Abstract Offshore fresh groundwater (OFG) is regarded as a significant, albeit typically non‐renewable, unconventional water resource stored within continental shelves. Many models of offshore freshwater emplacement use present‐day stratigraphy while applying Pleistocene sea‐level fluctuations. However, these static‐grid models do not account for sediment deposition or erosion, processes that could affect the volume and distribution of OFG. Here, we couple a sediment transport model (Sequence) with a variable‐density groundwater flow and solute transport model to reconstruct OFG sequestration over the last 500 kyr BP in response to late Pleistocene sea‐level changes. Our simulations incorporate the dynamic evolution of continental margin stratigraphy and associated hydrogeological processes. We examine six scenarios, including varying sea‐level fluctuations, deposition/erosion dynamics, subsidence rate, sediment flux, and offshore sediment diffusivity. Our findings demonstrate that overpressure generation due to relatively rapid sedimentation inhibits offshore freshwater emplacement. We also found that static‐grid models can significantly overestimate the volume of OFG. A second key observation is that the largest volumes of fresh groundwater are emplaced within the shallowest confined aquifer. Progressive burial of this unit by overlying confining units promotes salinization of the fresh groundwater through isolation from direct recharge and variable‐density flow effects. Our simulated OFG volumes range from 0.07 to 6.18 km3 per km length of coastline, consistent with field‐based estimates from continental shelf margins around the world. These findings showcase the importance of incorporating sediment dynamics into coastal hydrogeologic modeling to accurately predict the OFG distribution and support sustainable water management practices.

Regions of intra-sedimentary fluid injections, such as wastewater disposal, CO2 storage, and enhanced geothermal systems, commonly record near- and far-field basement-hosted earthquakes, highlighting complex fluid migration pathways and triggering mechanisms. Here, we present results from a rapid-response magnetotelluric geophysical survey of the 2016 M(w)5.8 Pawnee, Oklahoma earthquake epicenter region, the largest recorded injection-induced event, that aimed to illuminate the fluid migration pathways from the sedimentary Arbuckle carbonate injection zones into the intrabasement hypocentral zones. Intriguingly, our resistivity model reveals a broadly resistive seismogenic basement, relatively conductive shallow basement and Arbuckle intervals, overlain by discontinuous but highly conductive post-Arbuckle units. Porosity estimates indicate extremely low (<3%) basement porosity at >2 km depths, contrasting with intermediate porosity in the fractured top-basement and Arbuckle unit, and highly porous (10%-30%) post-Arbuckle sequences. Arbuckle-to-basement fracture distribution in core samples shows a prominent decrease in fracture intensities across the basement-sedimentary interface, representing a fluid flow barrier, and greater intensities in the Upper-Arbuckle and deeper basement levels. These observations present new evidence suggesting large-scale confinement of injected saline fluids to flow units within the Arbuckle, and potentially significant lateral dissipation of the fluids than previously thought. The results support a remote triggering model for the 2016 event and provide useful insights for understanding seismic triggering in similar fluid-injection settings.

The Shumagin Gap, a creeping segment of the Alaska subduction zone characterized by tsunamigenic structures, experienced a deep rupture during the July 2020 M7.8 earthquake. However, shallow slip behavior and the upper boundary of the rupture remain poorly understood. Here we utilize controlled-source electromagnetic data to image subsurface electrical resistivity, investigating the role of fluids in modulating megathrust locking state within the Shumagin Gap. Results reveal pronounced trench-normal heterogeneity in electrical resistivity both along the shallow plate interface and within the overriding plate, showing fluid presence but low overall porosity at the interface. An observed conductive channel extending into the overriding plate may facilitate upward fluid drainage. Our findings suggest that the volumes of fluids and inferred pore pressures are not sufficient to explain megathrust creep at the Shumagin Gap. Rather, the intricate interplay between heterogeneous structure and fluid distribution contributes to the region's seismogenic behavior and tsunami hazards, particularly in the shallow portion of the megathrust.

Aquifer overexploitation and saltwater intrusion threaten freshwater resources in coastal regions worldwide. In Bangladesh, arsenic contamination further reduces shallow freshwater availability. In the coastal zone, much of the shallow groundwater is saline, while the availability of deeper fresh groundwater remains poorly understood. Here, we utilize deep-sensing magnetotelluric soundings to image contrasts in electrical resistivity between fresh and saline groundwater along the Pusur River in the Ganges-Brahmaputra Delta. Our data reveal two distinct deep freshwater bodies separated by a high-salinity zone. We propose that these aquifers formed during the Last Glacial Maximum sea-level lowstand and are protected by overlying fine-grained sediments, whereas the Ganges paleovalley incision, followed by marine transgression and deposition, created the saline gap. Our work maps potential resources for this water-stressed region and suggests that the interplay between past sea-level cycles, sedimentation, and hydrogeological processes demonstrated here may also control the distribution of fresh groundwater in other deltas.

Recent advances in marine electromagnetic surveys have allowed geophysicists to interpret and map offshore freshwater resources with unprecedented resolution and to test inferences regarding onshore-offshore hydrologic connections. To date, however, little is known about the timing or isotopic composition of this unconventional water resource. Here, we reconstructed the Pleistocene hydrogeology of the U.S. Atlantic continental shelf using a cross-sectional paleo-hydrogeologic model to explore possible mechanisms and timing of freshwater emplacement offshore Martha's Vineyard, Massachusetts. We considered two scenarios in which the Laurentide ice sheet extended different distances offshore, and a third scenario without any ice sheet. The hydrostratigraphic framework was constructed by integrating borehole lithology data, seismic data, and formation resistivity data. Model results were compared to formation resistivity data as well as borehole salinity, groundwater residence time, and stable isotope profiles. Neither of the ice-sheet scenarios provided a significantly better fit to the onshore isotopic and offshore salinity observations than the third scenario. All three model scenarios predicted freshwater emplacement within Tertiary and Cretaceous units. Pleistocene deposits were largely devoid of freshened groundwater. Simulated groundwater residence times for the midshelf region ranged between 10(4) and 10(6) yr at depths of <500 m. Simulated groundwater ages from wells completed within Pleistocene confined aquifers are consistent with measured groundwater ages within confined aquifers of Martha's Vineyard and Nantucket Island (2750-5900 yr). Analysis of onshore H-3/He-3 dating data indicates that some wells contain a mixture of old and modern (<60 yr) groundwater. Calculated fossil groundwater in the midshelf region that included ice-sheet loading retained relatively low delta O-18 values, consistent with glacial meltwater recharge. Model results suggest that much of the freshwater emplacement occurred within the last glacial cycle and that the island and offshore hydrogeologic systems appear to be connected.

Aquifer overexploitation and saltwater intrusion threaten freshwater resources in coastal regions worldwide1,2. In Bangladesh, arsenic contamination and droughts further impact shallow freshwater availability in parts of the country3,4. In the coastal zone, much of the shallow groundwater is saline while availability of deeper fresh groundwater is poorly understood. Here, we utilize deep-sensing magnetotelluric soundings to image the contrast in electrical resistivity between resistive fresh and conductive saline groundwater along a 120 km profile adjacent to the Pusur River in the Ganges-Brahmaputra Delta, Bangladesh. Our data reveal two distinct deep freshwater bodies: a seaward-dipping wedge extending to 600 m depth in the northern transect, and a smaller fresh-to-brackish zone located near the coastline. We propose that these aquifers are remnants of preserved Pleistocene groundwater, supported by 14C ages up to 30,000 years5. An intervening high-salinity zone between the two freshwater bodies likely formed by Ganges paleovalley incision during the last glacial maximum, followed by marine transgression and deposition of conductive muddy sediments. Our results provide a framework for location and distribution of fossil groundwater reserves in Bangladesh’s riverine coastline, offering a potential resource for this water-stressed region. The interplay between past sea-level cycles, sedimentation, and hydrogeological processes may also control the distribution of deep fresh groundwater at other deltas.

The role of fluids in earthquake rupture is key to understanding seismic hazards, particularly at subduction zones. The Shumagin Gap, Alaska, is notable due to a paucity of large earthquake nucleation and weak coupling between the overriding and subducting plates. Fluids have been hypothesized to explain these observations, but the source of the fluids remains unclear. Here we present an image of the subsurface electrical resistivity derived from marine magnetotelluric data collected in the Shumagin segment. The model reveals an approximately 50-km-wide conductive (that is, fluid-rich) zone near the plate interface with fluids sourced from the dehydration of slab mantle (15–25 km beneath the crust–mantle boundary). We find that the July 2020 megathrust earthquake—which nucleated near the Semidi segment and propagated westwards into the Shumagin segment—only ruptured the conductive portion of the plate interface. This suggests that slab mantle fluids can influence the seismogenic zone by, for example, creating patches that are prone to dynamic rupture. In contrast, updip of the slip patch is simultaneously resistive and weakly coupled, suggesting that fluids alone are not responsible for weak coupling and that plate roughness plays a role. More broadly, these results suggest that slab mantle fluids could be an underappreciated fluid source in the water budgets of forearc subduction zones.

The dynamics of accretionary prisms and the processes that take place along subduction interfaces are controlled, in part, by the porosity and fluid overpressure of both the forearc wedge and the sediments transported to the system by the subducting plate. The Hikurangi Margin, located offshore the North Island of New Zealand, is a particularly relevant area to investigate the interplay between the consolidation state of incoming plate sediments, dewatering and fluid flow in the accretionary wedge and observed geodetic coupling and megathrust slip behaviour along the plate interface. In its short geographic extent, the margin hosts a diversity of properties that impact subduction processes and that transition from north to south. Its southernmost limit is characterized by frontal accretion, thick sediment subduction, the absence of seafloor roughness, strong interseismic coupling and deep slow slip events. Here we use seafloor magnetotelluric (MT) and controlled-source electromagnetic (CSEM) data collected along a profile through the southern Hikurangi Margin to image the electrical resistivity of the forearc and incoming plate. Resistive anomalies in the shallow forearc likely indicate the presence of gas hydrates, and we relate deeper forerarc resistors to thrust faulting imaged in colocated seismic reflection data. Because MT and CSEM data are highly sensitive to fluid phases in the pore spaces of seafloor sediments and oceanic crust, we convert resistivity to porosity to obtain a representation of fluid distribution along the profile. We show that porosity predicted by the resistivity data can be well fit by an exponential sediment compaction model. By removing this compaction trend from the porosity model, we are able to evaluate the second-order, lateral changes in porosity, an approach that can be applied to EM data sets from other sedimentary basins. Using this porosity anomaly model, we examine the consolidation state of the incoming plate and accretionary wedge sediments. A decrease in porosity observed in the sediments approaching the trench suggests that a protothrust zone is developing ∼25 km seaward of the frontal thrust. Our data also imply that sediments deeper in the accretionary wedge are slightly underconsolidated, which may indicate incomplete drainage and elevated fluid overpressures of the deep wedge.

Electromagnetic (EM) geophysical techniques offer the possibility of monitoring subsurface CO2 in saline reservoirs due to the fact that CO2 has high electrical resistivity compared to the surrounding geologic materials. In this paper we first discuss the underlying physics of two different borehole-based EM monitoring techniques; electrical resistivity tomography (ERT) and crosswell EM. This discussion is followed by the description of an experiment at the Carbon Management Canada CaMI FRS test site where time-lapse single well ERT and crosswell EM data have been acquired to image CO2 injection into a shallow aquifer. Resistivity imaging results from inversion of the two data types separately and jointly will be compared and contrasted, and interpretation of the extent of the injected CO2 provided.

SUMMARYWe present a method for computing a meaningful uncertainty quantification (UQ) for regularized inversion of electromagnetic (EM) geophysical data that combines the machineries of regularized inversion and Bayesian sampling with a ‘randomize-then-optimize’ (RTO) approach. The RTO procedure is to perturb the canonical objective function in such a way that the minimizers of the perturbations closely follow a Bayesian posterior distribution. In practice, this means that we can compute UQ for a regularized inversion by running standard inversion/optimization algorithms in a parallel for-loop with only minor modification of existing codes. Our work is split into two parts. In Part I, we review RTO and extend the methodology to estimate the regularization penalty weight on the fly, not unlike in the Occam inversion. We call the resulting algorithm the RTO-TKO and explain that it samples from a biased distribution which we numerically demonstrate to be nearby the Bayesian posterior distribution. In return for accepting this small bias, the advantage of RTO-TKO over asymptotically unbiased samplers is that it significantly accelerates convergence and leverages computational parallelism, which makes it highly scalable to 2-D and 3-D EM problems. In Part II, we showcase the versatility and computational efficiency of RTO-TKO and apply it to a variety of EM inversions in 1-D and 2-D, carefully comparing the RTO-TKO results to established UQ estimates using other methods. We further investigate scalability to 3-D, and discuss the influence of prior assumptions and model parametrizations on the UQ.

This paper is Part II of a two-part series on a mathematical and computational framework for computing a meaningful uncertainty quantification (UQ) for regularized inversions of electromagnetic data. In Part I, we explained the theory behind a sampling algorithm, which we call RTO-TKO, and in Part II, we showcase RTO-TKO in practice. We individually and jointly invert seafloor magnetotelluric (MT) and surface-towed controlled source electromagnetic field data, collected for imaging offshore freshened groundwater beneath the US Atlantic margin. We also invert seafloor MT data collected for subsalt imaging to produce 2D resistivity models and uncertainty estimates that characterize the salt body geometry and surrounding sediments. We compare the UQ of the RTO-TKO with results from trans-dimensional sampling, and explain the differences arising from different underlying (prior) assumptions of the two algorithms. We also discuss the practical implications of these findings. Most importantly, however, the 2D case study unambiguously demonstrates the computational advantages of RTO-TKO and its ability to make use of massive parallelism.

Plate tectonics requires a low-viscosity layer beneath the lithosphere–asthenosphere boundary (LAB), yet the origin of this ductile transition remains debated 1 , 2 . Explanations include the weakening effects of increasing temperature 3 , 4 , mineral hydration 5 or partial melt 6 . Electrical resistivity is sensitive to all three effects 7 , including melt volatile content 8 , but previous LAB constraints from magnetotelluric soundings did not simultaneously consider the thermodynamic stability of the inferred amount of melt and the effect of uncertainty in the estimated resistivity 8 – 14 . Here we couple an experimentally constrained parameterization of mantle melting in the presence of volatiles 15 , 16 with Bayesian resistivity inversion 17 and apply this to magnetotelluric data sensitive to a LAB channel beneath the Cocos Plate 9 . Paradoxically, we find that the conductive channel requires either anomalously large melt fractions with moderate volatile contents or moderate melt fractions with anomalously large volatile contents, depending on the assumed mantle temperature. Large melt fractions are unlikely to be mechanically stable and conflict with melt-migration models 18 . As large volatile contents require a highly enriched mantle source inconsistent with mid-ocean-ridge estimates 19 , our results indicate that a mantle plume emplaced volatile-rich melts in the LAB channel. This requires the presence of a previously undetected nearby plume or the influence of the distant Galápagos hotspot. Plumes that feed thin, hydrous melt channels 9 , 14 , 20 may be an unrecognized source of LAB anomalies globally.

In the past decade, rapid advances in distributed optical fibre sensing technologies have made it possible to record various geophysical data (e.g. strain, temperature and pressure) continuously in both time and space along the fibre, providing an unprecedented quantity and spatial density of data compared to traditional geophysical measurements as well as reducing data acquisition cost. To date, no distributed fibre‐based electromagnetic field sensing system has been implemented although electromagnetic sensing could have a broad range of applications to geophysical imaging and monitoring in borehole environments. The goal of this paper is to provide a theoretical feasibility study regarding the design and use of an electromagnetic sensing optical fibre for geophysical applications. First, we present the sensitivity analysis of a ‘hypothetical’ optical fibre coated with polyvinylidene fluoride, a polymer that provides relatively high piezoelectric properties, yet unlike ceramics, is flexible. Using a two‐dimensional electromagnetic modelling algorithm, we simulate the earth electric‐field‐to‐fibre‐strain transfer function and estimate the theoretical sensitivity of the optical fibre to electric fields. Given the state‐of‐the‐art distributed acoustic sensing strain sensitivities in the picometres strain range, our numerical modelling analysis suggests that a perfectly coupled polyvinylidene fluoride–coated optical fibre can measure electric field values in the mV/m to V/m amplitude range. We then apply a cylindrically symmetric modelling algorithm to simulate numerical models demonstrating the applicability of such a fibre in an oilfield environment. Scenarios investigated employ an electric field source and suggest that the measurements can be used to distinguish the oil versus water ratio with a fibre mounted inside a producing steel cased oil well as well as distinguishing between brine and hydrocarbon filled reservoir zones with a fibre located outside of the casing.

Subduction of hydrated oceanic lithosphere can carry water deep into the Earth, with consequences for a range of tectonic and magmatic processes. Most of the fluid is released in the forearc where it plays a critical role in controlling the mechanical properties and seismic behaviour of the subduction megathrust. Here we present results from three-dimensional inversions of data from nearly 400 long-period magnetotelluric sites, including 64 offshore, to provide insights into the distribution of fluids in the forearc of the Cascadia subduction zone. We constrain the geometry of the electrically resistive Siletz terrane, a thickened section of oceanic crust accreted to North America in the Eocene, and the conductive accretionary complex underthrust along the margin. We find that fluids accumulate over timescales exceeding 1 My above the plate in metasedimentary units, while the mafic rocks of Siletzia remain dry. Fluid concentrations tend to peak at slab depths of 17.5 and 30 km, suggesting control by metamorphic processes, but also concentrate around the edges of Siletzia, suggesting that this mafic block is impermeable, with dehydration fluids escaping up-dip along the megathrust. Our results demonstrate that the lithology of the overriding crust can play a critical role in controlling fluid transport in a subduction zone. The lithology of the overriding plate plays a critical role in determining fluid transport in subduction zones, according to magnetotelluric imaging of the impact of the dry, mafic Siletzia terrane on fluids in the Cascadia subduction zone, North America.

Antarctica's fast-flowing ice streams drain the ice sheet, with their velocity modulated by subglacial water systems. Current knowledge of these water systems is limited to the shallow portions near the ice-bed interface, but hypothesized deeper groundwater could also influence ice streaming. Here, we use magnetotelluric and passive seismic data from Whillans Ice Stream, West Antarctica, to provide the first observations of deep sub-ice stream groundwater. Our data reveal a volume of groundwater within a >1-kilometer-thick sedimentary basin that is more than an order of magnitude larger than the known subglacial system. A vertical salinity gradient indicates exchange between paleo seawater at depth and contemporary basal meltwater above. Our results provide new constraints for subglacial water systems that affect ice streaming and subglacial biogeochemical processes.

We develop an inverted long-baseline (ILBL) acoustic navigation system for determining the position of deep-towed instruments, such as controlled-source electromagnetic transmitters and receivers. The ILBL system uses a deep-tow mounted acoustic transceiver system to measure travel times to a pair of surface transponders towed on paravanes behind the survey vessel. The travel times, transponder positions and pressure depth data are inverted for the lateral position of the deep-tow vehicle, as well as the positions of any relay transponders on the antenna and receiver array that are towed horizontal behind the deep-tow vehicle. Three example applications demonstrate position accuracies of about 5 and 37 m in the inline and crossline directions for 3 km water depths and around 6 m for 1 km depth. The portability and generality of the system make it suitable for deep-tow applications for geophysical, geochemical and geological surveying purposes. We have shown that the accuracy of the ILBL system is similar to that of commercial USBL systems, but is considerably more cost effective than even portable USBL systems, and can be used on vessels lacking permanently installed USBL transceiver heads. Further, it can be used in water depths of 5,000 m or more. It could also be readily modified for further improving its position accuracy if desired.

First reported in the 1960s, offshore freshened groundwater (OFG) has now been documented in most continental margins around the world. In this review we compile a database documenting OFG occurrences and analyze it to establish the general characteristics and controlling factors. We also assess methods used to map and characterize OFG, identify major knowledge gaps, and propose strategies to address them. OFG has a global volume of 1 × 10 6 km 3 ; it predominantly occurs within 55 km of the coast and down to a water depth of 100 m. OFG is mainly hosted within siliciclastic aquifers on passive margins and recharged by meteoric water during Pleistocene sea level lowstands. Key factors influencing OFG distribution are topography‐driven flow, salinization via haline convection, permeability contrasts, and the continuity/connectivity of permeable and confining strata. Geochemical and stable isotope measurements of pore waters from boreholes have provided insights into OFG emplacement mechanisms, while recent advances in seismic reflection profiling, electromagnetic surveying, and numerical models have improved our understanding of OFG geometry and controls. Key knowledge gaps, such as the extent and function of OFG, and the timing of their emplacement, can be addressed by the application of isotopic age tracers, joint inversion of electromagnetic and seismic reflection data, and development of three‐dimensional hydrological models. We show that such advances, combined with site‐specific modeling, are necessary to assess the potential use of OFG as an unconventional source of water and its role in sub‐seafloor geomicrobiology.

The role of subducting topography on the mode of fault slip—particularly whether it hinders or facilitates large megathrust earthquakes—remains a controversial topic in subduction dynamics 1 – 5 . Models have illustrated the potential for subducting topography to severely alter the structure, stress state and mechanics of subduction zones 4 , 6 ; however, direct geophysical imaging of the complex fracture networks proposed and the hydrology of both the subducting topography and the associated upper plate damage zones remains elusive. Here we use passive and controlled-source seafloor electromagnetic data collected at the northern Hikurangi Margin, New Zealand, to constrain electrical resistivity in a region of active seamount subduction. We show that a seamount on the incoming plate contains a thin, low-porosity basaltic cap that traps a conductive matrix of porous volcaniclastics and altered material over a resistive core, which allows 3.2 to 4.7 times more water to subduct, compared with normal, unfaulted oceanic lithosphere. In the forearc, we image a sediment-starved plate interface above a subducting seamount with similar electrical structure to the incoming plate seamount. A sharp resistive peak within the subducting seamount lies directly beneath a prominent upper plate conductive anomaly. The coincidence of this upper plate anomaly with the location of burst-type repeating earthquakes and seismicity associated with a recent slow slip event 7 directly links subducting topography to the creation of fluid-rich damage zones in the forearc that alter the effective normal stress at the plate interface by modulating the fluid overpressure. In addition to severely modifying the structure and physical conditions of the upper plate, subducting seamounts represent an underappreciated mechanism for transporting a considerable flux of water to the forearc and deeper mantle.

The 3D electrical resistivity model derived from magnetotelluric data inversion. The model file is in netCDF format, which can be viewed using any standard netCDF plotting tools (e.g. Panoply).