scholarly journals A seismic vertical vibrator driven by linear synchronous motors

Geophysics ◽  
2015 ◽  
Vol 80 (2) ◽  
pp. EN57-EN67 ◽  
Author(s):  
Rik Noorlandt ◽  
Guy Drijkoningen ◽  
Johan Dams ◽  
Rob Jenneskens
Keyword(s):  
Low Frequency ◽  
Field Tests ◽  
Seismic Source ◽  
Neutral Position ◽  
Low Frequencies ◽  
Synchronous Motors ◽  
Frequency Regime ◽  
Linear Synchronous Motors ◽  
Spring Mechanism ◽  
The Individual

A linear synchronous motor (LSM) is an electric motor that can produce large controllable forces and is therefore suitable as a driving engine for a seismic vibrator. This motor consists of two independent elements, a magnet track and a coil track, allowing practically unlimited motor displacements. This makes the LSM very suitable for expanding the source frequency band to the lower frequencies in which larger strokes are needed. In contrast to hydraulic engines, the LSM performs equally well over the whole frequency range, making possible a smaller amount of signal distortion, especially at the low frequencies. To find the feasibility of an LSM-driven vibrator, we successfully designed and built a multi-LSM prototype vibrator of some 1200 kg. We addressed the synchronization between the individual motor tracks and the different motors. To lower the energy consumption, a spring mechanism was implemented that delivered the force needed to lift the vibrator mass to its neutral position. The resonance belonging to this spring mechanism was successfully suppressed with the help of a position feedback control that also suppressed the temperature effects. The seismic data acquired in the field tests proved that the prototype LSM vibrator acted very well as a seismic source. It has no trouble generating pseudorandom sweeps, and even given its limited size, it generated signals within the low-frequency regime, down to 2 Hz, rather easily.

scholarly journals ACOUSTIC BAND GAP FORMATION IN METAMATERIALS

2010 ◽  
Vol 24 (25n26) ◽  
pp. 4935-4945 ◽  
Author(s):  
D. P. ELFORD ◽  
L. CHALMERS ◽  
F. KUSMARTSEV ◽  
G. M. SWALLOWE
Keyword(s):  
Band Gap ◽  
Lattice Constant ◽  
Low Frequency ◽  
Band Gaps ◽  
Gap Formation ◽  
Individual Band ◽  
Resonance Band ◽  
Frequency Regime ◽  
Sonic Crystal ◽  
The Individual

We present several new classes of metamaterials and/or locally resonant sonic crystal that are comprised of complex resonators. The proposed systems consist of multiple resonating inclusion that correspond to different excitation frequencies. This causes the formation of multiple overlapped resonance band gaps. We demonstrate theoretically and experimentally that the individual band gaps achieved, span a far greater range (≈ 2kHz) than previously reported cases. The position and width of the band gap is independent of the crystal's lattice constant and forms in the low frequency regime significantly below the conventional Bragg band gap. The broad envelope of individual resonance band gaps is attractive for sound proofing applications and furthermore the devices can be tailored to attenuate lower or higher frequency ranges, i.e., from seismic to ultrasonic.

scholarly journals Dipolar Excitation of a Perfectly Electrically Conducting Spheroid in a Lossless Medium at the Low-Frequency Regime

2018 ◽  
Vol 2018 ◽  
pp. 1-20 ◽  
Author(s):  
Panayiotis Vafeas
Keyword(s):  
Electric Fields ◽  
Vector Fields ◽  
Dimensional Space ◽  
Scattering Problem ◽  
Low Frequency ◽  
Main Idea ◽  
Wave Number ◽  
Low Frequencies ◽  
Static Approximation ◽  
Frequency Regime

The electromagnetic vector fields, which are scattered off a highly conductive spheroid that is embedded within an otherwise lossless medium, are investigated in this contribution. A time-harmonic magnetic dipolar source, located nearby and operating at low frequencies, serves as the excitation primary field, being arbitrarily orientated in the three-dimensional space. The main idea is to obtain an analytical solution of this scattering problem, using the appropriate system of spheroidal coordinates, such that a possibly fast numerical estimation of the scattered fields could be useful for real data inversion. To this end, incident and scattered as well as total fields are written in a rigorous low-frequency manner in terms of positive integral powers of the real-valued wave number of the exterior environment. Then, the Maxwell-type problem is converted to interconnected Laplace’s or Poisson’s equations, complemented by the perfectly conducting boundary conditions on the spheroidal object and the necessary radiation behavior at infinity. The static approximation and the three first dynamic contributors are sufficient for the present study, while terms of higher orders are neglected at the low-frequency regime. Henceforth, the 3D scattering boundary value problems are solved incrementally, whereas the determination of the unknown constant coefficients leads either to concrete expressions or to infinite linear algebraic systems, which can be readily solved by implementing standard cut-off techniques. The nonaxisymmetric scattered magnetic and electric fields follow and they are obtained in an analytical compact fashion via infinite series expansions in spheroidal eigenfunctions. In order to demonstrate the efficiency of our analytical approach, the results are degenerated so as to recover the spherical case, which validates this approach.

TEMPORAL FLUCTUATIONS IN BIORHYTHMS: EXPRESSION OF SELF-ORGANIZED CRITICALITY?

Fractals ◽  
1993 ◽  
Vol 01 (03) ◽  
pp. 380-387 ◽  
Author(s):  
KLAUS-D. KNIFFKI ◽  
WOLFGANG MANDEL ◽  
PHUOC TRAN-GIA
Keyword(s):  
Point Process ◽  
Critical State ◽  
Degrees Of Freedom ◽  
Low Frequency ◽  
Well Being ◽  
Scale Invariant ◽  
Low Frequencies ◽  
Temporal Fluctuations ◽  
Self Organized ◽  
The Individual

Recently, a general organizing principle has been reported connecting 1/f-noise with the self-similar scale-invariant ‘fractal’ properties in space, hence reflecting two sides of a coin, the so-called self-organized critical state. The basic idea is that dynamical systems with many degrees of freedom operate persistently far from equilibrium at or near a threshold of stability at the border of chaos. Temporal fluctuations which cannot be explained as consequences of statistically independent random events are found in a variety of physical and biological phenomena. The fluctuations of these systems can be characterized by a power spectrum density S(f) decaying as f−b at low frequencies with an exponent b<1.5. We present a new approach to describe the individual biorhythm of humans using data from a colleague who has kept daily records for two years of his state of well-being applying a fifty-point magnitude category scale. This time series was described as a point process by introducing two discriminating rating levels R for the occurrence of R≥40 and R≤10. For b<1 a new method to estimate the low frequency part of S(f) was applied using counting statistics without applying Fast Fourier Transform. The method applied reliably discriminates these types of fluctuations from a random point process, with b=0.0. It is very tempting to speculate that the neural mechanisms at various levels of the nervous system underlying the perception of different values of the subjective state of well-being, are expressions of a self-organized critical state.

scholarly journals Blazars in the LOFAR Two-Metre Sky Survey first data release

2019 ◽  
Vol 622 ◽  
pp. A14 ◽  
Author(s):  
S. Mooney ◽  
J. Quinn ◽  
J. R. Callingham ◽  
R. Morganti ◽  
K. Duncan ◽  
...  
Keyword(s):  
Spectral Index ◽  
Low Frequency ◽  
Value Added ◽  
Low Frequencies ◽  
Sample Distribution ◽  
Sky Survey ◽  
Data Release ◽  
Frequency Regime ◽  
Source Confusion ◽  
Bl Lacs

Historically, the blazar population has been poorly understood at low frequencies because survey sensitivity and angular resolution limitations have made it difficult to identify megahertz counterparts. We used the LOFAR Two-Metre Sky Survey (LoTSS) first data release value-added catalogue (LDR1) to study blazars in the low-frequency regime with unprecedented sensitivity and resolution. We identified radio counterparts to all 98 known sources from the Third Fermi-LAT Point Source Catalogue (3FGL) or Roma-BZCAT Multi-frequency Catalogue of Blazars (5th edition) that fall within the LDR1 footprint. Only the 3FGL unidentified γ-ray sources (UGS) could not be firmly associated with an LDR1 source; this was due to source confusion. We examined the redshift and radio luminosity distributions of our sample, finding flat-spectrum radio quasars (FSRQs) to be more distant and more luminous than BL Lacertae objects (BL Lacs) on average. Blazars are known to have flat spectra in the gigahertz regime but we found this to extend down to 144 MHz, where the radio spectral index, α, of our sample is −0.17 ± 0.14. For BL Lacs, α = −0.13 ± 0.16 and for FSRQs, α = −0.15 ± 0.17. We also investigated the radio-to-γ-ray connection for the 30 γ-ray-detected sources in our sample. We find Pearson’s correlation coefficient is 0.45 (p = 0.069). This tentative correlation and the flatness of the spectral index suggest that the beamed core emission contributes to the low-frequency flux density. We compare our sample distribution with that of the full LDR1 on colour-colour diagrams, and we use this information to identify possible radio counterparts to two of the four UGS within the LDR1 field. We will refine our results as LoTSS continues.

scholarly journals The Power of Low Frequencies: Faraday Tomography in the Sub-GHz Regime

Galaxies ◽  
2018 ◽  
Vol 6 (4) ◽  
pp. 112 ◽  
Author(s):  
Cameron Van Eck
Keyword(s):  
Magnetic Fields ◽  
Interstellar Medium ◽  
Frequency Dependence ◽  
Faraday Rotation ◽  
Low Frequency ◽  
Low Frequencies ◽  
Polarized Emission ◽  
Frequency Regime ◽  
Nearby Galaxies ◽  
Murchison Widefield Array

Faraday tomography, the study of the distribution of extended polarized emission by strength of Faraday rotation, is a powerful tool for studying magnetic fields in the interstellar medium of our Galaxy and nearby galaxies. The strong frequency dependence of Faraday rotation results in very different observational strengths and limitations for different frequency regimes. I discuss the role these effects take in Faraday tomography below 1 GHz, emphasizing the 100–200 MHz band observed by the Low Frequency Array and the Murchison Widefield Array. With that theoretical context, I review recent Faraday tomography results in this frequency regime, and discuss expectations for future observations.

On the generation of low frequencies with modern seismic vibrators

Geophysics ◽  
2013 ◽  
Vol 78 (2) ◽  
pp. WA91-WA97 ◽  
Author(s):  
Zhouhong Wei ◽  
Thomas F. Phillips
Keyword(s):  
Seismic Data ◽  
Harmonic Distortion ◽  
Low Frequency ◽  
Field Tests ◽  
Data Acquisition System ◽  
Limiting Factor ◽  
Hydraulic Pump ◽  
Force Output ◽  
Low Frequencies ◽  
Distortion Reduction

Acquiring low-frequency seismic data using Vibroseis techniques has gained attention in recent years. Successful application of low-frequency Vibroseis acquisition requires evaluating each element of the data acquisition system and ensuring that each part of the system contributes to the success of the method. We focus on generating low frequencies with Vibroseis sources rather than recording and preserving them. To generate the low-frequency signals using seismic vibrators, it is better to know (1) the hydraulic and mechanical system of the vibrator; (2) the design of a low-frequency sweep that can deliver the vibrator ground force maximally; and (3) the vibrator control system that can minimize the harmonic distortion of the vibrator ground force output at low frequencies. Field tests show that hydraulic pump flow is not a limiting factor for the generation of the low frequencies for most modern seismic vibrators. In addition, with harmonic distortion reduction control, the vibrator performance at low frequencies is improved.

Microparticle Trapping in Streaming Flows in Open Rectangular Chambers Undergoing Low Frequency Vertical Vibrations

2014 ◽  
Author(s):  
Prashant Agrawal ◽  
Prasanna S. Gandhi ◽  
Adrian Neild
Keyword(s):  
High Frequency ◽  
Low Frequency ◽  
Mode Switching ◽  
Small Particles ◽  
Low Frequencies ◽  
Different Types ◽  
Frequency Regime ◽  
Vertical Vibrations ◽  
Rectangular Chamber ◽  
Streaming Flows

Microparticle collection in microfluidic systems via mechanical vibrations has been demonstrated in both low frequency systems (in the range of 100Hz) and in the high frequency regime (in the range of 1MHz). However, in most systems, collection of particles with lower inertia is hindered by second order time-averaged streaming flows. In our approach, we experimentally demonstrate collection of small particles (about 3 μm in diameter) by trapping them in streaming flows in a liquid filled open rectangular chamber undergoing vertical vibrations at low frequencies. The proposed method is then utilized to separate two different types of particles in distinct patterns through mode-switching.

The Dielectric Properties of Systems Containing Straight Polar Chains

1952 ◽  
Vol 5 (1) ◽  
pp. 135
Author(s):  
RA Sack
Keyword(s):  
Dielectric Properties ◽  
Low Frequency ◽  
Hydroxyl Groups ◽  
Total Moment ◽  
Low Frequencies ◽  
Elementary Transition ◽  
A Chain ◽  
The Individual ◽  
High Dielectric ◽  
Mathematical Derivation

A mathematical derivation is given of the dielectric properties of systems containing straight polar chains such that to each value of the total moment of a chain there corresponds only one arrangement of its dipoles. If the moments of the individual dipoles and the probability of an elementary transition are fixed, both the total dielectric loss and the effective relaxation time of the system increase in proportion to the square of the number of states of each chain. These conclusions are not valid for kinked chains and apply only qualitatively if the chains are branched. The theory provides an explanation for the high dielectric losses at low frequencies observed in many solids containing hydroxyl groups. It can further explain the low frequency absorption found in ionic crystals containing lattice imperfections ; in this interpretation the theory is related to Jaffe's theory of conductivity in polarizable media.

What controls the initial peak of an air-gun source signature?

Geophysics ◽  
2019 ◽  
Vol 84 (2) ◽  
pp. P27-P45 ◽  
Author(s):  
Leighton M. Watson ◽  
Jonatan Werpers ◽  
Eric M. Dunham
Keyword(s):  
New York ◽  
Rise Time ◽  
Acoustic Waves ◽  
High Frequency ◽  
Low Frequency ◽  
Field Tests ◽  
Operating Pressure ◽  
Low Frequencies ◽  
Air Gun ◽  
Initial Peak

Seismic air guns are broadband sources that generate acoustic waves at many frequencies. The low-frequency waves can be used for imaging, whereas the high-frequency waves are attenuated and/or scattered before they can reflect from targets of interest in the subsurface. It is desirable to reduce the amplitude of the high-frequency acoustic waves because they are thought to be disruptive, and potentially damaging, to marine life and are not useful for geophysical purposes. The high-frequency acoustic waves are primarily associated with the initial expansion of the air-gun bubble and associated peak in the acoustic pressure time series, which is commonly referred to as the source signature of the air gun. We have developed a quasi-1D model of a seismic air gun coupled to a spherical bubble that accounts for gas dynamics and spatially variable depressurization inside the firing chamber to investigate controls on the initial peak of the source signature. The model is validated against data collected during field tests in Lake Seneca, New York. Simulations and field data show that the initial peak is primarily dependent on the operating pressure. A lower gun pressure results in a smaller peak amplitude and a slower rise time. The slope, the amplitude of the initial peak divided by the rise time, is used as a proxy for environmental impact and can decrease by as much as 50% when the air-gun pressure is reduced from 2000 to 1000 psi. The low frequencies are controlled by the total discharged mass, which is dependent upon the gun volume and pressure. Decreasing the operating pressure while simultaneously increasing the gun volume will reduce the high frequencies while maintaining the desirable low-frequency signals.

scholarly journals Earless toads sense low frequencies but miss the high notes

2017 ◽  
Vol 284 (1864) ◽  
pp. 20171670 ◽  
Author(s):  
Molly C. Womack ◽  
Jakob Christensen-Dalsgaard ◽  
Luis A. Coloma ◽  
Juan C. Chaparro ◽  
Kim L. Hoke
Keyword(s):  
High Frequency ◽  
Species Differences ◽  
Low Frequency ◽  
Auditory Brainstem ◽  
Low Frequencies ◽  
Hearing Sensitivity ◽  
Sensory Pathways ◽  
Airborne Sound ◽  
Vibrational Sensitivity ◽  
Species Specific

Sensory losses or reductions are frequently attributed to relaxed selection. However, anuran species have lost tympanic middle ears many times, despite anurans' use of acoustic communication and the benefit of middle ears for hearing airborne sound. Here we determine whether pre-existing alternative sensory pathways enable anurans lacking tympanic middle ears (termed earless anurans) to hear airborne sound as well as eared species or to better sense vibrations in the environment. We used auditory brainstem recordings to compare hearing and vibrational sensitivity among 10 species (six eared, four earless) within the Neotropical true toad family (Bufonidae). We found that species lacking middle ears are less sensitive to high-frequency sounds, however, low-frequency hearing and vibrational sensitivity are equivalent between eared and earless species. Furthermore, extratympanic hearing sensitivity varies among earless species, highlighting potential species differences in extratympanic hearing mechanisms. We argue that ancestral bufonids may have sufficient extratympanic hearing and vibrational sensitivity such that earless lineages tolerated the loss of high frequency hearing sensitivity by adopting species-specific behavioural strategies to detect conspecifics, predators and prey.

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