Experiments for multibeam Backscatter Adjustments on the NOAA Ship FAIRWEATHER

A series of experiments were conducted to adjust and normalize the acoustic backscatter acquired by Reson 8111 and 8160 systems. The dependency of the backscatter on the receiver gain, transmit power, pulse width and acquisition mode was analyzed. Empirical beam patterns are calculated as the difference between the backscatter measured by the sonars and the expected backscatter. Expected acoustic backscatter is estimated based on a mathematical model

Clustering Acoustic Backscatter in the Angular Response Space

Backscatter mosaicking is a necessary step in the analysis and interpretation of sidescan and multibeam sonar records. However, due to limitations intrinsic to the mosaicking technique, backscatter mosaics are restricted in their capacity to unambiguously discriminate seafloor properties. A more adequate technique to characterize the seafloor is the analysis of backscatter angular responses, since those responses are intrinsic properties of the seafloor. This technique sometimes lacks spatial resolution, however, as the analysis is limited to the swath width of the sonar. In this paper, we propose an approach to combine mosaicking and angular response analysis techniques in an attempt to take advantage of both the spatial resolution of the mosaic, and the angular resolution derived from the angular response analysis. In order to test these ideas, we used acoustic backscatter acquired by a Reson 8101 (240kHz) multibeam sonar during normal survey operations conducted on the NOAA Ship FAIRWEATHER around Cape Decision, Alaska in spring 2005. First, we defined parameters that uniquely described the angular responses, and treated those parameters as a feature vector in a multidimensional space. The parameters were then clustered with a simple unsupervised clustering algorithm. The result of the clustering analysis defined areas on the seafloor which had similar angular responses, which we called themes. We then used these themes to develop more robust indicators of angular response from their coverage areas, which were finally used as Angle Varying Gain correction tables to assemble an enhanced mosaic

DC magnetic field generation in unmagnetized shear flows

The generation of DC magnetic fields in unmagnetized plasmas with velocity shear is predicted for non relativistic and relativistic scenarios either due to thermal effects or due to the onset of the Kelvin-Helmholtz instability (KHI). A kinetic model describes the growth and the saturation of the DC field. The predictions of the theory are confirmed by multidimensional particle-in-cell simulations, demonstrating the formation of long lived magnetic fields ($t \sim 100s \omega_{pi}^{-1}$) along the full longitudinal extent of the shear layer, with transverse width on the electron length scale ($\sqrt{\gamma_0}c/\omega_{pe}$), reaching magnitudes $eB_{\mathrm{DC}}/m_ec\omega_{pe}\sim \beta_0\sqrt{\gamma_0}$

A conceptual design of an advanced 23 m diameter IACT of 50 tons for ground-based gamma-ray astronomy

A conceptual design of an advanced Imaging Air Cherenkov Telescope with a 23 m diameter mirror and of 50 tons weight will be presented. A system photon detection efficiency of 15-17%, averaged over 300-600 nm, is aimed at to lower the threshold to 10-20 GeV. Prospects for a second generation camera with Geiger-mode Avalanche Photo Diodes will be discussed.Comment: 4 pages, 1 figure, to appear in the proceedings of the 31th International Cosmic Ray Conference, Lodz, Poland, 200

The high-frequency backscattering angular response of gassy sediments: Model/data comparison from the Eel River Margin, California

A model for the high-frequency backscatter angular response of gassy sediments is proposed. For the interface backscatter contribution we adopted the model developed by Jackson et al. @J. Acoust. Soc. Am. 79, 1410–1422 ~1986!#, but added modifications to accommodate gas bubbles. The model parameters that are affected by gas content are the density ratio, the sound speed ratio, and the loss parameter. For the volume backscatter contribution we developed a model based on the presence and distribution of gas in the sediment. We treat the bubbles as individual discrete scatterers that sum to the total bubble contribution. This total bubble contribution is then added to the volume contribution of other scatters. The presence of gas affects both the interface and the volume contribution of the backscatter angular response in a complex way that is dependent on both grain size and water depth. The backscatter response of fine-grained gassy sediments is dominated by the volume contribution while that of coarser-grained gassy sediments is affected by both volume and interface contributions. In deep water the interface backscatter is only slightly affected by the presence of gas while the volume scattering is strongly affected. In shallow water the interface backscatter is severely reduced in the presence of gas while the volume backscatter is only slightly increased. Multibeam data acquired offshore northern California at 95 kHz provides raw measurements for the backscatter as a function of grazing angle. These raw backscatter measurements are then reduced to scattering strength for comparison with the results of the proposed model. The analysis of core samples at various locations provides local measurements of physical properties and gas content in the sediments that, when compared to the model, show general agreement

Hybridizing Non-dominated Sorting Algorithms: Divide-and-Conquer Meets Best Order Sort

Many production-grade algorithms benefit from combining an asymptotically efficient algorithm for solving big problem instances, by splitting them into smaller ones, and an asymptotically inefficient algorithm with a very small implementation constant for solving small subproblems. A well-known example is stable sorting, where mergesort is often combined with insertion sort to achieve a constant but noticeable speed-up. We apply this idea to non-dominated sorting. Namely, we combine the divide-and-conquer algorithm, which has the currently best known asymptotic runtime of $O(N (\log N)^{M - 1})$, with the Best Order Sort algorithm, which has the runtime of $O(N^2 M)$ but demonstrates the best practical performance out of quadratic algorithms. Empirical evaluation shows that the hybrid's running time is typically not worse than of both original algorithms, while for large numbers of points it outperforms them by at least 20%. For smaller numbers of objectives, the speedup can be as large as four times.Comment: A two-page abstract of this paper will appear in the proceedings companion of the 2017 Genetic and Evolutionary Computation Conference (GECCO 2017

Electron-scale shear instabilities: magnetic field generation and particle acceleration in astrophysical jets

Strong shear flow regions found in astrophysical jets are shown to be important dissipation regions, where the shear flow kinetic energy is converted into electric and magnetic field energy via shear instabilities. The emergence of these self-consistent fields make shear flows significant sites for radiation emission and particle acceleration. We focus on electron-scale instabilities, namely the collisionless, unmagnetized Kelvin-Helmholtz instability (KHI) and a large-scale dc magnetic field generation mechanism on the electron scales. We show that these processes are important candidates to generate magnetic fields in the presence of strong velocity shears, which may naturally originate in energetic matter outburst of active galactic nuclei and gamma-ray bursters. We show that the KHI is robust to density jumps between shearing flows, thus operating in various scenarios with different density contrasts. Multidimensional particle-in-cell (PIC) simulations of the KHI, performed with OSIRIS, reveal the emergence of a strong and large-scale dc magnetic field component, which is not captured by the standard linear fluid theory. This dc component arises from kinetic effects associated with the thermal expansion of electrons of one flow into the other across the shear layer, whilst ions remain unperturbed due to their inertia. The electron expansion forms dc current sheets, which induce a dc magnetic field. Our results indicate that most of the electromagnetic energy developed in the KHI is stored in the dc component, reaching values of equipartition on the order of $10^{-3}$ in the electron time-scale, and persists longer than the proton time-scale. Particle scattering/acceleration in the self generated fields of these shear flow instabilities is also analyzed

Geocoder: An Efficient Backscatter Map Constructor

The acoustic backscatter acquired by multibeam and sidescan sonars carries important information about the seafloor morphology and physical properties, providing valuable data to aid the difficult task of seafloor characterization, and important auxiliary information for a bathymetric survey. One necessary step towards this characterization is the assemblage of more consistent and more accurate mosaics of acoustic backscatter. For that, it is necessary to radiometrically correct the backscatter intensities registered by these sonars, to geometrically correct and position each acoustic sample in a projection coordinate system and to interpolate properly the intensity values into a final backscatter map. Geocoder is a software tool that implements the ideas discussed above. Initially, the original backscatter time series registered by the sonar is corrected for angle varying gains, for beam pattern and filtered for speckle removal. All samples of the time series are preserved during all the operations, ensuring that the full data resolution is used for the final mosaicking. The time serie s is then slant-range corrected based on a bathymetric model, in the case of sidescan, or based on beam bathymetry, in the case of the multibeam. Subsequently, each backscatter sample of the series is geocoded in a projected coordinate system in accordance to an interpolation scheme that resembles the acquisition geometry. An anti-aliasing algorithm is applied in parallel to the mosaicking procedure, which allows the assemblage of mosaics at any required resolution. Overlap among parallel lines is resolved by a priority table based on the distance of each sample from the ship track; a blending algorithm is applied to minimize the seams between overlapping lines. The final mosaic exhibits low noise, few artifacts, reduced seams between parallel acquisition lines and reduced clutter in the near-nadir region, while still preserving regional data continuity and local seafloor features