Mean, variance, and temporal coherence of the 3D acoustic field forward propagated through random inhomogeneities in continental-shelf and deep ocean waveguides

Thesis (Ph. D.)--Massachusetts Institute of Technology, Dept. of Mechanical Engineering, 2009.Includes bibliographical references (p. 165-175).When an acoustic field propagates through a multimodal waveguide, the effect of variations in medium properties induced by 3D random inhomogeneities accumulates by multiple forward scattering over range. This causes significant random fluctuations in the received field and greatly affects underwater acoustic sensing and communication systems, such as Ocean Acoustic Waveguide Remote Sensing(OAWRS). In order to characterize this effect, analytical expressions are derived for the mean, variance and temporal covariance of the acoustic field forward propagated through an ocean waveguide containing internal waves, fish shoals, wind-generated bubble clouds and krill. These expressions account for the accumulated effects of multiple forward scattering through temporally and spatially varying scatter function densities of the 3D inhomogeneities. In order to quantify the statistics of the scatter function densities, physical models and statistical descriptions of these inhomogeneities are developed.Acoustic field transmission through internal waves in both continental shelf and deep ocean waveguides is investigated. Stratified ocean models are used to describe physical and statistical internal waves properties. Simulations for a typical continental-shelf environment show that when the standard deviation of the internal wave displacement exceeds the acoustic wavelength, the acoustic forward field becomes so randomized that the expected total intensity is dominated by the variance field and lacks a the coherent interference structure beyond moderate ranges. This leads to an effectively saturated field that decays monotonically. It is found that 3D scattering effects become pronounced when the acoustic Fresnel width exceeds the cross-range coherence length of the internal waves. This leads to frequency and range-dependent power losses in the forward field that explains some of the attenuation observed in acoustic transmission through typical continental shelf and deep ocean waveguides.(cont.) A general analytical expression is derived for the temporal coherence of an acoustic signal propagating through an ocean waveguide with random 3D inhomogeneities.Advance knowledge of this coherence time scale is often essential in the design of ocean acoustic experiments and subsequent data analysis. This is because it determines the number of fluctuations in a given measurement period and the time window within which the coherent processing techniques essential to ocean acoustic data reduction and analysis can be applied. The analytic approach is found to explain the time scale of acoustic field fluctuations observed both at mega meters ranges in the deep ocean, as well as at kilometer ranges in continental shelf environments. The acoustic time scale is found to be much shorter than the coherence time scale of ocean internal waves. This is shown to be a consequence of multiple forward scattering of the acoustic waves through the internal waves. Analytical expressions are derived for the attenuation and dispersion of the acoustic field forward propagated through fish shoals and wind-generated bubble clouds in an ocean waveguide. It is found that at swim bladder resonance, fish shoals may sometimes lead to measurable attenuation in the forward field. The attenuation at off-resonant OAWRS frequencies, however, is typically negligible as shown both by the present theory and experimental data. The modeled attenuation due to random wind-generated bubble clouds is found to be highly sensitive to the choice of cutoff radius, which determines whether resonant bubbles are included in the bubble spectra. It is also found that bubble clouds generated under high wind speeds lead to additional dispersion and attention of the transmitted signal. These expected distortions can significantly degrade standard coherent processing techniques in ocean acoustics, such as the match filter, if not taken into account.(cont.) Antarctic krill play a key role in the marine food chain as the primary source of sustenance for many species of whales, seals, birds, squid and fish. This makes knowledge of the distribution and abundance of krill essential to ecological research in the southern oceans. It is shown that swarms of Antarctic krill with typical packing densities can be instantaneously imaged by OAWRS over thousands of square kilometers in both deep and shallow water environments given properly designed experiments.by Tianrun Chen.Ph.D

Simulating Shear Wave Propagation in Two-Dimensional Fractured Heterogeneous Media by Coupling Boundary Element and Finite Difference Methods

A hybrid method to model the shear wave (SH) scattering from 2D fractures embedded in a heterogeneous medium is developed by coupling Boundary Element Method (BEM) and Finite Different Method (FDM) in the frequency domain. FDM is used to propagate an SH wave from a source through heterogeneities to localized homogeneous domains where fractures are embedded within artificial boundaries. According to Huygens’ Principle, the boundary points can be regarded as “secondary” point sources and their values are determined by FDM. Given the incident fields from these point sources, BEM is applied to model scatterings from fractures and propagate them back to the artificial boundaries. FDM then takes the boundaries as secondary sources and continues propagating the scattered field into the heterogeneous medium. The hybrid method utilizes both the advantage of BEM and FDM. A numerical iterative scheme is also presented to account for the multiple scattering between different sets of fractures. The results calculated from this hybrid method with pure BEM method are first compared to show the accuracy of the hybrid approach and the iterative scheme. This method is then applied to calculate the wave scattered from fractures embedded in complex media

Temporal coherence of the acoustic field forward propagated through a continental shelf with random internal waves

An analytical model derived from normal mode theory for the accumulated effects of range-dependent multiple forward scattering is applied to estimate the temporal coherence of the acoustic field forward propagated through a continental-shelf waveguide containing random three-dimensional internal waves. The modeled coherence time scale of narrow band low-frequency acoustic field fluctuations after propagating through a continental-shelf waveguide is shown to decay with a power-law of range to the −1/2 beyond roughly 1 km, decrease with increasing internal wave energy, to be consistent with measured acoustic coherence time scales. The model should provide a useful prediction of the acoustic coherence time scale as a function of internal wave energy in continental-shelf environments. The acoustic coherence time scale is an important parameter in remote sensing applications because it determines (i) the time window within which standard coherent processing such as matched filtering may be conducted, and (ii) the number of statistically independent fluctuations in a given measurement period that determines the variance reduction possible by stationary averaging

Sensitivity analysis of fracture scattering

The understanding of seismic scattering of a finite fracture is very important in reservoir fracture characterizations, but the analytical solution of this problem is not available. Thus, in this paper, we present an approach for numerical study of the seismic response of a finite fracture.Eni-MIT Energy Initiative Founding Member Program (Eni Multiscale Reservoir Science Project)Massachusetts Institute of Technology. Earth Resources Laborator

Continual Semantic Segmentation with Automatic Memory Sample Selection

Continual Semantic Segmentation (CSS) extends static semantic segmentation by incrementally introducing new classes for training. To alleviate the catastrophic forgetting issue in CSS, a memory buffer that stores a small number of samples from the previous classes is constructed for replay. However, existing methods select the memory samples either randomly or based on a single-factor-driven handcrafted strategy, which has no guarantee to be optimal. In this work, we propose a novel memory sample selection mechanism that selects informative samples for effective replay in a fully automatic way by considering comprehensive factors including sample diversity and class performance. Our mechanism regards the selection operation as a decision-making process and learns an optimal selection policy that directly maximizes the validation performance on a reward set. To facilitate the selection decision, we design a novel state representation and a dual-stage action space. Our extensive experiments on Pascal-VOC 2012 and ADE 20K datasets demonstrate the effectiveness of our approach with state-of-the-art (SOTA) performance achieved, outperforming the second-place one by 12.54% for the 6stage setting on Pascal-VOC 2012.Comment: Accepted to CVPR202

Asymmetric source acoustic LWD for the improved formation shear velocity estimation

Most acoustic LWD tools generate a single pure borehole mode (e.g., dipole or quadrupole) to estimate the formation shear velocity. We propose an approach where many borehole modes are generated and all the modes are used simultaneously to obtain a better shear estimate. In this approach we find the best fit to the dispersion characteristics of a number of modes, rather than one mode. We propose using an asymmetric source, that is a single source on one side of the tool, together with arrays of receivers distributed azimuthally around the tool to allow different modes to be identified and analyzed. We investigate such an approach using synthetic and laboratory data. The lab data uses a scale-model LWD tool with one active sources transducer mounted on the side of the tool. This source geometry generates monopole, dipole, and quadrupole modes simultaneously. Four sets of receiver arrays, each separated by 90 degrees azimuthally, are used to isolate and analyze each of these modes by adding and subtracting the signals received from different arrays. Based on the dispersion analysis and the method of least square fitting, we find that the by simultaneously using both dipole and quadrupole modes, we can reduce the residual error of the best fit shear velocity. It should be noted that higher order modes (e.g., hexapole, etc) will also be generated by an asymmetric source, and these modes could also be utilized with the appropriate azimuthal receiver configuration

Covid-19 Diagnosis Based on CT Images Through Deep Learning and Data Augmentation

Coronavirus disease 2019(Covid-19) has made people around the world suffer. And there are many researchers make efforts on deep learning methods based on CT imgaes, but the limitation of  this work is the lackage of the dataset, which is not easy to obtain. In this study, we try to use data augmentation to compensate this weakness. In the first part, we use traditional DenseNet-169, and the result shows that data augmentation can help improve the calculating speed and the accuracy. In the second part, we combine Self-trans and DenseNet-169, and the result shows that when doing data augmentation, many model performance metrics have been improved. In the third part, we use UNet++, which reaches accuracy of 0.8645. Apart from this, we think GAN and CNN may also make difference

Asymmetric source acoustic LWD for the improved formation shear velocity estimation

Most acoustic logging while drilling (LWD) tools generate a single pure borehole mode (e.g., dipole or quadrupole) to estimate the formation shear velocity. We propose an approach where multipole modes are generated simultaneously and used to obtain a better shear estimation. This approach uses an asymmetric source with arrays of receivers distributed azimuthally around the tool to generate and identify signals from different modes. We investigate such an approach using both synthetic and laboratory data. The laboratory data are collected from a scale-model LWD tool with one active source transducer mounted on the side of the tool. Four sets of receiver arrays, each separated by 90 degrees azimuthally, are used to isolate monopole, dipole and quadrupole modes by coherently adding and subtracting received arrivals. A method is then apply to perform dispersion analysis on these arrivals. With least square fitting, formation shear velocities are estimated from both dipole and quadrupole modes' arrivals. We find that, by averaging the estimates obtained independently from dipole and quadrupole modes, we can reduce the uncertainty and improve the confidence of the estimation for the formation shear velocity

A robust method for fracture orientation and density detection from seismic scattered energy

The measurements of fracture parameters, such as fracture orientation, fracture density and fracture compliance, in a reservoir is very important for field development and exploration. Traditional seismic methods for fracture characterization include shear wave birefringence (Gaiser and Dok, 2001; Dok et al., 2001; Angerer et al., 2002; Vetri et al., 2003) and amplitude variations with offset and azimuth (AVOA) (Ruger, 1998; Shen et al., 2002; Hall et al., 2003; Liu et al., 2010; Lynn et al., 2010). These methods are based on the equivalent medium theory with the assumption that fracture dimension and spacing are small relative to the seismic wave length, so a fracture zone behaves like an equivalent anisotropic medium. But fractures on the order of seismic wave length are also very important for enhanced oil recovery, and they are one of the important subsurface scattering sources that generate scattered seismic waves. Willis et al. (2006) developed the Scattering Index method to extract the fracture scattering characteristics by calculating the transfer funtion of a fracture zone. This method has two sources of uncertainty: (1) calculation of the transfer function is sensitive to the analysis time window; (2) the interpretation of the transfer function is based on the assumption that the background reflectivity of the medium is white. Here we propose a modification of the SI methods that addresses these issues and leads to a more robust fracture characterization.Eni-MIT Energy Initiative Founding Member Progra