scholarly journals Differentiating tidal and groundwater dynamics from barrier island framework geology: Testing the utility of portable multifrequency electromagnetic induction profilers

Geophysics ◽  
2016 ◽  
Vol 81 (5) ◽  
pp. E347-E361 ◽  
Author(s):  
Bradley A. Weymer ◽  
Mark E. Everett ◽  
Chris Houser ◽  
Phil Wernette ◽  
Patrick Barrineau
Keyword(s):  
Electromagnetic Induction ◽  
Large Scale ◽  
Low Frequency ◽  
Barrier Island ◽  
Step Response ◽  
Small Scale ◽  
Near Surface ◽  
Groundwater Dynamics ◽  
Dry Conditions ◽  
Text Signal

Electromagnetic induction (EMI) techniques are becoming increasingly popular for near-surface coastal geophysical applications. However, few studies have explored the capabilities and limitations of portable multifrequency EMI profilers for mapping large-scale ([Formula: see text]) barrier island hydrogeology. The purpose of this study is to investigate the influence of groundwater dynamics on apparent conductivity [Formula: see text] to separate the effects of hydrology and geology from the [Formula: see text] signal. Shore-normal and alongshore surveys were performed within a highly conductive barrier island/wind-tidal flat system at Padre Island National Seashore, Texas, USA. Assessments of instrument calibration and signal drift suggest that [Formula: see text] measurements are stable, but vary with height and location across the beach. Repeatability tests confirm [Formula: see text] values using different boom orientations collected during the same day are reproducible. Measurements over a 12 h tidal cycle suggest that there is a tide-dependent step response in [Formula: see text], complicating data processing and interpretation. Shore-normal surveys across the barrier/wind-tidal flats show that [Formula: see text] is roughly negatively correlated with topography and these relationships can be used for characterizing different coastal habitats. For all surveys, [Formula: see text] increases with decreasing frequency. Alongshore surveys performed during different seasons and beach states reveal a high degree of variability in [Formula: see text]. Here, it is argued that surveys collected during dry conditions characterize the underlying framework geology, whereas these features are somewhat masked during wet conditions. Differences in EMI signals should be viewed in a relative sense rather than as absolute magnitudes. Small-scale heterogeneities are related to changing hydrology, whereas low-frequency signals at the broadest scales reveal variations in framework geology. Multiple surveys should be done at different times of the year and tidal states before geologic interpretations can confidently be made from EMI surveys in coastal environments. This strategy enables the geophysicist to separate the effects of hydrology and geology from the [Formula: see text] signal.

scholarly journals Physical Transport Processes that Affect the Distribution of Oil in the Gulf of Mexico: Observations and Modeling

Oceanography ◽  
2021 ◽  
Vol 34 (1) ◽  
pp. 58-75
Author(s):  
Michel Boufadel ◽  
◽  
Annalisa Bracco ◽  
Eric Chassignet ◽  
Shuyi Chen ◽  
...  
Keyword(s):  
Gulf Of Mexico ◽  
Physical Environment ◽  
Large Scale ◽  
Transport Processes ◽  
Small Scale ◽  
Surface Mixed Layer ◽  
Physical Processes ◽  
Mixing Processes ◽  
Near Surface ◽  
Wide Range

Physical transport processes such as the circulation and mixing of waters largely determine the spatial distribution of materials in the ocean. They also establish the physical environment within which biogeochemical and other processes transform materials, including naturally occurring nutrients and human-made contaminants that may sustain or harm the region’s living resources. Thus, understanding and modeling the transport and distribution of materials provides a crucial substrate for determining the effects of biological, geological, and chemical processes. The wide range of scales in which these physical processes operate includes microscale droplets and bubbles; small-scale turbulence in buoyant plumes and the near-surface “mixed” layer; submesoscale fronts, convergent and divergent flows, and small eddies; larger mesoscale quasi-geostrophic eddies; and the overall large-scale circulation of the Gulf of Mexico and its interaction with the Atlantic Ocean and the Caribbean Sea; along with air-sea interaction on longer timescales. The circulation and mixing processes that operate near the Gulf of Mexico coasts, where most human activities occur, are strongly affected by wind- and river-induced currents and are further modified by the area’s complex topography. Gulf of Mexico physical processes are also characterized by strong linkages between coastal/shelf and deeper offshore waters that determine connectivity to the basin’s interior. This physical connectivity influences the transport of materials among different coastal areas within the Gulf of Mexico and can extend to adjacent basins. Major advances enabled by the Gulf of Mexico Research Initiative in the observation, understanding, and modeling of all of these aspects of the Gulf’s physical environment are summarized in this article, and key priorities for future work are also identified.

scholarly journals Internal tide energy flux over a ridge measured by a co-located ocean glider and moored ADCP

2019 ◽  
Author(s):  
Rob Hall ◽  
Barbara Berx ◽  
Gillian Damerell
Keyword(s):  
Energy Flux ◽  
Large Scale ◽  
Low Frequency ◽  
Internal Tide ◽  
Small Scale ◽  
Potential Density ◽  
The North ◽  
Model Study ◽  
Continental Shelves ◽  
Alternative Approach

Abstract. Internal tide energy flux is an important diagnostic for the study of energy pathways in the ocean, from large-scale input by the surface tide, to small-scale dissipation by turbulent mixing. Accurate calculation of energy flux requires repeated full-depth measurements of both potential density (ρ) and horizontal current velocity (u) over at least a tidal cycle and over several weeks to resolve the internal spring-neap cycle. Typically, these observations are made using full-depth oceanographic moorings that are vulnerable to being fished-out by commercial trawlers when deployed on continental shelves and slopes. Here we test an alternative approach to minimise these risks, with u measured by a low-frequency ADCP moored near the seabed and ρ measured by an autonomous ocean glider holding station by the ADCP. The method is used to measure the M2 internal tide radiating from the Wyville Thompson Ridge in the North Atlantic. The observed energy flux (4.2 ± 0.2 kW m−1) compares favourably with historic observations and a previous numerical model study. Error in the energy flux calculation due to imperfect co-location of the glider and ADCP is estimated by sub-sampling potential density in an idealised internal tide field along pseudorandomly distributed glider paths. The error is considered acceptable (

Unsteady Structure Measurement of Cloud Cavitation on a Foil Section Using Conditional Sampling Technique

1989 ◽  
Vol 111 (2) ◽  
pp. 204-210 ◽  
Author(s):  
A. Kubota ◽  
H. Kato ◽  
H. Yamaguchi ◽  
M. Maeda
Keyword(s):  
High Speed ◽  
Large Scale ◽  
Laser Doppler Anemometry ◽  
Low Frequency ◽  
Sampling Technique ◽  
Experimental Result ◽  
Small Scale ◽  
Conditional Sampling ◽  
Cloud Cavitation ◽  
Structure Of Flow

The structure of flow around unsteady cloud cavitation on a stationary two-dimensional hydrofoil was investigated experimentally using a conditional sampling technique. The unsteady flow velocity around the cloud cavitation was measured by a Laser Doppler Anemometry (LDA) and matched with the unsteady cavitation appearance photographed by a high-speed camera. This matching procedure was performed using data from pressure fluctuation measurements on the foil surface. The velocities were divided into two components using a digital filter, i.e., large-scale (low-frequency) and small-scale (high frequency) ones. The large-scale component corresponds with the large-scale unsteady cloud cavitation motion. In this manner, the unsteady structure of the cloud cavitation was successfully measured. The experimental result showed that the cloud cavitation observed at the present experiment had a vorticity extremum at its center and a cluster containing many small cavitation bubbles. The convection velocity of the cavitation cloud was much lower than the uniform velocity. The small-scale velocity fluctuation was not distributed uniformly in the cavitation cloud, but was concentrated near its boundary.

scholarly journals Limitations of using a thermal imager for snow pit temperatures

2014 ◽  
Vol 8 (2) ◽  
pp. 387-394 ◽  
Author(s):  
M. Schirmer ◽  
B. Jamieson
Keyword(s):  
Surface Structure ◽  
Large Scale ◽  
Structural Changes ◽  
Thermal Structure ◽  
Large Temperature ◽  
Small Scale ◽  
Thermal Imager ◽  
Snow Layer ◽  
Ir Camera ◽  
Near Surface

Abstract. Driven by temperature gradients, kinetic snow metamorphism plays an import role in avalanche formation. When gradients based on temperatures measured 10 cm apart appear to be insufficient for kinetic metamorphism, faceting close to a crust can be observed. Recent studies that visualised small-scale (< 10 cm) thermal structures in a profile of snow layers with an infrared (IR) camera produced interesting results. The studies found melt-freeze crusts to be warmer or cooler than the surrounding snow depending on the large-scale gradient direction. However, an important assumption within these studies was that a thermal photo of a freshly exposed snow pit was similar enough to the internal temperature of the snow. In this study, we tested this assumption by recording thermal videos during the exposure of the snow pit wall. In the first minute, the results showed increasing gradients with time, both at melt-freeze crusts and artificial surface structures such as shovel scours. Cutting through a crust with a cutting blade or shovel produced small concavities (holes) even when the objective was to cut a planar surface. Our findings suggest there is a surface structure dependency of the thermal image, which was only observed at times during a strong cooling/warming of the exposed pit wall. We were able to reproduce the hot-crust/cold-crust phenomenon and relate it entirely to surface structure in a temperature-controlled cold laboratory. Concave areas cooled or warmed more slowly compared with convex areas (bumps) when applying temperature differences between snow and air. This can be explained by increased radiative and/or turbulent energy transfer at convex areas. Thermal videos suggest that such processes influence the snow temperature within seconds. Our findings show the limitations of using a thermal camera for measuring pit-wall temperatures, particularly during windy conditions, clear skies and large temperature differences between air and snow. At crusts or other heterogeneities, we were unable to create a sufficiently planar snow pit surface and non-internal gradients appeared at the exposed surface. The immediate adjustment of snow pit temperature as it reacts with the atmosphere complicates the capture of the internal thermal structure of a snowpack with thermal videos. Instead, the shown structural dependency of the IR signal may be used to detect structural changes of snow caused by kinetic metamorphism. The IR signal can also be used to measure near surface temperatures in a homogenous new snow layer.

Asian Summer Precipitation over the Past 544 Years Reconstructed by Merging Tree Rings and Historical Documentary Records

2018 ◽  
Vol 31 (19) ◽  
pp. 7845-7861 ◽  
Author(s):  
Hui Shi ◽  
Bin Wang ◽  
Edward R. Cook ◽  
Jian Liu ◽  
Fei Liu
Keyword(s):  
El Niño ◽  
Large Scale ◽  
El Nino ◽  
Rainfall Variability ◽  
Low Frequency ◽  
Summer Precipitation ◽  
Small Scale ◽  
Central Pacific El Niño ◽  
Asian Summer ◽  
Mode A

Sparse long-term Asian monsoon (AM) records have limited our ability to understand and accurately model low-frequency AM variability. Here we present a gridded 544-yr (from 1470 to 2013) reconstructed Asian summer precipitation (RAP) dataset by weighted merging of two complementary proxies including 453 tree-ring-width chronologies and 71 historical documentary records. The RAP dataset provides substantially improved data quality when compared with single-proxy-type reconstructions. Skillful reconstructions are obtained in East and North China, northern India and Pakistan, the Indochina Peninsula, midlatitude Asia, the Maritime Continent, and southern Japan. The RAP faithfully illustrates large-scale regional rainfall variability but has more uncertainties in representing small-scale local rainfall anomalies. The RAP reproduces a realistic climatology and captures well the year-to-year rainfall variability averaged over monsoon Asia, arid central Asia, and all of Asia during the twentieth century. It also shows a general agreement with other proxies (speleothems and ice cores) during the period of 1470–1920. The RAP captures the remarkably abrupt change during the 1600s recorded in the upwelling proxy over the Arabian Sea. Four major modes of variability of the Asian summer precipitation are identified with the long record of the RAP, including a biennial El Niño–Southern Oscillation (ENSO) mode, a low-frequency ENSO mode, a central Pacific El Niño–like decadal mode, and an interdecadal mode. In sum, the RAP provides a valuable dataset for study of the large-scale Asian summer precipitation variability, especially the decadal–centennial variability that is caused by external forcing and internal feedback processes within the Earth climate system.

scholarly journals Stochastic modeling of the oceanic mesoscale eddies

2021 ◽  
Author(s):  
Long Li ◽  
Bruno Deremble ◽  
Noé Lahaye ◽  
Etienne Mémin
Keyword(s):  
Large Scale ◽  
Mesoscale Eddy ◽  
Low Frequency ◽  
Stochastic Approach ◽  
Coarse Grained ◽  
Small Scale ◽  
Oceanic Circulation ◽  
Stochastic Representation ◽  
Stochastic Framework ◽  
Gyre Circulation

&lt;p&gt;In this work, a stochastic representation [Bauer2020a, Bauer2020b] based on a physical transport principle is proposed to account for mesoscale eddy effects on the the large-scale oceanic circulation. This stochastic framework [M&amp;#233;min2014] arises from a decomposition of the Lagrangian velocity into a time-smooth component and a highly oscillating noise term. One important characteristic of this random model is that it conserves the energy of any transported tracer. Such an energy-preserving representation has been successfully implemented in a well established multi-layered quasi-geostrophic dynamical core (http://www.q-gcm.org). The empirical spatial correlation of the small-scale noise is estimated from the eddy-resolving simulation data. In particular, a sub-grid correction drift has been introduced in the noise due to the bias ensuing from the coarse-grained procedure. This non intuitive term seems quite important in reproducing on a coarse mesh the meandering jet of the wind-driven double-gyre circulation. In addition, a new projection method has been proposed to constrain the noise living along the iso-surfaces of the vertical stratification. The resulting noise enables us to improve the intrinsic low-frequency variability of the large-scale current. From some statistical studies and energy transfers analysis, this improvement is well demonstrated.&lt;/p&gt;&lt;ul&gt;&lt;li&gt;&lt;span&gt;[&lt;/span&gt;Bauer2020a] W. Bauer, P. Chandramouli, B. Chapron, L. Li, and E. M&amp;#233;min. Deciphering the role&amp;#160;of small-scale inhomogeneity on geophysical flow structuration: a stochastic approach. Journal of Physical Oceanography, 50(4):983-1003, 2020a.&lt;span&gt;&amp;#160; &amp;#160; &amp;#160; &amp;#160;&lt;/span&gt;&lt;/li&gt; &lt;li&gt;&lt;span&gt;[&lt;/span&gt;Bauer2020b] W. Bauer, P. Chandramouli, L. Li, and E. M&amp;#233;min. Stochastic representation of mesoscale&amp;#160;eddy effects in coarse-resolution barotropic models. Ocean Modelling, 151:101646 (2020b). &lt;span&gt;&amp;#160; &amp;#160;&lt;/span&gt;&lt;/li&gt; &lt;li&gt;[M&amp;#233;min2014] E. M&amp;#233;min. Fluid flow dynamics under location uncertainty. Geophysical &amp; Astrophysical Fluid Dynamics, 108(2):119-146, 2014.&lt;span&gt;&amp;#160; &amp;#160; &amp;#160;&lt;/span&gt;&lt;/li&gt; &lt;/ul&gt;

Vertical and small-scale horizontal distribution of cephalopods in the Charlie-Gibbs Fracture Zone of the Mid-Atlantic Ridge

2020 ◽  
Vol 96 (2) ◽  
pp. 341-356
Author(s):  
Jared Richards ◽  
Michael Vecchione
Keyword(s):  
Fracture Zone ◽  
Diel Vertical Migration ◽  
Large Scale ◽  
Common Species ◽  
Horizontal Distribution ◽  
Small Scale ◽  
Near Surface ◽  
Mid Atlantic Ridge ◽  
Migratory Patterns

In summer 2009, NOAA surveyed the nekton fauna of the fracture zone on the Mid-Atlantic Ridge halfway between Iceland and the Azores as a small-scale follow-up to a previous large-scale Norwegian expedition. Midwater sampling with a Norwegian Krill Trawl resulted in 64 discrete-depth samples from 12 stations at depths from near-surface to 3000 m. Seven additional bottom samples were collected with a large trawl at depths of 2000–3500 m. The expedition collected 416 cephalopods in ca. 19 species in the vicinity of the fracture zone. Over 50 hrs of ROV video from the Norwegian expedition was also viewed to determine diel migratory patterns of the most common species of cephalopod in the region, Gonatus steenstrupi, for comparison with the NOAA trawl data. We found that trawl stations southeast of the Subpolar Front were generally most diverse. Cluster analysis showed that midwater trawls were more similar in species composition than bottom trawls. Unlike in the ROV observations, the small G. steenstrupi from trawl samples did not appear to participate in diel vertical migration, suggesting that trawl-caught juveniles are ecologically distinct from those visible in submersible videos.

Seismo-acoustic ground coupling: Wave types, transfer efficiency, and near-surface structure

2020 ◽  
Author(s):  
Florian Fuchs ◽  
Artemii Novoselov ◽  
Götz Bokelmann
Keyword(s):  
Soil Properties ◽  
Acoustic Waves ◽  
Coupling Efficiency ◽  
Pressure Sensors ◽  
Low Frequency ◽  
High Amplitude ◽  
Small Scale ◽  
Near Surface ◽  
Acoustic Coupling ◽  
Acoustic Sources

&lt;p&gt;Pressure perturbations such as e.g. impulsive acoustic waves can couple into solid earth through the long-known phenomenom of seismo-acoustic coupling. Yet, the associated mechanisms are not always clear. Most studies investigate seismo-acoustic through low-frequency and high amplitude signals generated by e.g. natural or man-made explosions.&lt;/p&gt;&lt;p&gt;We conducted a small-scale field experiment with firecrackers as acoustic sources and hundred 3-component nodal geophones as receivers in a 20m diameter ring layout, some of them co-located with seismically decoupled Hyperion IFS-5111 infrasound sensors. This allowed us to investigate seismo-acousting coupling for higher frequencies and very small (meter scale) offsets.&lt;/p&gt;&lt;p&gt;The large receiver density enabled us to observe and distinguish different wave types induced by acoustic sources, including direct air waves, air-coupled Rayleigh waves, and possibly slow Biot waves. Having co-located seismic and pressure sensors additionally allowed us to investigate the coupling efficiency, which is in the order of 10&lt;sup&gt;-7&lt;/sup&gt; and thus similar to many of the low-frequency and large-offset studies. Furthermore, we can deduct soil properties such as rigidity, bulk modulus, and density from the co-located sensors, and efficiently infer near-surface (soil) properties using cheap acoustic sources.&lt;/p&gt;

Influence of non-monochromaticity on zonal-flow generation by magnetized Rossby waves in the ionospheric E-layer

2009 ◽  
Vol 75 (3) ◽  
pp. 345-357 ◽  
Author(s):  
T. D. KALADZE ◽  
H. A. SHAH ◽  
G. MURTAZA ◽  
L. V. TSAMALASHVILI ◽  
M. SHAD ◽  
...  
Keyword(s):  
Large Scale ◽  
Laboratory Experiments ◽  
Low Frequency ◽  
Zonal Flow ◽  
Present Theory ◽  
Resonant Interaction ◽  
Finite Amplitude ◽  
Small Scale ◽  
Zonal Flows ◽  
Flow Generation

AbstractThe influence of non-monochromaticity on low-frequency, large-scale zonal-flow nonlinear generation by small-scale magnetized Rossby (MR) waves in the Earth's ionospheric E-layer is considered. The modified parametric approach is used with an arbitrary spectrum of primary modes. It is shown that the broadening of the wave packet spectrum of pump MR waves leads to a resonant interaction with a growth rate of the order of the monochromatic case. In the case when zonal-flow generation by MR modes is prohibited by the Lighthill stability criterion, the so-called two-stream-like mechanism for the generation of sheared zonal flows by finite-amplitude MR waves in the ionospheric E-layer is possible. The growth rates of zonal-flow instabilities and the conditions for driving them are determined. The present theory can be used for the interpretation of the observations of Rossby-type waves in the Earth's ionosphere and in laboratory experiments.

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