A method for correcting acoustic finite-difference amplitudes for elastic effects

Geophysics ◽  
2014 ◽  
Vol 79 (4) ◽  
pp. T243-T255 ◽  
Author(s):  
James W. D. Hobro ◽  
Chris H. Chapman ◽  
Johan O. A. Robertsson
Keyword(s):  
Wave Equation ◽  
Finite Difference ◽  
Effective Means ◽  
Cost Effective ◽  
P Wave ◽  
S Waves ◽  
Velocity Models ◽  
Acoustic Simulation ◽  
Wide Range ◽  
Elastic Simulation

We present a new method for correcting the amplitudes of arrivals in an acoustic finite-difference simulation for elastic effects. In this method, we selectively compute an estimate of the error incurred when the acoustic wave equation is used to approximate the behavior of the elastic wave equation. This error estimate is used to generate an effective source field in a second acoustic simulation. The result of this second simulation is then applied as a correction to the original acoustic simulation. The overall cost is approximately twice that of an acoustic simulation but substantially less than the cost of an elastic simulation. Because both simulations are acoustic, no S-waves are generated, so dispersed converted waves are avoided. We tested the characteristics of the method on a simple synthetic model designed to simulate propagation through a strong acoustic impedance contrast representative of sedimentary geology. It corrected amplitudes to high accuracy for reflected arrivals over a wide range of incidence angles. We also evaluated results from simulations on more complex models that demonstrated that the method was applicable in realistic sedimentary models containing a wide range of seismic contrasts. However, its accuracy was reduced for wide-angle reflections from very high impedance contrasts such as a shallow top-salt interface. We examined the influence of modeling at coarse grid resolutions, in which converted S-waves in the equivalent elastic simulation are dispersed. These results provide some validation for the accuracy of the method when applied using finite-difference grids designed for acoustic modeling. The method appears to offer a cost-effective means of modeling elastic amplitudes for P-wave arrivals in a useful range of velocity models. It has several potential applications in imaging and inversion.

Prestack scalar reverse-time depth migration of 3D elastic seismic data

Geophysics ◽  
2006 ◽  
Vol 71 (5) ◽  
pp. S199-S207 ◽  
Author(s):  
Robert Sun ◽  
George A. McMechan ◽  
Chen-Shao Lee ◽  
Jinder Chow ◽  
Chen-Hong Chen
Keyword(s):  
Wave Equation ◽  
Finite Difference ◽  
Seismic Data ◽  
P Wave ◽  
Scalar Wave ◽  
Reverse Time ◽  
S Wave ◽  
Absolute Value ◽  
S Waves ◽  
Finite Difference Solution

Using two independent, 3D scalar reverse-time depth migrations, we migrate the reflected P- and S-waves in a prestack 3D, three-component (3-C), elastic seismic data volume generated with a P-wave source in a 3D model and recorded at the top of the model. Reflected P- and S-waves are extracted by divergence (a scalar) and curl (a 3-C vector) calculations, respectively, during shallow downward extrapolation of the elastic seismic data. The imaging time for the migrations of both the reflected P- and P-S converted waves at each point is the one-way P-wave traveltime from the source to that point.The divergence (the extracted P-waves) is reverse-time extrapolated using a finite-difference solution of the 3D scalar wave equation in a 3D P-velocity modeland is imaged to obtain the migrated P-image. The curl (the extracted S-waves) is first converted into a scalar S-wavefield by taking the curl’s absolute value as the absolute value of the scalar S-wavefield and assigning a positive sign if the curl is counterclockwise relative to the source or a negative sign otherwise. This scalar S-wavefield is then reverse-time extrapolated using a finite-difference solution of the 3D scalar wave equation in a 3D S-velocity model, and it is imaged with the same one-way P-wave traveltime imaging condition as that used for the P-wave. This achieves S-wave polarity uniformity and ensures constructive S-wave interference between data from adjacent sources. The algorithm gives satisfactory results on synthetic examples for 3D laterally inhomogeneous models.

Two robust imaging methodologies for challenging environments: Wave-equation dispersion inversion of surface waves and guided waves and supervirtual interferometry + tomography for far-offset refractions

2018 ◽  
Vol 6 (4) ◽  
pp. SM27-SM37 ◽  
Author(s):  
Jing Li ◽  
Kai Lu ◽  
Sherif Hanafy ◽  
Gerard Schuster
Keyword(s):  
Wave Equation ◽  
Velocity Distribution ◽  
Surface Waves ◽  
Guided Waves ◽  
Velocity Model ◽  
Dispersion Curves ◽  
P Wave ◽  
Head Wave ◽  
Near Surface ◽  
Velocity Models

Two robust imaging technologies are reviewed that provide subsurface geologic information in challenging environments. The first one is wave-equation dispersion (WD) inversion of surface waves and guided waves (GW) for the shear-velocity (S-wave) and compressional-velocity (P-wave) models, respectively. The other method is traveltime inversion for the velocity model, in which supervirtual refraction interferometry (SVI) is used to enhance the signal-to-noise ratio of far-offset refractions. We have determined the benefits and liabilities of both methods with synthetic seismograms and field data. The benefits of WD are that (1) there is no layered-medium assumption, as there is in conventional inversion of dispersion curves. This means that 2D or 3D velocity models can be accurately estimated from data recorded by seismic surveys over rugged topography, and (2) WD mostly avoids getting stuck in local minima. The liability is that WD for surface waves is almost as expensive as full-waveform inversion (FWI) and, for Rayleigh waves, only recovers the S-velocity distribution to a depth no deeper than approximately 1/2 to 1/3 wavelength of the lowest-frequency surface wave. The limitation for GW is that, for now, it can estimate the P-velocity model by inverting the dispersion curves from GW propagating in near-surface low-velocity zones. Also, WD often requires user intervention to pick reliable dispersion curves. For SVI, the offset of usable refractions can be more than doubled, so that traveltime tomography can be used to estimate a much deeper model of the P-velocity distribution. This can provide a more effective starting velocity model for FWI. The liability is that SVI assumes head-wave first arrivals, not those from strong diving waves.

A hybrid method applied to a 2.5D scalar wave equation

2008 ◽  
Vol 45 (12) ◽  
pp. 1517-1525
Author(s):  
P. F. Daley ◽  
E. S. Krebes ◽  
L. R. Lines
Keyword(s):  
Wave Equation ◽  
Finite Difference ◽  
Heterogeneous Medium ◽  
Integral Transform ◽  
P Wave ◽  
Acoustic Wave Equation ◽  
Seismic Modeling ◽  
Subsurface Structures ◽  
Spatial Dimensions ◽  
Finite Integral

The 3D acoustic wave equation for a heterogeneous medium is used for the seismic modeling of compressional (P-) wave propagation in complex subsurface structures. A combination of finite difference and finite integral transform methods is employed to obtain a “2.5D” solution to the 3D equation. Such 2.5D approaches are attractive because they result in computational run times that are substantially smaller than those for the 3D finite difference method. The acoustic parameters of the medium are assumed to be constant in one of the three Cartesian spatial dimensions. This assumption is made to reduce the complexity of the problem, but still retain the salient features of the approach. Simple models are used to address the computational issues that arise in the modeling. The conclusions drawn can also be applied to the more general fully inhomogeneous problem. Although similar studies have been carried out by others, the work presented here is new in the sense that (i) it applies to subsurface models that are both vertically and laterally heterogeneous, and (ii) the computational issues that need to be addressed for efficient computations, which are not trivial, are examined in detail, unlike previous works. We find that it is feasible to generate true-amplitude synthetic seismograms using the 2.5D approach, with computational run times, storage requirements, and other factors, being at reduced and acceptable levels.

Elastic reflection traveltime inversion with decoupled wave equation

Geophysics ◽  
2018 ◽  
Vol 83 (5) ◽  
pp. R463-R474 ◽  
Author(s):  
Guanchao Wang ◽  
Shangxu Wang ◽  
Jianyong Song ◽  
Chunhui Dong ◽  
Mingqiang Zhang
Keyword(s):  
Wave Equation ◽  
Wave Velocity ◽  
Waveform Inversion ◽  
P Wave ◽  
Model Parameters ◽  
Local Minima ◽  
S Wave ◽  
Traveltime Inversion ◽  
Velocity Models ◽  
Elastic Reflection

Elastic full-waveform inversion (FWI) updates high-resolution model parameters by minimizing the residuals of multicomponent seismic records between the field and model data. FWI suffers from the potential to converge to local minima and more serious nonlinearity than acoustic FWI mainly due to the absence of low frequencies in seismograms and the extended model domain (P- and S-velocities). Reflection waveform inversion can relax the nonlinearity by relying on the tomographic components, which can be used to update the low-wavenumber components of the model. Hence, we have developed an elastic reflection traveltime inversion (ERTI) approach to update the low-wavenumber component of the velocity models for the P- and S-waves. In our ERTI algorithm, we took the P- and S-wave impedance perturbations as elastic reflectivity to generate reflections and a weighted crosscorrelation as the misfit function. Moreover, considering the higher wavenumbers (lower velocity value) of the S-wave velocity compared with the P-wave case, optimizing the low-wavenumber components for the S-wave velocity is even more crucial in preventing the elastic FWI from converging to local minima. We have evaluated an equivalent decoupled velocity-stress wave equation to ERTI to reduce the coupling effects of different wave modes and to improve the inversion result of ERTI, especially for the S-wave velocity. The subsequent application on the Sigsbee2A model demonstrates that our ERTI method with the decoupled wave equation can efficiently update the low-wavenumber parts of the model and improve the precision of the S-wave velocity.

Light Rail Lite or Cost-Effective Improvements to Bus Service?

2005 ◽  
Vol 1927 (1) ◽  
pp. 22-30 ◽  
Author(s):  
Daniel B. Hess ◽  
Brian D. Taylor ◽  
Allison C. Yoh
Keyword(s):  
High Speed ◽  
Low Cost ◽  
Effective Means ◽  
Cost Effective ◽  
Light Rail ◽  
Rapid Transit ◽  
Service Characteristics ◽  
Wide Range ◽  
Bus Service ◽  
The Cost

Bus rapid transit (BRT) is growing rapidly in popularity because it is viewed widely as an efficient and effective means to improve both transit service and patronage. This paper argues that two distinct views of BRT are emerging: ( a) BRT as a new form of high-speed, rubber-tired, rail-like rapid transit and ( b) BRT as a cost-effective way to upgrade both the quality and image of traditional fixed-route bus service. These two views carry different price tags because the cost of planning, constructing, and operating BRT depends on the complexity of new service features and on rises for BRT that offer service characteristics approaching those of light rail. This study fills a gap in the literature on the costs of BRT by examining in detail component costs–-actual costs for recently implemented services and projected costs for planned new services–-for a sample of BRT systems in North American cities. The study examined BRT costs of 14 planned and recently opened BRT systems to determine how the wide range of BRT service and technology configurations affect costs. The study found that although some of the most successful and popular new BRT systems are high-quality services operating in mixed traffic and implemented at relatively low cost, most BRT projects on the drawing boards are more elaborate, more expensive systems than many currently in service. Most new BRT projects emphasize elaborate LRT-type improvements to lines and stations in one or a few corridors rather than less splashy improvements (such as next-bus monitors, signal preemption, queue-jump lanes, and so forth) affecting more lines and modes in local transit networks. Among the 14 systems examined here, most could be characterized as light rail lite.

Development of a General Use Quarter-Vehicle Test Rig

2007 ◽  
Author(s):  
Justin Langdon ◽  
Steve C. Southward
Keyword(s):  
Effective Means ◽  
Academic Research ◽  
Cost Effective ◽  
Functional Requirements ◽  
Test Rig ◽  
Wide Range ◽  
Vehicle Test ◽  
And Performance ◽  
Improved Design ◽  
Vehicle Testing

This paper discusses the development of an improved design for a tire-coupled quarter-vehicle testing rig. The use of indoor-based simulation tools has become a mainstay in vehicle testing for the automotive and motorsports industries. Testing on a quarter-vehicle rig provides a cost effective means for making accurate and repeatable measurements that enables the user to perform a relatively large number of tests in a short amount of time. A review of current quarter-vehicle test platforms, both commercially available and in academic research labs, indicated that many desired functional requirements were not available. The goal of this effort was to develop a new quarter-vehicle rig with expanded capabilities that are not simultaneously present in the current state-of-the-art. The desired functional requirements are: accommodation of a wide range of actual vehicle suspension components including the tire and wheel, weight transfer due to braking and acceleration, aerodynamic forces, and vehicle roll. The test rig was constructed and tested using a Porsche 996 suspension. The suspension dynamics were characterized by fitting the parameters of a linear dynamic model to experimental response data from the rig. The design and performance of this new quarter-vehicle test rig is shown to be a cost effective solution for meeting the broad range of functional requirements.

The Role of Ultrasound-Guided Therapeutic Paracentesis in an Outpatient Transitional Care Program: A Case Series

2018 ◽  
Vol 35 (9) ◽  
pp. 1256-1260 ◽  
Author(s):  
Jeffrey Wang ◽  
Shahida Khan ◽  
Paige Wyer ◽  
Jessica Vanderwilp ◽  
Justin Reynolds ◽  
...  
Keyword(s):  
Transitional Care ◽  
Effective Means ◽  
Case Series ◽  
Care Program ◽  
Cost Effective ◽  
Refractory Ascites ◽  
Ultrasound Guided ◽  
Care Clinic ◽  
Wide Range ◽  
Ed Visits

Background: Patients with ascites suffer from distressing symptoms and are at high risk for readmission after hospitalization. Timely paracentesis is an important palliative tool in managing this vulnerable population. At our institution, we have developed a multidisciplinary transitional care program for patients discharged from the hospital with a wide range of complex conditions including refractory ascites. Methods: We present a case series of 10 patients with symptomatic ascites who were enrolled in our transitional care program and treated with ultrasound-guided therapeutic paracentesis in our clinic. Patient medical records were retrospectively reviewed to collect procedure details, outcomes, and follow-up data on emergency department (ED) visits and readmissions. Cost data were obtained from the hospital financial system. Results: Over the span of 9 months (September 2016 to July 2017), 22 total therapeutic paracenteses were performed on 10 unique patients in the transitional care clinic. Median age of the patient cohort was 52.5 years (range: 27-71 years). All patients reported immediate relief of ascites-related discomfort following the procedure. We did not observe any major adverse effects due to the in-clinic procedure. Nine of the 10 patients did not have any ED visits or readmissions within 30 days of discharge. The cost of performing ultrasound-guided paracentesis in the transitional care clinic was US$546.77 compared to US$978.32 when performed in the hospital. Conclusion: Our experience suggests that outpatient paracentesis may be a safe, feasible, and cost-effective means of providing symptom management for patients with ascites during their transition from hospital to home.

Sign in / Sign up

Export Citation Format

Share Document