RADAR DIFFRACTION HORIZONS IN SNOW AND FIRN DUE TO A SURFICIAL VERTICAL TRANSFER OF MASS

ABSTRACT. We focus here on three horizons conspicuously embedded in the rich radar stratigraphy revealed on the fixed-offset radar data obtained Plateau Detroit, Antarctic Peninsula. Spatial filtering removed the more energetic reflection field and the surface wave arrivals at the earlier time, leaving only the diffracted field. This is particularly striking for the early time horizon where the direct wave arrivals had shrouded the diffractions before filtering. The density estimates and the photographic datasets from a centrally located well at depths compatible with the diffraction horizons suggested they share a common origin: a vertical transfer of mass associated with the formation of surficial hoar from a strong vertical temperature gradient in the snow cover, followed by a quick burial by fresh snow in a high accumulation environment. We have inverted the fundamental mode of the phase velocity dispersion of the surface waves to obtain a group velocity estimate and its depth range, used to improve the 1–D velocity model from a CMP gather by correcting its first velocity estimate. The same inversion solved an apparent ambiguity in our data by associating the surficial horizon with a specific density residual. We have also shown through modeling that the diffraction horizons seen in our data can be explained by the existence of large coarse–grained faceted crystals which became denser with depth than the surrounding firn.Keywords: GPR, radar, campo difratado, modelo de velocidade, guia de ondas, estratigrafia polar, depth hoar, AntárticaRESUMO. Concentramo-nos aqui em três horizontes conspicuamente embutidos na rica estratigrafia revelada nos dados de radar de afastamento constante obtidos Platô Detroit, Península Antártica. Uma filtragem espacial removeu o campo de reflexão mais energético e as chegadas de onda de superfície das primeiras chegadas, deixando apenas o campo difratado. Isso é notável para o horizonte de primeiras chegadas, onde as ondas diretas encobriam as difrações antes da filtragem. As estimativas de densidade e os conjuntos de dados fotográficos do poço localizado no centro da aquisição mostra em profundidades cristais compatíveis com os horizontes de difração dos dados de GPR sugerindo uma origem comum: uma transferência vertical de massa associada à formação de hoar devido a um forte gradiente vertical de temperatura na cobertura de neve fresca em um ambiente de alta acumulação. Nós invertemos o modo fundamental da dispersão de velocidade de fase das ondas de superfície para obter uma estimativa de velocidade de grupo e sua faixa de profundidade usada para melhorar o modelo de velocidade 1-D a partir de um CMP, corrigindo sua primeira estimativa de velocidade. A mesma inversão resolveu uma ambiguidade aparente em nossos dados ao associar o horizonte superficial a uma densidade específica residual. Também mostramos através de modelagem que os horizontes de difração observados em nossos dados podem ser explicados pela existência de grandes cristais facetados de granulação grossa que se tornaram mais densos com a profundidade do que o firn circundante.Palavras-chave: GPR, radar, campo de difração, modelo de velocidade, guia de onda, estratigrafia polar, depth hoar, Antarctica

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Mean-Field Models for EEG/MEG: From Oscillations to Waves

Brain Topography ◽

10.1007/s10548-021-00842-4 ◽

2021 ◽

Author(s):

Áine Byrne ◽

James Ross ◽

Rachel Nicks ◽

Stephen Coombes

Keyword(s):

Gap Junction ◽

Large Scale ◽

Mean Field ◽

Coarse Grained ◽

Neural Mass Model ◽

Neuron Network ◽

Mass Model ◽

Mean Field Model ◽

Neural Mass ◽

The Rich

AbstractNeural mass models have been used since the 1970s to model the coarse-grained activity of large populations of neurons. They have proven especially fruitful for understanding brain rhythms. However, although motivated by neurobiological considerations they are phenomenological in nature, and cannot hope to recreate some of the rich repertoire of responses seen in real neuronal tissue. Here we consider a simple spiking neuron network model that has recently been shown to admit an exact mean-field description for both synaptic and gap-junction interactions. The mean-field model takes a similar form to a standard neural mass model, with an additional dynamical equation to describe the evolution of within-population synchrony. As well as reviewing the origins of this next generation mass model we discuss its extension to describe an idealised spatially extended planar cortex. To emphasise the usefulness of this model for EEG/MEG modelling we show how it can be used to uncover the role of local gap-junction coupling in shaping large scale synaptic waves.

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Greenland ice velocity maps from the PROMICE project

Earth System Science Data ◽

10.5194/essd-13-3491-2021 ◽

2021 ◽

Vol 13 (7) ◽

pp. 3491-3512

Author(s):

Anne Solgaard ◽

Anders Kusk ◽

John Peter Merryman Boncori ◽

Jørgen Dall ◽

Kenneth D. Mankoff ◽

...

Keyword(s):

Ice Sheet ◽

Greenland Ice Sheet ◽

Radar Data ◽

Temporal Consistency ◽

Gps Measurements ◽

High Accumulation ◽

Synthetic Aperture Radar Data ◽

Spatial Coverage ◽

Ice Velocity ◽

The Difference

Abstract. We present the Programme for Monitoring of the Greenland Ice Sheet (PROMICE) Ice Velocity product (https://doi.org/10.22008/promice/data/sentinel1icevelocity/greenlandicesheet, Solgaard and Kusk, 2021), which is a time series of Greenland Ice Sheet ice velocity mosaics spanning September 2016 through to the present. The product is based on Sentinel-1 synthetic aperture radar data and has a 500 m grid spacing. A new mosaic is available every 12 d and spans two consecutive Sentinel-1 cycles (24 d). The product is made available within ∼ 10 d of the last acquisition and includes all possible 6 and 12 d pairs within the two Sentinel-1A cycles. We describe our operational processing chain from data selection, mosaicking, and error estimation to final outlier removal. The product is validated against in situ GPS measurements. We find that the standard deviation of the difference between satellite- and GPS-derived velocities (and bias) is 20 m yr−1 (−3 m yr−1) and 27 m yr−1 (−2 m yr−1) for the components in an eastern and northern direction, respectively. Over stable ground the values are 8 m yr−1 (0.1 m yr−1) and 12 m yr−1 (−0.6 m yr−1) in an eastern and northern direction, respectively. This is within the expected values; however, we expect that the GPS measurements carry a considerable part of this uncertainty. We investigate variations in coverage from both a temporal and spatial perspective. The best spatial coverage is achieved in winter due to the comprehensive data coverage by Sentinel-1 and high coherence, while summer mosaics have the lowest coverage due to widespread melt. The southeast Greenland Ice Sheet margin, along with other areas of high accumulation and melt, often has gaps in the ice velocity mosaics. The spatial comprehensiveness and temporal consistency make the product ideal both for monitoring and for studying ice-sheet-wide and glacier-specific ice discharge and dynamics of glaciers on seasonal scales.

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A survey of the repository of groundwater potential and distribution using geoelectrical resistivity method in Itu Local Government Area (L.G.A), Akwa Ibom State, southern Nigeria

Open Geosciences ◽

10.2478/s13533-012-0152-5 ◽

2013 ◽

Vol 5 (4) ◽

Cited By ~ 8

Author(s):

J. Ibuot ◽

G. Akpabio ◽

N. George

Keyword(s):

Local Government ◽

Local Government Area ◽

Least Square ◽

Coarse Grained ◽

Depth Range ◽

Average Depth ◽

Current Electrode ◽

Wide Range ◽

Southern Nigeria ◽

Maximum Current

AbstractVertical electrical sounding (VES), employing a Schlumberger electrode configuration, was used to investigate the sediments and aquifer repositories in Itu Local Government Area of Akwa Ibom state, southern Nigeria. This was done in sixteen (16) locations/communities with the maximum current electrode spread ranging between 800–1000m. The field data were interpreted using forward and iterative least square inversion modeling, which gives a resolution with 3–5 geoelectric layers. The observed frequencies in curve types include 31.25% of AKH, 18.8% of AAK and HK and 6.25% of K, QHK, AKH, KA and KHQ, respectively. These sets of curves show a wide range of variabilities in resistivities between and within the layers penetrated by current. The presence of K and H curve types in the study area indicates the alteration of the geomaterials with limited hydrologic significance to the prolific groundwater repository. A correlation of the constrained nearby borehole lithology logs with the VES results shows that the layers were all sandy formations (fine and well sorted sands to gravelly sands or medium to coarse-grained sands as described by nearby lithology logs) with some wide ranges of electrical resistivity values and thicknesses caused by electrostratigraphic inhomogeneity. The geologic topsoil (motley topsoil) is generally porous and permeable and as such the longitudinal conductance (S) values for the covering/protective layer is generally less than unity of Siemens (S < 1Ω−1), the value considered for efficient protection of the underlying aquifers by the topmost and overlying layer. The spatial orientations and the leveling patterns of the most economically viable potential groundwater repository within the maximum current electrode separations has been delineated in 2-D and 3-D contoured maps. The estimated depth range for the desired groundwater repository is 32.6–113.1m and its average depth value is 74.30m. The thickness of this layer ranges from 27.9–103m while its average depth has been evaluated to be 63.02m. Also, its resistivity range and average value have been estimated to be 507–5612m and 3365.125Ωm

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TTZ-SOUTH seismic experiment

Geofizicheskiy Zhurnal ◽

10.24028/gzh.v43i2.230189 ◽

2021 ◽

Vol 43 (2) ◽

pp. 28-44

Author(s):

T. Janik ◽

V. Starostenko ◽

P. Aleksandrowski ◽

T. Yegorova ◽

W. Czuba ◽

...

Keyword(s):

High Velocity ◽

Velocity Model ◽

Data Cube ◽

P Wave ◽

Depth Range ◽

Time Inversion ◽

Crust And Upper Mantle ◽

Moho Interface ◽

East European ◽

First Arrival

The wide-angle reflection and refraction (WARR) TTZ-South transect carried out in 2018 crosses the SW region of Ukraine and the SE region of Poland. The TTZ-South profile targeted the structure of the Earth’s crust and upper mantle of the Trans-European Suture Zone, as well as the southwestern segment of the East European Craton (slope of the Ukrainian Shield). The ~550 km long profile (~230 km in Poland and ~320 km in western Ukraine) is an extension of previously realized projects in Poland, TTZ (1993) and CEL03 (2000). The deep seismic sounding study along the TTZ-South profile using TEXAN and DATA-CUBE seismic stations (320 units) made it possible to obtain high-quality seismic records from eleven shot points (six in Ukraine and five in Poland). This paper presents a smooth P wave velocity model based on first-arrival travel-time inversion using the FAST (First Arrival Seismic Tomography) code. The obtained image represents a preliminary velocity model which, according to the P wave velocities, consists of a sedimentary layer and the crystalline crust that could comprise upper, middle and lower crustal layers. The Moho interface, approximated by the 7.5 km/s isoline, is located at 45—47 km depth in the central part of the profile, shallowing to 40 and 37 km depth in the northern (Radom-Łysogуry Unit, Poland) and southern (Volyno-Podolian Monocline, Ukraine) segments of the profile, respectively. A peculiar feature of the velocity cross-section is a number of high-velocity bodies distinguished in the depth range of 10—35 km. Such high-velocity bodies were detected previously in the crust of the Radom-Łysogуry Unit. These bodies, inferred at depths of 10—35 km, could be allochthonous fragments of what was originally a single mafic body or separate mafic bodies intruded into the crust during the break-up of Rodinia in the Neoproterozoic, which was accompanied by considerable rifting. The manifestations of such magmatism are known in the NE part of the Volyno-Podolian Monocline, where the Vendian trap formation occurs at the surface.

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Seismic observations with broadband instruments at Santorini volcano

Bulletin of the Geological Society of Greece ◽

10.12681/bgsg.16844 ◽

2018 ◽

Vol 40 (3) ◽

pp. 1150 ◽

Cited By ~ 3

Author(s):

A. Kolaitis ◽

P. Papadimiriou ◽

I. Kassaras ◽

K. Makropoulos

Keyword(s):

Low Frequency ◽

Volcanic Tremor ◽

Velocity Model ◽

Dominant Frequency ◽

Depth Range ◽

Traveltime Inversion ◽

Time Frequency ◽

Inversion Procedure ◽

Santorini Volcano ◽

Better Than

Two arrays equipped with broadband sensors were installed for a period of 10 months, in order to study the seismic activity in the area of Santorini (Thira) volcano. During these periods, about 330 earthquakes were recorded and located within a radius of 50 km from the center of the caldera. An iterative damped traveltime inversion procedure yielded a local 1-D Ρ-wave velocity model and improved locations with an accuracy better than 5 Km in both horizontal and vertical components for 135 earthquakes. Those are mainly distributed within a depth range 5-18 Km, in the vicinity of the submarine Kolumbo Reef (NE of Santorini Island). Signal analysis of the recorded volcanic earthquakes including typical Fourier transformations and several operations in the time-frequency domain, allowed their dominant frequency determination and their classification into three groups based on waveform appearance and frequency content: (1) highfrequency events; (2) low-frequency events; and (3) volcanic tremor. Frequencytime analysis of tremor, detected at three stations, revealed two kinds of harmonic tremor with one sharp peak, at 3-5 Hz and 8.5-10 Hz.

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Design optimisation of permanently installed monitoring system for polycrystalline materials

Structural Health Monitoring ◽

10.1177/1475921720950197 ◽

2020 ◽

pp. 147592172095019

Author(s):

Yuan Liu ◽

Peter B. Nagy ◽

Peter Cawley

Keyword(s):

Surface Area ◽

Monitoring System ◽

Signal To Noise Ratio ◽

Coarse Grained ◽

Polycrystalline Materials ◽

Depth Range ◽

Bulk Wave ◽

Signal To Noise ◽

Trade Off ◽

Noise Ratio

This article presents a design procedure for structural health monitoring systems based on bulk wave ultrasonic sensors for structures fabricated from polycrystalline materials. When designing a monitoring system, maximum coverage per transducer is a general requirement in order for the system to be economic. For coarse-grained polycrystalline materials, monitoring is often made challenging by low signal-to-noise ratios caused by grain scattering. Therefore, when designing a monitoring system for these materials, in addition to the economic requirement, it needs to be ensured that an adequate signal-to-noise ratio can be obtained throughout the monitoring volume. This typically introduces a trade-off between volume coverage per transducer and sensitivity that must be investigated. In this article, this trade-off is studied and a methodology using signal-to-noise maps is presented to design the system, that is, choose the optimal transducer parameters and placement. First, a combined analytical and numerical approach is used to generate a signal-to-noise map. Then, the influence of various factors on signal-to-noise ratio is investigated. Finally, two representative examples, with different criteria, are given to illustrate the methodology. In one example, the full surface area of the testpiece is covered with transducers and the optimum gives the deepest coverage. The other one aims to achieve the minimum fractional surface area that has to be covered with transducers to monitor a narrow depth range far from the surface, which has a potential application in weld monitoring. Results show that the optimum is likely to be at much lower frequency than typically used in inspection, as tracking signals with time gives sensitivity gains. Experiments were carried out to illustrate that higher volume coverage can be obtained at lower frequencies.

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Crosshole reflection imaging with ground-penetrating radar data: Applications in near-surface sedimentary settings

Geophysics ◽

10.1190/geo2019-0558.1 ◽

2020 ◽

Vol 85 (4) ◽

pp. H61-H69

Author(s):

Niklas Allroggen ◽

Stéphane Garambois ◽

Guy Sénéchal ◽

Dominique Rousset ◽

Jens Tronicke

Keyword(s):

Ground Penetrating Radar ◽

Field Data ◽

Electromagnetic Property ◽

Velocity Model ◽

Radar Data ◽

Near Surface ◽

Velocity Models ◽

Reflection Image ◽

Detailed Interpretation ◽

Ground Penetrating

Crosshole ground-penetrating radar (GPR) is applied in areas that require a very detailed subsurface characterization. Analysis of such data typically relies on tomographic inversion approaches providing an image of subsurface parameters. We have developed an approach for processing the reflected energy in crosshole GPR data and applied it on GPR data acquired in different sedimentary settings. Our approach includes muting of the first arrivals, separating the up- and the downgoing wavefield components, and backpropagating the reflected energy by a generalized Kirchhoff migration scheme. We obtain a reflection image that contains information on the location of electromagnetic property contrasts, thus outlining subsurface architecture in the interborehole plane. In combination with velocity models derived from different tomographic approaches, these images allow for a more detailed interpretation of subsurface structures without the need to acquire additional field data. In particular, a combined interpretation of the reflection image and the tomographic velocity model improves the ability to locate layer boundaries and to distinguish different subsurface units. To support our interpretations of our field data examples, we compare our crosshole reflection results with independent information, including borehole logs and surface GPR data.

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Confirmation and Characterization of the Rupture Model of the 2017 Ms 7.0 Jiuzhaigou, China, Earthquake

Seismological Research Letters ◽

10.1785/0220200466 ◽

2021 ◽

Author(s):

Xu Zhang ◽

Li-Sheng Xu ◽

Lei Yi ◽

Wanpeng Feng

Keyword(s):

Strong Motion ◽

Early Time ◽

Pulse Mode ◽

P Wave ◽

Fault System ◽

Seismic Gap ◽

Depth Range ◽

Seismogenic Fault ◽

Slip Pulse ◽

Rupture Model

Abstract On 8 August 2017, an Ms 7.0 earthquake struck the Jiuzhaigou town, Sichuan Province, China, rupturing an unmapped fault, which is adjacent to the Maqu seismic gap in the Min Shan uplift zone in the easternmost part of the Bayan Har block. Having summarized the previous studies on the source of this earthquake, we confirmed the rupture model by jointly inverting the teleseismic P-wave and SH-wave data, Interferometric Synthetic Aperture Radar line-of-sight displacement data, and the near-field seismic and strong-motion data, a most complete dataset until now. The confirmation showed that a scalar seismic moment of 6.6×1018 N·m was released (corresponding to a moment magnitude of Mw 6.5), and 95% of the release occurred in the first 10 s. The slip area was composed of two asperities, with a horizontal extension of ∼20 km and a depth range of ∼2–15 km. A bilateral extending occurred at shallow depths, but the rupturing upward from deep depth dominated in the early time. The rupture process was found generally featuring the slip-pulse mode, which was related to the weak prestress condition. The aftershocks almost took place in gaps of the mainshock slip because of the coulomb stress change. Combining the aftershock relocations, aftershock focal mechanism solutions, and our confirmed rupture model, we suggest that the seismogenic fault was a northward extension of the mapped Huya fault. The occurrence of this earthquake made the Maqu seismic gap at a higher level of seismic risk, in addition to the moderate to high strain accumulation on the easternmost tip of the Kunlun fault system and the weak lower crust below.

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The nature of the Albstadt Shear Zone, Germany

10.5194/egusphere-egu2020-7165 ◽

2020 ◽

Author(s):

Sarah Mader ◽

Klaus Reicherter ◽

Joachim Ritter ◽

the AlpArray Working Group

Keyword(s):

Shear Zone ◽

Seismic Velocity ◽

Active Regions ◽

Velocity Model ◽

Strike Slip ◽

Fault System ◽

Earthquake Catalog ◽

Depth Range ◽

Study Region ◽

The Town

The region around the town of Albstadt, SW Germany, is one of the most seismically active regions in Central Europe. In the last century alone three earthquakes with a magnitude greater than five happened and caused major damage. The ruptures occur along the Albstadt Shear Zone (ASZ), an approx. 20-30 km long, N-S striking fault with left-lateral strike slip. As there is no evidence for surface rupture the nature of the Albstadt Shear Zone can only be studied by its seismicity.To characterize the ASZ we continuously complement the earthquake catalog of the State Earthquake Service of Baden-W&#252;rttemberg with additional seismic phase onsets. For the latter we use the station network of AlpArray as well as 5 additional, in 2018/2019 installed seismic stations from the KArlsruhe BroadBand Array. We inverted for a new minimum 1D seismic velocity model of the study region. We use this seismic velocity model to relocalize the complemented catalog and to calculate focal mechanisms. The majority of the seismicity happens between the towns T&#252;bingen and Albstadt at around 9&#176;E in a depth range of about 1.5 to 16 km and aligns north-south. Additionally, we see a clustering of events at the towns Hechingen and Albstadt. The dominating focal mechanism is strike-slip, but we also observe minor components of normal and reverse faulting. Our results image the ASZ by its mainly micro-seismic activity between 2011 and 2018 confirming the N-S striking character, but also indicating a more complex fault system.We thank the State Earthquake Service in Freiburg for using their data (Az. 4784//18_3303). &#160;

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