scholarly journals Towards improving the physical basis for ice-dynamics models

1997 ◽  
Vol 25 ◽  
pp. 177-182 ◽  
Author(s):  
J. A. Richter-Menge
Keyword(s):  
High Frequency ◽  
Low Frequency ◽  
The Arctic ◽  
Frequency Content ◽  
Ice Dynamics ◽  
Relative Significance ◽  
Thermally Induced ◽  
Induced Stress ◽  
Dynamics Models

In situ measurements of ice stress were made on a multi-year floe in the Alaskan Beaufort Sea over a 6 month period, beginning in October 1993. The data suggest that, in this region of the Arctic during this experiment, there were two main sources of stress: a thermally induced stress caused by changes in air temperature, and a stress generated by ice motion. Due to the natural damping of the snow and ice above the sensor, the thermally-induced stresses are low frequency (order of days). Stresses associated with periods of ice motion have both a high-frequency (order of hours), and low-frequency, content. The relative significance of these sources of stress is seasonal, reflecting the changes in the strength and continuity of the pack.

scholarly journals Towards improving the physical basis for ice-dynamics models

1997 ◽  
Vol 25 ◽  
pp. 177-182 ◽  
Author(s):  
J. A. Richter-Menge
Keyword(s):  
High Frequency ◽  
Low Frequency ◽  
The Arctic ◽  
Frequency Content ◽  
Ice Dynamics ◽  
Relative Significance ◽  
Thermally Induced ◽  
Induced Stress ◽  
Dynamics Models

In situ measurements of ice stress were made on a multi-year floe in the Alaskan Beaufort Sea over a 6 month period, beginning in October 1993. The data suggest that, in this region of the Arctic during this experiment, there were two main sources of stress: a thermally induced stress caused by changes in air temperature, and a stress generated by ice motion. Due to the natural damping of the snow and ice above the sensor, the thermally-induced stresses are low frequency (order of days). Stresses associated with periods of ice motion have both a high-frequency (order of hours), and low-frequency, content. The relative significance of these sources of stress is seasonal, reflecting the changes in the strength and continuity of the pack.

Broadband seismic over/under sources and their designature-deghosting

Geophysics ◽  
2017 ◽  
Vol 82 (5) ◽  
pp. P61-P73 ◽  
Author(s):  
Lasse Amundsen ◽  
Ørjan Pedersen ◽  
Are Osen ◽  
Johan O. A. Robertsson ◽  
Martin Landrø
Keyword(s):  
Seismic Data ◽  
High Frequency ◽  
Low Frequency ◽  
Point Sources ◽  
High Frequency Side ◽  
Frequency Content ◽  
Source Depth ◽  
Deep Sources ◽  
The Cost ◽  
High Frequency Content

The source depth influences the frequency band of seismic data. Due to the source ghost effect, it is advantageous to deploy sources deep to enhance the low-frequency content of seismic data. But, for a given source volume, the bubble period decreases with the source depth, thereby degrading the low-frequency content. At the same time, deep sources reduce the seismic bandwidth. Deploying sources at shallower depths has the opposite effects. A shallow source provides improved high-frequency content at the cost of degraded low-frequency content due to the ghosting effect, whereas the bubble period increases with a lesser source depth, thereby slightly improving the low-frequency content. A solution to the challenge of extending the bandwidth on the low- and high-frequency side is to deploy over/under sources, in which sources are towed at two depths. We have developed a mathematical ghost model for over/under point sources fired in sequential and simultaneous modes, and we have found an inverse model, which on common receiver gathers can jointly perform designature and deghosting of the over/under source measurements. We relate the model for simultaneous mode shooting to recent work on general multidepth level array sources, with previous known solutions. Two numerical examples related to over/under sequential shooting develop the main principles and the viability of the method.

In Situ Measurement of Ground Vibration Induced by Inter-City Express Train

2012 ◽  
Vol 204-208 ◽  
pp. 502-507 ◽  
Author(s):  
Quan Min Liu ◽  
Xun Zhang ◽  
Zhi Jun Zhang ◽  
Xiao Zhen Li
Keyword(s):  
High Frequency ◽  
Low Frequency ◽  
Ground Vibration ◽  
Vertical Vibration ◽  
Test Results ◽  
Low Frequency Vibration ◽  
Express Train ◽  
Environmental Vibration ◽  
Logarithmic Relationship

On the basis of the measured ground borne vibration of some inter-city express railway viaduct, test results analysis shows that: the test environmental vibration is under the limit set by standard of environmental vibration in urban area; whether transverse or vertical vibration, a rapid attenuation of the peak acceleration with the distance to the up-track center is observed, however the vibration at 7.5m appears to be amplified; the ground vibration with the distance conforms to a logarithmic relationship; the horizontal ground vibration at 7.5m caused by the elevated rail transit is larger 3.6cm/s2 than the vertical vibration; low-frequency vibration transfers farther than high frequency vibration; the eccentric effect of two-track viaduct is obvious.

Effects of averaging and sampling on the statistics of reflection coefficients

Geophysics ◽  
1991 ◽  
Vol 56 (1) ◽  
pp. 50-58 ◽  
Author(s):  
K. Hsu ◽  
R. Burridge
Keyword(s):  
Reflection Coefficient ◽  
High Frequency ◽  
Low Frequency ◽  
Seismic Exploration ◽  
Reflection Coefficients ◽  
Continuous Formation ◽  
Sampling Intervals ◽  
Frequency Components ◽  
Analytical Formulas

The reflection coefficients derived from sonic and density logs are frequently used in seismic exploration. Even though they measure the in‐situ formation slowness and density, sonic and density tools do not measure the exact, continuous formation properties but locally averaged properties sampled at discrete depth points. Furthermore, the logs are frequently reinterpolated to form a Goupillaud medium for many applications such as synthetic seismogram computation. Both the logging tools and the Goupillaud interpolation introduce averaging and sampling effects into the reflection coefficients and significantly alter the autocorrelation of the reflection coefficient sequence. Analytical formulas are derived to show how the autocorrelation is altered and to calculate how the autocorrelation depends on the averaging and sampling intervals. Essentially, these effects impose sincsquared envelopes on the power spectrum of the reflection coefficient sequence and alias high‐frequency components to low‐frequency components in the spectral domain. These findings are verified using synthetic and real examples.

scholarly journals Extreme event occurrences and impacts in coastal waters of western Europe

2020 ◽  
Author(s):  
Coline Poppeschi ◽  
Maximilian Unterberger ◽  
Guillaume Charria ◽  
Peggy Rimmelin-Maury ◽  
Eric Goberville ◽  
...  
Keyword(s):  
Extreme Events ◽  
Coastal Waters ◽  
High Frequency ◽  
Western Europe ◽  
Extreme Event ◽  
Low Frequency ◽  
In Situ Observations ◽  
Bay Of Brest ◽  
The Relationship

<p><strong>Extreme event occurrences and impacts in coastal waters of western Europe</strong></p><p><strong> </strong></p><p>Coline Poppeschi<sup>1</sup>, Maximilian Unterberger<sup>1</sup>, Guillaume Charria<sup>1</sup>, Peggy Rimmelin-Maury<sup>2</sup>, Eric Goberville<sup>3</sup>, Nicolas Barrier<sup>5</sup>, Emilie Grossteffan<sup>2</sup>, Michel Repecaud<sup>6</sup>, Loïc Quemener<sup>6</sup>, Sébastien Theetten<sup>1</sup>, Sébastien Petton<sup>7</sup>, Jean-François Le Roux<sup>1</sup>, Paul Tréguer<sup>4</sup></p><p><sup> </sup></p><p><sup>1</sup> Ifremer, Univ. Brest, CNRS, IRD, Laboratoire d'Océanographie Physique et Spatiale (LOPS), IUEM, 29280 Brest, France.</p><p><sup>2 </sup>OSU-Institut Universitaire Européen de la Mer (IUEM), UMS3113, F-29280, Plouzané, France.</p><p><sup>3 </sup>Muséum National d’Histoire Naturelle, UMR 7208 BOREA, Sorbonne Université, CNRS, UCN, UA, IRD, Paris, France.</p><p><sup>4 </sup>IUEM, UMR-CNRS 6539 Laboratoire de l’Environnement Marin (LEMAR), OSU IUEM, F-29280, Plouzané, France.</p><p><sup>5 </sup>MARBEC, Université de Montpellier, Centre National de la Recherche Scientifique (CNRS), Ifremer, Institut de Recherche pour le Développement (IRD), F-34203 Sète, France.</p><p><sup>6 </sup>Ifremer, Centre de Brest, REM/RDT/DCM, F-29280, Plouzané, France.</p><p><sup>7 </sup>Ifremer, Centre de Brest, RBE/PFOM/LPI, F-29840, Argenton en Landunvez, France.</p><p> </p><p><strong>Abstract</strong></p><p>            The occurrence and the impact of the atmospheric extreme events in coastal waters of western Europe is evolving. Responses of the coastal environment to those events and evolutions need to be explored and explained. In this framework, the hydrodynamical and biogeochemical processes driven by extreme events in the bay of Brest are studied to better estimate their impacts on the local ecosystem. We are analyzing long-term in situ observations (since 2000), sampled at high and low frequencies, from the COAST-HF and SOMLIT network sites, located at the entrance to the bay of Brest. This study is divided into two main parts: the detection and characterization of extreme events, followed by the analysis of a realistic numerical simulation of these events to understand the underlying ocean processes. We focus on freshwater events during the winter months (December, January, February and March), considering the season with most of extreme event occurrence. The relationship between local extreme events and variability at larger scales, considering climate indices such as the North Atlantic Oscillation (NAO), is detailed. A comparison between the low frequency data from the SOMLIT network and the high frequency data from the COAST-HF network is carried out, highlighting the potential of high frequency measurements for the detection of extreme events. A comparison between in situ data and two numerical simulations of different resolutions is also performed over salinity time series. The interannual variability of extreme event occurrences and features in a context of climate change is also discussed. The link between these extreme low salinity events and the winter nitrate levels in the bay of Brest is shown. Then, we investigate the relationship between extreme events and biology in the coastal environment.</p><p> </p><p><strong>Keywords </strong></p><p>In-situ observations, High and low frequency measurements, Extreme events, Numerical simulations, Bay of Brest, Weather regimes.</p>

scholarly journals Simulation of Broadband Ground Motion by Superposing High-Frequency Empirical Green’s Function Synthetics on Low-Frequency Spectral-Element Synthetics

Geosciences ◽  
2020 ◽  
Vol 10 (9) ◽  
pp. 339
Author(s):  
Ramses Mourhatch ◽  
Swaminathan Krishnan
Keyword(s):  
Ground Motion ◽  
Green's Function ◽  
Seismic Wave ◽  
High Frequency ◽  
Low Frequency ◽  
Spectral Element ◽  
Time Shift ◽  
Frequency Content ◽  
Empirical Green's Function ◽  
High Frequency Content

Seismic wave-propagation simulations are limited in their frequency content by two main factors: (1) the resolution of the seismic wave-speed structure of the region in which the seismic waves are propagated through; and (2) the extent of our understanding of the rupture process, mainly on the short length scales. For this reason, high-frequency content in the ground motion must be simulated through other means. Toward this end, we adopt a variant of the classical empirical Green’s function (EGF) approach of summing, with suitable time shift, recorded seismograms from small earthquakes in the past to generate high-frequency seismograms (0.5–5.0 Hz) for engineering applications. We superimpose these seismograms on low-frequency seismograms, computed from kinematic source models using the spectral element method, to produce broadband seismograms. The non-uniform time- shift scheme used in this work alleviates the over-estimation of high-frequency content of the ground motions observed. We validate the methodology by simulating broadband motions from the 1999 Hector Mine and the 2006 Parkfield earthquakes and comparing them against recorded seismograms.

scholarly journals Dynamical and Thermodynamical Impacts of High- and Low-Frequency Atmospheric Eddies on the Initial Melt of Arctic Sea Ice

2017 ◽  
Vol 30 (3) ◽  
pp. 865-883 ◽  
Author(s):  
Bradley M. Hegyi ◽  
Yi Deng
Keyword(s):  
Sea Ice ◽  
Heat Transport ◽  
High Frequency ◽  
Low Frequency ◽  
Longwave Radiation ◽  
Arctic Sea Ice ◽  
The Arctic ◽  
Positive Anomaly ◽  
Meridional Heat Transport ◽  
Arctic Sea

Abstract The role of high-frequency and low-frequency eddies in the melt onset of Arctic sea ice is investigated through an examination of eddy effects on lower-tropospheric (1000–500 hPa) meridional heat transport into the Arctic and local surface downwelling shortwave and longwave radiation. Total and eddy components of the meridional heat transport into the Arctic from 1979 to 2012 are calculated from reanalysis data, and surface radiation data are acquired from the NASA Clouds and the Earth’s Radiant Energy System (CERES) project dataset. There is a significant positive correlation between the mean initial melt date and the September sea ice minimum extent, with each quantity characterized by a negative trend. Spatially, the earlier mean melt onset date is primarily found in a region bounded by 90°E and 130°W. The decline in this region is steplike and not associated with an increase in meridional heat transport but with an earlier appearance of above-freezing temperatures in the troposphere. In most years, discrete short-duration episodes of melt onset over a large area occur. In an investigation of two of these melt episodes, a positive total meridional heat transport is associated with the peak melt, with the product of high-frequency eddy wind and mean temperature fields being the most important contributor. Additionally, there is a key positive anomaly in surface downwelling longwave radiation immediately preceding the peak melt that is associated with increased cloud cover and precipitable water. These results suggest the importance of carefully considering and properly representing atmospheric eddies when modeling the melt onset of Arctic sea ice.

The Waveform Characteristics and Classification of Intermediate-depth Earthquakes in Ryukyu Subduction Zone

2020 ◽  
Author(s):  
Yu-Jhen Lin ◽  
Tai-Lin Tseng ◽  
Wen-Tzong Liang
Keyword(s):  
Subduction Zone ◽  
High Frequency ◽  
Time Window ◽  
Source Region ◽  
Focal Depth ◽  
Low Frequency ◽  
Frequency Content ◽  
Low Velocity ◽  
Waveform Data ◽  
Intermediate Depth

<p>Using intraslab earthquakes shallower than 150 km in the southernmost Ryukyu subduction zone, previous studies in Taiwan found the wave guide effect that typically shows a low-frequency (<2Hz) first P arrival followed by sustained high-frequency (3–10 Hz) wave trains. Recently occurred deeper events at depth 150-300 km allow us to better quantify the properties of those seismic waves traveling in the subduction zone. In this study, we aim to systematically scan through the local broadband waveforms of the intermediate depth earthquakes with M>5 between 1997 and 2016. Event are classified based on the waveform characteristics and their frequency contents.</p><p>To detect events with similar properties, we applied sliding-window cross-correlation (SCC)​ using three components of P waveform data simultaneously for a set of stations​. The time window used here was 10 s and traces were bandpass filtered in the frequency range 0.5–10 Hz. After the degree of similarity are calculated, e​vents containing comparable waveforms can be sorted into families. The events within a family would have been triggered because they came from the same source region and their paths to a particular receiver should produce similar waveforms. Our results show that most earthquakes are low in waveform similarity, implying no “repeating” behavior for those intermediate intraslab events. However, some events (cc>0.6 threshold) present enough charterers that can be grouped as a family.</p><p>One important property is the frequency content of the arrivals that may be related to the speed of structure traveled. We have developed a work scheme to determine the delayed time of higher-frequency energy. On family of events show beautiful dispersion with arrival time smoothly increasing with frequency between 0.5 and 6 Hz.​ Another type of dispersive waveforms appear as two distinct arrivals: low frequency and then high-frequency energy, separated by around 1 s. The time delay seems to be independent of focal depth. The latter case has been reported in the previous study for shallower event and it was interpreted as effect from low-velocity layer or heterogeneity of the subducted slab. On the other hand, the continuous dispersion is a new feature observed by our study, which may infer a thinner layer and/or longer propagation for some kind of reflecting waves to develop such interference.</p><p>In addition, we will classify the waveforms according to the frequency content and decay of coda. The variations in P coda properties can be associated with the way in which the seismic energy gets ducted into the stochastic waveguide associated with the lithosphere. With sufficient amount of data, it is possible to further identify the earthquakes with unusual source properties or structure anomaly along specific propagation paths. We expect the classification results can provide a reference for future numerical simulation analysis. </p><p>Keywords: Ryukyu subduction zone, SCC, guide wave, waveform classification, intermediate-depth earthquakes</p>

scholarly journals Optimizing Mooring Placement to Constrain Southern Ocean Air–Sea Fluxes

2020 ◽  
Vol 37 (8) ◽  
pp. 1365-1385
Author(s):  
Yanzhou Wei ◽  
Sarah T. Gille ◽  
Matthew R. Mazloff ◽  
Veronica Tamsitt ◽  
Sebastiaan Swart ◽  
...  
Keyword(s):  
Southern Ocean ◽  
Numerical Weather Prediction ◽  
High Frequency ◽  
Weather Prediction ◽  
Low Frequency ◽  
Heat Fluxes ◽  
Test Case ◽  
Numerical Weather ◽  
Ocean Observing

AbstractProposals from multiple nations to deploy air–sea flux moorings in the Southern Ocean have raised the question of how to optimize the placement of these moorings in order to maximize their utility, both as contributors to the network of observations assimilated in numerical weather prediction and also as a means to study a broad range of processes driving air–sea fluxes. This study, developed as a contribution to the Southern Ocean Observing System (SOOS), proposes criteria that can be used to determine mooring siting to obtain best estimates of net air–sea heat flux (Qnet). Flux moorings are envisioned as one component of a multiplatform observing system, providing valuable in situ point time series measurements to be used alongside satellite data and observations from autonomous platforms and ships. Assimilating models (e.g., numerical weather prediction and reanalysis products) then offer the ability to synthesize the observing system and map properties between observations. This paper develops a framework for designing mooring array configurations to maximize the independence and utility of observations. As a test case, within the meridional band from 35° to 65°S we select eight mooring sites optimized to explain the largest fraction of the total variance (and thus to ensure the least variance of residual components) in the area south of 20°S. Results yield different optimal mooring sites for low-frequency interannual heat fluxes compared with higher-frequency subseasonal fluxes. With eight moorings, we could explain a maximum of 24.6% of high-frequency Qnet variability or 44.7% of low-frequency Qnet variability.

The Increasing Prevalence of High Frequency Internal Waves in an Arctic Ocean With Declining Sea Ice Cover

2019 ◽  
Author(s):  
Tom Rippeth ◽  
Vasyl Vlasenko ◽  
Nataliya Stashchuk ◽  
Igor E. Kozlov ◽  
Brian Scannell ◽  
...  
Keyword(s):  
Sea Ice ◽  
Arctic Ocean ◽  
Internal Waves ◽  
High Frequency ◽  
Ice Cover ◽  
The Arctic ◽  
Sea Ice Extent ◽  
Road Map ◽  
The Arctic Ocean

Abstract Receding seasonal sea ice extent over the Arctic Ocean is increasing access to what was a largely inaccessible region. At lower latitudes the complex vertical current structure associated with large amplitude, high frequency non-linear internal waves, sometimes referred to as solitons, present a significant challenge to the safe engineering design and operation of offshore infrastructure. In this paper we examine the prevalence this type of internal wave in the Arctic Ocean. To do so we will draw on both in situ and remotely sensed oceanographic data. This will be combined with state-of-the-art numerical modelling to demonstrate a link between the geographical occurrence of these waves and the tide. Whilst the link implies that these features are geographically limited, it is also likely that the geographical limits will change with declining sea ice cover. These results will then be used to provide a road map towards a methodology for forecasting the prevalence of these phenomena in a future Arctic Ocean.

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