Advective Flow Research Articles

Zonally dominated dynamics and Dimits threshold in curvature-driven ITG turbulence

The saturated state of turbulence driven by the ion-temperature-gradient instability is investigated using a two-dimensional long-wavelength fluid model that describes the perturbed electrostatic potential and perturbed ion temperature in a magnetic field with constant curvature (a $Z$ -pinch) and an equilibrium temperature gradient. Numerical simulations reveal a well-defined transition between a finite-amplitude saturated state dominated by strong zonal-flow and zonal temperature perturbations, and a blow-up state that fails to saturate on a box-independent scale. We argue that this transition is equivalent to the Dimits transition from a low-transport to a high-transport state seen in gyrokinetic numerical simulations (Dimits et al., Phys. Plasmas, vol. 7, 2000, 969). A quasi-static staircase-like structure of the temperature gradient intertwined with zonal flows, which have patch-wise constant shear, emerges near the Dimits threshold. The turbulent heat flux in the low-collisionality near-marginal state is dominated by turbulent bursts, triggered by coherent long-lived structures closely resembling those found in gyrokinetic simulations with imposed equilibrium flow shear (van Wyk et al., J. Plasma Phys., vol. 82, 2016, 905820609). The breakup of the low-transport Dimits regime is linked to a competition between the two different sources of poloidal momentum in the system – the Reynolds stress and the advection of the diamagnetic flow by the $\boldsymbol {E}\times \boldsymbol {B}$ flow. By analysing the linear ion-temperature-gradient modes, we obtain a semi-analytic model for the Dimits threshold at large collisionality.

Analyzing the ignition differences between conventional spark discharges and nanosecond-pulsed high-frequency discharges

New methods of energy deposition using nanosecond-pulsed high-frequency discharges (NPHFD) have shown promise in igniting fuel-lean conditions. This work compared an NPHFD ignition system to a conventional, capacitive discharge system in a flowing environment under matching total energy and average power conditions. The results showed that, in every case, matching the average power and deposited energy between the two systems resulted in similar trends in kernel radius growth, time to minimum growth rate, and kernel radius at which minimum growth rate occurs. These results were used as a baseline for comparison for the lower average power NPHFD test conditions. Decreasing the power of the NPHFD system while maintaining the total energy deposited allowed kernel growth to be aided by flow advection, resulting in a ∼38% increase in the streamwise radius as compared to the baseline. The larger kernel size comes at the expense of the kernel taking ∼20% longer to transition to a self-propagating flame that occurs at a radius that is ∼72% larger than the respective values for the baseline condition. This behavior is due to the long duration of the discharge and the low energy density per unit volume in the fluid, respectively. The benefit of the larger kernel size comes at the cost of reliability. Therefore, in combustor conditions with strong external quenching physics, depositing the most energy in the shortest time will be optimal. Conversely, for kernels in the presence of mild turbulence, the average power can be decreased, and flow advection can be utilized to grow the kernel over a longer duration without risk of extinction.

Immersive diagnostics of reservoirs under WAG injection, Part II – Effect of depositional settings and dynamic spatial correlations

Immersive visualization and diagnostics have become essential tools in the arsenal of engineers and scientists in developing innovative concepts, products, and techniques. In reservoir engineering applications, these approaches can be immensely helpful in detecting the dynamics occurring within the reservoir under complex and evolving conditions. In Part I of this study, we presented several novel immersive diagnostics quantifying displacement measures and unraveled some interesting dynamics around the injector in reservoirs undergoing Water-Alternating-Gas (WAG) injection. This article is Part II of the study on the utility of immersive diagnostics to ascertain the effect of geological formation characteristics on WAG reservoir dynamics.WAG injection can yield incremental recovery around 10% in mature oilfields. In this study, using immersive metrics, we critically investigated WAG performance of four different injection-fluid streams (pure CO2, pure methane, lean gas, and enriched gas) and three WAG injection-cycle strategies in several depositional systems. Spatial clusters of displacement behavior appeared. The relationship of oil displacement efficiency with various driving forces may depend on multiple elements. These elements include compositional variation in the residual oil close to the injector and the producers and the corresponding changes in thermophysical and multiphase flow properties. The pressure level, distance to the injector, formation permeability, and depositional units contribute to the flood performance. Over time with pressure depletion and frontal movement from the injector to producers, the distinction of these clusters dissipates.Spatial correlational analyses of displacement measures reveal interesting observations. The fluid flow of the oleic phase in channelized/turbiditic formations tends to be more inertial than diffusive. Oil displacement efficiency in the channelized/turbiditic formations depends strongly on the injector-producer connectivity via these flow conduits. In most formations, we can expect a reduction in water viscosity or an increase in the inertial force in the water, yielding more efficient oil displacement.At late times, diffusive flow dictates oil displacement in oil-water zones, wherein the advective flow of oil predominates in pockets where all three phases (oil, gas, and water) are present. In general, reservoir locations with high water-phase inertial to viscous forces wield higher oil-displacement efficiency. However, in high-permeable formations at late times, reservoir locations closer to the producers having low-permeability experience low-water inertial-to-viscous forces and relatively less oil displacement efficiency, which gets worse with the presence of all three phases.

A quasi-geostrophic diagnosis of the zonal flow associated with cut-off lows over South Africa and surrounding oceans

The zonal flow associated with cut-off lows (COLs) comprises two jet streaks of different spatial extents. The smaller scale jet streak, located north of the COLs, forms as a result of meridional divergence of vorticity advection and it is quasi-stationary, relative to the COLs. It dissipates as the COLs do the same. The larger scale jet streak gives rise to anticyclonic and equatorward Rossby wave breaking (RWB) as it propagates southeasterly to the base of the ridge, south of the COL and then northeasterly beyond that point. As the jet streak propagates it brings with it the anticyclonic barotropic shear that causes the Rossby waves to break. Its propagation is caused by zonal momentum advection by the zonal flow from jet streak entrance to its exit. As it propagates, its northwesterly/southeasterly orientation changes to one that is more zonal to become south-westerly/northeasterly at the end of the COL life cycle. This change in orientation is due to meridional advection of zonal momentum,where the meridional flow advects momentum southward (northward) at the jet streak entrance (exit). The jet streaks form a split jet structure and the winds between the streaks is decelerated by vorticity advection convergence. Because the flow and COL (and RWB) life cycle are coupled, understanding the dynamics that underlie the changes in the COL ambient flow contributes to resolving the outstanding RWB/COL causality problem.

Cells in New Light: Ion Concentration, Voltage, and Pressure Gradients across a Hydrogel Membrane.

The ionic compositions of the intra- and extracellular environments are distinct from one another, with K+ being the main cation in the cytosol and Na+ being the most abundant cation outside of the cell. Specific ions can permeate into and out of the cell at different rates, bringing about uneven distribution of charges and development of negative electric potential inside the cell. Each healthy cell must maintain a specific ion concentration gradient and voltage. To account for these functions, various ionic pumps and channels located within the cell membrane have been invoked. In this work, we use a porous alginate hydrogel as a model gelatinous network representing the plant cell wall or cytoskeleton of the animal cell. We show that the gel barrier is able to maintain a stable separation of ionic solutions of different ionic strengths and chemical compositions without any pumping activity. For the Na+/K+ concentration gradient sustained across the barrier, a negative electric potential develops within the K+-rich side. The situation is reminiscent of that in the cell. Furthermore, also the advective flow of water molecules across the gel barrier is restricted, despite the gel’s large pores and the osmotic or hydrostatic pressure gradients across it. This feature has important implications for osmoregulation. We propose a mechanism in which charge separation and electric fields developing across the permselective (gel) membrane prevent ion and bulk fluid flows ordinarily driven by chemical and pressure gradients.

Bifurcation and patterns induced by flow in a prey-predator system with Beddington-DeAngelis functional response.

In this paper, a prey-predator system described by a couple of advection-reaction-diffusion equations is studied theoretically and numerically, where the migrations of both prey and predator are considered and depicted by the unidirectional flow (advection term). To investigate the effect of population migration, especially the relative migration between prey and predator, on the population dynamics and spatial distribution of population, we systematically study the bifurcation and pattern dynamics of a prey-predator system. Theoretically, we derive the conditions for instability induced by flow, where neither Turing instability nor Hopf instability occurs. Most importantly, linear analysis indicates the instability induced by flow depends only on the relative flow velocity. Specifically, when the relative flow velocity is zero, the instability induced by flow does not occur. Moreover, the diffusion-driven patterns at the same flow velocity may not be stationary because of the contribution of flow. Numerical bifurcation analyses are consistent with the analytical results and show that the patterns induced by flow may be traveling waves with different wavelengths, amplitudes, and speeds, which are illustrated by numerical simulations.

A More General Paradigm for Understanding the Decoupling of Stratocumulus‐Topped Boundary Layers: The Importance of Horizontal Temperature Advection

AbstractMost prior studies on decoupling of a stratocumulus‐topped boundary layer (STBL) are focused on subtropics where cold air advection with moderate strength is dominant. This study expands across a wider spectrum of temperature advection spanning from moderately strong warm air advection to extremely strong cold air advection. A STBL undergoing warm advection is found to be more mixed than a STBL undergoing cold advection. This finding is consistent with the cold advection facilitating turbulent mixing in the boundary layer. When cold advection becomes sufficiently strong (<−5 K/day), the STBL becomes more stably stratified again because of emergence of the cumulus‐coupled STBL regime induced by the “deepening‐warming” mechanism. Such a “deepening‐warming” induced STBL decoupling, however, is still much weaker than that caused by warm advection flows (even weak ones), suggesting that the direction and strength of temperature advection must be considered for any STBL decoupling studies.

A self-similarity transport model of gas desorption inside high maturity shale under thermal enhanced conditions

Thermal stimulation methods have been investigated to enhance gas extraction from the high maturity shale recently. In this paper, a Longmaxi shale core sample, with vitrinite reflectance = 2.8%, was selected for canister tests to investigate the gas desorption process first at 55 °C and then switched to 110 °C. A mathematical model (self-similarity) was proposed to mimic the gas decaying process in nano-sized pore system. Gas adsorption/desorption incorporated advection and Knudsen flow were considered in this model. For high Knudsen number flow, the effective diffusion coefficient of the gas in the model was assumed to be a power law of density gradient. From the experiment, when heating temperature was raised from 55 °C to 110 °C, more adsorbed gas started desorbing to the porous media. It was found that the gas production rate increased significantly (20 times) and the cumulative gas production also raised considerably (11 times). By selecting an appropriate exponent m for effective diffusion coefficient, the gas production rate decayed as 1t at 55 °C and declined as 1t2 at 110 °C, which fit well with experimental data. The scaling results indicated that Knudsen flow was the dominant source of the gas flow in the whole heating process. In conclusion, Knudsen flow is the dominant gas transport form during the heating process. Thermal stimulation can significantly promote gas production rate. Moreover, the developed mathematical model can well predict the gas production rate in the heating process.

Biogeochemistry in an intertidal pocket beach

Sandy beaches are places of active organic matter mineralization due to water renewal providing organic matter and electron acceptors in the porous and permeable sands. Recycled biogenic compounds are efficiently transferred to the coastal marine environment via wave and tidal-driven advective flows. The biogeochemical processes in beach aquifers were mainly studied in semi enclosed systems with low tidal amplitude, and with a connection to continental aquifers contributing to solute fluxes to the coast from terrestrial groundwater. We present here the study of a pocket beach isolated from terrestrial aquifers with a high tidal amplitude and a medium energy wave regime. In situ measurements, cross-shore profiles and vertical sampling were conducted during several tidal cycles in spring and autumn. Cross-shore transects, obtained at low tide from holes that represent a mixture of the upper 20 cm of the water saturated zone, showed concentration gradients of redox and recycled compounds. Increase in pCO2, dissolved phosphate and ammonium concentrations downslope revealed that more products from organic matter mineralization accumulated in the lower beach. The related increase in total alkalinity downslope indicated that the part of anaerobic processes in organic matter oxidation was higher in the lower beach. Concentration and δ13C of dissolved inorganic carbon in pore waters suggested that the carbon mineralized in pore waters came from marine plant debris that were mixed with the sand. Continuous probe records of dissolved oxygen saturation and vertical profiles revealed a tidally-driven dynamics of pore water in the first centimetres of the lower beach aquifer. Ventilation of pore waters corresponded to wave pumping and swash-induced infiltration of seawater in the upper 10–20 cm of sediment. Nutrients and reduced compounds produced through organic matter mineralization remained stored in pore water below the layer disturbed by wave. The flux of these components to seawater is possible when this interface is eroded, for example when wave energy increases after a less energetic period. The low extension of the studied aquifer, typical of pocket beaches, limits the connection with continental groundwater. Both tidally-driven and wave-driven recirculation of seawater allows pocket beaches to be efficient bioreactors for marine organic matter mineralization. As such, they provide the coastal environment with recycled nutrients, and not new nutrients.

Accretion Flow Evolution of a New Black Hole Candidate MAXI J1348–630 during the 2019 Outburst

Abstract Galactic black hole (BH) candidate MAXI J1348–630 was recently discovered by MAXI and Swift/BAT satellites during its first outburst in 2019 January, which continued for ∼4 months. We study the spectral and timing properties of the source in detail. The combined 1–150 keV Swift/XRT, Swift/BAT, and MAXI/GSC spectra are investigated with the two-component advective flow (TCAF) solution. Physical flow parameters of TCAF, such as the Keplerian disk accretion rate, the sub-Keplerian halo accretion rate, the shock location, and the shock compression ratio, are estimated from our spectral fits. Based on the variation of flux in soft and hard X-ray ranges, the hardness ratio, TCAF model fitted accretion rates, and the accretion rate ratio, we show how the source evolved through four spectral states, viz., hard, hard-intermediate, soft-intermediate, and soft, in rising and declining states. Low-frequency quasi-periodic oscillations are observed in two observations during the rising phase of the outburst. From the spectral analysis, we estimate the mass of the BH to be M ⊙. We also find that the viscous timescale in this outburst is ∼3.5 days. The distance of the source is also estimated as 5–10 kpc from state transition luminosity.

Mineral equilibrium constraints on the feasibility of diffusive H2O‐fluxed melting in the continental crust

AbstractGeneration of granitic melt is believed to occur predominantly by melting through the breakdown of hydrous minerals. However, melting due to the influx of H2O has been recognized in anatectic amphibolite facies tonalitic grey gneisses, metagreywackes and low‐P metapelites, and has consequently been proposed as an alternative mechanism for the generation of granitic melt. Melting induced by H2O addition is recognized from voluminous melt production at relatively low temperature, where hydrous minerals are stable and anhydrous minerals are preferentially consumed during melting. Mineral equilibrium modelling to determine the P–T conditions, melt volumes, melting reactions and viable H2O sources reveals that the process is not restricted to specific compositions or P–T conditions, although lower pressure and lithologies with a low hydrous mineral content are more favourable. Melting reactions in all lithologies primarily consume quartz and feldspars to yield 5–6 mol.% melt for each mol.% of H2O added. remains constant at ~0.70 to 0.77 during progressive melting as long as alkali feldspar is present. Once alkali feldspar is exhausted, plagioclase becomes the main reactant, producing more tonalitic melt compositions with gradually higher . Our results demonstrate that, at the site of melting, melting is driven by diffusion of H2O into the target rock along chemical potential gradients, rather than the advective flow of a mechanically distinct water‐rich fluid phase. Melting will initiate and proceed as long as a gradient exists between the H2O source and target lithology. Our calculations show that an ordinary magma, such as an I‐type magma with typical H2O content, has a high enough to be a viable H2O source, allowing diffusive H2O‐fluxed melting to produce melt proportions and fertility comparable to that of dehydration melting. However, high degrees of partial melting require a considerable amount of H2O, which necessitates a continuously advecting H2O source such as a magma conduit or melt‐bearing shear zone. A magmatic H2O source at emplacement level will undergo a similar amount of crystallization as the melt fraction produced in the target rock such that there will be no net melt production. Considering that shear‐zone hosted magma conduits are localized features, diffusive H2O‐fluxed melting is likely to only be viable in a small fraction of the anatectic orogenic crust. Although it may play an important role in locally raising melt volumes and modifying magma chemistry through mingling and hybridization, it does not appear to, of itself, be able to generate significant volumes of granitic melt.

Properties of two-temperature magnetized advective accretion flow around rotating black hole

ABSTRACT We study the two-temperature magnetized advective accretion flow around the Kerr black holes (BHs). During accretion, ions are heated up due to viscous dissipation, and when Coulomb coupling becomes effective, they transfer a part of their energy to the electrons. On the contrary, electrons lose energy due to various radiative cooling processes, namely bremsstrahlung, synchrotron, and Comtonization processes, respectively. To account for the magnetic contribution inside the disc, we consider the toroidal magnetic fields which are assumed to be dominant over other components. Moreover, we adopt the relativistic equation of state to describe the thermal characteristics of the flow. With this, we calculate the global transonic accretion solutions around the rotating BHs. We find that accretion solution containing multiple critical points may harbour shock wave provided the standing shock conditions are satisfied. Further, we investigate the shock properties, such as shock location (xs) and compression ratio (R) that delineate the post-shock corona (hereafter PSC) and find that the dynamics of PSC is controlled by the flow parameters, such as accretion rate (${\dot{m}}$) and magnetic fields (β, defined as the ratio of gas pressure to the magnetic pressure), etc. Finally, we calculate the emission spectra of the accretion flows containing PSC and indicate that both ${\dot{m}}$ and β play the pivotal roles in explaining the spectral state transitions commonly observed for BH X-ray binaries.

Structural Incorporation of Sorbed Molybdate during Iron(II)-Induced Transformation of Ferrihydrite and Goethite under Advective Flow Conditions

Aqueous Fe(II) can induce recrystallization of ferrihydrite and goethite [α-FeOOH] to their more crystalline or molecularly homogeneous counterparts. Despite common association with these and other...

Accretion flows in nonmagnetic white dwarf binaries as observed in X-rays

Cataclysmic Variables (CVs) are compact binaries with white dwarf (WD) primaries. CVs and other accreting WD binaries (AWBs) are useful laboratories for studying accretion flows, gas dynamics, outflows, transient outbursts, and explosive nuclear burning under different astrophysical plasma conditions. They have been studied over decades and are important for population studies of galactic X-ray sources. Recent space- and ground-based high resolution spectral and timing studies, along with recent surveys indicate that we still have observational and theoretical complexities yet to answer. I review accretion in nonmagnetic AWBs in the light of X-ray observations. I present X-ray diagnostics of accretion in dwarf novae and the disk outbursts, the nova-like systems, and the state of the research on the disk winds and outflows in the nonmagnetic CVs together with comparisons and relations to classical and recurrent nova systems, AM CVns and Symbiotic systems. I discuss how the advective hot accretion flows (ADAF-like) in the inner regions of accretion disks (merged with boundary layer zones) in nonmagnetic CVs explain most of the discrepancies and complexities that have been encountered in the X-ray observations. I stress how flickering variability studies from optical to X-rays can be probes to determine accretion history and disk structure together with how the temporal and spectral variability of CVs are related to that of LMXBs and AGNs. Finally, I discuss the nature of accretion in nonmagnetic WDs in terms of ADAF-like accretion flows, and elaborate on the solutions it brings and its complications, constructing an observational framework to motivate new theoretical calculations that introduce this flow-type in disks, outflow and wind models together with disk-instability models of outbursts and nova outbursts in AWBs and WD physics, in general.

Wintertime Arctic Oscillation and North Atlantic Oscillation and their impacts on the Northern Hemisphere climate in E3SM

The characteristics of the wintertime Arctic Oscillation (AO) and North Atlantic Oscillation (NAO) and their impacts on climate variability over the Northern Hemisphere are important metrics for evaluating a climate system model. Observational analyses reveal that the horizontal and vertical structures in the AO and NAO exhibit a meridional dipole and a large-scale barotropic pattern between the Arctic and mid-latitudes. Historical model simulations from the Energy Exascale Earth System Model (E3SM-HIST) are used to identify how well it captures these major climate modes. It is found that the simulated AO and NAO modes have spatial structures similar to the observed features. In addition, the observed frequency bands in the AO and NAO-related time variability are captured well in the E3SM-HIST simulation. Associated with the positive phase in wintertime AO and NAO, zonal flow and warm advection in mid-latitude continents are enhanced, along with stronger cold flow from enhanced northerly winds over high latitudes. These features are linked to the atmospheric circulation pattern reflected by lower SLP anomalies over the Arctic and higher SLP anomalies over the mid‐latitudes. In E3SM-HIST, these spatial associations and main structural features are analogous to those in observations. In the time-height evolution related to winter AO and NAO modes, it can also be seen that the simulations reproduce the downward propagating patterns in observations. Nevertheless, the vertical structures associated with AO and NAO in E3SM-HIST exhibit substantial biases in the lower stratosphere. The cause of these stratospheric biases is investigated using the strength of climatological stratospheric polar vortex (SPV) and wave activity fluxes. The results herein suggest that E3SM-HIST has a reasonable skill in reproducing the observed characteristics related to the winter AO and NAO, although there exist systematic biases in the associated climate variability.

Role of magnetically dominated disc-outflow symbiosis on bright hard-state black hole sources: ultra-luminous X-ray sources to quasars

ABSTRACTWe present optically thin solutions for magnetized, advective disc-outflow symbiosis around black holes (BHs). The main objective is to explain the bright, hard-state observations of accreting systems with stellar mass-to-supermassive BHs. We include the effects of magnetic fields and radiation counterpart in entropy gradient based on the first law of thermodynamics to represent energy advection. The cooling process includes bremsstrahlung, synchrotron radiation, and inverse Comptonization process. One of our main ventures is to explain some long-standing issues of ultra-luminous X-ray sources (ULXs). The existing physical scenarios to explain their unusual high luminosity are either the existence of the missing class of intermediate-mass BH (IMBH) or super-Eddington accretion around a stellar-mass BH. However, most ULXs with steep power-law spectrum can be well explained through super-Eddington accretion, while the existence of IMBH is indeed disputed extensively. Nevertheless, the interpretation of ULXs with a hard power-law-dominated state remains mysterious. Here we show that our magnetically dominated disc-outflow symbiosis around rapidly spinning stellar-mass BHs can achieve such large luminosity even for sub-Eddington accretion rate. The magnetic field at the outer zone of the advective flow is more than the corresponding Eddington limit. Such a field becomes dynamically dominant near the BH through continuous accretion process due to flux freezing, but maintaining its Eddington limit. This unique field configuration enhances the synchrotron and synchrotron self-Comptonization process to achieve very large luminosity. Through the same mechanism, our solutions for supermassive BHs can explain the unusual large luminosity of ultra-luminous quasars.

Decomposing Barotropic Transport Variability in a High‐Resolution Model of the North Atlantic Ocean

AbstractA method using a linear shallow water model is presented for decomposing the temporal variability of the barotropic stream function in a high‐resolution ocean model. The method is based on the vertically averaged momentum equations and is applied to the time series of annual mean stream function from the model configuration VIKING20 for the northern North Atlantic. An important result is the role played by the nonlinear advection terms in VIKING20 for driving transport. The method is illustrated by examining how the Gulf Stream transport in the recirculation region responds to the winter North Atlantic Oscillation (NAO). While no statistically significant response is found in the year overlapping with the winter NAO index, there is a tendency for the Gulf Stream transport to increase as the NAO becomes more positive. This becomes significant in lead years 1 and 2 when the mean flow advection and eddy momentum flux contributions, associated with nonlinear momentum advection, dominate. Only after 2 years, does the potential energy term, associated with the density field, start to play a role and it is only after 5 years that the transport dependence on the NAO ceases to be significant. It is also shown that the potential energy contribution to the transport stream function has significant memory of up to 5 years in the Labrador and Irminger Seas. However, it is only around the northern rim of these seas that VIKING20 and the transport reconstruction exhibit similar memory. This is due to masking by the mean flow advection and eddy momentum flux contributions.

ONE-DIMENSIONAL CHAOTIC LAMINAR FLOW WITH COMPETITIVE EXOTHERMIC AND ENDOTHERMIC REACTIONS

AbstractWe consider the numerical solution of competitive exothermic and endothermic reactions in the presence of a chaotic advection flow. The resulting behaviour is characterized by a strong dependence on the competitive reaction history. The burnt temperature is not immediately connected to simple enthalpy calculations, so there is a subtlety in the interplay between the major parameters, notably the Damköhler number, the ratio of the heats of exothermic and endothermic reactions, as well as the ratio of their respective activation energies. This paper seeks to explore the way these parameters affect the steady states of these reaction fronts and their stability.

An Experimental Method to Measure Gaseous Diffusivity in Tight and Partially Saturated Porous Media via Continuously Monitored Mass Spectrometry

Detection of radioxenon and radioargon produced by underground nuclear explosions is one of the primary methods by which the Comprehensive Nuclear-Test–Ban Treaty (CTBT) monitors for nuclear activities. However, transport of these noble gases to the surface via barometric pumping is a complex process relying on advective and diffusive processes in a fractured porous medium to bring detectable levels to the surface. To better understand this process, experimental measurements of noble gas and chemical surrogate diffusivity in relevant lithologies are necessary. However, measurement of noble gas diffusivity in tight or partially saturated porous media is challenging due to the transparent nature of noble gases, the lengthy diffusion times, and difficulty maintaining consistent water saturation. Here, the quasi-steady-state Ney–Armistead method is modified to accommodate continuous gas sampling via effusive flow to a mass spectrometer. An analytical solution accounting for the cumulative sampling losses and induced advective flow is then derived. Experimental results appear in good agreement with the proposed theory, suggesting the presence of retained groundwater reduces the effective diffusivity of the gas tracers by 10–1000 times. Furthermore, by using a mass spectrometer, the method described herein is applicable to a broad range of gas species and porous media.

Complex Eyed Pockmarks and Submarine Groundwater Discharge Revealed by Acoustic Data and Sediment Cores in Eckernförde Bay, SW Baltic Sea

AbstractSubmarine groundwater discharge into coastal areas is a common global phenomenon and is rapidly gaining scientific interest due to its influence on marine ecology, the coastal sedimentary environment, and its potential as a future freshwater resource. We conducted an integrated study of hydroacoustic surveys combined with geochemical pore water and water column investigations at a well‐known groundwater seep site in Eckernförde Bay (Germany). We aim to better constrain the effects of shallow gas and submarine groundwater discharge on high‐frequency multibeam backscatter data and to present acoustic indications for submarine groundwater discharge. Our high‐quality hydroacoustic data reveal hitherto unknown internal structures within the pockmarks in Eckernförde Bay. Using precisely positioned sediment core samples, our hydroacoustic‐geochemical approach can differentiate intrapockmark regimes that were formerly assigned to pockmarks of a different nature. We demonstrate that high‐frequency multibeam data, in particular the backscatter signals, can be used to detect shallow free gas in areas of enhanced groundwater advection in muddy sediments. Intriguingly, our data reveal relatively small (typically <15 m across) pockmarks within the much larger, previously mapped pockmarks. The small pockmarks, which we refer to as “intrapockmarks,” have formed due to the localized ascent of gas and groundwater; they manifest themselves as a new type of “eyed” pockmarks, revealed by their acoustic backscatter pattern. Our data suggest that, in organic‐rich muddy sediments, morphological lows combined with a strong multibeam backscatter signal can be indicative of free shallow gas and subsequent advective groundwater flow.