Tube Wave Generation At A Layer Boundary For An Incident Compressional Plane Wave

An approximate theory for the scattering of an incident plane P wave into tube waves in a fluid-filled borehole drilled through two homogeneous half-spaces is proposed in this paper. This theory is in excellent agreement with the zero frequency formulation (White, 1983) for frequencies below hundreds of Hertz (in the range of conventional crosshole or VSP experiments) and finite difference simulation at high frequencies. At low frequency the excited tube wave is found to be independent of the borehole radius and shows stronger sensitivity to the formation shear velocity contrast across the layer boundary. The sensitivity towards the compressional velocity perturbation is opposite to that of the shear wave and density such that little tube wave can be generated if the compressional and shear velocities are both increased or decreased accordingly. Unlike the tube wave excited in the borehole when an incident plane wave hits a fracture, the reflected and transmitted tube waves generated at a layer boundary show opposite polarities.Massachusetts Institute of Technology. Borehole Acoustics and Logging ConsortiumnCUBE (Graduate Student Fellowship

Acoustic Wave Propagation In And Around A Fluid-Filled Borehole Of Irregular Cross-Section

Boreholes with 10% or more ellipticity are not uncommon. In this paper, we consider the coupling of an incident elastic wave into a borehole of irregular cross-section and investigate the cross-mode coupling phenomenon in sonic well logging in the presence of borehole irregularity. The mode-matching method is used. Different from its original formulation, we employ the Reichel et al. algorithm to obtain the discrete least square approximation by trigonometric polynomials, a technique closely related to the fast Fourier transform (FFT). Our method not only yields great accuracy but also gains computational speed. Our study shows that the pressure in the borehole fluid is sensitive to the irregularity of the borehole cross-section, it is larger if the incident wave is along the effective minor axis and smaller if the incident wave is along the effective major axis. In the frequency range of a typical borehole experiment, the solid displacement in the formation is much less affected by the borehole irregularity. In an elliptical borehole, a monopole source excites dipole wave trains that are characteristic of the tube waves, and a centered dipole source excites monopole wave trains that are characteristic of the flexural waves.Massachusetts Institute of Technology. Borehole Acoustics and Logging Consortiu

Background Subtraction for Night Videos

Motion analysis is important in video surveillance systems and background subtraction is useful for moving object detection in such systems. However, most of the existing background subtraction methods do not work well for surveillance systems in the evening because objects are usually dark and reflected light is usually strong. To resolve these issues, we propose a framework that utilizes a Weber contrast descriptor, a texture feature extractor, and a light detection unit, to extract the features of foreground objects. We propose a local pattern enhancement method. For the light detection unit, our method utilizes the finding that lighted areas in the evening usually have a low saturation in hue-saturation-value and hue-saturation-lightness color spaces. Finally, we update the background model and the foreground objects in the framework. This approach is able to improve foreground object detection in night videos, which do not need a large data set for pre-training

Pressure in a fluid‐filled borehole caused by a seismic source in stratified media

A method for numerically simulating hydrophone vertical seismic profiles (VSP) and crosswell data measured in a fluid-filled borehole (either open or cased) embedded in stratified media is presented. The method makes use of both the borehole coupling theory and the global matrix formulation for computing synthetic seismograms in a stratified medium. The global matrix formulation is used to calculate the stress field at the borehole location. Borehole coupling theory is then employed to obtain the pressure in the borehole fluid. Comparisons with exact solutions for an open borehole in a homogeneous and unbounded formation show that this method is accurate for frequencies below 2 kHz. This method is used to model the Kent Cliffs hydrophone VSP data, where good agreement between the numerical simulations and the field measurements has been found, in both traveltimes and rms amplitudes of the direct P-wave. Examples show that this method is efficient and accurate, and can be applied to model VSP and crosswell experiments using an array of hydrophones.United States. Department of Energy (Grant No. DE-FG02-86ER1363)Reservoir Delineation ConsortiumMassachusetts Institute of Technology. Earth Resources Laboratory (nCUBE/ERL Geophysical Center for Parallel Processing

Cased Borehole Effect On Downhole Seismic Measurements

Approximate and exact formulations are presented for the interaction of an incident wave with a cased borehole. In the approximate method, the borehole coupling theory is used to compute pressure in the fluid at a low frequency. The results are simple and explicit. They are useful in the study of cased borehole coupling and as well as borehole radiation. In the exact method, elastic potentials in each annulus are represented as a superposition of fundamental solutions to the Helmholtz equations. Continuity of displacements and stresses across layer boundaries are used to determine unknown coefficients. The global matrix method is employed to simultaneously compute these coefficients in individual layers. This method is advantageous over the Thomson Haskell propagator matrix method in handling evanescent waves. Our results show that, in a cased borehole, the borehole effects on downhole seismic measurements are more significant than those in an open borehole, especially when the formation is soft and the casing is steel. For hard formations and frequency below 1 kHz, cased borehole influence on downhole geophone measurement is minimal, while at high frequencies, large discrepancies occur, especially at grazing incidence. For soft formations, both the pressure in the fluid and the solid displacement on the borehole wall show strong dependence on frequency and incidence angle, even at very low frequencies. Strong resonance occurs in the fluid for an SV incidence at angle δ = cos[superscript -1]β/C[subscript T] where CT is the tube wave velocity in a cased borehole. This resonance is prominent even at a very high frequency and large incidence angle because the tube wave velocity is raised well above the formation shear velocity by the steel pipe. This behavior is very different from that in an open borehole. At a particular angle of incidence of a plane P wave, the pressure in the fluid is near zero at low frequencies. This angle is dependent on the casing thickness and can be computed exactly. In general the casing behaves like a shield in such a way that the amplitude of both pressure in the fluid and solid motion on the borehole wall are reduced compared to those in an open borehole.ERL/nCUBE Geophysical Center for Parallel PrecessingMassachusetts Institute of Technology. Borehole Acoustics and Logging Consortiu

Collective human mobility pattern from taxi trips in urban area.

We analyze the passengers' traffic pattern for 1.58 million taxi trips of Shanghai, China. By employing the non-negative matrix factorization and optimization methods, we find that, people travel on workdays mainly for three purposes: commuting between home and workplace, traveling from workplace to workplace, and others such as leisure activities. Therefore, traffic flow in one area or between any pair of locations can be approximated by a linear combination of three basis flows, corresponding to the three purposes respectively. We name the coefficients in the linear combination as traffic powers, each of which indicates the strength of each basis flow. The traffic powers on different days are typically different even for the same location, due to the uncertainty of the human motion. Therefore, we provide a probability distribution function for the relative deviation of the traffic power. This distribution function is in terms of a series of functions for normalized binomial distributions. It can be well explained by statistical theories and is verified by empirical data. These findings are applicable in predicting the road traffic, tracing the traffic pattern and diagnosing the traffic related abnormal events. These results can also be used to infer land uses of urban area quite parsimoniously

Regional Differential Information Entropy for Super-Resolution Image Quality Assessment

PSNR and SSIM are the most widely used metrics in super-resolution problems, because they are easy to use and can evaluate the similarities between generated images and reference images. However, single image super-resolution is an ill-posed problem, there are multiple corresponding high-resolution images for the same low-resolution image. The similarities can't totally reflect the restoration effect. The perceptual quality of generated images is also important, but PSNR and SSIM do not reflect perceptual quality well. To solve the problem, we proposed a method called regional differential information entropy to measure both of the similarities and perceptual quality. To overcome the problem that traditional image information entropy can't reflect the structure information, we proposed to measure every region's information entropy with sliding window. Considering that the human visual system is more sensitive to the brightness difference at low brightness, we take $\gamma$ quantization rather than linear quantization. To accelerate the method, we reorganized the calculation procedure of information entropy with a neural network. Through experiments on our IQA dataset and PIPAL, this paper proves that RDIE can better quantify perceptual quality of images especially GAN-based images.Comment: 8 pages, 9 figures, 4 table

Large scale stochastic inventory routing problems with split delivery and service level constraints

A stochastic inventory routing problem (SIRP) is typically the combination of stochastic inventory control problems and NP-hard vehicle routing problems, which determines delivery volumes to the customers that the depot serves in each period, and vehicle routes to deliver the volumes. This paper aims to solve a large scale multi-period SIRP with split delivery (SIRPSD) where a customer’s delivery in each period can be split and satisfied by multiple vehicle routes if necessary. This paper considers SIRPSD under the multi-criteria of the total inventory and transportation costs, and the service levels of customers. The total inventory and transportation cost is considered as the objective of the problem to minimize, while the service levels of the warehouses and the customers are satisfied by some imposed constraints and can be adjusted according to practical requests. In order to tackle the SIRPSD with notorious computational complexity, we first propose an approximate model, which significantly reduces the number of decision variables compared to its corresponding exact model. We then develop a hybrid approach that combines the linearization of nonlinear constraints, the decomposition of the model into sub-models with Lagrangian relaxation, and a partial linearization approach for a sub model. A near optimal solution of the model found by the approach is used to construct a near optimal solution of the SIRPSD. Randomly generated instances of the problem with up to 200 customers and 5 periods and about 400 thousands decision variables where half of them are integer are examined by numerical experiments. Our approach can obtain high quality near optimal solutions within a reasonable amount of computation time on an ordinary PC