A Bipolar-Bisection Piecewise Encoding Scheme for Multi-Source Reverse Time Migration

Conventional shot-record reverse time migration (RTM) suffers from a highcomputational burden when dealing with massive data. The computational cost ofRTM can be reduced by shot-encoding techniques, and plane-wave encoding is a commonly used and effective shot-encoding scheme. However, plane-wave encoding requires long time padding to avoid information loss, which decreases the efficiencyof the time-domain wavefield extrapolator, and the time padding becomes longer withthe increasing distance between multiple sources. The piecewise plane-wave encodingscheme cuts multiple sources into several segments prior to implementing plane-waveencoding, hence reduces the time padding, but brings new crosstalk due to the mutualinterference between shots in different source segments. We suppress the crosstalkartifacts by a new bipolar-bisection amplitude encoding method, in which half of theencoding array of each migration is different from that of any other migrations to reduce the number of crosstalk terms with as few migrations as possible. We embed thebipolar-bisection method into piecewise plane-wave encoding. Compared with plane-wave encoding, the proposed scheme requires considerably shorter time padding andthus works more efficiently and can generate a qualified imaging result. The feasibilityof the proposed method is tested on the 2D SEG/EAGE salt model and the Marmousimodel.     

Making True Amplitude Angle Domain Common Image Gathers Using Invertible Radon Transform

Converting subsurface offset domain common image gathers (ODCIGs) toangle domain common image gathers (ADCIGs) through a Radon Transform (RT) ineither the spatial or wavenumber domain is efficient and valid except for the distortion of both frequency spectrum and amplitude versus angle (AVA) effect. This paperpresents two modifications to the existing method to keep the frequency spectrum ofthe resultant ADCIGs the same as the input data and to preserve the relative amplitudes. The spectrum invariance is achieved by replacing the conventional RT or slantslack by an invertible RT. Amplitude preservation is obtained by applying an amplitude correction factor in the angle domain. Tests on both synthetic and field datasetsvalidate the accuracy of these modifications.     

Kinetic Slip Boundary Condition for Isothermal Rarefied Gas Flows Through Static Non-Planar Geometries Based on the Regularized Lattice-Boltzmann Method

The simulation of rarefied gas flows through complex porous media is challenging due to the tortuous flow pathways inherent to such structures. The Lattice Boltzmann method (LBM) has been identified as a promising avenue to solve flows through complex geometries due to the simplicity of its scheme and its high parallel computational efficiency. It has been proposed to model the stress-strain relationship with the extended Navier-Stokes equations rather than attempting to directly solve the Boltzmann equation. However, a regularization technique is required to filter out non-resolved higher-order components with a low-order velocity scheme. Although slip boundary conditions (BCs) have been proposed for the non-regularized multiple relaxation time LBM (MRT-LBM) for planar geometries, previous slip BCs have never been verified extensively with the regularization technique. In this work, following an extensive literature review on the imposition of slip BCs for rarefied flows with the LBM, it is proven that earlier values for kinetic parameters developed to impose slip BCs are inaccurate for the regularized MRT-LBM and differ between the D2Q9 and D3Q15 schemes. The error was eliminated for planar flows and good agreement between analytical solutions for arrays of cylinders and spheres was found with a wide range of Knudsen numbers.     

A Two-Dimensional Second Order Conservative Front-Tracking Method with an Original Marker Advection Approach Based on Jump Relations

A two-dimensional front-tracking method is developed for handling complex shape interfaces satisfying the volume conservation. In order to validate the proposed front-tracking method, a complete convergence study is carried out on severalanalytical test cases for which the interface is widely stretched and deformed. Comparisons to different existing approaches show that our front-tracking method is secondorder accurate in space with lower errors than existing interface tracking techniques ofthe literature.We also propose an original marker advection method which takes into account thejump relations valid at interface in order to deal with the contrast of physical propertiesencountered in two-phase flow simulations. The conservative front-tracking methodcomputed in this work is shown to be able to describe interfaces with high accuracyeven for poorly resolved Eulerian grids.     

A Non-Ersatz Material Approach for the Topology Optimization of Elastic Structures Based on Piecewise Constant Level Set Method

The topology optimization of a linearized elasticity system with the area(volume) constraint is investigated. A non-ersatz material approach is proposed. Byintroducing a fixed background domain, the linearized elasticity system is extendedinto the background domain by a characteristic function. The piecewise constant levelset (PCLS) method is applied to represent the original material region and the voidregion. A quadratic function of PCLS function is proposed to replace the characteristicfunction. The functional derivative of the objective functional with respect to PCLSfunction is derived, which is zero in the void region and nonzero in the original material region. A penalty gradient algorithm is proposed. Four numerical experiments of2D and 3D elastic structures with different boundary conditions are presented, illustrating the validity of the proposed algorithm.     

Distal Gastric Cancer and Extensive Surgery: A New Evaluation Method Based on the Study of the Status of Residual Lymph Nodes After Limited Surgery

Background: Curative resection (R0) is the treatment of choice for distal gastric cancer, but it is unclear whether this operation should include a total gastrectomy (TG) with splenectomy and extended (D2) lymph node dissection. A new concept was developed based on the fact that residual metastatic lymph nodes after a limited (D1) subtotal gastrectomy (SG) may be the source of fatal relapse. We conducted a prospective study on patients who had undergone a D2 TG to evaluate whether certain stations left behind after a D1 SG contain metastasis.Methods: We studied 1207 nodes obtained from 35 eligible patients who underwent a TG within 2 years. Of these patients, 29 fulfilled the criterion for a D2 dissection with curative potential. Numbers of retrieved and tumor-containing nodes by each station according to the Japanese Research Society for Gastric Cancer were documented prospectively in a standardized protocol. All lymph nodes were studied in sections smaller than 2 mm, but emphasis was given to the study of nodes from stations 1 and 2 (paracardial right and left), station 10 (splenic hilum), and stations 7 through 12 (around celiac axis, and in hepatoduodenal ligament) that can be dissected with a TG, splenectomy, and D2 dissection, respectively. For quality control of D2 dissection, the numbers ofnodes retrieved by each compartment II nodal station (7–12) documented by a pathologist were used and compared with proposed reference values. Long-term survival and cumulative risk of relapse were calculated in terms of lymph node status and presence of metastasis in compartment II nodes.Results: A mean total node yield of 37.4 from stations 1–12 and 11.4 from compartment II (stations 7–12) was obtained from 29 patients who had a D2 TG with curative intent. A substantial variation in node yields was found, and sometimes several stations contained no lymph nodes, which suggested an important cause of noncompliance (no yield of lymph nodes detected by the pathologist from that indicated for dissection stations) and difficulties for quality control. No positive node was detected in stations 1, 2, and 10 among patients who had a curative TG with splenectomy. However, substantially high was the incidence of metastasis in compartment II nodes, which was detected in one third of patients with node-positive disease. After 10 years of follow-up, overall survival and relapse rates among R0 D2 patients with negative compartment II nodes (pN0/pN1 disease) were 47% and 44%, respectively.Conclusions: Our results suggest the necessity of D2 dissection, but not of TG with splenectomy, to achieve an R0 resection for patients with distal gastric carcinoma. A large prospective study based on our protocol and findings may clarify whether a D2 R0 resection would result in a survival benefit.