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Point source

About: Point source is a research topic. Over the lifetime, 5077 publications have been published within this topic receiving 94091 citations.


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TL;DR: In this article, an interval average and iterative approach is proposed to estimate the normal mode parameter Km (the horizontal component of mode wavenumber) by using a modified Prony method.
Abstract: Horizontal samples of a point source in a shallow water waveguide are processed by the Prony method. First, when the source position is known, a high resolution estimation of the normal mode parameter Km (the horizontal component of mode wavenumber) can be obtained. Owing to the weak damping process, βm, the modal attenuation coefficient, can be obtained by using a modified Prony method. In this article, an ‘‘interval average and iterative’’ approach is proposed. Numerical examples illustrate that a 10−4 resolution of Km (150‐Hz frequency) is provided by a 500‐m data length, roughly an order of magnitude smaller than the data length required by the linear spectrum analysis technique. Second, when the source position is unknown but the waveguide characters (Km, βm) are known, the same procedure can be used for source position estimation. Some numerical examples in typical shallow water are presented.

29 citations

Journal ArticleDOI
TL;DR: It is found that all methane plumes were detectable in all satellite simulations and that coarsening spatial resolution had the largest impact on the sensitivity of the results.
Abstract: This study evaluates a new generation of satellite imaging spectrometers to measure point source methane emissions from anthropogenic sources. We used the Airborne Visible and Infrared Imaging Spectrometer Next Generation(AVIRIS-NG) images with known methane plumes to create two simulated satellite products. One simulation had a 30 m spatial resolution with ~200 Signal-to-Noise Ratio (SNR) in the Shortwave Infrared (SWIR) and the other had a 60 m spatial resolution with ~400 SNR in the SWIR; both products had a 7.5 nm spectral spacing. We applied a linear matched filter with a sparsity prior and an albedo correction to detect and quantify the methane emission in the original AVIRIS-NG images and in both satellite simulations. We also calculated an emission flux for all images. We found that all methane plumes were detectable in all satellite simulations. The flux calculations for the simulated satellite images correlated well with the calculated flux for the original AVIRIS-NG images. We also found that coarsening spatial resolution had the largest impact on the sensitivity of the results. These results suggest that methane detection and quantification of point sources will be possible with the next generation of satellite imaging spectrometers.

29 citations

Journal ArticleDOI
TL;DR: In this paper, the spectra were well-fitted by an absorbed power law, with photon indices r = 154 + 0 4 2 - 0 3 7 (readout streak) and r = 142 + 0 0 1 4 2 − 0 0 5 7 (annulus) consistent with a black hole low/hard state.
Abstract: In 2000 August, we observed the black hole candidate 1E 17407-2942, the brightest persistent hard X-ray source within a few degrees of the Galactic Centre, for 10 ks with Chandra (ACIS-I) Attempting to compensate for pile-up effects, we found that the spectra were well-fitted by an absorbed power law, with photon indices r = 154 + 0 4 2 - 0 3 7 (readout streak) and r = 142 + 0 1 4 - 0 1 4 (annulus), consistent with a black hole low/hard state We have analysed a public observation performed by Chandra which utilized short frames in order to avoid severe pile-up effects Subtracting the core point spread function from the whole image, we did not find evidence for any elongated feature perpendicular to the radio jet axis, as reported in a recent analysis of the same data Moreover, comparing the radial profiles with those of an unscattered X-ray point source, we found an indication of an extended, previously undetected, X-ray scattering halo The measured halo fractional intensity at 3 keV is between 30 and 40 per cent within 40 arcsec but drops below detectable levels at 5 keV Finally, by placing a limit on the X-ray flux from the radio-emitting lobe, which has been identified as the hotspot at the end of the northern jet of 1E 17407-2942, we are able to constrain the magnetic energy density in that region

29 citations

Journal ArticleDOI
TL;DR: In this article, the intensity covariance of a point source of waves propagating in a random medium is calculated by using the asymptotic theory (strong scattering regime) and a Kolmogorov refractive-index spectrum with an inner-scale cutoff.
Abstract: The intensity covariance of a point source of waves propagating in a random medium is calculated by using the asymptotic theory (strong-scattering regime) and a Kolmogorov refractive-index spectrum with an inner-scale cutoff. The variance and small-scale correlation scale are both larger than predicted for zero inner scale. The behavior of the small-scale structure is dominated by irregularities near the receiver, whereas the behavior of the large-scale structure is dominated by irregularities near the transmitter. The theoretical predictions are compared with experimental results. Higher terms in the asymptotic theory are required for intensity covariance to be predicted adequately. Saturation of intensity fluctuations (variance of unity) is unlikely for laser propagation in typical atmospheric conditions. The calculation of intensity covariance in an extended medium cannot be simplified by substituting an equivalent thin screen in the midpoint of the propagation path.

29 citations

Journal ArticleDOI
TL;DR: In this paper, a method of determining simultaneously the moment tensor and source time function in the point source approximation is presented, where a good correlation of both synthetic and observed records is reached.
Abstract: A method of determining simultaneously the moment tensor and source time function in the point source approximation is presented. For trial values of the moment tensor components and of the source time function, parametrized by the sum of overlapping triangles delayed in time, theoretical seismograms can be synthetized and compared with the recorded ones. The iterative procedure determines the adjustment of source parameters until a good correlation of both synthetic and observed records is reached. The Green functions in a horizontally stratified medium are constructed with the use of a modal summation method. The limits of applicability of the algorithm are illustrated by the inversion of four synthetic seismograms constructed for two horizontally stratified models of the structure in Friuli area, Italy. The records constructed for the same structural model as for which the Green functions were computed can be inverted even in the high-frequency range. In the opposite case, when the records and Green functions used corresponded to different structural models, a good correlation of the input records with the final synthetics was obtained for low - pass filtered data only. Additional tests performed with input seismograms contaminated with random noise yielded good resolution of the moment tensor and the duration of the source time function even for a high noise to signal ratio.

29 citations


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Performance
Metrics
No. of papers in the topic in previous years
YearPapers
202350
2022133
2021103
2020135
2019123
2018133