Wyniki wyszukiwania - Biblioteka Nauki

1

Shape of dispersion curves in the Rayleigh--Lamb spectrum

100%

Ahmad F.

Archives of Mechanics

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2004

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tom Vol. 56, nr 2

157-165

EN

The following features of the symmetric Lamb modes in an elastic waveguide are well known: 1. There exists no mode with phase speed less than cR. 2. There is only one mode whose speed asymptotically approaches cR. 3. A horizontal line above c = cT (including the line c = cL) cannot be an asymptote to any of the modes. 4. Phase speed of all modes, except the lowest mode, approaches cT as the frequency becomes very large. The above features characterize the spectrum which is obtained numerically or experimentally but are not fully understood analytically. We analyze the Rayleigh-Lamb equation and provide analytical explanation for the above features.

2

Expanding seismic surface waves measurements towards low periods with gravity measurements

72%

Wilde-Piórko M. , Dykowski P. , Polkowski M. , Olszak T. , Grad M. , Kryński J. , Sękowski M. , Krankowski A. , Rajner M.

Geoinformation Issues

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2017

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tom Vol. 9, no. 1(9)

5--13

EN

Seismic events in gravity measurements are considered as disturbances and are usually removed from the records. However, the physical properties of tidal gravimetric instruments allow researchers to record seismic surface waves of very long periods. In the case of a superconducting gravimeter, periods of even up to 400 s can be determined. Simultaneous seismic and gravity records at the same locations allow the study of a wider response for incoming seismic waves by using two quite different instruments. For test purposes 4 seismometer-gravimeter pairs were temporarily deployed in Poland at three locations: Borowa Gora Geodetic-Geophysical Observatory (BG), Jozefoslaw Astro-Geodetic Observatory (JO), and Lamkowko Satellite Observatory (LA). During the test period from December 2016 to May 2017 several large teleseismic events were observed with well-formed surface waves. Group velocity dispersion curves for long surface waves, as well as periods of free oscillations are presented for selected events. The correlation of a broadband seismometer signal with different types of gravimetric sensors signals gives the opportunity to analyse gravimeter noise components, in the instrumental and micro-seismic domains.

PL

Zjawiska sejsmiczne w pomiarach grawimetrycznych są traktowane jako zakłócenia i zwykle ich efekty są usuwane z zapisów grawimetrycznych. Jednakże, grawimetry dzięki swojej konstrukcji umożliwiają rejestrację sejsmicznych fal powierzchniowych o bardzo długich okresach. W przypadku grawimetru nadprzewodnikowego, możliwe jest zaobserwowanie fal powierzchniowych, generowanych przez trzęsienia ziemi, o okresach nawet do 400 s. Przeprowadzenie równoczesnych rejestracji sejsmicznych i grawimetrycznych instrumentami zlokalizowanymi w tym samym miejscu, powinno umożliwić przeanalizowanie szerszego zakresu częstości sygnału sejsmicznego niż w przypadku użycia tylko jednego typu instrumentu. W celu sprawdzenia prawdziwości powyższego stwierdzenia, 4 pary instrumentów: sejsmometrów i grawimetrów zostały zainstalowane w Polsce w trzech lokalizacjach, w Obserwatorium Geodezyjno-Geofizycznym Borowa Góra (BG), Obserwatorium Astronomiczno-Geodezyjnym Józefosław (JO) oraz Obserwatorium Satelitarnym Lamkówko (LA). W czasie projektu pilotażowego, trwającego od grudnia 2016 do maja 2017 roku, zarejestrowano kilka dużych trzęsień ziemi z dobrze wykształconymi falami powierzchniowymi. W pracy zaprezentowano krzywe dyspersji grupowych prędkości fal powierzchniowych, jak również okresy oscylacji swobodnych dla wybranych zjawisk. Korelacja szerokopasmowego sygnału sejsmicznego z sygnałem zarejestrowanym przez różnego typu grawimetry umożliwi analizę szumu grawimetrycznego w zakresie częstości pływowych instrumentów, jak i w zakresie mikrosejsmicznym.

3

Barely visible impact damage identificationin a 3D core sandwich structure

72%

Sikdar S. , Ostachowicz W. , Kudela P. , Radzieński M.

Computer Assisted Methods in Engineering and Science

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2017

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tom Vol. 24, no. 4

259--268

EN

3D core sandwich structure (3DCSS) is a popular lightweightconstruction material in the automotive,aerospace and marine industries. However, barely visible low-speed impact-damage (BVLID) may occurin the 3DCSS due to foreign-object-impact that can significantly reduce the load-bearing capacity of thestructure. This paper presents a guided wave (GW) propagation based BVLID identification technique forthe 3DCSS. A global-matrix formulation based semi-analytical model is applied to generate the dispersioncurve for the GW propagation in the 3DCSS. It is observed thatthe GW propagation in the 3DCSS ismulti-modal in nature. Finite-element numerical simulation of GW propagation in the 3DCSS is carriedout in Abaqus. A significant increment in the primary antisymmetric mode is noticed due to the presenceof BVLID region in the structure. Experiments are then conducted on a 3DCSS sample to validate thesimulation results. There is a good agreement between the simulation and experimental results in allthe cases.

4

Determination of near-surface structures from multi-channel surface wave data using multi-layer perceptron neural network (MLPNN) algorithm

72%

Caylak C. , Kaftan I.

Acta Geophysica

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2014

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tom Vol. 62, no. 6

1310--1327

EN

This study proposes the use of multi-layer perceptron neural networks (MLPNN) to invert dispersion curves obtained via multi-channel analysis of surface waves (MASW) for shear S-wave velocity profile. The dispersion curve used in inversion includes the fundamental-mode dispersion data. In order to investigate the applicability and performance of the proposed MLPNN algorithm, test studies were performed using both synthetic and field examples. Gaussian random noise with a standard deviation of 4 and 8% was added to the noise-free test data to make the synthetic test more realistic. The model parameters, such as S-wave velocities and thicknesses of the synthetic layered-earth model, were obtained for different S/N ratios and noise-free data. The field survey was performed over the natural gas pipeline, located in the Germencik district of Aydın city, western Turkey. The results show that depth, velocity, and location of the embedded natural gas pipe are successfully estimated with reasonably good approximation.

5

A semi-analytical method for identification of thin elastic plate parameters basing on LWM

58%

Pabisek E. , Waszczyszyn Z. , Ambroziński Ł.

Computer Assisted Methods in Engineering and Science

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2014

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tom Vol. 21, no. 1

5--14

EN

A new semi-analytical method, discussed in the presented paper, is composed of two stages. Stage A corresponds to the direct analysis, in which the Lamb Waves Measurements (LWM) technique enables obtaining an experimental set of points D(fj , kj) Jj =1, where f and k are frequency and wavenumber, respectively. After the preprocessing in the transform space an experimental approximate curve kexp(f | D) can be formulated. In Stage B the identification procedure is simulated as a sequence of direct analyses. The dimensionless Lamb Dispersion curves are computed by means of the dimensionless simulation curve ksim(f | par), where the vector of plate parameters par = {E,ν, d, ρ} is adopted, in which Young modulus E, Poisson ratio ν, plate thickness d and density ρ are used. The main idea of the proposed approach is similar to that in the classical method of error minimization. In our paper we propose to apply the zero error value of relative criterion Reky = 0, cf. formula (15). The formula can be applied for the identification of a single plate parameter, assuming a fixed value of the other plate parameters. This approach was used in a case study, in which Stages A and B were analysed for an aluminum plate.

6

Analytical and numerical identification of lamb waves modes for hybrid composites

58%

Barski M. , Pająk P.

Journal of KONES

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2017

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tom Vol. 24, No. 1

31--38

EN

The current work is devoted to the problem of analytical and numerical identification of fundamental elastic waves' modes, namely symmetric mode S0 and antisymmetric mode A0, in the case of hybrid composite. The investigated material consists of one layer made of aluminum alloy Pa38 and six layers made of glass fabric/epoxy resin. At the very beginning, the dispersion curves are determined with the use of stiffness matrix method. The calculated values of phase velocities are verified by numerical simulation. The semi – analytical finite element method is applied. Next, the numerical simulations of elastic waves propagation are performed. In the studied model, the plane state of strain is assumed. These simulations are carried out with the use of finite element method. The excitation signal is a sine wave modulated by Hanning window. The simulation is repeated for different excitation frequency. The group velocities of wave modes S0 and A0 are estimated and compared with the analytical results. The evaluation of the group velocities is based on the analysis of the appropriate components of displacement. The two different method are employed, namely: cross – correlation method and envelope extraction by Hilbert transform. Generally, the obtained results are in a good agreement. However, the method based on envelope extraction by Hilbert transform provides better correlation between analytical and numerical results. The significant discrepancy is observed in the case of symmetric mode S0 for relatively high values of frequency. It is caused by the dispersion phenomena. The analytical calculations are performed with the use of SCILAB 5.5.2 free software and the numerical simulations are carried out with the use of finite element system ANSYS 13.0.

7

Application of Multichannel Analysis of Surface Waves to S-Phase Wave Anisotropy Estimation

58%

Stan-Kłeczek I. , Mendecki M. J.

Acta Geophysica

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2016

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tom Vol. 64, no. 5

1593--1604

EN

The Multichannel Analysis of Surface Waves (MASW) is an increasingly used technique for recognition of a shallow geological structure and estimation of geotechnical parameters, e.g., S-wave velocity, layer density, layer thickness, shear modulus, estimated P-wave velocity, and estimated Poisson ratio. MASW surveys were carried out in two limestone quarries in the southern part of Poland. The experimental areas are characterised by a simple geological structure: consolidated Triassic limestone. Measurement profiles were arranged as a shapely six-pointed star. For each survey line, 12 geophones with 2-meter (Deposit 1) and 3-meter (Deposit 2) spacing were applied. The research allowed to compare P- and S-wave velocity changes with the main crack systems in the studied rock masses.

8

Subsurface profiling of a region in Jamshedpur city using active MASW: a case study

58%

Imam A. , Sharma K. K. , Kumar V. , Singh N.

Acta Geophysica

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2022

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tom Vol. 70, no 4

1601--1617

EN

This paper deals specifically with the active MASW method, which was applied for subsurface exploration of a region in Jamshedpur city, India, to study the various lithological and stiffness properties of subsurface materials. The study investigates the impact of data acquisition parameters on obtaining a high-resolution dispersion image, based on the ongoing MASW survey. A linear array of 24 numbers of 4.5 Hz geophones was used to collect raw wavefield traces generated by a 10 kg sledgehammer. Wavefields were regulated using a range of sampling frequencies (500 Hz, 1000 Hz, 2000 Hz, 4000 Hz, and 8000 Hz), as well as offset distances (1, 2 m, 4 m, 6 m, 8 m, 10 m, and 12 m) and inter receiver spacing (1 m and 2 m). Based on the results, the best data collection parameters for a high signal-to-noise ratio were determined to be: 1000 Hz sampling frequency, 8 m offset distance, and 1 m inter receiver spacing, resulting in a sufficient resolution dispersion image. Moreover, 1D and 2D shear-wave velocity profiles for the chosen site were derived. The stiff silty clay soil (up to a depth of 5 m) and dense to very dense weathered mica schist was found (at variable locations and depths from 8 to 30 m or beyond). The average Vs30 is 402 m/s, and the site is classed as Type C as per NEHRP Site Classification. The shear-wave velocity profiles show a high level of agreement with borehole data, demonstrating the effectiveness of the non-invasive technology for sub-surface investigation.