CWP Seminars  2014 Fall
CWP seminars discuss topics pertaining to our broad areas of research interests. These seminars are led by CWP faculty, students and, on occasion, by guest presenters. CWP seminars are held every Monday at 4 p.m. in the Green Center on the Colorado School of Mines campus. Click here to see previous CWP Seminars.
Note: To view weekly seminar schedules of individual CWP teams, click a link below:
CTeam seminars 
Steam seminars 
Fall 2014 CWP seminars
Date  Speaker(s)  Title  Abstract 
12/8  No seminar  CSM Exams Week 

12/1  The optimized based low rank method for wavefield extrapolation 
Spectral methods are fast becoming an indispensable tool for wavefield extrapolation, especially in anisotropic media, because it tends to be dispersion and artifact free, as well as highly accurate, when solving the wave equation. However, for inhomogenous media, we face difficulties in dealing with the mixed spacewave number domain extrapolation operator efficiently. To solve this problem, we propose an optimized expansion method that can approximate this operator with a low rank variable separation representation. The rank defines the number of inverse Fourier transforms for each time extrapolation step, and thus, the lower the rank the faster the extrapolation. The method uses optimization instead of matrix decomposition to find the optimal wavenumbers and velocities needed to approximate the full operator with its explicit low rank representation. As a result, we obtain lower rank representations compared with the standard low rank method within reasonable accuracy, and thus cheaper extrapolations. 

11/24  Myth, magic or Marchenko: Seeing an invisible medium 
Frequently, I have been told that Marchenko looks promising from the results we have seen; however, we are still not sure how you exactly implement the Marchenko iterative scheme. I will discuss Marchenko equations intuitively, why it works, and its shortfalls/limitations. I will also briefly mention my future work and current developments in using Marchenkotype equations in geophysics. 

11/17 
CWP students 
Review and reflection: 2014 SEG Annual Meeting 
CWP students will provide reviews of the presentations they attended during the 2014 SEG Annual Meeting. 
11/10  Estimating Qfactors by varying the seismic source strength 
A new method is proposed to estimate seismic Qfactors as a function of depth from two seismic surveys acquired over the same area. These surveys should be acquired using different source strengths. Explicit and simple equations are derived based on conventional seismic Qtheory. It is suggested to use these equations to directly estimate the average Qvalue for a seismic interface that is mapped at a given twoway travel time or depth. From a field example using two 2D lines acquired over a North Sea field it is shown that 2D cross sections of Q can be estimated. Furthermore, this method can be used both in the prestack and poststack domain, and can be used for multiple reflectors in the subsurface. In this way, it is possible to estimate 3D Qcubes using two succeeding seismic surveys acquired over a given area. Alternatively, it is possible to reshoot selected 2Dlines in a given area, in order to save acquisition costs. A third alternative is to vary the source depth during a survey, and exploit the source diversity achieved by this to estimate 3D Qcubes. 

11/3 

Depth imaging and reflectioncoefficient estimation 
A depthimaging extended image gather gives a blurred estimate of an interface refection operator containing planewave reflection coefficients. Hence the subsurface gather in principle allows amplitude versus offset or angle analysis for those situations where depth imaging is necessary. We present the basis of a workflow for such analysis. The method assumes a good migration velocity model in which modelling/demigration is required as well as migration, in particular for socalled receiverside blurring functions. The talk will also cover poststack pointspread function approaches to dealing with ghost and attenuation effects. 
10/27  No seminar  2014 SEG Annual Meeting week 

10/20  SEG Annual Meeting discussion (CWP administrative topics)  
Spectral sensitivities technique for fullwaveform inversion 
I extend a technique originally proposed for diffraction tomography spectral coverage analysis (Devaney, 1984) to sensitivity analysis. After investigating the spectral resolution possibilities of fullwavefield in several horizontally uniform models I show some general consequences on roles of several types of waves for fullwaveform inversion applications. The results on the role of free surface multiples obtained I discuss in details and suggest a preconditioning algorithm exploiting the spectral sensitivities technique. 

10/13  No seminar  CSM Fall Break 

10/6  A scalar imaging condition for elastic reversetime migration 
Polarity changes in convertedwave images constructed by elastic reversetime migration cause destructive interference when stacking images computed for different experiments in a seismic survey. We derive a simple imaging condition for converted waves to correct the image polarity and reveal the conversion strength from one wave mode to another. Compared to alternative methods for correcting polarity reversals in PS and SP images, our imaging condition is simple and robust, and does not add significantly to the cost of reversetime migration. Our imaging condition requires computing the divergence and curl of source P and S wavefields, respectively, which provide information regarding the directionality of the wavefields. However, computing the gradient and curl distorts the amplitude spectrum as well as the phase of the wavefields, and as a result, it distorts the amplitude and phase of the migrated images. We design a simple filter to correct for this amplitude and phase distortion. In addition, this filter can also be applied to correct for the amplitude and phase distortion resulting from the Helmholtz decomposition, which also involves the computation of the divergence and curl of wavefields. 

9/29  Error propagation with synthetic aperture for CSEM 
Controlledsource electromagnetics (CSEM) is a geophysical method used to derisk and find hydrocarbon reservoirs in marine settings. The size of the target, low resolution, and the depth of the target limit the detection of the target by the method. I have previously shown how the technique of optimally weighted synthetic aperture applied to CSEM responses increases the detectability and resolution of reservoirs. In the seminar, I will show how we have incorporated error propagation theory into our optimization method to decrease the level of noise while still increasing the anomaly from the reservoir. I will demonstrate the benefits of the optimization with synthetic electromagnetic responses.


9/22  Horizon volumes and unconformities 
I will discuss two methods for constructing seismic horizons. The first method generates horizons one at a time; the second generates an entire volume of horizons at once. Rather than gradually building a horizon by extending one or more seed points to a surface along seismic reflectors, both of the two methods generate horizons by solving partial differential equations. The most significant new aspect of both methods is the ability to interactively specify a small number of control points to more accurately construct horizons. I will also discuss a 3D method to automatically detect unconformities from a seismic image, then use the detected unconformities as constraints to more accurately estimate seismic normal vectors at unconformities, and better flatten seismic images containing unconformities. All methods were implemented when I worked at Transform Software/Drillinginfo last summer. 

9/15  FWI+MVA the natural way 
Integrating migration velocity analysis (MVA) and full waveform inversion (FWI) can help reduce the high nonlinearity of the classic FWI objective function. The combination of inverting for the long and short wavelength components of the velocity model using a dual objective function that is sensitive to both components is still very expensive and have produced mixed results. We develop an approach that includes both components integrated to complement each other. We specifically utilize the image to generate reflections in our synthetic data only when the velocity model is not capable of producing such reflections. As a result, we get the MVA working when we need it, and mitigate it's influence when the velocity model produces accurate reflections (possibly first for the low frequencies). This is achieved using a novel objective function that includes both objectives. Applications to a layered model, as well as, the Marmousi model demonstrate the approach main features. 

9/8  Prestack exploding reflector modeling and migration in TI media 
Prestack depth migration in anisotropic media, especially those that exhibit tilt, can be costy using reverse time migration (RTM). I will present a twoway spectral wavefield extrapolation using prestack exploding reflector model (PERM) in acoustic transversely isotropic (TI) media. This technique can naturally export the common image gathers compared to crosscorrelation imaging condition and poynting vectors approaches. I construct systematic ways to evaluate phase angles and phase velocities in dip oriented TI (DTI), vertical TI (VTI) and tilted TI (TTI) media, which are required by the spectral timesteppings of wavefields. I will show migration results from the Marmousi VTI model and the BP2007 TTI model. 

9/1  Sensitivity to timelapse velocity change using multiply elastic scattered waves 
Multiply scattered waves provide the potential for increased illumination or increased detectability of the weak perturbations presence within a medium. One of the proven use of the multiply scattered waves is in the detection of the weak timelapse changes, such as velocity changes due geomechanical processes within the subsurface or localized defects/cracks within mechanical structures. However, using multiply scattered waves to resolve localized weak changes within a medium requires the characterization of the distribution of the scattered waves within the medium. In a couple of meetings back we explored the characteristics of acoustic sensitivity kernel that can be used to resolve weak changes within a scattering medium. In this presentation, we will look at the computation of the elastic sensitivity kernels and explore their characteristics. 

8/25  1st CWP seminar of the Fall 2014 semester (CWP administrative topics) 
Previous CWP Seminars
2014 


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2012 