scholarly journals Approbation of the Method for Examining the Performance of Seafloor Observatory Sensors Using Distant Earthquakes Records

2021 ◽  
Vol 9 ◽  
Author(s):  
Mikhail Nosov ◽  
Viacheslav Karpov ◽  
Kirill Sementsov ◽  
Sergey Kolesov ◽  
Hiroyuki Matsumoto ◽  
...  
Keyword(s):  
Linear Relationship ◽  
Direct Consequence ◽  
Forced Oscillations ◽  
Vertical Acceleration ◽  
Frequency Range ◽  
Calibration Accuracy ◽  
Seafloor Pressure ◽  
Seafloor Observatory ◽  
Acceleration Component

An algorithm is presented for testing the calibration accuracy of both z-accelerometers and pressure gauges (PG) installed in seafloor observatories. The test is based on the linear relationship between the vertical acceleration component of the seafloor movement and variations of the seafloor pressure, which is a direct consequence of Newton's 2-nd law and holds valid in the frequency range of “forced oscillations.” The operability of the algorithm is demonstrated using signals registered by 28 observatories of the DONET-2 system during 4 earthquakes of magnitude Mw ~ 8 that took place in 2018-2019 at epicentral distances from 55° up to 140°.

Analysis of Glacier Run-Off and Meteorological Observations

1972 ◽  
Vol 11 (63) ◽  
pp. 303-318 ◽  
Author(s):  
Gudmundur Gudmundsson ◽  
Guttormur Sigbjarnarson
Keyword(s):  
Spectral Analysis ◽  
Linear Relationship ◽  
Impulse Response ◽  
Linear Models ◽  
Time Lag ◽  
Diurnal Variations ◽  
Frequency Range ◽  
Western Margin ◽  
Run Off ◽  
Meteorological Observations

Abstract Linear models of the relationships between meteorological observations and the flow of river Tungnaá at the western margin of glacier Vatnajökull were investigated by means of spectral analysis and estimation of the impulse response. Most of the variation of Tungnaá is confined to the lowest frequencies and the diurnal variations. The temperature has most effect on the rapid variations around 1 cycle/day whereas the largest coherences with the precipitation are in the lowest frequencies. The wind explains over 20% of the variations in the frequency range from 0–1 cycle/day, but this is partly due to its coherence with the precipitation. The time lag between changes in the temperature and the river is about 2 h, but the time lag between precipitation and the river is longer. Analysis of longer records of daily observations from Ϸjórsá shows that the coherence of the run-off and temperature increases at frequencies too low to be estimated from these data. At frequencies over 1 cycle/day most of the observed variations of the river cannot be explained by means of a linear relationship with the meteorological series.

When the Hydrophone Works as an Accelerometer

2020 ◽  
Vol 92 (1) ◽  
pp. 365-377
Author(s):  
Giovanni Iannaccone ◽  
Giuseppe Pucciarelli ◽  
Sergio Guardato ◽  
Gian Paolo Donnarumma ◽  
Giovanni Macedonio ◽  
...  
Keyword(s):  
Shallow Water ◽  
Water Pressure ◽  
Vertical Component ◽  
Forced Oscillations ◽  
Campi Flegrei ◽  
Frequency Range ◽  
Ground Acceleration ◽  
Seismic Sensors ◽  
Wide Range ◽  
Amplitude Spectra

Abstract We show the equivalence of earthquake-induced ground acceleration and water-pressure waveforms for the case of collocated hydrophones and seafloor seismometers installed in shallow water. In particular, the comparison of the waveforms and amplitude spectra of the acceleration and water-pressure signals confirms the existence of a frequency range of “forced oscillations” in which the water-pressure variations are proportional to the vertical component of the ground acceleration. We demonstrate the equivalence of the acceleration and water-pressure signals for a set of local earthquakes (epicenter distance of a few tens of kilometers) and regional earthquakes with a wide range of magnitude (2.7<Mw<6.8), recorded by seismometers and hydrophones operating in shallow water (depth less than 80 m) in the Campi Flegrei caldera (southern Italy). We describe the “forced oscillations” theory, and we demonstrate the signals equivalence in the frequency range 0.1–10 Hz, thus extending the frequency range of application of the hydrophones as accelerometers. The high correlation between the ground acceleration, derived from the ground velocity, and hydrophone pressure signals in the mentioned frequency range enables the use of the hydrophone waveforms for standard seismological studies (i.e., earthquake source). The calibration of hydrophones by comparison with collocated accelerometers, or seismometers, is also enabled in a range of frequencies that is very difficult to reproduce in a laboratory. The results of our work also open the possibility of hydrophones being more extensively used in place of accelerometers in marine environments where accurate installation of seismic sensors is difficult or unaffordable.

scholarly journals Method for testing the calibration of acceleration and pressure gauges installed at the ocean bottom

2019 ◽  
Author(s):  
Mikhail Nosov ◽  
Viacheslav Karpov ◽  
Sergey Kolesov ◽  
Kirill Sementsov ◽  
Hiroyuki Matsumoto ◽  
...  
Keyword(s):  
Linear Relationship ◽  
Frequency Band ◽  
Power Spectra ◽  
Forced Oscillations ◽  
Ocean Bottom ◽  
Bottom Pressure ◽  
Ocean Bottom Pressure ◽  
Mean Pressure ◽  
Explicit Formulae ◽  
Gravity Acceleration

Abstract. A method is proposed for testing pressure gauges and z-accelerometers, installed in ocean-bottom observatories. The method is based on the linear relationship between variations of the ocean-bottom pressure and the z-acceleration, observed during seismic movements of the bottom within the frequency band of "forced oscillations". Calculation of the boundaries of this frequency band is based on the ocean depth at the observatory site making use of explicit formulae. In the case of correct calibration of the gauges calculation of the ratios of power spectra of bottom pressure variations and the z-accleration within the band of "forced oscillations" yields constant values equal to the square ratio of the total mean pressure and the gravity acceleration. The conditions for application of the proposed method are formulated.

Lagrangian frequency spectra of vertical velocity and vorticity in high-Reynolds-number oceanic turbulence

1998 ◽  
Vol 362 ◽  
pp. 177-198 ◽  
Author(s):  
REN-CHIEH LIEN ◽  
ERIC A. D'ASARO ◽  
GEOFFREY T. DAIRIKI
Keyword(s):  
Turbulent Flows ◽  
Frequency Spectrum ◽  
Finite Size ◽  
Inertial Subrange ◽  
Substantial Part ◽  
Spatial Average ◽  
Vertical Acceleration ◽  
Frequency Range ◽  
Frequency Spectra ◽  
Large Eddy

Lagrangian properties of oceanic turbulent boundary layers were measured using neutrally buoyant floats. Vertical acceleration was computed from pressure (depth) measured on the floats. An average vertical vorticity was computed from the spin rate of the float. Forms for the Lagrangian frequency spectra of acceleration, ϕa(ω), and the Lagrangian frequency spectrum of average vorticity are found using dimension analysis. The flow is characterized by a kinetic energy dissipation rate, ε, and a large-eddy frequency, ω0. The float is characterized by its size. The proposed non-dimensionalization accurately collapses the observed spectra into a common form. The spectra differ from those expected for perfect Lagrangian measurements over a substantial part of the measured frequency range owing to the finite size of the float. Exact theoretical forms for the Lagrangian frequency spectra are derived from the corresponding Eulerian wavenumber spectra and a wavenumber–frequency distribution function used in previous numerical simulations of turbulence. The effect of finite float size is modelled as a spatial average. The observed non-dimensional acceleration and vorticity spectra agree with these theoretical predictions, except for the high-frequency part of the vorticity spectrum, where the details of the float behaviour are important, but inaccurately modelled. A correction to the exact Lagrangian acceleration spectra due to measurement by a finite-sized float is thus obtained. With this correction, a frequency range extending from approximately one decade below ω0 to approximately one decade into the inertial subrange can be resolved by the data. Overall, the data are consistent with the proposed transformation from the Eulerian wavenumber spectrum to the Lagrangian frequency spectrum. Two parameters, ε and ω0, are sufficient to describe Lagrangian spectra from several different oceanic turbulent flows. The Lagrangian Kolmogorov constant for acceleration, βa≡ϕa/ε, has a value between 1 and 2 in a convectively driven boundary layer. The analysis suggests a Lagrangian frequency spectrum for vorticity that is white at all frequencies in the inertial subrange and below, and a Lagrangian frequency spectrum for energy that is white below the large-eddy scale and has a slope of −2 in the inertial subrange.

Computation of Added Mass and Damping Coefficients of a Horizontal Circular Cylinder in OpenFOAM

2016 ◽  
Author(s):  
Hao Chen ◽  
Erik Damgaard Christensen
Keyword(s):  
Free Surface ◽  
Analytical Solution ◽  
Numerical Computation ◽  
Added Mass ◽  
Forced Oscillations ◽  
Surface Zone ◽  
Computational Domain ◽  
Frequency Range ◽  
Two Phase ◽  
Damping Coefficients

This paper presents numerical computation of added mass and damping coefficients of a slender horizontal cylinder in the free surface zone, which typically serves as a fish cage floater. A fully viscous two phase flow solver in OpenFOAM was employed in the numerical computation. The purpose was to validate the capability of this solver and dynamic mesh functionality. A two dimensional numerical wave tank was set up, and two wave relaxation zones were used to reduce the size of the computational domain. Harmonic forced oscillations of the cylinder were performed at different frequencies and amplitudes. The mesh at free surface zone was refined based on the radiated wave heights at different oscillation frequencies in order to properly resolve the radiated waves. The result shows that in most frequency ranges, the numerical computation agreed well with the experimental data and analytical solution. However at low frequency range for added mass coefficient in heave motion, deviations were observed, and it was due to the effect of finite water depth. In addition for sway motion at high frequency range, the damping coefficient was underestimated comparing with analytical solution. This was believed to be as a result of high steepness of the radiated waves.

scholarly journals Analysis of Glacier Run-Off and Meteorological Observations

1972 ◽  
Vol 11 (63) ◽  
pp. 303-318 ◽  
Author(s):  
Gudmundur Gudmundsson ◽  
Guttormur Sigbjarnarson
Keyword(s):  
Spectral Analysis ◽  
Linear Relationship ◽  
Impulse Response ◽  
Linear Models ◽  
Time Lag ◽  
Diurnal Variations ◽  
Frequency Range ◽  
Western Margin ◽  
Run Off ◽  
Meteorological Observations

AbstractLinear models of the relationships between meteorological observations and the flow of river Tungnaá at the western margin of glacier Vatnajökull were investigated by means of spectral analysis and estimation of the impulse response. Most of the variation of Tungnaá is confined to the lowest frequencies and the diurnal variations. The temperature has most effect on the rapid variations around 1 cycle/day whereas the largest coherences with the precipitation are in the lowest frequencies. The wind explains over 20% of the variations in the frequency range from 0–1 cycle/day, but this is partly due to its coherence with the precipitation. The time lag between changes in the temperature and the river is about 2 h, but the time lag between precipitation and the river is longer. Analysis of longer records of daily observations from Ϸjórsá shows that the coherence of the run-off and temperature increases at frequencies too low to be estimated from these data. At frequencies over 1 cycle/day most of the observed variations of the river cannot be explained by means of a linear relationship with the meteorological series.

A Novel Method of Stereo Camera Calibration Using BP Neural Network

2010 ◽  
Vol 29-32 ◽  
pp. 2692-2697
Author(s):  
Jiu Long Xiong ◽  
Jun Ying Xia ◽  
Xian Quan Xu ◽  
Zhen Tian
Keyword(s):  
Neural Network ◽  
Linear Relationship ◽  
Camera Calibration ◽  
Bp Neural Network ◽  
Calibration Method ◽  
Two Stage ◽  
Non Linear ◽  
Novel Method ◽  
Calibration Accuracy ◽  
The Relationship

Camera calibration establishes the relationship between 2D coordinates in the image and 3D coordinates in the 3D world. BP neural network can model non-linear relationship, and therefore was used for calibrating camera by avoiding the non-linear factors of the camera in this paper. The calibration results are compared with the results of Tsai’s two stage method. The comparison show that calibration method based BP neural network improved the calibration accuracy.

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