Modeling High-Frequency Seafloor Backscattering of Gassy Sediments: The Eel River Margin Case

Models of acoustic backscatter typically take into account two different processes: interface scattering and volume scattering. What happens to these two contributions when the sediment is charged with gas bubbles? For the interface backscatter contribution we adopted the model developed by Jackson et al. (1986), but added modifications to accommodate gas bubbles, which when present, even in very small quantities, can dominate the acoustic characteristics of the sediment. The model parameters that are affected by gas content are the density ratio, the sound speed ratio and the loss parameter. To a first approximation, the model roughness parameters are not influenced by the presence of gas. 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 point scatters that sum to the bubble contribution. This bubble contribution is then added to the volume contribution of other scatters. A potential area to test the ideas outlined above is the highly sedimented, tectonically active, Eel River margin offshore Northern California. This continental margin reveals evidence of abundant subsurface gas and numerous seafloor expulsion features, where a large volume of marine data has been acquired as part of the STRATAFORM project. Two different sets of multibeam backscatter data acquired at 30kHz and 95kHz provide raw measurements for the backscatter as a function of grazing angle. These raw backscatter measurements are then radiometrically corrected in order to be compared with the results of the proposed model. Radiometric corrections include the removal of the time varying and angle varying gains applied during acquisition, calculation of the true grazing angle with respect to a bathymetric model, and correction for footprint size. Results of core data analysis at various sampling locations provide local measurements of gas content in the sediments that when compared to the model show general agreement

A Patient-Centered Framework for Evaluating Digital Maturity of Health Services: A Systematic Review

© Kelsey Flott, Ryan Callahan, Ara Darzi, Erik Mayer.Background: Digital maturity is the extent to which digital technologies are used as enablers to deliver a high-quality health service. Extensive literature exists about how to assess the components of digital maturity, but it has not been used to design a comprehensive framework for evaluation. Consequently, the measurement systems that do exist are limited to evaluating digital programs within one service or care setting, meaning that digital maturity evaluation is not accounting for the needs of patients across their care pathways. Objective: The objective of our study was to identify the best methods and metrics for evaluating digital maturity and to create a novel, evidence-based tool for evaluating digital maturity across patient care pathways. Methods: We systematically reviewed the literature to find the best methods and metrics for evaluating digital maturity. We searched the PubMed database for all papers relevant to digital maturity evaluation. Papers were selected if they provided insight into how to appraise digital systems within the health service and if they indicated the factors that constitute or facilitate digital maturity. Papers were analyzed to identify methodology for evaluating digital maturity and indicators of digitally mature systems. We then used the resulting information about methodology to design an evaluation framework. Following that, the indicators of digital maturity were extracted and grouped into increasing levels of maturity and operationalized as metrics within the evaluation framework. Results: We identified 28 papers as relevant to evaluating digital maturity, from which we derived 5 themes. The first theme concerned general evaluation methodology for constructing the framework (7 papers). The following 4 themes were the increasing levels of digital maturity: resources and ability (6 papers), usage (7 papers), interoperability (3 papers), and impact (5 papers). The framework includes metrics for each of these levels at each stage of the typical patient care pathway. Conclusions: The framework uses a patient-centric model that departs from traditional service-specific measurements and allows for novel insights into how digital programs benefit patients across the health system

Seafloor Characterization Through the Application of AVO Analysis to Multibeam Sonar Data

In the seismic reflection method, it is well known that seismic amplitude varies with the offset between the seismic source and detector and that this variation is a key to the direct determination of lithology and pore fluid content of subsurface strata. Based on this fundamental property, amplitude-versus-offset (AVO) analysis has been used successfully in the oil industry for the exploration and characterization of subsurface reservoirs. Multibeam sonars acquire acoustic backscatter over a wide range of incidence angles and the variation of the backscatter with the angle of incidence is an intrinsic property of the seafloor. Building on this analogy, we have adapted an AVO-like approach for the analysis of acoustic backscatter from multibeam sonar data. The analysis starts with the beam-by-beam time-series of acoustic backscatter provided by the multibeam sonar and then corrects the backscatter for seafloor slope (i.e. true incidence angle), time varying and angle varying gains, and area of insonification. Once the geometric and radiometric corrections are made, a series of “AVO attributes” (e.g. near, far, slope, gradient, fluid factor, product, etc.) are calculated from the stacking of consecutive time series over a spatial scale that approximates half of the swath width (both along track and across track). Based on these calculated AVO attributes and the inversion of a modified Williams, K. L. (2001) acoustic backscatter model, we estimate the acoustic impedance, the roughness, and consequently the grain size of the insonified area on the seafloor. The inversion process is facilitated through the use of a simple, interactive graphical interface. In the process of this inversion, the relative behavior of the model parameters is constrained by established inter-property relationships. The approach has been tested using a 300 kHz Simrad EM3000 multibeam sonar in Little Bay, N.H., an area that we can easily access for ground-truth studies. AVO-derived impedance estimates are compared to in situ measurements of sound speed and AVO-derived grain-size estimates are compared to the direct measurement of grain size on grab samples. Both show a very good correlation indicating the potential of this approach for robust seafloor characterization

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

Das Wort sie sollen lassen stahn

Das Wort sie sollen lassen stahn (The word they should let sting

Swath Mapping on the Continental Shelf and Slope: The Eel River Basin, Northern California

First Paragraph The STRATAFORM program sponsored by the Office of Naval Research (Nittrouer and Kravitz, 1996, this issue) seeks to understand how sedimentary processes lead to the formation of the stratigraphic sequences on continental margins. A central challenge facing this effort is to understand the transport of sediments in shore-parallel as well as shore-perpendicular directions• Multidimensionality is necessary to describe, for example, the accumulation of sediments from river inputs, the distribution of gullies and canyons on the slope, the meandering of channels, and the structure of slumps and slides

Correction of Bathymetric Survey Artifacts Resulting from Apparent Wave-Induced Vertical Position of an AUV

Recent increases in the capability and reliability of autonomous underwater vehicles (AUVs) have provided the opportunity to conduct bathymetric seafloor surveys in shallow water (\u3c 50 m). Unfortunately, surveys of this water depth may contain artifacts induced by large amplitude wave motion at the surface. The artifacts occur when an onboard pressure sensor determines the depth of the AUV. Waves overhead induce small pressure fluctuations at depth, which modulate the AUV’s pressure sensor output without causing actual vertical movement of the AUV. Since bathymetric measurements are made with respect to the AUV’s depth, these pressure fluctuations, in turn, modulate the measurement of the seafloor. The result is a periodic across-track, vertical offset of the seafloor profile (similar to a heave artifact sometimes common in surface vessel surveys). In this paper we describe our experience with the “Gavia” model AUV (Hafmynd EHF, Iceland) in a recent bathymetric survey during which wave action overhead induced such an artifact with a peak-to-peak amplitude as large as 1 meter. A method for removing the artifact as well as recommendations for modifications to the sonar, INS and AUV to mitigate the effect in the future are provided