Tunable broadband low-frequency band gap of multiple-layer metastructure induced by time-delayed vibration absorbers
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A seismic velocity transducer (SVT) is the commonly used pickup in engineering. However it is unsuitable for low frequency vibration measurement due to the limitation set by the mechanical natural frequency. It is showed that connecting a capacitor in parallel to the output terminal of SVT means the equivalent mass and damping coefficient are increased. The reliability and measurement range are increased and the natural frequency is lowered at the cost of the decrement of sensitivity at high frequency band. And the decrement of sensitivity of the pick up at low frequency band can not been visibly observed. A circuit network is used to further compensate the low frequency characteristics and a novel type of inertial extremely low frequency transducer is obtained.
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THE STUDY OF THE LOW-FREQUENCY SECTION IN THE NEAR-SURFACE MICRO-LOGGING RECORD OF THE TAHE OILFIELD
There generally exists the record of a low-frequency band characterized by low frequency,strong energy and fat wave form in the near-surface micro-logging data of the Tahe oilfield,which usually occurs near the top of the high-velocity layer.The reason for the formation of this special low-frequency band and its influence on the selection of IP well depth are important problems that geologists have to face in seismic data acquisition in this area.The low-frequency band was previously considered to be produced by lithological change,and hence in well depth design,the low-frequency band was intentionally avoided.Based on practical data,theroretical analysis and well depth test,the authors hold that the low-frequency band results from ghosting,and that the IP in the low-frequency band will not change the single gun quality.The characteristics and the formation mechanism of the low-frequency band as well as its impact on the selection of IP well depth are analyzed in detail,and it is considered that the influence of the low-frequency band can be ignored in the IP well depth design.
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Ghosting
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Summary There has been an increasing interest in the industry for powerful very low frequency (VLF) seismic sources. The lowest frequency range is important for deriving the elastic properties of the subsurface by seismic full wave field inversion (FWI). Accordingly, there has been a need for powerful low frequency marine sound sources operating in the frequency band of 1 to 5 Hz. This paper describes a new low frequency source that is easy to deploy and operate from a small vessel. The new very low frequency source is built on a novel source concept using resonance to increase acoustic efficiency and at the same time decrease complexity of the source. The resonance frequency of the source can easily be adjusted to a certain frequency or adaptive in case of running sweeps in the frequency band of 1 to 5 Hz. This less complex source can be deployed at a water depth of 5 m or less and will give a SPL of more than 190 dB rel. 1 micro-Pa in the frequency range from 2 to 5 Hz.
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Radio spectrum
Seismic vibrator
Extremely low frequency
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Abstract This paper investigates the design method for absorbing and isolating low-frequency noise by utilizing the acoustic metamaterials, which consist of mass and membrane vibrators embedded into acoustic materials. Firstly, the unit cell model of the metamaterials is established to analyze characteristics of band-gaps, which is verified further by frequency domain response analysis. And then, the influence of geometric and material parameters of unit cells on the band-gap properties is investigated. Finally, the wave propagation in the metamaterials with finite periodic structures is simulated for the studies of the low-frequency noise isolation, and the effect of different group patterns of unit cells on the isolation of low-frequency noise is discussed. The results show that the greater the density difference between the mass and the base material is, the greater the number of band-gaps and each bandwidth become. And also, as the thickness of membrane increases, the number of band-gaps increases but the band-gaps are narrow and scattered. The finite periodic structures composed of uniform unit cells can hardly widen the low-frequency bandwidth by adjusting the parameters of the components. However, the non-uniform finite periodic structures composed of different unit cells can widen the band-gaps of the whole structure by adjusting parameters of different components.
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Acoustic Metamaterials
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ABSTRACT The very low‐frequency (VLF) electromagnetic method utilizes primary signals (field) transmitted from worldwide distant transmitters located in coastal areas. These transmitters are meant for long distance marine communication. VLF transmitters operate at a low communication frequency band (between 5–30 kHz) and the transmitted signal travels a long distance. Transmitted signals penetrate the Earth’s subsurface and produce electromagnetic induction in the subsurface even several thousands of kilometres away from the transmitters. The VLF method is quite simple and frequently used in the delineation of near‐surface conducting structures of various practical applications. Several conducting structures lying along a measured profile with different conductivities can be properly induced at distinct frequencies that yield the maximum response. Therefore, such conductors may not be identified or resolved well using single frequency VLF measurement. A 2D numerical modelling study was carried out over a wide frequency range (1–500 kHz) to find the frequency that produces the maximum response for a given conductor. Results show that a particular frequency (focusing frequency) produces the maximum (peak) response for a conductor. When the measuring frequency either increases or decreases with respect to the focusing frequency, then the peak response always decreases. The focusing frequency remains almost similar with an increase in target depth and host resistivity. An increase in the overburden conductivity shows a decline in the focusing frequency. Two or more targets of different conductivity present in the subsurface yield peak responses at corresponding focusing frequencies. This shows that they will be resolved well at corresponding focusing frequencies. In such circumstances, inversion using single frequency VLF data yields inaccurate results. However, the use of multi‐frequency VLF data yields better results. Inversion of multi‐frequency VLF data is presented to show the efficacy of the approach. A field measurement is also presented and the effectiveness of multi‐frequency VLF measurement is highlighted. Since the numerical modelling study is performed over a broad frequency range covering the VLF and radiomagnetotelluric signal, the focusing study is valid for radiomagnetotelluric applications as well.
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Overburden
Extremely low frequency
Radio spectrum
Economic geology
SIGNAL (programming language)
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A theoretical model of a new type of two-degree-of-freedom local resonance mass-spring system with the resonator considering vertical and rotational vibration (the moment of inertia) is proposed to generate a negative effective mass over specific frequency ranges. Based on the theoretical analysis, a novel elastic metamaterial plate is designed. The flexural vibration band structures as well as the transmission spectrum of the metamaterial plate are investigated by using the finite element method. Subsequently, the formation mechanism of the band gaps is analyzed by studying the displacement field of the eigenmodes at the band gap edges. At last, the effects of the geometrical parameters on the flexural vibration band gaps (FVBGs) are studied and discussed in detail. The related results well confirm that the two-degree-of-freedom metamaterial shows negative mass density in two distinctive asymptotic regions, and the proposed elastic metamaterial plate has two FVBGs with the total width of 94.45 Hz below 200 Hz. The magnitude of torques introduced into the local resonance system can obviously affect the locations of the FVBGs. For the elastic metamaterial plate with thick local resonance lead plate and weak rubber is appropriate to obtain a lower gap, but the total width of the FVBGs becomes narrow. However, it does just the opposite for the condition with thin local resonance lead plate and strong rubber. For the double-side stubbed plate, it can enhance the coupling of the flexural traveling wave and the local resonance modes, which can greatly enlarge the bandwidths. The new working mechanism and the related calculation results of the designed structures would provide an effective way for elastic metamaterial plate to broaden the FVBGs at low frequencies, which has potential applications in low-frequency vibration and noise attenuation.
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This paper designs a miniaturized high-transmission and low-absorption frequency selection Rasorber based on X-band. The frequency selection Rasorber (FSR) is composed of a periodic array of cell structures, and each cell is composed of an upper resistance layer and a lower band-pass frequency selection surface. . At low frequencies, the FSR acts as an absorber, with a 10dB absorption bandwidth of 2.1-6.2GHZ. At high frequencies, the transmission band is almost transparent, allowing signals to pass through, and a 1dB transmission bandwidth is 7.9-12.8GHZ. The FSR in this paper has a wide high-frequency transmission band and low-frequency absorption band. In addition, the miniaturized design makes the FSR have better angular stability.
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Center frequency
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Although several researchers have reported that focal brain cooling (FBC) terminates epileptic seizure, a frequency analysis about an electroencephalogram (EEG) has not yet been precisely performed and it is not clear whether a dominant frequency band may exist or not during FBC. In this paper, we examined the frequency bands about EEG during both the epileptic discharges and FBC. The results for the frequency analysis of EEG in a cortical seizure model and patients with epilepsy showed that Alpha waves and Beta waves significantly increased in the power spectrums of epileptic waves. In analyses of EEG power spectrums in a cortical seizure model, we found that such a high-frequency component as Alpha wave and Beta wave decreases earlier than a low-frequency component. These results denoted that control of a frequency band may be related to the region of FBC. Simulation results using Pennes's Bio-heat equation suggested that one can limit a cooling region under a setting temperature to realize control of a frequency band using FBC.
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Radio spectrum
Frequency analysis
Alpha (finance)
BETA (programming language)
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The phenomenon that frequency decreases and amplitude increases near the bottom of a gas layer on a seismic profile is called a low-frequency shadow, but this phenomenon may not occur in all gas reservoirs. When the tight gas reservoir is thick enough, spectral decomposition data after Fourier transformation will indicate characteristics similar to those of low-frequency shadows, which we call generalized low-frequency shadows. Compared to the dominant frequency of the nongas-bearing zone spectral, the dominant frequency of a gas zone moves toward the low end of the frequency range and the low-frequency amplitude increases accordingly. By analyzing known gas reservoirs such as the Sulige and Yanchang tight sandstones in the Ordos Basin and tight carbonate rocks in the Tarim Basin, we can see that, with the visual dominant seismic frequency close to 30 Hz, the peak frequency of the gas-bearing tight sandstones and tight dolomite reservoirs will move from approximately 30 to 10–15 Hz. There is a certain correlation among the drop of the dominant frequency of a tight gas reservoir, the attenuation energy difference, and the thickness and productivity of the gas layer. Several cases indicate that nearly all tight gas layers thicker than 15 m exhibit attenuation characteristics of generalized low-frequency shadows.
Tight gas
Frequency band
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