Pre-stack full waveform inversion of ultra-high-frequency marine seismic reflection data

The full waveform inversion (FWI) of seismic reflection data aims to reconstruct a detailed physical properties model of the subsurface, fitting both the amplitude and the traveltime of the reflections generated at physical discontinuities in the propagation medium. Unlike reservoir-scale seismic exploration, where seismic inversion is a widely adopted remote characterization tool, ultrahigh-frequency (UHF, 0.2–4.0 kHz) multichannel marine reflection seismology is still most often limited to a qualitative interpretation of the reflections’ architecture. Here we propose an elastic FWI methodology, custom-tailored for pre-stack UHF marine data in vertically heterogeneous media to obtain a decimetric-scale distribution of P-impedance, density and Poisson’s ratio within the shallow subseabed sediments. We address the deterministic multiparameter inversion in a sequential fashion. The complex trace instantaneous phase is first inverted for the P-wave velocity to make up for the lack of low frequency in the data and reduce the nonlinearity of the problem. This is followed by a short-offset P-impedance optimization and a further step of full offset range Poisson’s ratio inversion. Provided that the seismogram contains wide reflection angles (>40°), we show that it is possible to invert for density and decompose a posteriori the relative contribution of P-wave velocity and density to the P-impedance. A broad range of synthetic tests is used to prove the potential of the methodology and highlights sensitivity issues specific to UHF seismic. An example application to real data is also presented. In the real case, trace normalization is applied to minimize the systematic error deriving from an inaccurate source wavelet estimation. The inverted model for the top 15 m of the subseabed agrees with the local lithological information and core-log data. Thus, we can obtain a detailed remote characterization of the shallow sediments using a multichannel sub-bottom profiler within a reasonable computing cost and with minimal pre-processing. This has the potential to reduce the need of extensive geotechnical coring campaigns

Gravitational Lensing Statistics in Universes Dominated by Dark Energy

We study lens statistics in flat, low-density universes with different equations of state $w=p_Q/\rho_Q$ for the dark energy component. Dark energy modifies the distance-redshift relation and the mass function of dark matter halos leading to changes in the lensing optical depth as a function of image separation. Those effects must, however, be distinguished from effects associated with the structure of dark matter halos. Baryonic cooling causes galaxy-mass halos to have different central density profiles than group- and cluster-mass halos, which causes the distribution of normal arcsecond-scale lenses to differ from the distribution of ``wide-separation'' (\Delta\theta \gtrsim 4\arcsec) lenses. Fortunately, the various parameters related to cosmology and halo structure have very different effects on the overall image separation distribution: (1) the abundance of wide-separation lenses is exremely sensitive (by orders of magnitude) to the distribution of ``concentration'' parameters for massive halos modeled with the Navarro-Frenk-White profile; (2) the transition between normal and wide-separation lenses depends mainly on the mass scale where baryonic cooling ceases to be efficient; and (3) dark energy has effects at all image separation scales. While current lens samples cannot usefully constrain all of the parameters, ongoing and future imaging surveys should discover hundreds or thousands of lenses and make it possible to disentangle the various effects and constrain all of the parameters simultaneously. (abridged)Comment: 15 pages, 11 figures, accepted for publication in Ap

Redshifts of CLASS Radio Sources

Spectroscopic observations of a sample of 42 flat-spectrum radio sources from the Cosmic Lens All-Sky Survey (CLASS) have yielded a mean redshift of $= 1.27$ with an RMS spread of 0.95, at a completeness level of 64%. The sample consists of sources with a 5-GHz flux density of 25-50 mJy, making it the faintest flat-spectrum radio sample for which the redshift distribution has been studied. The spectra, obtained with the Willam Herschel Telescope (WHT), consist mainly of broad-line quasars at $z>1$ and narrow-line galaxies at $z<0.5$. Though the mean redshift of flat-spectrum radio sources exhibits little variation over more than two orders of magnitude in radio flux density, there is evidence for a decreasing fraction of quasars at weaker flux levels. In this paper we present the results of our spectroscopic observations, and discuss the implications for constraining cosmological parameters with statistical analyses of the CLASS survey.Comment: 10 pages, AJ accepte

The UCSD Radio-Selected Quasar Survey for Damped Lyman alpha System

As large optical quasar surveys for damped Lya become a reality and the study of star forming gas in the early Universe achieves statistical robustness, it is now vital to identify and quantify the sources of systematic error. Because the nature of optically-selected quasar surveys makes them vulnerable to dust obscuration, we have undertaken a radio-selected quasar survey for damped Lya systems to address this bias. We present the definition and results of this survey. We then combine our sample with the CORALS dataset to investigate the HI column density distribution function f(N) of damped Lya systems toward radio-selected quasars. We find that f(N) is well fit by a power-law f(N) = k_1 N^alpha_1, with log k_1 = 22.90 and alpha_1 = -2.18. This power-law is in excellent agreement with that of optically-selected samples at low N(HI), an important yet expected result given that obscuration should have negligible effect at these gas columns. However, because of the relatively small size of the radio-selected sample, 26 damped Lya systems in 119 quasars, f(N) is not well constrained at large N(HI) and the first moment of the HI distribution function, Omega_g, is, strictly speaking, a lower limit. The power-law is steep enough, however, that extrapolating it to higher column densities implies only a modest, logarithmic increase in Omega_g. The radio-selected value of Omega_g = 1.15 x 10^-3, agrees well with the results of optically-selected surveys. While our results indicate that dust obscuration is likely not a major issue for surveys of damped Lya systems, we estimate that a radio-selected sample of approximately 100 damped Lya systems will be required to obtain the precision necessary to absolutely confirm an absence of dust bias.Comment: 12 pages, 9 Figures. Accepted to ApJ April 11, 200

“Contagious Accountability”: A Global Multisite Randomized Controlled Trial on the Effect of Police Body-Worn Cameras on Citizens’ Complaints Against the Police

The use of body-worn cameras (BWCs) by the police is rising. One proposed effect of BWCs is reducing complaints against police, which assumes that BWCs reduce officer noncompliance with procedures, improve suspects’ demeanor, or both, leading to fewer complaints. We report results from a global, multisite randomized controlled trial on whether BWC use reduces citizens’ complaints. Seven discrete tests ( N = 1,847 officers), with police shifts as the unit of analysis ( N = 4,264), were randomly assigned into treatment and control conditions. Using a prospective meta-analytic approach, we found a 93% before–after reduction in complaint incidence ( Z = −3.234; p &lt; .001), but no significant differences between trial arms in the studies ( d = .053, SE = .11; 95% confidence interval [CI] = [−.163, .269]), and little between-site variation ( Q = 4.905; p = .428). We discuss these results in terms of an “observer effect” that influences both officers’ and citizens’ behavior and assess what we interpret as treatment diffusion between experimental and control conditions within the framework of “contagious accountability.” This is the author accepted manuscript. The final version is available from SAGE Publications via http://dx.doi.org/10.1177/009385481666821

A multi-epoch VLBI survey of the kinematics of CFJ sources

Context. This is the second in a series of papers presenting VLBI observations of the 293 Caltech-Jodrell Bank Flat-spectrum (hereafter CJF) sources and their analysis. Aims. We obtain a consistent motion dataset large enough to allow the systematic properties of the population to be studied. Methods. We present detailed kinematic analysis of the complete flux-density limited CJF survey. We computed 2D kinematic models based on the optimal model-fitting parameters of multi-epoch VLBA observations. This allows us to calculate not only radial, but also orthogonal motions, and thus to study curvature and acceleration. Statistical tests of the motions measured and their reliability were performed. A correlation analysis between the derived apparent motions, luminosities, spectral indices, and core dominance and the resulting consequences is described. Results. With at least one velocity in each of the 237 sources, this sample is much larger than any available before, so it allows a meaningful statistical investigation of apparent motions and any possible correlations with other parameters in AGN jets. The main results to emerge are as follows: - In general motions are not consistent with a single uniform velocity applicable to all components along a jet. - We find a slight trend towards a positive outward acceleration and also adduce some evidence for greater acceleration in the innermost regions. - We find a lack of fast components at physical distances less than a few pc from the reference feature. - Only ~4% of the components from galaxies and <2% of those from quasars undergo large bends i.e. within 15° of ± 90°. - The distribution of radial velocities shows a broad distribution of velocities (apparent velocities up to 30 c). Fifteen percent of the best-sampled jet components exhibit low velocities that may need to be explained in a different manner to the fast motions. - Some negative superluminal motions are seen, and in 15 cases (6%) these are definitely significant. - We find a strong correlation between the 5 GHz luminosity and the apparent velocity. - The CJF galaxies, on average, show slower apparent jet-component velocities than the quasars. - The mean velocity in the VLBA 2 cm survey (Kellermann et al. 2004, ApJ, 609, 539) is substantially higher than in the CJF survey, the ratio could be roughly a factor of 1.5-2. This supports the observed trend toward increasing apparent velocity with increasing observing frequency. Conclusions. This AGN survey provides the basis for any statistical analysis of jet and jet-component properties

Wearing body cameras increases assaults against officers and does not reduce police use of force: Results from a global multi-site experiment

Police use of force is at the forefront of public awareness in many countries. Body-worn videos (BWVs) have been proposed as a new way of reducing police use of force, as well as assaults against officers. To date, only a handful of peer-reviewed randomised trials have looked at the effectiveness of BWVs, primarily focusing on use of force and complaints. We sought to replicate these studies, adding assaults against police officers as an additional outcome. Using a prospective meta-analysis of multi-site, multi-national randomised controlled trials from 10 discrete tests with a total population of +2 million, and 2.2 million police officer-hours, we assess the effect of BWVs on the rates of (i) police use of force and (ii) assaults against officers. Averaged over 10 trials, BWVs had no effect on police use of force ( d = 0.021; SE = 0.056; 95% CI: –0.089–0.130), but led to an increased rate of assaults against officers wearing cameras (d = 0.176; SE = 0.058; 95% CI: 0.061–0.290). As there is evidence that cameras may increase the risk of assaults against officers, more attention should be paid to how these devices are implemented. Likewise, since other public-facing organisations are considering equipping their staff with BWVs (e.g. firefighters, private security, traffic wardens), the findings on risks associated with BWVs are transferrable to those occupations as well. This is the final version of the article. It first appeared from Sage via https://doi.org/10.1177/1477370816643734

Contrasting Decollement and Prism Properties over the Sumatra 2004-2005 Earthquake Rupture Boundary

Styles of subduction zone deformation and earthquake rupture dynamics are strongly linked, jointly influencing hazard potential. Seismic reflection profiles across the trench west of Sumatra, Indonesia, show differences across the boundary between the major 2004 and 2005 plate interface earthquakes, which exhibited contrasting earthquake rupture and tsunami generation. In the southern part of the 2004 rupture, we interpret a negative-polarity sedimentary reflector ~500 meters above the subducting oceanic basement as the seaward extension of the plate interface. This predécollement reflector corresponds to unusual prism structure, morphology, and seismogenic behavior that are absent along the 2005 rupture zone. Although margins like the 2004 rupture zone are globally rare, our results suggest that sediment properties influence earthquake rupture, tsunami hazard, and prism development at subducting plate boundaries