Submarine hydrothermal vents associated with active volcanoes provide valuable insight into the shallow fluid migration pathways and volatile transport from magmatic systems. The distribution and activity of these vents are controlled by the subseafloor structure such as faulting and zones of hydrothermal alteration. These structural features govern fluid pathways and influence long-term volcanic behavior making the location, geometry, and variability of these structural and alteration features important information for volcanic hazard assessment. Additionally, hydrothermal vents provide sources of heat and chemically distinct substrates that support benthic ecosystems. Identifying and characterizing these features aids better understanding and management of these ecosystems.Controlled-source electromagnetic (CSEM) methods are well suited for investigating these systems due to their sensitivity to changes in electrical resistivity associated with pore fluid composition and hydrothermal alteration. Here, we present results from a surface-towed CSEM survey conducted offshore Whakaari/White Island in the Taupō Volcanic Zone, New Zealand. The survey targeted regions containing previously mapped vent sites and was designed to image hydrothermal pathways within the upper few hundreds of meters of the seafloor. Additionally, survey lines extended from the island to the area surrounding Te Paepae o Aotea/Volkner Rocks, a site of inferred structural and magmatic connectivity, to capture active hydrothermal vent sites or shallow subseafloor alteration associated with prior venting activity.Preliminary inversions reveal electrical resistivity signatures that we interpret as zones of volcanically altered material, fluid and/or gas flow along fault structures, and individual vent features. These results provide a detailed view of near-seafloor electrical structure associated with active and relict hydrothermal processes offshore Whakaari. Our inversions complement deeper constraints on magmatic systems from ocean-bottom EM surveys and regional airborne EM data by resolving shallow conductive and resistive features linked to fluid flow and alteration. These data improve characterization of shallow subsurface structure and hydrothermal pathways in an active volcanic setting and demonstrate the sensitivity of surface-towed CSEM to vent-related processes in the shallow seafloor.

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.

Development of offshore infrastructure, such as wind farms, requires detailed geotechnical information of the seafloor and the identification of potential hazards, such as shallow hydrocarbons. This information can be supplied by electromagnetic surveys optimized for high-resolution data collection through the use of high source frequencies, short source-receiver offsets, and close proximity to the seafloor. We developed such a system, the compact undersea electromagnetic source instrument (CUESI), which transmits a 10-100 Hz, 2-10 amp current on a horizontal dipole and records three-axis electric field signals at offsets of 10-30 m. Unlike bottom-dragged systems, which cannot be used in protected areas, the CUESI system is neutrally buoyant and is flown 1-3 m above the seafloor, maintaining altitude by buoyancy feedback provided by a short counterweight cable. CUESI is capable of operating at water depths up to 200 m. Tests offshore Santa Barbara, California, demonstrate the capability of the system to detect lateral resistivity changes that are consistent with sediment characteristics observed in coring. Inversions of data over the Santa Rosa Fault resolve the fault trace much more sharply than from a similar system towed on the sea surface. Resistors imaged at the seafloor coincide with tar-like features captured by a camera mounted on the CUESI depressor weight. Based on these tests, CUESI would be a useful tool for offshore development because of its ability to measure lateral changes in seafloor character and detect hazards, such as faults and hydrocarbons.

We present new numerical tools for geophysical inversion and uncertainty quantification (UQ), with an emphasis on blocky (piecewise-constant) layered models that can reproduce sharp contrasts in geophysical or geological properties. The new tools are inspired by an 'old' and very successful inversion tool: regularized, nonlinear inversion. We combine Occam's inversion with total variation regularization and a split Bregman method to obtain an inversion algorithm that we call blocky Occam, because it determines the blockiest model that fits the data adequately. To generate an UQ, we use a modified randomize-then-optimize approach (RTO) and call the resulting algorithm RamBO (randomized blocky Occam), because it essentially amounts to running blocky Occam in a randomized parallel for-loop. Blocky Occam and RamBO inherit computational advantages and stability from the combination of Occam's inversion, split Bregman and RTO, and, therefore, can be expected to be robustly applicable across geophysics.

The Gulf of California Rift (GCR) represents a transitional boundary between the North American and Pacific plates, linking continental breakup along the San Andreas fault system with oceanic spreading centers at the East Pacific Rise. This study focuses on the northern GCR, where pull-apart basins such as Wagner and Consag overlay a basement suspected to be a transitional crust composed of stretched continental material, sediments, and intrusive bodies. Rapid sedimentation from the Colorado River, combined with thermal and magmatic processes, has influenced crustal evolution in this region, potentially delaying the onset of a complete continental breakup. We present the first 3D inversion of marine magnetotelluric (MT) data in Mexico, analyzing 13 observation sites across two profiles spanning a 100 × 120 km area. We register electromagnetic fields at periods ranging from 2 to 5000 by 18 days. The resulting 3D resistivity model highlights two prominent conductive anomalies associated with crustal processes. Shallow conductive zones (< 6 km depth) correspond to fault-related seawater infiltration and sedimentary structures. A deeper anomaly (3–5 Ohm-m) between 10 and 20 km depth in the southern Wagner and northern Consag basins is interpreted as caused by saline fluids in the continental crust, likely derived from serpentinized peridotite at the top of the mantle. This conductive anomaly and other geophysical observations, including earthquake hypocenters, seismic velocities, and gravity anomalies, support the hypothesis of ongoing crustal modification through magmatism and sedimentation. We present an electrical resistivity model that adds independent information to the seismic-wave velocity and mass density derived from previous geophysical studies. The resistivity distribution supports the hypothesis of a transitional crust in the northern GCR, whose trans-tensional deformation causes fractures and upwelling of saline fluids from the top of the mantle.

During the final stages of breakup at magma-poor rifted margins, mantle rocks are commonly exhumed and altered to serpentinite due to the ingress of ocean water. This mantle exhumation phase is followed by an increase in magmatism as new oceanic crust begins to form. However, the degree to which serpentinisation is focused at faults and whether the onset of magmatism is abrupt or gradual are both unclear. These processes are difficult to untangle with seismic data alone because the P wave velocities of mafic crustal rocks and partially serpentinised mantle rocks can be similar. However, serpentinised mantle rocks are generally more conductive, often by about an order of magnitude, than mafic crustal rocks, so controlled source electromagnetic (CSEM) and magnetotelluric (MT) techniques provide a promising route to resolve controversies around the structure of lithosphere formed during the onset of seafloor spreading.To take advantage of the complementary information provided by seismic and electromagnetic data, in September 2023 we acquired a coincident and densely sampled wide-angle seismic, CSEM and MT datasets across the continent-ocean transition at Goban Spur, southwest of the UK. Our c. 200-km profile is coincident with a pre-existing high-quality seismic reflection profile. It extends from thinned continental crust, whose nature is confirmed by drilling, across a broad zone that is inferred on the basis of a previous wide-angle seismic experiment to be composed of exhumed and serpentinised mantle, and into oceanic crust, evidenced by the presence of the prominent seafloor-spreading magnetic anomaly A34. Along this profile, we deployed 49 seafloor instruments at c. 4-km spacing that were each capable of recording seismic, electric field and magnetometer data, plus an additional two instruments recording the inline electric field on 200-m dipoles. These instruments were on the seafloor for about two weeks. During this time we acquired two wide-angle seismic profiles: one using a 5200 cu. in. airgun array shot at 90-s intervals and a second using a 3900 cu. in. airgun array shot at 30-s intervals. We also acquired a frequency-domain CSEM profile using a transmitter towed c. 100 m above the seabed that powered a 300-m electric dipole with a c. 100-A current at a fundamental frequency of 0.25 Hz. Preliminary data analysis showed that seismic signals were recorded to c. 90 km offset and CSEM signals to c. 8 km offset, while high-quality MT data were recorded at periods of 20-10000 s.Thus we expect to recover coincident high-resolution images of the seismic velocity and resistivity structure of the upper few km of the basement, sufficient to image patterns of serpentinisation and mafic intrusion. We also expect to recover lower-resolution images of the resistivity to tens of km below the seabed and thus to distinguish continental mantle lithosphere from depleted oceanic lithosphere. We will present examples of the data acquired and the results of some preliminary analysis.

SUMMARY The Southern San Andreas fault (SSAF) poses one of the largest seismic risks in California. Yet, there is much ambiguity regarding its deeper structural properties around Coachella Valley, in large part due to the relative paucity of everyday seismicity. Here, we image a multistranded section of the SSAF using a non-seismic method, namely magnetotelluric (MT) soundings, to help inform depth-dependent fault zone geometry, fluid content and porosity. The acquired MT data and resultant inversion models highlight a conductive column encompassing the SSAF zone that includes a 2–3 km wide vertical to steeply northeast dipping conductor down to ∼4 km depth (maximum of ∼1 Ω·m at 2 km depth) and another prominent conductor in the ductile crust (∼1 Ω·m at 12 km depth and slightly southwest of the surface SSAF). We estimate porosities of 18–44 per cent for the conductive uppermost 500 m, a 10–15 per cent porosity at 2 km depth and that small amounts (0.1–3 per cent) of interconnected hypersaline fluids produce the deeper conductor. Located northeast of this conductive region is mostly resistive crust indicating dry crystalline rock that extends down to ∼20 km in places. Most of the local seismicity is associated with this resistive region. Located farther northeast still is a conductive region at >13 km depth and separate from the one to the southwest. The imaged anomalies permit two interpretations. The SSAF zone is vertical to steeply northeast dipping in the upper crust and (1) is near vertical at greater depth creating mostly an impermeable barrier for northeast fluid migration or (2) continues to dip northeast but is relatively dry and resistive up to ∼13 km depth where it manifests as a secondary deep ductile crustal conductor. Taken together with existing knowledge, the first interpretation is more likely but more MT investigations are required.

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.

Understanding the depth of investigation for electromagnetic and electrical methods is important in experimental design and for interpreting inversions. Many studies have defined the depth of investigation for electromagnetic sounding methods, but none have included continuously towed controlled-source electromagnetic methods. Nodal controlled-source electromagnetic surveys using ocean-bottom electromagnetic receivers have generally been found to have a depth of investigation limited to about half the maximum source-receiver spacing, but experience using continuously towed arrays suggests sensitivity to targets at depths approaching the source-receiver spacing. We test this on two-dimensional synthetic data using two methods. A rigorous approach is to re-invert data as a highly conductive or resistive basement is included at successively shallower depths. When the data misfit becomes unacceptably high we can conclude that the maximum depth of inference has been passed. Rather than rely on overall misfit, we note that it is more realistic to preferentially examine those data most sensitive to largest depths (longest offsets and lowest frequencies). A more practical approach is to determine the depth at which a conductivity contrast can be imaged by inversion, noting that knowledge of a contrast is geologically useful even if the actual conductivities cannot be recovered. Both approaches confirm that the increased data density of towed electromagnetic systems at longest offset increases the depth of investigation to about the maximum offset distance.

The spectrum of geomagnetic variations is broad and reflects multiple internal and external physical Earth processes. The geodynamo generates an internal magnetic field, dominated by the low frequency contribution of the axial dipole whose temporal variations range from geomagnetic reversals, excursions, and large-scale paleosecular variations, down to decadal and sub-annual changes. Variations in the external field arise from high frequency interactions of the solar wind with Earth's magnetosphere and ionosphere, along with excitation within the atmospheric cavity by lightning, power systems, and radio transmissions. External variations induce a field in the conductive Earth that adds to the internal signal. A spherical harmonic analysis of dipole terms in over 100 years of observatory data allows us to show that the external field is stronger than the internal field at periods of the 11-year sunspot cycle and shorter. Using spectral estimates derived from this and other data sets by adaptive multi-spectral time series analysis, we can create a composite power spectrum that spans frequencies from 10-15 Hz to 20 kHz (periods ranging from 10 million years to 5 x 10-5 s), and powers ranging from 10-9 (nT)2/Hz to 1021 (nT)2/Hz. The different processes contributing to the spectrum are characterized by various inverse power laws across frequency bands. This Grand Spectrum quantifies the successive dominance of the many different geophysical processes with frequency, but importantly provides a compelling graphic to illustrate the complexity of the geomagnetic field.

&lt;p&gt;Seafloor fracture zones are the inactive extensions of transform faults that represent a discontinuity in seafloor age, temperature, and bathymetry. Their structures and tectonic features provide important information about lithospheric evolution. The age contrast across the fracture zone produces differential subsidence resulting in lithospheric flexure with uplift on the young side and subsidence on the old side. There is still debate on whether a fracture zone is significantly weaker than the surrounding lithosphere.&amp;#160; Some models predict small-scale convection beneath the FZ due to the sharp thermal contrast. A magnetotelluric (MT) survey over the Mendocino fracture zones (MFZ) was carried out 600 km away from the US west coast in 2018. The primary objective is to determine whether changes in the electrical resistivity across the MFZ are caused by temperature variations or other factors at the LAB. There is a strong crust age contrast at different sides of the MFZ. The northern side, younger with a crust age of 6.5 Ma, is formed at the Gorda Ridge with depths of about 3 km. The southern side, older with a crust age of 33 Ma, is part of the Pacific plate with depths of about 4 km. On both sides of MFZ, seamounts are observed in the study region, which indicates off-axis magma upwelling. Two-dimensional (2D) inversion of MT data was done to construct a 2D resistivity model across MFZ. The inversion can fit the data to an RMS of 1.8 with an error floor of 5 % on the MT impedance tensor. The model shows strong resistivity contrasts at the depths of 30-80 km on both sides of MFZ. The resistivity contrast at such depths is likely attributed to partial melt and temperature since composition is most likely similar at both sides of MFZ. Other conductive anomalies are also observed at shallow depths along the profile, which collocates with seamounts at the study region, especially near the southern end of our MT profile. Therefore, the conductive anomalies at the shallow depths likely present partial melts driven by upwelling magma.&lt;/p&gt;

The marine controlled-source electromagnetic (CSEM) method has been used for offshore natural resource exploration for a few decades, and it has the potential to be used for offshore CO2 storage monitoring. Airwaves, previously treated as distortions, often dominate marine CSEM data when the offshore seawater is shallower than a couple of kilometers. Therefore, different methods have been proposed to distinguish and then correct the airwaves in marine CSEM data. In this study, we analyzed the airwave features by different model parameter perturbations with two-dimensional (2D) modeling. After a thorough study of differentiated EM fields and Poynting vectors for each single model component, we summarize the airwave propagation features and sensitivities regarding different modeling parameters. Particularly, the scenario with and without an oil reservoir or CO2 storage is carefully studied. It turns out that the airwave can provide useful information at certain transmitter and receiver offsets. Nevertheless, we propose to model the airwave as what it is in the marine CSEM data rather than correct it before feeding the offshore CSEM data to inversion. This idea is demonstrated with a field example from offshore of Svalbard, Norway.

Marine hydrocarbon seeps are found on all continental margins and release significant amounts of greenhouse gases and hydrocarbons into the atmosphere and hydrosphere. Most methods of studying seeps rely on seafloor or near-seafloor observations, but seepage rates and seep locations can be variable, leading to uncertainty. We exploit the fact that hydrocarbons are electrically resistive compared with surrounding sediments and use marine electromagnetic methods to study the deeper sources and accumulation sites, which are controlled by local geology and should be more stable than the seabed expressions. Our surface towed marine controlled source electromagnetic system used a horizontal electric dipole transmitter and floating electric dipole receivers spaced 100-400 m from the transmitter, collecting frequency domain amplitude and phase data at ~2 Hz and harmonics. The survey targeted known and inferred hydrocarbon seeps within the Coal Oil Point seep field offshore Santa Barbara, California, USA. Two dimensional inversions of the data indicate that the method is sensitive to the shallowly buried (<400 m) structure of the marine hydrocarbon seeps and is an efficient and effective tool in their identification and characterization. The results show spatial variability of seafloor hydrocarbons along the Coal Oil Point seep field and indicate at least two previously unidentified intermediate depth accumulation sites. The depth and lateral extent of these hydrocarbon accumulation sites may improve seep emission models for the Coal Oil Point seep field.

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.

To investigate the underlying magmatic systems that drive high-temperature geothermal fields and volcanism in the north-central Taupo Volcanic Zone (TVZ), we use a combination of land and lake-bottom magnetotelluric (MT) measurements. In total, MT data from 399 sites (including 34 lake-bottom locations) across 2800 km(2) covering the Rotorua Caldera and Okataina Volcanic Centre are inverted to image the 3-D resistivity structure of the crust to 20 km depth. Beneath this region, a singular conductive zone in the mid-crust is shown, inferred to represent a complex of relatively mafic magma, consistent with petrologic and other geophysical data. Discrete low-resistivity (conductive) plumes rise from the margins of this magmatic complex and connect 1-to-1 with the locations of high-temperature geothermal fields at the surface. These plumes are inferred to represent pathways of heat transfer through the crust via magmatic intrusion and exsolved saline fluids, and inform connections with other geothermal areas located nearby to the major fields. An additional conductive plume rises beneath the area where the most voluminous recent volcanism has occurred in the Okataina Volcanic Centre; the summit region (e.g. Makatiti dome) in the Haroharo Volcanic Complex that has erupted 39 km(3) (magma equivalent) in the past 9 ka. While hot springs locate around this conductive plume at the base of the rhyolite domes, the plume itself is arrested at 1-2 km depth beneath the resistive (inferred impermeable) young (5 ka old) rhyolite lava domes, and is considered to represent a deep 'blind' geothermal system in the TVZ.

Climate change has a pronounced effect on water resources in many semiarid climates, causing populated areas such as San Diego County (USA), to become more vulnerable to water shortages in the coming decades. To prepare for decreased water supply, San Diego County is adopting policies to decrease water use and to develop additional local sources of water. One new local source of freshwater is produced by a desalination facility that purifies brackish groundwater from the coastal San Diego Formation. This formation has been studied extensively onshore, but little is known about the geology or groundwater quality offshore in the adjacent continental shelf. Because most groundwater systems are interconnected and complex, further analysis is needed to identify offshore geology, possible sequestration of freshwater in the shelf, and potential pathways for saltwater intrusion. This comprehensive understanding is important because seawater intrusion may limit use of the San Diego Formation and longevity of desalination facilities. Controlled-source electromagnetic methods are uniquely suited to detecting offshore groundwater as they are sensitive to changes in pore fluids such as the transition from fresh to brackish groundwater. This paper describes results from surface-towed electromagnetic surveys that mapped the pore-fluid salinity and possible fluid pathways in the continental shelf off the coast of San Diego. The results indicate a considerable volume of fresh-to-brackish groundwater sequestered in the shelf, both in continuous lenses and isolated pockets, that appear influenced by fault systems and shallow stratigraphy.