CWP Seminars  2015 Spring
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 
Spring 2015 CWP seminars
Date  Speaker(s)  Title  Abstract 
4/27 
Umair bin Waheed 
Modeling and inversion of firstarrival traveltimes in anisotropic media 
Traveltime tomography using transmission data has been widely used for static corrections and for obtaining nearsurface models for seismic depth imaging. More recently, traveltime tomography is also employed to build initial models for full waveform inversion. The classical traveltime tomography approach based on ray tracing has serious limitations in handling large data sets resulting from current seismic acquisition surveys. Some of these difficulties can be overcome by using the adjointstate method, due to its lowmemory requirement and numerical efficiency. The need for explicit computation of the Frechet derivative matrix is avoided by coupling the gradient computation to nonlinear optimization. Furthermore, the computation cost is equivalent to twice the solution of the forward modeling problem, irrespective of the size of the input data. Despite these attractive features of the adjointstate method for first arrival traveltime tomography, its development has been limited to isotropic medium due to unavailability of efficient eikonal solvers for anisotropic media. In this presentation, I will first develop an iterative, fast sweeping based eikonal solver for anisotropic media. Subsequently, I will discuss extension of the adjointstate method to first arrival traveltime tomography in transversely isotropic media. 
4/20 
François 
Full waveform inversion of ground penetrating radar data: Towards multiparameter imaging from surface data, and beyond 
Full waveform inverson (FWI) is a quantitative imaging technique inherited from seismic exploration which has been successfully adapted to ground penetrating radar data (GPR) for producing maps of the subsurface electrical properties (permittivity and conductivity), that can be in turn related to its petrophysical parameters (e.g. porosity, water content or permeability). First, FWI algorithms for GPR data should be improved by designing efficient 3D modelling tools and by considering the antennas characteristics in the simulations. Second, FWI of GPR data would greatly benefit of being combined with lowfrequency electromagnetic measurements for better constraining the conductivity models. Combining these different data as a prior information or within joint inversion schemes raises the question of the parameterization of the problem, both in terms of the petrophysical description of the material, and of the mathematical representation of the image. 
4/13  No seminar  CWP preparations for Project Review Meeting 

4/6  2015 Project Review Meeting 
Preparation with Madagascar 

Preparation with Matlab 

Preparation with Mines JTK 

3/30  Timedomain finitedifference modeling for attenuative anisotropic media 
Accurate and efficient modeling of seismic wavefields that accounts for both attenuation and anisotropy is essential for further development of processing methods. Here, we present a 2D timedomain finitedifference algorithm for simulating multicomponent data in viscoelastic transversely isotropic media with a vertical symmetry axis (VTI). The generalized standard linear solid (GSLS) model is employed to extend the definitions of the relaxation function and the tauparameter (which quantifies the difference between the stress and strain relaxation times) to anisotropic media. This approach produces nearly constant values of all components of the qualityfactor matrix within a specified frequency band. The developed numerical implementation is based on a set of anisotropic viscoelastic wave equations parameterized by memory variables. Application of the spectralratio method to synthetic data generated by our algorithm for a layered VTI medium confirms the accuracy of the proposed scheme. Synthetic examples for TI models with different structural complexity illustrate the influence of attenuation and attenuation anisotropy on multicomponent wave fields. 

Separation of diffracted waves in TI media 
Imaging diffracted waves can provide useful information about complex subsurface geology and fracture networks. Separation of diffractions from typically more intensive reflected waves can be done based on specularity, which measures deviation from Snell's law. Here, we analyze two formulations of specularity and their applicability to diffraction processing for anisotropic media.We show that the most common definition of specularity, originally introduced for pure modes in isotropic media, remains valid for both pure and converted waves in arbitrarily anisotropic models. The other formulation operates directly with the difference between the slowness projections onto the reflector for the incident and reflected waves. Testing on a VTI (transversely isotropic with vertical symmetry axis) diffraction ramp model demonstrate that both formulations produce satisfactory results for anisotropic media with appropriate tapering of the specularity gathers. 

3/23  Datadomain tomography using localcorrelations 
Wavefield tomography in the data domain is usually formulated by using the data difference as the misfit criterion. The data difference however, is very susceptible to cycle skipping which leads to convergence into local minima. To overcome the strong nonlinearity, a multiscale approach to the inversion is often needed. The success of this approach relies on the lowfrequency content of the data. Here, we define the data domain tomographymisfit criterion using local (windowed) correlations. Less correlation crosstalk leads to cleaner adjoint sources, and hence, cleaner gradients. We further improve the gradients using a penalty function that is consistent with the bandwidth of the seismic data, which is more realistic than linear penalty functions that annihilate infinite bandwidth data. 

3/16  Moving faults while unfaulting 3D seismic images 
Unfaulting seismic images to correlate seismic reflectors across faults is helpful for seismic interpretation, especially for seismic horizon extraction. Most unfaulting methods assume the fault geometry does not change when unfaulting a seismic image. This assumption is not valid for images with multiple faults, especially intersecting faults, and often yields unnecessary distortions in the corresponding unfaulted images. I will discuss two methods to compute vector shifts that move fault blocks and faults themselves simultaneously to obtain an unfaulted image with minimal distortions. 

3/9  No seminar  CSM Spring Break  
3/2 
Nori Nakata 
Ambientnoise bodywave tomography at Long Beach, California 
In contrast to surfacewave extraction, bodywave extraction is much more difficult from ambientnoise fields because of smaller energy of body waves and strict stationary locations. To characterize elastic media even in the isotropic case, however, we need to determine both shear and compressional moduli. Therefore, I develop a signalprocessing technique to extract P diving waves from ambient seismic noise and use them for tomography to estimate 3D Pwave velocities. The technique is a postcorrelation process and contains a waveform matching filter and a noise suppression filter. This is the first body wave tomography result obtained from only ambient noise recorded at the ground surface. The estimated velocity structure has higher resolution than using surface waves. 
2/23  Anisotropy signature in RTM extended images for VTI media 
I will discuss the defocusing in RTM extended images due to inaccuracy in an anisotropy model. I will show the influence of the anellipticity parameter eta, and the Thomsen coefficient delta. I will discuss the sensitivity of differential semblance optimization (DSO) and stackpower objective functions to eta and delta. Using a model with a single homogeneous layer, I will show DSO sensitivity kernels and gradients for the anellipticity parameter eta.


2/16  No seminar  Presidents' Day holiday  
2/9  Gradient computation and regularization for elastic waveform tomography 
Elastic waveform tomography uses multicomponent waveform recordings and elastic wave equation modeling to reconstruct models of subsurface properties. Equations needed for elastic waveform tomography to reconstruct P and Swave velocity models are derived using the adjointstate method. Synthetic tests demonstrate that, for a given acquisition geometry, P and Swaves in general will update different areas of the subsurface. Moreover, certain areas might be updated by only one wave mode, and, as a result, the velocity model corresponding to the missing wave mode will not be well constrained. To better constrain the inversion, we introduce a regularization term that penalizes unrealistic relations between Vp and Vs. 

Anisotropic waveform tomography with pseudoacoustic equations 
Multiparameter seismic waveform inversion especially with anisotropic parameters is intriguing but challenging due to the computational cost and the huge nullspace introduced by the imperfect model parameterizations. I will briefly discuss the theory and the implementation of the gradients using the wellknown adjointstate method. Some computational cost issues associated with the modeling and adjoint modeling will be addressed. I will also illustrate the sensitivity kernels of different parameters using some synthetic models. If the time allows, I will introduce one possible set of inequality constraints to the inversion which can also be treated as the regularization term. 

2/2  No seminar  CSM Department of Geophysics presentation  
1/26  Walking in the footsteps of Michael Berry 
I will give an overview of research that I did during my sabbatical. This includes superresolution without evanescent waves, seismic waves as a Foucault pendulum (or not), interferometry in caustics, and timelapse changes. 

1/19  Internships with our sponsors and hackathon fun 
BHP Billiton Petroleum Geoscience Internship ExxonMobil Exploration Company Geophysics Internship SEG 2014 Geophysics Hackathon


1/12  1st CWP seminar of the Spring 2015 semester (CWP administrative topics) 
Previous CWP Seminars
2014 

2013 

2012 