Session 6B: Geophysics

An airborne magnetic survey has offered new insights into the structural setting of the Puna Geothermal Venture (PGV) on the Island of Hawaii. Previous magnetic investigations of the eastern flank of Kilauea predominantly relied on the ‘Puna Forest’ airborne magnetic survey conducted by the United States Geological Survey in 1978. Ormat’s 2024 airborne magnetic survey utilized denser line spacing (100m flight lines), advanced magnetic detection equipment and processing techniques to delineate smaller-scale fissures and fractures at and near PGV. The collected magnetic data have been correlated, where applicable, with magnetic susceptibility measurements, lithologic interpretations from drilling, and paleointensity data previously collected across the area of interest. The Lower East Rift Zone (LERZ) is characterized by a magnetic high anomaly extending from Kilauea's summit down its southeastern flank, making a notable left step near PGV to form a region with enhanced permeability that hosts fluid movement in the geothermal system.

The magnetic signatures captured by this survey reveal pronounced magnetic anomalies along the major N63E LERZ trend, with cross-cutting features that strike perpendicular to the regional LERZ trend. Although the N63E-trending fissures and fractures are well-characterized and are permeable, the structural nature of the cross-cutting features are not well understood. However, NW/SE structures identified in PGV well image logs merit further investigation for their geothermal significance (Spake, 2024). Tracer tests conducted at PGV corroborate the presence of cross cutting structures at PGV, potentially serving as pathways for fluid flow. In a geologically homogeneous environment, a largely uniform magnetic signature would be expected; however, the tholeiitic basalts deposited across the LERZ present a contrast of strong magnetic highs juxtaposed with adjacent magnetic lows (±6000 nT). This survey represents a significant advancement in understanding the complex magnetic and structural environment at PGV.

This study was conducted to investigate the potential for a hot sedimentary aquifer geothermal system within the NEOM area in the northwest of Saudi Arabia using regional geophysics and 2D seismic lines covering the main known basins along the Red Sea coast. The thickness of each basin was investigated to determine the maximum depth to basement within each basin. The Red Sea sedimentary basin deposition started as part of the Red Sea rift sequence, and is mainly influenced by tectonic activity within each subregion of the early rift. The Red Sea rift began ~32 Ma and has created the basin that includes the modern-day Red Sea and the Gulf of Suez. The rift presents itself as a series of half grabens that extend from Bab Al-Mandab Strait in the south to the Mediterranean in the north. This tectonic arrangement was superimposed in the northern Red Sea by the strike-slip tectonics of the Gulf of Aqaba in which began movement ~12 Ma as part of the left-lateral Dead Sea Fault system.

This is the first attempt at geothermal exploration in these sedimentary basins, where currently no known systems exist. The basis for designing this exploration work is on the premise of conduction heat flow, with no impact of convection or magmatic processes that would diverge away from the background heat gradient. Using the known heat gradient in the area from oil and gas wells, the intent is to drill into a reservoir at a depth that delivers the necessary temperature for use as geothermal power, heating, and refrigeration. Using a greenfield exploration approach, the workflow has included the analysis of regional geology, heritage well data, published technical papers, geologic map data, 2D imagery, airborne magnetic and gravity data, along with the acquisition and interpretation of 180 km of 2D seismic. Of the mapping completed thus far, several targets were identified in the Midyan area where the investigation suggests deep basins which could host Hot Sedimentary Aquifer geothermal systems. The targets in the area lie in syn-rift sedimentary reservoir-seal pairs that are deeper than 3 km at present and are either within marine turbidite sands, or early-rift continental sands capped by marine shales. The 2D seismic interpretation along with the assessment of other existing data will form the basis of discussion as to how the identified prospects were chosen.

Surprise Valley, located in the northwestern Great Basin, forms as an asymmetric extensional basin that marks a major tectonic transition between the relatively un-extended volcanic Modoc Plateau to the west, and the Basin and Range to the east that has undergone 10-15% extension. In addition, it sits just north of the Walker Lane which accommodates up to 20% of dextral slip associated with Pacific-North American plate interactions.

Thermal springs issue from eight areas within Surprise Valley. Most of these occur within the basin and are not situated on the main basin forming range-front faults. As a result, efforts to resolve the structural setting of the valley’s hydrothermal system has relied on geophysics to characterize basin structure and geology.

Extensive efforts to map the basin with ground and airborne magnetics has revealed a >35 km-long linear, intra-basin magnetic high, interpreted as a buried dike swarm. Geothermal springs on the eastern side of the valley, including Seifert Hot Springs, Leonards Hot Springs, and Surprise Valley Hot Springs (SVHS), are all situated along the high and occur at local breaks and bends in the anomaly, suggesting that fracture permeability is enhanced along the feature and particularly at these discontinuities.

Recent studies, including drilling over the anomaly near SVHS that likely intersected dike material, as well as mapping and sampling along the anomaly south of SVHS that reveal dikes outcropping on the playa surface, confirm (as previously inferred) that mafic intrusives are the principal source of the anomaly. Similar interpretations made in two other valleys (in southern Oregon and northwestern Nevada), where inferred intra-basin dikes appear to be spatially correlated with hot springs or prospective geothermal resource areas, suggest that the impact of pre-existing basement structure on hydrothermal activity may pertain more generally to other hydrothermal settings throughout the Great Basin. If so, efforts to map basement may prove crucial to understanding structural controls on some geothermal systems.

Furthermore, similarities across these disparate sites suggest that magmatism may play a much larger role in accommodating extension and influencing basin evolution across the western Great Basin than previously recognized.

Blind geothermal systems are believed to be common in the Basin and Range province and represent an underutilized source of renewable green energy. Their discovery has historically been by chance but more methodological strategies for exploration of these resources are being developed. One characteristic of blind systems is that they are often overlain by near-surface zones of low-resistivity caused by alteration of the overlying sediments to swelling clays. These zones can be imaged by resistivity-based geophysical techniques to facilitate their discovery and characterization.

Here we present a side-by-side comparison of resistivity models produced from helicopter transient electromagnetic (HTEM) and ground-based broadband magnetotelluric (MT) surveys over a previously discovered blind geothermal system with measured shallow temperatures of ~100°C in East Hawthorne, NV. The HTEM and MT data were collected as part of the BRIDGE project, an initiative for improving methodologies for discovering blind geothermal systems. HTEM data were collected and modelled along profiles, and the results suggest the method can resolve the resistivity structure 300 – 500 m deep. A 61-station MT survey was collected on an irregular grid with ~800 m station spacing and modelled in 3D on a rotated mesh aligned with HTEM flight directions. Resistivity models are compared with results from potential fields datasets, shallow temperature surveys, and available temperature gradient data in the area of interest.

We find that the superior resolution of the HTEM can reveal near-surface details often missed by MT. However, MT is sensitive to several km deep, can resolve 3D structures, and is thus better suited for single-prospect characterization. We conclude that HTEM is a more practical subregional prospecting tool than is MT, because it is highly scalable and can rapidly discover shallow zones of low resistivity that may indicate the presence of a blind geothermal system. Other factors such as land access and ground disturbance considerations may also be decisive in choosing the best method for a particular prospect. Resistivity methods in general cannot fully characterize the structural setting of a geothermal system, and so we used potential fields and other datasets to guide the creation of a diagrammatic structural model at East Hawthorne.

The northeastern portion of the Reese River basin in north-central Nevada is the focus of detailed geophysical and geological studies as part of the INGENIOUS project, which aims to identify new, commercially viable hidden geothermal systems in the Great Basin region of the western U.S. This location, herein referred to as Argenta Rise, occupies a broad (~15km wide) left-step between major range-front fault systems along the northwestern edge of the Shoshone Range and Argenta Rim, with numerous ENE-striking intra-basin faults presumably accommodating sinistral-normal oblique slip across the step-over. Four discrete regions have been identified within the study area that have favorable structural settings for hosting a blind hydrothermal system. However, with no definitive or extensive surface manifestations of an active hydrothermal system (e.g., geysers, steam vents, sinter, etc.), detailed geophysical studies are necessary to resolve subsurface geology and structure, and identify zones of enhanced structural complexity that may promote hydrothermal fluid flow. Hence, we collected high-resolution gravity, MT, and rock property data (density, magnetic susceptibility), and analyzed the recently acquired GeoDAWN aeromagnetic data to characterize potential geothermal resources in this region. Using the new geophysical datasets, we jointly modeled gravity and magnetic data along a series of intersecting 2D profiles that integrated information from recent, local-scale fault mapping. Rock property measurements performed on outcrops and hand samples throughout the study area constrained the models. The MT data were used to construct a 3D resistivity model that highlights the location of inferred alteration and fluids in the subsurface. Combined MT and potential field results reveal which structures may be most important for controlling hydrothermal fluid migration, as well as which geologic units may host hydrothermal fluids. Our gravity derived depth to basement surface coincides well with the base of shallow conductive anomalies, suggesting hydrothermal fluids may be confined to basin fill sediments and volcanics. This work supports our development of 3D geophysical and geologic models that are focused along the western flank of the northern Shoshone Range and aids the process of selecting sites for temperature gradient drilling.

Abstract Submission: are also located along contacts between resurgent rhyolite, mafic lavas, and surficial deposits which may provide additional pathways for gas and fluid migration in the shallow subsurface.

Characterizing structure and lithology using geophysical anomalies is critical to determining primary structural controls on the hydrothermal system and the extent of subsurface alteration at these sites. We conducted ground and airborne-based potential field geophysical surveys to map gravity and magnetic anomalies. These anomalies are then used to model subsurface geology, structure, and hydrothermal alteration. Here we present our preliminary geophysical mapping and modelling results at both tree-kill locations. Gravity and magnetic data suggest complex structural intersections are coincident with heated ground and gas emissions at the SRTKA and BCTKA. Hydrothermal systems are often observed or interpreted to exploit fault intersections which can serve as highly permeable pathways for hydrothermal fluid and gas discharge, enabling economic geothermal energy production. Geophysical mapping and modelling are an effective means of investigating such structural complexity at Mammoth Lakes due to the presence of unidentified and concealed structures.