A constrained-based optimization approach for seismic data recovery problems

Random and structured noise both affect seismic data, hiding the reflections of interest (primaries) that carry meaningful geophysical interpretation. When the structured noise is composed of multiple reflections, its adaptive cancellation is obtained through time-varying filtering, compensating inaccuracies in given approximate templates. The under-determined problem can then be formulated as a convex optimization one, providing estimates of both filters and primaries. Within this framework, the criterion to be minimized mainly consists of two parts: a data fidelity term and hard constraints modeling a priori information. This formulation may avoid, or at least facilitate, some parameter determination tasks, usually difficult to perform in inverse problems. Not only classical constraints, such as sparsity, are considered here, but also constraints expressed through hyperplanes, onto which the projection is easy to compute. The latter constraints lead to improved performance by further constraining the space of geophysically sound solutions.Comment: International Conference on Acoustics, Speech and Signal Processing (ICASSP 2014); Special session "Seismic Signal Processing

A Primal-Dual Proximal Algorithm for Sparse Template-Based Adaptive Filtering: Application to Seismic Multiple Removal

Unveiling meaningful geophysical information from seismic data requires to deal with both random and structured "noises". As their amplitude may be greater than signals of interest (primaries), additional prior information is especially important in performing efficient signal separation. We address here the problem of multiple reflections, caused by wave-field bouncing between layers. Since only approximate models of these phenomena are available, we propose a flexible framework for time-varying adaptive filtering of seismic signals, using sparse representations, based on inaccurate templates. We recast the joint estimation of adaptive filters and primaries in a new convex variational formulation. This approach allows us to incorporate plausible knowledge about noise statistics, data sparsity and slow filter variation in parsimony-promoting wavelet frames. The designed primal-dual algorithm solves a constrained minimization problem that alleviates standard regularization issues in finding hyperparameters. The approach demonstrates significantly good performance in low signal-to-noise ratio conditions, both for simulated and real field seismic data

Lapped transforms and hidden Markov models for seismic data filtering

International audienceSeismic exploration provides information about the ground substructures. Seismic images are generally corrupted by several noise sources. Hence, efficient denoising procedures are required to improve the detection of essential geological information. Wavelet bases provide sparse representation for a wide class of signals and images. This property makes them good candidates for efficient filtering tools, allowing the separation of signal and noise coefficients. Recent works have improved their performance by modelling the intra- and inter-scale coefficient dependencies using hidden Markov models, since image features tend to cluster and persist in the wavelet domain. This work focuses on the use of lapped transforms associated with hidden Markov modelling. Lapped transforms are traditionally viewed as block-transforms, composed of M pass-band filters. Seismic data present oscillatory patterns and lapped transforms oscillatory bases have demonstrated good performances for seismic data compression. A dyadic like representation of lapped transform coefficient is possible, allowing a wavelet-like modelling of coefficients dependencies. We show that the proposed filtering algorithm often outperforms the wavelet performance both objectively (in terms of SNR) and subjectively: lapped transform better preserve the oscillatory features present in seismic data at low to moderate noise levels

Distant horizontal gene transfer is rare for multiple families of prokaryotic insertion sequences

Horizontal gene transfer in prokaryotes is rampant on short and intermediate evolutionary time scales. It poses a fundamental problem to our ability to reconstruct the evolutionary tree of life. Is it also frequent over long evolutionary distances? To address this question, we analyzed the evolution of 2,091 insertion sequences from all 20 major families in 438 completely sequenced prokaryotic genomes. Specifically, we mapped insertion sequence occurrence on a 16S rDNA tree of the genomes we analyzed, and we also constructed phylogenetic trees of the insertion sequence transposase coding sequences. We found only 30 cases of likely horizontal transfer among distantly related prokaryotic clades. Most of these horizontal transfer events are ancient. Only seven events are recent. Almost all of these transfer events occur between pairs of human pathogens or commensals. If true also for other, non-mobile DNA, the rarity of distant horizontal transfer increases the odds of reliable phylogenetic inference from sequence dat

Evolutionary dynamics of the LTR retrotransposons roo and rooA inferred from twelve complete Drosophila genomes

BACKGROUND: Roo is the most abundant retrotransposon in the fruit fly Drosophila melanogaster. Its evolutionary origins and dynamics are thus of special interest for understanding the evolutionary history of Drosophila genome organization. We here study the phylogenetic distribution and evolution of roo, and its highly diverged relative rooA in 12 completely sequenced genomes of the genus Drosophila. RESULTS: We identify a total of 164 roo copies, 57 of which were previously unidentified copies that occur in 9 of the 12 genomes. Additionally we find 66 rooA copies in four genomes and remnants of this element in two additional genomes. We further increased the number of elements by searching for individual roo/rooA sequence domains. Most of our roo and rooA elements have been recently inserted. Most elements within a genome are highly similar. A comparison of the phylogenetic tree of our roo and rooA elements shows that the split between roo and rooA took place early in Drosophila evolution. Furthermore there is one incongruency between the species tree and the phylogenetic tree of the roo element. This incongruency regards the placement of elements from D. mojavensis, which are more closely related to D. melanogaster than elements from D. willistoni. CONCLUSION: Within genomes, the evolutionary dynamics of roo and rooA range from recent transpositional activity to slow decay and extinction. Among genomes, the balance of phylogenetic evidence, sequence divergence distribution, and the occurrence of solo-LTR elements suggests an origin of roo/rooA within the Drosophila clade. We discuss the possibility of a horizontal gene transfer of roo within this clade

When formal institutions impede entrepreneurship: how and why refugees establish new ventures in the Dadaab refugee camps

For this paper we investigated refugee entrepreneurship in the Dadaab refugee camps, Kenya, a place where humanitarian aid practices and domestic legislation impede entrepreneurship, yet hundreds of new ventures have been established by refugees. The analysis finds that refugee camp entrepreneurs erode formal institutions, recombine conducive aspects of both formal and informal institutions, and exploit the advantages of institutional misalignment. We explain how entrepreneurs strategically maintain rather than overcome institutional misalignment for venture creation. Second, we show how self-determination, rather than mere subsistence or necessity, is an important yet often overlooked motivator for entrepreneurship in low and lower middle-income contexts

Hilbert pairs of M-band orthonormal wavelet bases

International audienceRecently, there has been a growing interest for wavelet frames corresponding to the union of an orthonormal wavelet basis and its dual Hilbert transformed wavelet basis. However, most of the existing works specifically address the dyadic case. In this paper, we consider orthonormal M-band wavelet decompositions, since we are motivated by their advantages in terms of frequency selectivity and symmetry of the analysis functions, for M > 2. More precisely, we establish phase conditions for a pair of critically subsampled M-band filter banks. The conditions we obtain generalize a previous result given in the two-band case. We also show that, when the primal filter bank and its wavelets have symmetry, it is inherited by their duals. Furthermore, we give a design example where the number of vanishing moments of the approximate dual wavelets is imposed numerically to be the same as for the primal ones