloride pools) that suggest differential mixing of hydrothermal and meteoric fluids at depth with distinct subsurface pathways. We use ground-based and airborne geophysical methods to connect local and regional scales, from shallow (<4 m) to large (~400 m) depths. We seek to image the shallow plumbing system that feeds these hydrothermal features, and the subsurface structures that contribute to mixing deep hydrothermal and shallow meteoric waters. Our investigations especially aim at: (a) delineating the lateral extent of the hydrothermal area and associated surface features; (b) estimating the dimensions of the upflow zone and identifying the main geological structures (e.g., fractures, faults or lava flows) controlling its extent; and (c) assessing
<figure><img src="image_4.png" /><figcaption>**Figure 1.** (a) Shaded-relief topographic map of Yellowstone National Park extracted from the USGS National Elevation Data set (United States Geological Survey, 2021), showing the rim of the Yellowstone caldera (black line), the edges of Mallard Lake (ML) and Sour Creek (SC) resurgent domes (black dashed lines) and the state boundaries (gray dashed lines). The Mud Volcano Thermal Area (MVTA, red square) is located north of the Elephant Back (EB) rhyolite flow. (b) Geologic map (after R. L. Christiansen & Blank, 1975) of the MVTA, with the location of hydrothermal areas. The extent of the ATEM survey used in this study (black rectangle) is also represented, along with the location of Mud Volcano (MV, red dot) and the Y-11 borehole (red star). The Obsidian Pool Thermal Area (OPTA, red square) is located in the southern part of the MVTA. The present and all subsequent maps are projected in the Universal Transverse Mercator (UTM) geographic coordinate system, zone 12 N.</figcaption></figure>

pathways along which fluids may circulate from deeper reservoirs to the surface in order to explain the diversity of hydrothermal features observed in OPTA.

## 2. Geological and Hydrothermal Setting

The Obsidian Pool Thermal Area lies on the eastern part of the Yellowstone caldera as part of the Mud Volcano Thermal Area (MVTA). It is located near the western edge of the Sour Creek resurgent dome and the northern edge of the Elephant Back rhyolite flow (Figure 1a). As described on the geologic map by R. L. Christiansen and Blank (1975), the area mainly consists of rhyolitic ash flow tuff covered with varying thicknesses of Quaternary deposits (Figure 1b). These deposits are mostly lacustrine or fluvial sediments and glacial tills of rhyolitic composition. Quaternary deposits have been identified down to approximately 18 m in the Y-11 borehole drilled near Mud Volcano (Bargar & Muffler, 1982), about 2 km north of OPTA. ATEM data highlighted a ~120 m thick low resistivity layer below these deposits corresponding to hydrothermal alteration in the upflow zone (Finn, Bedrosian, Holbrook, et al., 2022). Such a conductive layer was also evidenced in a previous resistivity model constructed on ground-based electrical soundings, overlaying a more resistive layer at depth interpreted as a vapor dominated region (Zohdy et al., 1973).

On the south, east and north sides, OPTA is surrounded by dense forests that cover the elevated terrains of Elephant Back Mountain and smaller hills formed by glacial deposits (Figure 2a). A small mountain lake (with no visible hydrothermal activity) lies northeast of the area. OPTA itself is mostly covered with wild grassland, except for areas surrounding thermal features where soils are bare (Figure 2b). The area is also characterized by extensive CO₂ diffuse degassing through the ground (Werner & Brantley, 2003; Werner et al., 2000) and high conductive heat flux (Hurwitz et al., 2012) with peaks corresponding to isolated thermal features. Although neutral-chloride thermal features have been observed in OPTA (Meyer-Dombard et al., 2005), the vast majority of them have acid-sulfate composition with low pH (around 2–3) and water temperatures ranging between 21.9°C and 84.0°C
<figure><img src="image_5.png" /><figcaption>**Figure 2.** (a) Aerial photograph (Google, Landsat/Copernicus) of MVTA showing the ATEM survey acquisition profiles (yellow dots, with associated line number) along with the location of OPTA (white square), Mud Volcano (MV, red dot) and the Y-11 borehole (red star). (b) Aerial photograph (Google, Landsat/Copernicus) of OPTA highlighting significant surface features: the Frying Pan (FP) (small shaded red area at the intersection between Line 1 and Line 2) and the surrounding heat flow anomaly (black line), the Mud Pot (MP) (shaded brown area) and the surrounding mud-filled depression (black line), the Figure Eight pool (F8) (shaded green area), the neighboring Obsidian Pool (OP) (shaded blue areas), and a fen (blue line). Locations for the ERT and seismic profiles (yellow lines), the GTEM soundings (yellow diamonds) and the extent of the EMI survey (yellow dashed line) are also shown, along with ATEM profiles (yellow dash-dot lines). Full coverage of EMI data is shown in Figure S1 of Supporting Information S1.</figcaption></figure>

(Hurwitz et al., 2012). Almost all of the pools are bubbling with temperatures below the boiling point of water, suggesting a strong influx of CO₂-rich gas.

Four main hydrothermal features have been identified at OPTA and are highlighted on Figure 2b. The first one is an intense degassing feature (referred to as the Frying Pan) located in the southern part of OPTA. It lies at the center of a 80 × 50 m heat flow anomaly previously highlighted by Hurwitz et al. (2012) and clearly delineated by bare soils with no vegetation or snow accumulation during winter. The active part of the Frying Pan is approximately 10 × 10 m and shows visible degassing, with some water bubbling at the surface with an average temperature of 89.0°C. The second feature is a large 100 × 50 m depression filled with mud (the Mud Pot) located at approximately 150 m north of the Frying Pan. The southern edge of the feature hosts two active and distinct pits of acid-sulfate composition. The third main feature, referred to as the Figure Eight pool, is located about 40 m east of the Mud Pot. It shows strong water composition variability in time, ranging between almost neutral-chloride to fully acid-sulfate compositions, which is expressed by marked changes of water color (opaque white to clear blue), pH (3.6–5.9) and temperature (50°C–68°C) throughout the year. Finally, the Obsidian Pool and a cluster of smaller pools and mud pits are present at about 20 m northeast of the Figure Eight pool. A fen (i.e., non-hydrothermal, peat-forming wetland) is located in the eastern woods outside of the hydrothermal area.

# 3. Material and Methods

## 3.1. Data Acquisition and Analysis

This study synthesizes the results of six distinct geophysical data sets that image OPTA's hydrothermal system at different depths and resolution: (a) electromagnetic induction (EMI); (b) electrical resistivity tomography (ERT); (c) seismic refraction tomography (SRT); (d) multichannel analysis of surface-wave (MASW); (e) ground-based transient electromagnetics (GTEM) and (f) airborne transient electromagnetics (ATEM). All the data were acquired in July 2016 at the exception of ATEM data (November 2016) and parts of the GTEM data (November 2015). Topography data were obtained from a 1-m resolution lidar-derived digital elevation model (Open-Topography, 2009) combined with a 30-m digital elevation model from the USGS National Elevation Data set
(United States Geological Survey, 2021). Here we present a brief overview of the data acquisition and methods used, working roughly from the shallowest to the deepest measurements. More details are given in the Supplementary Information.

### 3.1.1. Electromagnetic Induction

Ground-based electromagnetic induction data were acquired using a hand-carried, multi-frequency Geophex GEM-2™ instrument. This device has two coplanar coils separated by 1.66 m (H. Huang & Won, 2003) and is connected to a GPS system in order to ease data acquisition over large areas. EMI data were acquired in a random walk that covered about 20 ha at OPTA site (Figure 2b), with enough overlap between successive surveys to correct for instrumental drift and ensure lateral consistency (Figure S1 in Supporting Information S1). After calibration with coincident ERT models (refer to Supplementary Information), quadrature data were inverted using the EM1DFM software package (Farquharson et al., 2003). We obtained a map of shallow (<1 m) electrical resistivity at the site which is sensitive to temperature, clay content and fluid solute concentration, thus delineating the lateral extent of hydrothermal activity at the surface and its associated heat flux and fluid flow.

### 3.1.2. Electrical Resistivity Tomography

Electrical resistivity tomography data were acquired at OPTA along three transects with lengths 222 m (Lines 1 and 3) and 417.5 m (Line 2) (Figure 2b). Data were acquired with an AGI Supersting™ using several deployments of 112 electrodes spaced at 2 m along Line 1 and 3, and 2.5 m along Line 2, in a pole-dipole configuration. Apparent electrical resistivity data were filtered to remove data with low injected current, low voltage and high repeatability error. We then used the R2 software package to invert the data for electrical resistivity structure. Inversions were run using a homogeneous starting model with a resistivity of 1 Ω·m, and converged after a total of six iterations for all lines. The retrieved resistivity models intersect with the EMI surface mapping and image the extent of hydrothermal reservoirs and geological layers at depths down to ~25–50 m.

### 3.1.3. Seismic Refraction Tomography

Seismic refraction tomography data were collected along the same three transects as the ERT data (Figure 2b). Data were collected using 10, 24-channel Geometrics Geode™ seismic recorders and 240 4.5-Hz vertical-component geophones spaced at 1 m; sledgehammer shots on a metal plate every 10 m provided the energy source. First-arrival traveltimes were hand-picked on each recorded shot gather, then inverted using the pyGIMLi refraction tomography module (Rücker et al., 2017) to produce 2D cross-sectional estimates of subsurface compressional (P) wave velocity ($V_P$) extending down to depths of ~25–50 m. $V_P$ is sensitive to changes of mechanical properties in the subsurface that are driven by lithology, fracture density, weathering degree or water content.

### 3.1.4. Multichannel Analysis of Surface Waves

Surface-wave data were recorded alongside the SRT data by extending the recording time up to 750 ms for each shot gather. Multichannel analysis of surface-wave data was performed using the open software package SWIP (Pasquet & Bodet, 2017), which applies windowing and stacking techniques to extract lateral variations of surface-wave phase velocity along linear transects. These variations were extracted with a 30-m moving window, then inverted to retrieve successive one-dimensional (1D) estimates of shear (S) wave velocity ($V_S$) along each profile. These 1D inversions were eventually combined to form 2D cross-sections of $V_S$ that reach depths of ~10–15 m, and were then associated with $V_P$ SRT models to calculate Poisson's ratio ($\nu$):

$$ \nu = \frac{V_P^2 - 2V_S^2}{2(V_P^2 - V_S^2)} \qquad (1) $$

Poisson's ratio has proven to be a reliable proxy for below ground fluid saturation and can thus be used to delineate water ($\nu > 0.45$) and gas ($\nu < 0.45$) saturated parts of the subsurface across the hydrothermal area (Pasquet et al., 2016).
<table><caption>**Table 1**
Key Items Highlighted at Different Depths by Geophysical Properties</caption><thead><tr><td>Items</td><td>Area</td><td>Description</td></tr></thead><tbody><tr><td>1</td><td>Mud Pot</td><td>Large depression filled with mud characterized by very low resistivity and low P-wave velocity. Visible at shallow (&lt;1 m) and intermediate (between 1 and 25 m) depths.</td></tr><tr><td>2</td><td>Figure Eight pool</td><td>Hydrothermal pool with strong water composition variability characterized by low resistivity, low P-wave velocity and high Poisson's ratio. Visible at shallow and intermediate depths.</td></tr><tr><td>3</td><td>Frying Pan</td><td>Intense degassing feature characterized by high resistivity at the center and low resistivity on the edges, and overall low P-wave velocity and low Poisson's ratio. Visible at shallow and intermediate depths.</td></tr><tr><td>4</td><td>Upflow zone</td><td>Very low resistivity area visible at intermediate and large (&gt;25 m) depths, connected at the surface with items 1, 2 and 3.</td></tr><tr><td>5</td><td>Fens</td><td>Wetland area characterized by intermediate resistivity, intermediate P-wave velocity and high Poisson's ratio. Visible at shallow and intermediate depths.</td></tr><tr><td>6</td><td>Glacial tills</td><td>Small hills characterized by moderate resistivity, low P-wave velocity and low Poisson's ratio. Visible at shallow and intermediate depths.</td></tr><tr><td>7</td><td>West Thumb rhyolite flow</td><td>Very high resistivity, very high P-wave velocity and high Poisson's ratio. Visible at intermediate and large depths.</td></tr><tr><td>8</td><td>Unidentified lava flow</td><td>High resistivity. Visible at intermediate and large depths.</td></tr><tr><td>9</td><td>Lava Creek Tuff</td><td>Very high resistivity. Visible at intermediate and large depths.</td></tr></tbody></table>

### 3.1.5. Ground-Based Transient Electromagnetics

Ground-based transient electromagnetics data were acquired with an ABEM WalkTEM utilizing a 40 × 40 m square transmitting wire surrounding two, centered receiver square loops (10 × 10 m and 0.5 × 0.5 m, respectively). A total of 20 1D soundings were collected as part of two surveys, first in November 2015 with 6 soundings collected along Line 1, then in July 2016 with 14 soundings collected along Line 2 (Figure 2b). GTEM data processing was done with SPIA (Aarhus GeoSoftware). This software was used to perform smooth model (20 layers) 1D resistivity inversions using the AarhusInv inversion code (Auken et al., 2015). Inverted models describe electrical resistivity distributions at intermediate depths ranging from a few meters down to ~100–200 m (at best) below the surface, and thus help connecting surface features with larger scale geological structures.

### 3.1.6. Airborne Transient Electromagnetics

Airborne transient electromagnetics data were collected across MVTA in November 2016 as part of a larger survey aimed at understanding and mapping Yellowstone's hydrothermal systems at depth (Finn, Bedrosian, Holbrook, et al., 2022). ATEM data were acquired using the helicopter-based SkyTEM 312 system (Finn, Bedrosian, Bloss, et al., 2022; Sørensen & Auken, 2004). Continuous soundings along the helicopter's flight path provide numerous one-dimensional inversions that are combined into 2D sections of electrical resistivity reaching several hundred meters into the subsurface in resistive regions and less (~100 m) in conductive areas like OPTA (Bedrosian et al., 2022). We collected 11 survey profiles oriented NW–SE and spaced by 250 m across MVTA, with lengths of approximately 3.2 km (Figure 2a). The data set was then inverted using the AarhusInv inversion code (Auken et al., 2015) with full system response (Auken et al., 2020) as implemented in Aarhus Workbench. The vertical resolution and depth of investigation (DOI) of ATEM data is very comparable to GTEM, since they use similar acquisition parameters. Due to its large footprint (~100 m), ATEM has a lower horizontal resolution but offers the broadest spatial coverage, so as to provide a regional view of the hydrothermal system and its deeper connectivity.

## 4. Results

In the following, we describe the results of geophysical data inversion for the different methods used in the field, from the shallowest (EMI) to the deepest (ATEM). We especially highlight and label key items on each figure, and refer to their specific number in the text. A summarized description of these items is also given in Table 1.
<figure><img src="image_6.png" /><figcaption>**Figure 3.** Aerial photograph (Google, Landsat/Copernicus) of OPTA viewed from the southeast (a) and the northeast (b). It is overlain by EMI surface electrical resistivity ($ρ$) and cut by electrical resistivity cross-sections collected with ERT. The cross-sections are shown down to their respective DOI. We also show the locations of hydrothermal features (thin black lines), ERT profiles (thick black lines) and GTEM soundings (black dots). Numbering in the present and all subsequent figures corresponds to key items described in the main text, and summarized in Table 1.</figcaption></figure>

The electrical resistivity map obtained from the EMI inversion shows variations ranging from 2 to 1,600 Ω·m (Figure 3). The Mud Pot and its surroundings (item 1) are clearly delineated and are characterized by very low resistivity values (<5 Ω·m). Slightly higher resistivity values (~20 Ω·m) are observed in the northern part of the map between the Figure Eight pool and the lake. Similar resistivities are also visible in a horseshoe-shaped feature (item 3b) around the Frying Pan that circles a small (~25 × 30 m) resistive anomaly (item 3a). Outside the
hydrothermal area, the map is mostly characterized by resistivity values higher than 1,000 Ω·m, with the exception of two wetland areas located at both eastern and southern ends of OPTA (items 5a and 5b, respectively) that are delineated by intermediate resistivity values (~200 Ω·m). Small hills (items 6a and 6b) that cross the site in the N–S direction are also characterized by high resistivity values.

Electrical resistivity values obtained with ERT (Figures 4a–4c) vary between 2 and 2,000 Ω·m, and are remarkably consistent with surface electrical resistivity values retrieved with EMI. The surveyed profiles highlight a 5-10 m-thick, high resistivity layer (~1,000 Ω·m), that covers most of the site except for hydrothermal areas. In the center of the OPTA, this resistive layer lies on top of a conductive body (~20 Ω·m) that extends down to the maximum resolved depth (item 4). In the forested area, on the flanks of Elephant Back Mountain, high resistivities (>1,000 Ω·m) extend over the entire depth profile (item 7). At the center of the Frying Pan, the resistive anomaly observed at the surface (item 3a) extends down to about 6 m, and is surrounded by a ~20 Ω·m conductive ring (item 3b). Similar and even lower resistivities (<5 Ω·m) are also visible along Line 3, especially under the Mud Pot (item 1) and the Figure Eight pool. These very high conductivity values trap current lines close to the surface and thus lead to a shallower investigation depth (~10 m) compared to Line 1 and 2 (~25–50 m).

P-wave velocity values show fewer lateral heterogeneities than ERT results, and range between 200 m/s and 3,000 m/s (Figures 4d–4f). Mostly low velocities (<750 m/s) are observed across the hydrothermal area, and more particularly under the Mud Pot (item 1) and the Frying Pan (item 3a). Outside the hydrothermal area, toward the forested area (item 7), higher velocity (>2,000 m/s) is measured at depth and at the surface under the fen (item 5a). $V_P$ and $V_S$ velocities were then used to calculate Poisson's ratio along the three geophysical transects (Figures 4g–4i). The depth extent of the obtained Poisson's ratio is mostly limited by the lower DOI of $V_S$ profiles (shown in the Supplementary Information) that is inherent to MASW analysis (Pasquet et al., 2015). Poisson's ratio values are low (<0.45) around and below the Frying Pan (item 3a), roughly following the extent of the conductive anomaly observed on EMI and ERT data. Strong attenuation of seismic waves prevented us from estimating Poisson's ratio value in the Mud Pot (item 1), whereas high Poisson's ratio values (>0.45) can be observed between Figure Eight and Obsidian Pool (item 2). Away from the hydrothermal features and under the hills (items 6a and 6b), low Poisson's ratio values are only observed within the upper few meters, and followed at depth by higher Poisson's ratio, at the exception of the fen where high Poisson's ratio values are observed at the surface (item 5a).

GTEM soundings are less sensitive to the properties of near surface materials than ERT but have a larger investigation depth. They show a lower range of resistivity values both laterally and at depth (Figure 5b), but still point out similar superficial features identified on ERT. More specifically, soundings recorded near Figure Eight and Obsidian Pool highlight the rise of the conductive body (item 4) up to the surface. Similarly high conductivities are observed at depth in the eastern part of the area. GTEM data also point out the presence of a slightly more resistive (~150 Ω·m) layer between 50 and 100 m deep that separates the upper conductive body from a deeper one.

The inversion of ATEM data provided 11 parallel profiles of electrical resistivity constituting a three-dimensional (3D) model with dimensions of 3.2 km × 2.5 km × 0.5 km and variations of $\rho$ ranging from about 1.5 to 1,500 Ω·m (Figure 6). ATEM results are in good agreement with other resistivity-sensitive data, and show three strong resistive features ($\rho$ > 150 Ω·m) in the SE (item 7), SW (item 8) and NE (item 9) corners of the ATEM survey area. These features extend down to depths of about 200–300 m under Elephant Back Mountain, Hayden Valley and Sour Creek dome, respectively. A shallow resistive layer with thickness <40 m is also visible on the northwest corner of MVTA (item 6d). The center of the surveyed area is characterized by lower resistivity values (<20 Ω·m) which extend from Mud Volcano to OPTA (item 4) down to depths of about 100 m. This conductive zone narrows down toward OPTA, where it is enclosed by both SE (item 7) and SW (item 8) resistive bodies.

# 5. Discussion

## 5.1. Site Scale Interpretation

Combining the geophysical data, surface observation and topographic information provides detailed imaging of the shallow OPTA hydrothermal system. Figure 7 displays the topography of OPTA, overlain with the main contours of electrical resistivity identified in EMI (Figure 3 and Figure S2 in Supporting Information S1) and ATEM (Figure 8) results. Mapping of surface electrical resistivity at OPTA clearly highlights the boundaries of
<figure><img src="image_7.png" /><figcaption>**Figure 4.** Electrical resistivity ($ρ$) observed along Line 1 (a), Line 2 (b) and Line 3 (c). P-wave velocity ($V_p$) observed along Line 1 (d), Line 2 (e) and Line 3 (f). Poisson's ratio observed along Line 1 (g), Line 2 (h) and Line 3 (i). The cross-sections are shown down to their respective DOI. Vertical dashed lines show the intersection between Line 1 and Line 2.</figcaption></figure>

the conductive hydrothermal area and the lateral extent of each hydrothermal feature (Figure 3). These features correspond well to the heat flux anomalies observed by Hurwitz et al. (2012), although no specific hydrothermal feature was identified on EMI results near the lake in the NE part of the site (Figure 3a). At depth, the hydrothermal upflow zone shows up in ERT and GTEM results (Figure 5b) as a large conductive body (item 4).
<figure><img src="image_8.png" /><figcaption>**Figure 5.** (a) Poisson's ratio estimated from P- and S-wave velocities; (b) Electrical resistivity inferred from ERT and GTEM measurements. The cross-sections and soundings are shown down to their respective DOI. We also show the locations of hydrothermal features (thin black lines), geophysical profiles (thick black lines) and GTEM soundings (black dots).</figcaption></figure>

Following a careful analysis of borehole temperature and lithology logs in comparison to ATEM resistivity models at Yellowstone research holes, Dickey (2018) concluded that lithology and alteration are the primary controls on resistivity. Although acidic waters can contribute to low resistivities around acid-sulfate thermal features, they also induce significant clay alteration. This makes it difficult to isolate the individual effects, which is supported by the similarity in resistivity values between the active and hot MVTA and the dormant Ponuntpa Spring (Dickey, 2018; Finn, Bedrosian, Holbrook, et al., 2022). This suggests that, relative to water-saturated clays resulting from hydrothermal alteration, temperature has a negligible effect on resistivity. The upflow zone outcrops at the surface around the Figure Eight pool (item 2), the Obsidian Pool and the Mud Pot (item 1). Conversely, it lies under a ~10-m-thick resistive layer of glacial tills (item 6a) near the Frying Pan (item 3), which is consistent with stratigraphic observations in the Y-11 borehole (Bargar & Muffler, 1982). North to South aligned hills crossing the site (Figure 7), most likely formed by similar sediment accumulations, appear as a series
<figure><img src="image_9.png" /><figcaption>Figure 6.</figcaption></figure>
of resistive anomalies (items 6a and 6b) that overlie the upflow zone. Glacial tills usually appear as moderately conductive (10–50 Ω·m) due to fine grain size and clay content (refer to Figure 6a along the ATEM profile near Y11). The higher resistivity values observed at 6a and 6b points to a possible silicic sinter formation during a less acidic past. This impermeable sinter layer could trap gas below, as evidenced by the low Poisson's ratio. In the eastern part of the site, the hydrothermal area is limited by a high-resistivity (Figure 4c), high-velocity (Figure 4f) structure (item 7) that corresponds to the rhyolite flows forming the Elephant Back Mountain.

The Frying Pan is located on the south side of the most southern hill (Figure 7) and shows up as a 25 × 30 × 6 m resistive anomaly (item 3a) that highlights the presence of gas in the glacial tills, as also indicated by a clear decrease of Poisson's ratio under the Frying Pan (Figures 4g and 4i). Low Poisson's ratio extend deeper than the resistive body, highlighting the presence of gas in the underlying clays. The Poisson's ratio profile obtained along Line 2 (Figure 4i) also allows delineating the upper unsaturated area ($v < 0.45$) from the underlying perched water table ($v > 0.45$). We speculate that this shallow water table flows down from the flanks of Elephant Back Mountain in the overlaying glacial tills and meets upwelling hydrothermal fluids in OPTA's topographic low (Figure 7). Under the Frying Pan, phase separation probably takes place at the bottom of the resistive anomaly (i.e., depth = 6 m and elevation = 2,385 m), only allowing gas with very small amounts of water to reach the surface so as to form the observed fumaroles. Silicification of the tills, increasing their electrical resistivity and reducing permeability, would also support the proposed model. Although limited, hydrothermal alteration of rocks occurs along the path of the gas toward the surface, and the products of this weathering flow down around the Frying Pan to form the ring of conductive materials (item 3b) surrounding the degassing spot (item 3a).

The Figure Eight pool lies at an elevation of 2,387 m, directly east of the hills. It is characterized by both low resistivity (Figure 5b) and high Poisson's ratio (Figure 5a) which are consistent with the presence of hot hydrothermal water reaching up the surface (Pasquet et al., 2016). The topography characteristics and the shape of the NE low resistivity area between the Figure Eight pool and the lake suggest that products of hydrothermal alteration formed in the pool flow in the NE direction and spread out in a fan toward the lake (yellow arrow in Figure 7). Although water in the Figure Eight pool is mostly acidic, it shows substantial yearly variations of pH, temperature and color. Because Figure Eight is located at the convergence of run-off and subsurface flows of fresh water coming from Elephant Back Mountain and the surrounding hills of Quaternary deposits (blue arrows in Figure 7), we speculate that the observed changes in pool water composition are due to varying inputs of fresh, non-hydrothermal water throughout the year.

On the other side of the hill ridge, the Mud Pot lies at 2,385 m (item 1) and is characterized by extremely low resistivity values (Figures 3b and 4b) that are consistent with a close heat source and the accumulation of clays in the mud-filled depression. Neither ERT, GTEM nor ATEM results show a clear discontinuity in the deep conductive body suspected to support the hydrothermal activity of Mud Pot and Figure Eight. Although both features exhibit distinct hydrothermal behaviors, this result suggests that both Figure Eight and Mud Pot are connected to the same reservoir at depth. We hypothesize that Mud Pot's more intense activity and acid-sulfate composition are due to a combination of topographic and hydrological parameters. On the one hand, the lower elevation of Mud Pot puts it closer to the hydrothermal heat source than Figure Eight. On the other hand, the Figure Eight pool prevents most of the water flowing down from Elephant Back Mountain to reach the Mud Pot, which is mostly fed by deep hydrothermal waters and small amounts of meteoric water coming from both surrounding hills. Its lower elevation setting also favors the accumulation of hydrothermal alteration products within the mud pit.

## 5.2. Large Scale Interpretation

The large-scale ATEM survey performed over Yellowstone National Park in 2016 (Finn, Bedrosian, Bloss, et al., 2022) was aimed at imaging large portions of the caldera including most of the identified thermal areas. In

**Figure 6.** Examples of four ATEM profiles collected over the Y-11 borehole (a), Mud Volcano (MV) (b), the Mud Pot (MP) (c), and the Frying Pan (FP) (d). The estimated depth of investigation (DOI) is shown as a white dashed line. GTEM soundings collected along Line 2 are overlaid in (d). A stratigraphic description of the Y-11 borehole is shown on a locally magnified portion of the resistivity profile in (a). (e) 3D slices of electrical resistivity obtained from the complete ATEM survey. OPTA and the Yellowstone River are shown with a black square and a black line, respectively. Detailed 2D views of all ATEM profiles are shown in Figure S4 of Supporting Information S1.
<figure><img src="image_10.png" /><figcaption>**Figure 7.** Shaded-relief topographic map of OPTA (OpenTopography, 2009). Electrical resistivity contours at 80 Ω·m (thin white line) and 400 Ω·m (thick white line) extracted from the EMI results (Figure S2 of Supporting Information S1) are represented, along with the limits of surface hydrothermal features (black line). Main flow directions of run-off and subsurface water are shown with blue arrows, whereas flow directions of hydrothermal alteration products are shown with yellow arrows. Electrical resistivity contours at 15 Ω·m (thin red line) and 100 Ω·m (thick red line) extracted at 10 m depth from the large-scale 3D ATEM model (Figure 8) are also represented.</figcaption></figure>

their interpretation of the full data set, Finn, Bedrosian, Holbrook et al. (2022) have shown that electrical resistivity is a suitable parameter to differentiate: (a) unaltered and relatively dry rhyolite ($\rho$ > 450 Ω·m), (b) wet unaltered rhyolite ($15 < \rho < 450$ Ω·m), and (c) hydrothermal clays ($\rho$ < 15 Ω·m). They suggest that water saturation and alteration are the main factors controlling electrical resistivity, whereas the impact of temperature on resistivity is of second order. They also interpret upflow zones, which are characterized by significant hydrothermal fluid content and extensive alteration, with resistivities of less than 15 Ω·m. Outflow zones, on the other hand, have slightly higher resistivities (~15–30 Ω·m) as they contain thermal fluids exiting the main upflow area with only negligible alteration.

Here we analyze parts of the ATEM data set that were collected specifically across MVTA. These data provided a 12 × 10⁸ m³ model of electrical resistivity confidently resolved down to depths of about 100–150 m in conductive areas, and 400 m in resistive areas (Figure 6). ATEM resistivities interpreted at the Y-11 borehole location (Figure 6a) are consistent with those observed by Zohdy et al. (1973) with ground-based electrical resistivity measurements. They roughly highlight the succession of high-resistivity (>20 Ω·m) glacial tills, intermediate resistivity tuff of Uncle Tom's trail and low resistivity (<10 Ω·m) hydrothermally altered Lava Creek Tuff described by Bargar and Muffler (1982). The vapor-dominated zone observed in the borehole also shows up
<figure><img src="image_11.png" /><figcaption>**Figure 8.** (a) Electrical resistivity map extracted at a depth of 10 m in the ATEM inversion model, with resistivity contours at 15 Ω·m (thin black line) and 100 Ω·m (thick black line). (b) Electrical resistivity map extracted at a depth of 77 m in the ATEM model, with resistivity contours at 15 Ω·m (thin black line) and 100 Ω·m (thick black line). Both maps are underlaid by a composite shaded-relief topographic map (OpenTopography, 2009; United States Geological Survey, 2021). The red square corresponds to the location of OPTA. (c) 3D volume of electrical resistivity obtained from the ATEM survey and clipped between 15 and 100 Ω·m. Mapped faults are represented with red dashed-dotted lines, and interpreted faults with red dashed lines in (a, b). Yellowstone River's inferred paleo-channel is shown with a pink arrow in (b).</figcaption></figure>

as a slight increase in resistivity at depth >100 m, however the thickness of this zone cannot be resolved due to the screening effects of overlying conductive clay. Directly above Mud Volcano (Figure 6b), glacial tills appear as a moderate conductor overlying the highly conductive altered tuff. There, the upflow zone (item 4) seems to directly outcrop at the surface, which is expected in areas with such intense hydrothermal activity. In OPTA (Figures 6c and 6d), the upflow zone narrows down between two high-resistivity (>100 Ω·m) structures corresponding to the West Thumb rhyolite flow (item 7) and what seems to be a lava flow buried under Quaternary deposits in the Hayden Valley (item 8). ATEM data collected above the Frying Pan show a deeper upflow zone overlaid by thicker glacial tills, as observed on site-scale GTEM and ERT data (Figure 5b), and consistent with the less intense hydrothermal activity in this part of OPTA.

A depth slice of ATEM electrical resistivity at 10 m under the surface of MVTA allows us to highlight the shallow upflow zones with resistivity contours at 15 Ω·m (Figure 8a). These contours show a large conductive area around Mud Volcano (item 4) and two smaller areas, one just NW of Mud Volcano near the Y-11 borehole corresponding to Sulfur Caldron (Bargar & Muffler, 1982), and another at the southern limit of the ATEM survey that
<figure><img src="image_12.png" /><figcaption>**Figure 9.** (a) Shaded-relief topographic map of MVTA (United States Geological Survey, 2021) clipped between 2,385 and 2,392 m of elevation to better highlight topographic highs and lows. (b) Geologic map of MVTA (after R. L. Christiansen & Blank, 1975) with the location of hydrothermal areas. Electrical resistivity contours at 15 Ω·m (thin black line) and 100 Ω·m (thick black line) extracted at 10 m depth from the 3D ATEM model are represented (Figure 8). Mapped faults are represented with white (a) and red (b) dashed-dotted lines, and interpreted faults with white (a) and red (b) dashed lines. The red square corresponds to the location of OPTA.</figcaption></figure>

corresponds to the OPTA. These areas clearly correspond to identified hydrothermal features at the surface (Figure 9b) and lie within topographic lows (Figure 9a), where they are closer to the deep energy source. At a depth of 77 m (Figure 8b), resistivity contours at 15 Ω·m define a single conductive structure under Mud Volcano and OPTA (item 4), highlighting the lateral extent of the upflow zone that fuels the hydrothermal activity in the area (Figure 8c). The southeastern limit of this upflow zone (Figure 8b) seems to follow a NE-trending lineament that is lined up with the main normal faulting direction in the area (N 40°E), along which most of the thermal features are aligned (White et al., 1971). We speculate that the southwestern edge of the upflow zone, which strikes NNW, may also be a fault, as it follows a lineament derived from the magnetic data (Finn, Bedrosian, Holbrook, et al., 2022). We further suggest that these two features may be conjugate faults, potentially controlling the upwelling of hydrothermal fluids, the extent of the MVTA, and the location of surface features observed at OPTA.

In contrast, the 100 Ω·m resistivity contours (Figure 8a) delineate four main resistive units at the surface. The two units located northwest of MVTA and north of OPTA only extend down to ~40 m, whereas the other two stretch over several hundred meters (Figure 8c). The two shallow units (items 6c and 6d) are associated with topographic highs and are described as Quaternary deposits (mostly glacial tills) on the geologic map (Figure 9b) with no evidence of underlying volcanic rock. Apart from the hydrothermal areas already identified previously, the rest of MVTA is characterized by intermediate resistivity values. Even though the area is also mostly covered with glacial tills forming localized topographic highs, the proximity of the conductive upflow zone most likely leads to a lowering of resistivity close to the surface. The two other resistive bodies are located at the edges of the Sour Creek dome and the Elephant Back Mountain. They are both associated with larger topographic highs and correspond, according to the geologic map (Figure 9b), to: (a) alluvial deposits and Lava Creek Tuff (item 9), and (b) glacial tills and West Thumb rhyolite flow (item 7), respectively. The coarse vertical resolution of the ATEM survey precludes differentiating the unaltered, massive volcanic rock and the shallow Quaternary deposits as both lithologies are mostly resistive. Yet, we are able to identify a layering consistent with multiple stacked (resistive) rhyolite flows separated by flow-top breccias and interflow sediments (Figures 6c and 6d). Furthermore, the
inferred lower limits of the West Thumb rhyolite flow are in good agreement with those speculated by R. L. Christiansen (2001). The West Thumb rhyolite flow is distinctly cross-cut by a fault striking N 35°E which shows up both in the topography and the geologic map (Figure 9). In the southeastern part of the survey area, our results allow for delineating the deeper extent of the West Thumb rhyolite flow down to approximately 200 m in its thickest part (Figure 8c). Another resistive structure shows up at depth in the southwestern part of the area (item 8), and most likely corresponds to an old, unidentified lava flow buried under Quaternary sediments. There is also a lower resistivity area at depth running between the Lava Creek Tuff and the West Thumb rhyolite flow (pink arrow in Figure 8b). We speculate that this area could be a paleo-channel of the Yellowstone River, which was later shifted further northward after being blocked by the lava flows.

## 6. Conclusions

We present the results of a multi-method, multi-scale geophysical investigation of the Obsidian Pool Thermal Area in Yellowstone National Park (WY). Near-surface resistivity mapped from EMI clearly delineate the hydrothermal features previously mapped by heat flow (Hurwitz et al., 2012), with areas of high heat flow generally having low resistivity near the surface. At depth <50 m, electrical resistivity measured with ERT and GTEM permits delineating the upper limit of the upflow zone, while Poisson's ratio computed from $V_p$ and $V_s$ velocities highlights differences in gas and water content in the subsurface associated with hydrothermal activity and groundwater flowpaths. A joint analysis of these results with surface observation and topographic information allows us to speculate about the specific behavior of OPTA's hydrothermal features.

The Figure Eight pool is located in a topographic low and is surrounded by glacial tills hills to the west and by Elephant Back Mountain to the east. Our data indicate that the area is likely to be fed both by upflowing hydrothermal fluids and by run-off and subsurface fresh water flowing down the Elephant Back Mountain hillslopes. Variations in these distinct water inputs could explain the yearly fluctuations of water pH, color and temperature observed in the Figure Eight pool. The second main feature visible at the OPTA, a large acidic mud pot located on the western side of the glacial tills hills, forms a large depression filled with clays where no water can outflow. Due to its specific topographic setting, the Mud Pot appears to be mostly fed by deep, hot and acidic waters, explaining its particularly low pH and intense hydrothermal activity. Unlike the two previous features, the hydrothermal fluid pathway that feeds the Frying Pan degassing feature reaches the surface after going through unweathered and potentially silicified glacial tills, thus letting only fumaroles reach the surface. GTEM and ATEM data penetrate up to a few hundreds of meter in depth and suggest that a large upflow zone of clayey weathering products (at least $2.2 \times 10^8 \text{ m}^3$) underlies the entire area. This upflow zone reaches the surface in topographic lows where hydrothermal features have been identified in the MVTA, and more specifically in the OPTA. This upflow zone is surrounded by rhyolite flows forming the Sour Creek dome and Elephant Back Mountain, and seems to be confined by conjugate faults showing up in both ATEM results and topographic information.

Although our analysis is based on the joint interpretation of several data sets, joint or cooperative inversion approaches could be used to arrive at models that leverage the distinct yet overlapping sensitivities of each data set to produce, for example, a single resistivity model that honors the EMI, ERT, GTEM and ATEM data. Similarly, a joint inversion for resistivity and seismic velocities, applying model structure constraints such as the cross-gradient operator (e.g., Gallardo & Meju, 2004), may result in models with a greater degree of structural similarity. Petrophysical models and inversion strategies also need to be improved to convert geophysical parameters in rock and fluid volumetric fractions (Grana et al., 2022). Drone-based geophysical techniques also offer a unique opportunity to bridge the gap between traditional ground-based and airborne geophysical surveys (Mangel et al., 2022; Vilhelmsen et al., 2024). By providing high-resolution imaging capabilities at intermediate scales, drones can substantially enhance our understanding of hydrothermal systems. These innovative methods will enable detailed mapping of subsurface structures, thermal anomalies, and fluid flow pathways, ultimately leading to a more comprehensive assessment of geothermal potential and volcanic hazards.

Nonetheless, our results show that synoptic, multi-scale geophysical measurements can place important constraints on the extent of hydrothermal areas and their connectivity to depth. Deeper-sensing techniques, such as those employed in Finn, Bedrosian, Holbrook, et al. (2022), provide crucial insights into the broader geological context, revealing how faults control the distribution of large-scale thermal areas. However, these methods often lack the resolution to characterize site-specific thermal features such as those observed at OPTA. The variability
of these features, both spatially and temporally, arises from the complex interplay between deeper reservoirs, surface topography, fluctuating groundwater levels, and small-scale geological structures, such as the thin resistive layer overlying FP (Figure 3a). Multi-scale geophysical investigations thus provide a roadmap for understanding where and how geologic heterogeneity, fluid-gas separation, and the mixing of thermal and meteoric waters conspire to produce the wide variety of charismatic hydrothermal features at the surface. This understanding supports assessment of the risks associated with hydrothermal explosions, a major hazard within Yellowstone National Park. This research highlights the spatial variability of hydrothermal systems near the surface, where these threats are most prevalent, and underscores the importance of geophysical tools in their detection and characterization. This multi-scale investigation strategy is also highly relevant to geothermal exploration, as it can be effectively applied to improve reservoir characterization and enhance resource assessment.

## Data Availability Statement

The ground based geophysical data used in this study are available in Pasquet et al. (2023). The ATEM data can be found in Finn, Bedrosian, Bloss, et al. (2022). pyGIMLi (Rücker et al., 2017) is distributed under the terms of the Apache 2.0 license and published on GitHub (https://github.com/gimli-org/gimli). SWIP (Pasquet & Bodet, 2017) is distributed under the terms of the GPL-3 license and published on GitHub (https://github.com/spasquet/SWIP). The R2 program is available at http://www.es.lancs.ac.uk/people/amb/Freeware/R2/R2.htm. EM1DFM (Farquharson et al., 2003) is available upon request at https://gif.eos.ubc.ca/form/academic-license-form. Aarhus SPIA and Workbench are proprietary software and can be found at https://www.aarhusgeosoftware.dk/download-aarhus-spi and https://www.aarhusgeosoftware.dk/download-aarhus-workbench, respectively.

## Acknowledgments

The authors thank Matthew Provart and the WyCEHG Geophysics Field Team for their assistance in the field. The authors also acknowledge the U.S. Yellowstone National Park Service for granting access to the site (permit YELL-2016-SCI-6090) and thank Sean Scott for insightful discussions about Yellowstone geology and hydrothermal pools geochemistry. This publication was made possible by the Wyoming Experimental Program to Stimulate Competitive Research, the University of Wyoming Office of Research and Economic Development, and by the National Science Foundation under award EPS-1208909. Any use of trade, firm, or product names is for descriptive purposes only and does not imply endorsement by the U.S. Government.

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