Friday, June 26. 2015Tutorial on welltie calculus
The example in rsf/tutorials/welltie reproduces the tutorial from Evan Bianco on welltie calculus. The tutorial was published in the June 2014 issue of The Leading Edge.
Madagascar users are encouraged to try improving the results. Thursday, June 25. 2015Similarityweighted semblance
A new paper is added to the collection of reproducible documents:
Velocity analysis using similarityweighted semblance Weighted semblance can be used for improving the performance of the traditional semblance for specific datasets. We propose a novel approach for prestack velocity analysis using weighted semblance. The novelty comes from a different weighting criteria in which the local similarity between each trace and a reference trace is used. On one hand, low similarity corresponds to a noise point or a point indicating incorrect moveout, which should be given a small weight. On the other hand, high similarity corresponds to a point indicating correct moveout, which should be given a high weight. The proposed approach can also be effectively used for analyzing AVO anomalies with increased resolution compared with AB semblance. Both synthetic and field CMP gathers demonstrate higher resolution using the proposed approach. Applications of the proposed method on a prestack dataset further confirms that the stacked data using the similarityweighted semblance can obtain better energyfocused events, which indicates a more precise velocity picking. Wednesday, June 24. 2015Test case for PEF estimation
Another old paper is added to the collection of reproducible documents:
Test case for PEF estimation with sparse data II The twostage missing data interpolation approach of Claerbout (1998) (henceforth, the GEE approach) has been applied with great success (Fomel et al., 1997; Clapp et al., 1998; Crawley, 2000) in the past. The main strength of the approach lies in the ability of the prediction error filter (PEF) to find multiple, hidden correlation in the known data, and then, via regularization, to impose the same correlation (covariance) onto the unknown model. Unfortunately, the GEE approach may break down in the face of very sparselydistributed data, as the number of valid regression equations in the PEF estimation step may drop to zero. In this case, the most common approach is to simply retreat to regularizing with an isotropic differential filter (e.g., Laplacian), which leads to a minimumenergy solution and implicitly assumes an isotropic model covariance. Sunday, June 21. 2015Reproducible research and PDF files
Claerbout's principle of reproducible research, as formulated by Buckheit and Donoho (1995), states:
An article about computational science in a scientific publication is not the scholarship itself, it is merely advertising of the scholarship. The actual scholarship is the complete software development environment and the complete set of instructions which generated the figures. The geophysics class in the SEGTeX package features a new option: reproduce, which attaches SConstruct files or other appropriate code (Matlab scripts, Python scripts, etc.) directly to the PDF file of the paper, with a button under every reproducible figure for opening the corresponding script. Unfortunately, not every PDF viewer supports this kind of links. The screenshot below shows evince viewer on Linux, where clicking the button opens the file with gedit editor. Tuesday, June 16. 2015Doubleelliptic approximation in TI media
Another old paper is added to the collection of reproducible documents:
The doubleelliptic approximation in the group and phase domains Elliptical anisotropy has found wide use as a simple approximation to transverse isotropy because of a unique symmetry property (an elliptical dispersion relation corresponds to an elliptical impulse response) and a simple relationship to standard geophysical techniques (hyperbolic moveout corresponds to elliptical wavefronts; NMO measures horizontal velocity, and timetodepth conversion depends on vertical velocity). However, elliptical anisotropy is only useful as an approximation in certain restricted cases, such as when the underlying true anisotropy does not depart too far from ellipticity or the observed angular aperture is small. This limitation is fundamental, because there are only two parameters needed to define an ellipse: the horizontal and vertical velocities. (Sometimes the orientation of the principle axes is also included as a free parameter, but usually not.) Tuesday, June 2. 2015Literate programming with IPython notebooksLiterate programming is a concept promoted by Donald Knuth, the famous computer scientist (and the author of the Art of Computer Programming.) According to this concept, computer programs should be written in a combination of the programming language (the usual source code) and the natural language, which explains the logic of the program. When it comes to scientific programming, using comments for naturallanguage explanations is not always convenient. Moreover, it is limited, because such explanations may require figures, equations, and other common elements of scientific texts. IPython/Jupyter notebooks provide a convenient tool for combining different text elements with code. Related posts: Wednesday, May 27. 20151M
Madagascar passes a symbolic mark of one million lines of code. Black Duck Open Hub reports that
In a Nutshell, Madagascar... Saturday, May 9. 2015Amplitude balancing
Another old paper is added to the collection of reproducible documents:
Iterative leastsquare inversion for amplitude balancing Variations in source strength and receiver amplitude can introduce a bias in the final AVO analysis of prestack seismic reflection data. In this paper we tackle the problem of the amplitude balancing of the seismic traces from a marine survey. We start with a 2D energy map from which the global trend has been removed. In order to balance this amplitude map, we first invert for the correction coefficients using an iterative leastsquare algorithm. The coefficients are calculated for each shot position along the survey line, each receiver position in the recording cable, and each offset. Using these coefficients, we then correct the original amplitude map for amplitude variations in the shot, receiver, and offset directions. Thursday, May 7. 2015Twodimensional Hilbert transform
A new paper is added to the collection of reproducible documents:
Seismic dip estimation based on the twodimensional Hilbert transform and its application in random noise attenuation In seismic data processing, random noise seriously affects the seismic data quality and subsequently the interpretation. This study aims to increase the signaltonoise ratio by suppressing random noise and improve the accuracy of seismic data interpretation without losing useful information. Hence, we propose a structureoriented polynomial fitting filter. At the core of structureoriented filtering is the characterization of the structural trend and the realization of nonstationary filtering. First, we analyze the relation of the frequency response between twodimensional (2D) derivatives and the 2D Hilbert transform (Riesz transform). Then, we derive the noniterative seismic local dip operator using the 2D Hilbert transform to obtain the structural trend. Second, we select polynomial fitting as the nonstationary filtering method and expand the application range of the nonstationary polynomial fitting. Finally, we apply variableamplitude polynomial fitting along the direction of the dip to improve the adaptive structureoriented filtering. Model and field seismic data show that the proposed method suppresses the seismic noise while protecting structural information. Wednesday, May 6. 2015Structureconstrained acoustic impedance
A new paper is added to the collection of reproducible documents:
Structureconstrained relative acoustic impedance using stratigraphic coordinates Acoustic impedance inversion involves conversion of seismic traces to a reflection coefficient time series, and then into acoustic impedance. The usual assumption for the transformation of poststack seismic data into impedance , is that seismic traces can be modeled using the simple convolutional model. According to the convolutional model, a seismic trace is a normalincidence record, which is an assumption that is strictly true only if the earth structure is composed of horizontal layers. In the presence of dipping layers, such an assumption is violated, which introduces bias in the result of impedance inversion. I propose to implement impedance inversion in the stratigraphic coordinate system, where the vertical direction is normal to reflectors and seismic traces represent normalincidence seismograms. Tests on field data produce more accurate and detailed impedance results from inversion in the stratigraphic coordinate system, compared to impedance results using the conventional Cartesian coordinate system. Tuesday, May 5. 2015Summer events
Two exciting events are being planned for this summer.
The Madagascar School for Advanced Users is being planned for August 89, 2015. The school will take place in Qingdao, China, and will be hosted by the China University of Petroleum. Unlike previous Madagascar schools, the intended audience are not beginners but current users of Madagascar who want to learn more about advanced topics (parallel computing; graphical user interfaces; interfaces to C++, Python, and Matlab, etc.) More information is available at http://www.ahay.org/wiki/Qingdao_2015. The Working Workshop on 3D Seismic Data Processing is being planned for August 1922, 2015. The workshop will take place in Houston and will be hosted by Rice University. Unlike previous working workshops, the participations is not limited to Madagascar users. Users of other software packages with interest in seismic field data processing are encouraged to participate. The workshop is being organized by Karl Schleicher, with support of SEG. More information is available at http://www.ahay.org/wiki/SEG_3D_Seismic_Processing_Working_Workshop_Houston_2015_Land_3D Thursday, April 23. 2015Tutorial on seismic survey design
The example in rsf/tutorials/survey reproduces the tutorial from Evan Bianco on designing 3D seismic surveys. For more explanation, see Evian's blog post Laying out a seismic survey.
Madagascar users are encouraged to try improving the results. Tuesday, April 21. 2015Reproducible research in psychology
An article A Perfect Storm: The Record of a Revolution by EricJan Wagenmakers, a mathematical psychologist from the Unversity of Amsterdam, describes a reproducibility revolution, which is taking place in psychology:
The dynamics of political revolutions are in some ways similar to the academic revolution that has recently gripped the field of psychology. Over the last two decades, increasing levels of competition for scarce research funding have created a working environment that rewards productivity over reproducibility; this perverse incentive structure has caused some of the findings in the psychological literature to be spectacular and counterintuitive, but likely false [...] The general dissatisfaction with the state of the field was expressed in print only occasionally, until in 2011 two major events ignited the scientific revolution that is still in full force today. The article was published this month by the Inquisitive Mind (InMind) magazine. EricJan concludes: Although some researchers are less enthusiastic about the "replicability movement" than others, it is my prediction that the movement will grow until its impact is felt in other empirical disciplines including the neurosciences, biology, economy, and medicine. The problems that confront psychology are in no way unique, and this affords an opportunity to lead the way and create dependable guidelines on how to do research well. Such guidelines have tremendous value, both to individual scientists and to society as a whole. Program of the month: sfslant
sfslant is a TX implementation of slant stack, also known as Radon transform or taup transform.
The two middle panels in the example below from cwp/geo2006TimeShiftImagingCondition/zicig show a timeshift commonimage gather and its transformation by slant stack. sfslant is a linear operator and has an adjoint flag adj=: When adj=n, the transformation is from taup to tx. When adj=y, the transformation is from tx to taup. The sampling on the transformed coordinate is controlled in the two cases by nx=, dx=, x0= or, respectively np=, dp=, p0=. Antialiasing is enabled by default with anti=1 parameter. The central slope for antialiasing is given by p1=. A space integration is sfslant generally requires a corrective "rho filter" (halforder differentiation). It is enabled by rho= parameter. For an FX implementation of the Radon transform, see sfradon. 10 previous programs of the month:Friday, April 17. 2015Seisletbased MCA
A new paper is added to the collection of reproducible documents:
Seisletbased morphological component analysis using scaledependent exponential shrinkage Morphological component analysis (MCA) is a powerful tool used in image processing to separate different geometrical components (cartoons and textures, curves and points etc). MCA is based on the observation that many complex signals may not be sparsely represented using only one dictionary/transform, however can have sparse representation by combining several overcomplete dictionaries/transforms. In this paper we propose seisletbased MCA for seismic data processing. MCA algorithm is reformulated in the shapingregularization framework. Successful seisletbased MCA depends on reliable slope estimation of seismic events, which is done by planewave destruction (PWD) filters. An exponential shrinkage operator unifies many existing thresholding operators and is adopted in scaledependent shaping regularization to promote sparsity. Numerical examples demonstrate a superior performance of the proposed exponential shrinkage operator and the potential of seisletbased MCA in application to trace interpolation and multiple removal.
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