Entropy theory applied to hydrometric measurements enables establishing a relationship between the maximum and the mean velocities of flow passing a river section. In many developing countries, river ...discharge is still estimated by the velocity‐area method following the procedure established based on the point‐velocities measured at 0.6D depth from the water surface of many verticals of flow section. This study explores the establishment of a relationship between the maximum point‐velocity estimated at 0.6D depth of the vertical located at or nearer to the deepest flow depth, and the maximum point‐velocity occurring somewhere along the same vertical which in turn can be linked to the sectional mean flow velocity based on the established entropy theory. The appropriateness of the proposed two‐steps based approach of discharge estimation is first verified on the Tiber River and the Po River of Italy. The study finds that the values of the maximum point‐velocity estimated using the proposed approach closely matches with the recorded maximum flow velocity values along the same flow‐depths. After this verification, the proposed method was applied for discharge estimation at two Indian river gauging stations, where only the point‐velocity measurements at 0.6D depth locations of the verticals are recorded. The study finds that the discharges estimated by both the proposed approach and the velocity‐area method closely match with each other with the estimated Nash‐Sutcliffe Efficiency (NSE) values greater than 0.99. Thus the proposed two‐steps approach involving the entropy concept based relationship for discharge estimation enables to replace the tedious and time‐consuming velocity‐area approach.
Key Points
Proposes a new method to estimate discharge using u0.6D,max velocity measurement. Estimation of state equilibrium constant using the historical records of the velocity‐area method
It is establishing a relationship between u0.6D,max and umax
The potential of the approach is demonstrated through uncertainty analysis for the Indian Rivers
River discharge has to be monitored reliably for effective water management. As river discharge cannot be measured directly, it is usually inferred from the water level. This practice is unreliable ...at places where the relation between water level and flow velocity is ambiguous. In such a case, the continuous measurement of the flow velocity can improve the discharge prediction. The emergence of horizontal acoustic Doppler current profilers (HADCPs) has made it possible to continuously measure the flow velocity. However, the profiling range of HADCPs is limited, so that a single instrument can only partially cover a wide cross section. The total discharge still has to be determined with a model. While the limitations of rating curves are well understood, there is not yet a comprehensive theory to assess the accuracy of discharge predicted from velocity measurements. Such a theory is necessary to discriminate which factors influence the measurements, and to improve instrument deployment as well as discharge prediction. This paper presents a generic method to assess the uncertainty of discharge predicted from range‐limited velocity profiles. The theory shows that a major source of error is the variation of the ratio between the local and cross‐section‐averaged velocity. This variation is large near the banks, where HADCPs are usually deployed and can limit the advantage gained from the velocity measurement. We apply our theory at two gauging stations situated in the Kapuas River, Indonesia. We find that at one of the two stations the index velocity does not outperform a simple rating curve.
Key Points
Accuracy of discharge monitored with range‐limited velocity profilers theoretically explained
Correcting for the variation of the velocity profile over the hydrograph improves accuracy
Index velocity not necessarily superior to stage‐rating when measurements are restricted to the near‐bank region
This study examined changes in movement velocity and surface electromyographic (sEMG) activity of the pectoralis major (PM) and triceps brachii (TB) muscles during the bench press exercise to failure ...against different loads. Fourteen men performed a set to failure with maximum intended velocity, against low (40%‐1 repetition maximum‐RM), moderate (60%‐1RM), and heavy loads (80%‐1RM). Number of repetitions, volume load, mean and peak velocity, and total time increased with decreasing load (40% > 60% > 80%, p < 0.01). sEMG comparisons between different loads were performed by matching time under tension at the initial, middle, and last part of the set. sEMG was higher in the middle and last repetitions, compared with the initial, for all loads in both muscles (p < 0.001). sEMG activity of both muscles was higher in the 60% and 80%‐1RM conditions compared with the 40%1‐RM (p < 0.007). Also, sEMG of both muscles was similar for the 60%‐1RM and 80%‐1RM loads at the initial, middle, and last repetitions, with the exception of the last repetitions for the TB muscle. In contrast, sEMG integrated activity was higher for the 40% 1‐RM and 60% 1‐RM (p < 0.01) compared with the 80% 1‐RM load. Mean velocity loss at exhaustion and drop in sEMG median frequency were greater in the 40% and 60%‐1RM compared with the 80%‐1RM condition (p < 0.05). It was concluded that performing a set to exhaustion with maximum intended velocity using a load of 60% 1‐RM combines the characteristics of the high average sEMG activity of heavier loads, and the high total integrated sEMG observed at lighter loads.
Iron may critically influence the physical properties and thermochemical structures of Earth’s lower mantle. Its effects on thermal conductivity, with possible consequences on heat transfer and ...mantle dynamics, however, remain largely unknown. We measured the lattice thermal conductivity of lower-mantle ferropericlase to 120 GPa using the ultrafast optical pump-probe technique in a diamond anvil cell. The thermal conductivity of ferropericlase with 56% iron significantly drops by a factor of 1.8 across the spin transition around 53 GPa, while that with 8–10% iron increases monotonically with pressure, causing an enhanced iron substitution effect in the low-spin state. Combined with bridgmanite data, modeling of our results provides a self-consistent radial profile of lower-mantle thermal conductivity, which is dominated by pressure, temperature, and iron effects, and shows a twofold increase from top to bottom of the lower mantle. Such increase in thermal conductivity may delay the cooling of the core, while its decrease with iron content may enhance the dynamics of large low shear-wave velocity provinces. Our findings further show that, if hot and strongly enriched in iron, the seismic ultralow velocity zones have exceptionally low conductivity, thus delaying their cooling.
Controlling the group velocity of an optical pulse typically requires traversing a material or structure whose dispersion is judiciously crafted. Alternatively, the group velocity can be modified in ...free space by spatially structuring the beam profile, but the realizable deviation from the speed of light in vacuum is small. Here we demonstrate precise and versatile control over the group velocity of a propagation-invariant optical wave packet in free space through sculpting its spatio-temporal spectrum. By jointly modulating the spatial and temporal degrees of freedom, arbitrary group velocities are unambiguously observed in free space above or below the speed of light in vacuum, whether in the forward direction propagating away from the source or even traveling backwards towards it.
The sound speed of seawater is not constant. It varies with season, day time, depth and range. This variation was not considered in marine seismic data processing and imaging before deep-water ...seismic acquisition became a routine activity. As a result, non-ignorable errors may be contained in the final migrated-image of deep-water seismic data. To eliminate such errors, we propose here a scheme for inverting the seawater velocity under the condition that the seabed has a complex topography. In this scheme, the seabed topography is represented by step grids constrained by measured water depth data, and the sound profile is assumed to be the one satisfying the Munk formula. Thus, only the parameters appearing in the Munk formula need to be inverted. This is quite different from the conventional inversion schemes that take the velocity as the inversion target. We use the conjugate gradient method to minimize the objective function given as the sum of squares of traveltime differences. Tests on synthetic common-shot data set show that our scheme presented here works as expected. Applications to real marine data set further validates the reasonability and feasibility of our scheme under complex seabed topography. To investigate the reliability of our inversion scheme, we migrate the real 2D data set using the Kirchhoff migration with the inverted Munk profile as the velocity model. The corresponding results show that the seawater velocity obtained by our inversion scheme improves the imaging quality of underwater structures.
Dense hydrous magnesium silicate (DHMS) phase E is a potential water carrier in subducting slabs that can transport water to the Earth's deep mantle between the bottom of the upper mantle and the ...uppermost transition zone. Therefore, knowledge on the high pressure‒temperature (P‒T) full elastic moduli of phase E at relevant mantle conditions is important in deciphering the existence of DHMS phases and their influences on seismic profiles in the region; however, the high P‒T elasticity data of phase E still remains lacking. In this work, we determined the combined effect of P‒T on the single‐crystal elasticity of phase E up to 24 GPa and 900 K by in situ X‐ray diffraction and Brillouin scattering measurements in externally‐heated diamond anvil cells. The aggregate elastic moduli and compressional‐wave (VP) and shear‐wave (VS) velocities of phase E are then derived by analyzing the single‐crystal elasticity and density data using the third‐order finite‐strain equations. We found that phase E exhibits much lower bulk and shear moduli and acoustic velocities than the most abundant constituent minerals in the upper mantle and transition zone, such as olivine, clinopyroxene, garnet, and wadsleyite. The modeled results using the obtained elasticity results show that the existence of phase E in a hydrated pyrolite model can result in relatively lower Vp and Vs profiles and negative velocity anomalies in seismic observations. The existence of phase E with relatively lower velocity profiles could be a possible origin of the low‐velocity layers atop the 410‐km discontinuity in some cold and highly‐hydrated regions.
Plain Language Summary
Deep‐mantle water storage and circulation remains one of the most intriguing issues in geoscience. Serpentine and its high pressure‒temperature (P‒T) phases, namely high‐density magnesium silicates (phases A, superhydrous B, D, E, and H), are considered to be the dominant potential water carriers in subduction zones. Hence, their sound velocities and density at high P‒T conditions are of particular importance for interpreting seismic observations and understanding water circulation and geodynamic processes in subduction‐related environments. In this study, we report new experimental results on the high P‒T single‐crystal elasticity of phase E up to 24 GPa and 900 K obtained by in situ synchrotron X‐ray diffraction and Brillouin scattering measurements. The single‐crystal elasticity data of phase E are used to derive its aggregate sound velocities and build mantle velocity profiles for dry and hydrated pyrolite models. We found that the existence of phase E in a hydrated pyrolite model can result in relatively lower compressional‐wave and shear‐wave velocity profiles and negative velocity anomalies in seismic observations. This finding helps explain the origin of the low‐velocity layers atop the 410‐km discontinuity in some highly‐hydrated regions.
Key Points
Single‐crystal elasticity of the Mg‐endmember phase E has been measured at high pressure and temperature conditions
The aggregate sound velocities of phase E are much lower than those of the typical minerals in the upper mantle and mantle transition zone
The existence of phase E could contribute to the origin of the low‐velocity layers atop the 410 km discontinuity in some hydrated regions
Nitrogen Content in the Earth's Outer Core Bajgain, Suraj K.; Mookherjee, Mainak; Dasgupta, Rajdeep ...
Geophysical research letters,
16 January 2019, Letnik:
46, Številka:
1
Journal Article
Recenzirano
Odprti dostop
Using first principles molecular dynamic simulations, we explore the effects of nitrogen (N) on the density and sound velocity of liquid iron and evaluate its potential as a light element in the ...Earth's outer core. Our results suggest that Fe‐N melt cannot simultaneously explain the density and seismic velocity of the Earth's outer core. Although ~2.0 wt.% N can explain the bulk sound velocity of the outer core, such N content only lowers the density of liquid Fe by ~3%. Matching both the velocity and density by the other light elements limits the N in the core to ≪2.0 wt.%. Our finding suggests that nitrogen is a minor to trace element in the Earth's core and is consistent with the geochemical mass balance with terrestrial abundance of N and alloy‐silicate partitioning data, which suggest that there cannot be significant N in the core.
Plain Language Summary
Physical properties of liquid iron cannot explain the seismological observations of density and sound wave velocities in the liquid outer core. In this study we explore the effect of light element nitrogen on the physical properties of liquid iron and provide an upper bound of the amount of nitrogen in the outer core if nitrogen were the sole light element.
Key Points
Nitrogen decreases density and enhances bulk sound velocity of liquid iron
We provide an upper bound of nitrogen in the liquid outer core
The amount of the light elements required to explain density and velocity correlates with the atomic number of the light elements
Single‐crystal elasticity of both α‐ and β‐orthopyroxene was determined up to 20 GPa and 300 K by Brillouin scattering. Using the derived full elastic moduli (Cij), we investigated the contribution ...of the metastable pyroxene to the seismically observed 3%–5% low‐velocity anomalies along the subducting slab in the top transition zone. Our modeled results show that a harzburgite wedge with a 1000‐K colder geotherm and metastable α‐orthopyroxene and olivine displays compressional (VP) and shear‐wave (VS) velocities 3.0%–3.6(6)% and 2.0%–2.8(6)% lower than the surrounding mantle at 410–460 km depth, respectively. At deeper depth up to 520 km, VP and VS of this metastable wedge with β‐orthopyroxene and olivine are 3.6%–4.4(6)% and 2.8%–4.3(6)% lower than the pyrolitic mantle, respectively. The presence of both metastable orthopyroxene and olivine instead of metastable olivine alone helps better explain the origin of the low‐velocity anomalies within the subduction slab in the top transition zone.
Plain Language Summary
Subducting slab plays a significant role in transportation the surface material to the Earth's deep interior. It is normally imaged as a high‐velocity body compared to the surrounding mantle. However, seismic studies detected the existence of 3%–5% low compressional‐wave velocity anomalies accompanied with strong seismic shear‐wave anisotropies within the subducting slab at the top transition zone in various locations of the Earth which cannot be explained by the presence of metastable olivine alone. Besides olivine, orthopyroxene could also remain metastable in the coldest harzburgite layer of the slab. Here we report experimental results on the single‐crystal elasticity of both α‐ and β‐orthopyroxene up to 20 GPa and 300 K. These experimental results allow us to provide a comprehensive evaluation on the velocity profiles of the coldest harzburgite layer of the slab. We found that the coldest harzburgite layer with 22–37 vol.% orthopyroxene and 62–78 vol.% olivine has the VP and VS 3.0%–4.4(6)% and 2.0%–4.5(6)% lower than those of the pyrolitic mantle in the mantle transition zone, respectively. The observed 3%–5% low‐velocity anomalies within the slab in the top transition zone should be explained by the metastable orthopyroxene and olivine instead of metastable olivine alone.
Key Points
Single‐crystal elasticity of α‐ and β‐orthopyroxene was determined to 20 GPa and shows anomalous change across the phase transition
The obtained results were used to model the velocity profiles of the coldest harzburgite layer of the sinking subduction slab
We find that the low‐velocity anomalies within the slab at the top transition zone should be caused by metastable orthopyroxene and olivine
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