William Agudelo

Joint ray + Born least-squares migration and simulated annealing optimization for target-oriented quantitative seismic imaging

A seismic processing workflow based on iterative ray + Born migration/inversion and target-oriented postprocessing of the migrated image is developed for fine-scale quantitative characterization of reflectors. The first step of the workflow involves linear iterations of the ray + Born migration/inversion. The output of the first step is a true-amplitude migrated image parameterized by velocity perturbations. In a second step, postprocessing of the migrated image is performed through a random search with a very-fast simulated annealing (VFSA) algorithm. The forward problem of the global optimization is a simple convolutional model that linearly relates a vertical profile of the band-limited migrated image after depth-to-time conversion to a 1D velocity model composed of a stack of homogeneous layers of arbitrary velocity and thickness. The aim of the postprocessing is to eliminate the limited bandwidth effects of the source from the migrated image for resolution improvement and enhanced geological interpre...

Uncertainty analysis in statics corrections obtained by tomographic inversion : Application in a mountainous zone in Catatumbo [Colombia]

Seismic tomography techniques are frequently used to obtain velocity models of the weathering layer, which are then used to calculate static corrections times. The main idea in tomography is to use the misfit between synthetic and observed first-arrival (refraction) times to correct an initial velocity model. Due to the nature of the seismic method where, in general, sources and receivers are placed on only one edge of the medium, the Earth surface, it can be shown that inverting for velocities using the observed times does not give a unique solution. Instead, a family of solutions is obtained that explains the same data set. To give a meaning to a nonunique solution, uncertainty analysis techniques are necessary. They can give information about the family of solutions such as which zones have a larger error, which zones are better resolved, is there a dispersion measurement (such as variance) associated with the solution model family, and is there a more probable solution model. To give clues to the answ...

2D fine-scale imaging of the subduction channel in Gulf of Guayaquil by integrated iterative PSDM and simulated annealing optimization

To analyze the physical properties of seismic reflectors as possible indicators of the presence of fluids, we apply a processing sequence based on preserved amplitude prestack depth migration/inversion (PSDM) coupled with a post-processing of the depth migrated image. Aim of the post-processing is to remove the effects of the limited source bandwidth, that is, to build a structural velocity model and an impulsional migrated section from the limited-bandwidth migrated image and to estimate the uncertainties of velocities and layer thicknesses.We present here an application of this depth-domain processing sequence to a 2D multichannel seismic reflection dataset for the study of the decollement reflector in the Gulf of Guayaquil (Ecuadorian margin). Preliminary results show that the decollement mainly corresponds to a layer of 80–100 meters in thickness with a mean and maximum negative velocity contrasts of −50 m/s and −150m/s respectively on top of it. Along the decollement, these negative velocity contrasts result from the contrast in the physical properties between the subducted sediments and the overriding plate materials above the decollement. Very likely, this velocity decrease is also associated with changes in fluid pressure.

PSDM uncertainty estimation from illumination attributes analysis

This work proposes a methodology for getting information about uncertainty on PSDM images; it is based on the analysis of illumination attributes, in particular the seismic aperture in the wavenumber domain. Using this attribute it is possible to estimate the maximum diffraction angle distribution on CIG s and quantify uncertainty including information of limited illumination on RMO semblance computation. A synthetic dataset has been used for proving the methodology.

Seismic tomography for near surface: Uncertainty analysis

The low velocity propagation of the seismic waves in the first meters of the sub-surface and the topographic irregularities have an effect in the correct position of deep reflectors on seismic images. Velocity models builded using algorithms of tomographyc inversion have shown to be useful to correct this effect, but the solution has associated uncertainties. Three tecnhiques of uncertainty analysis are applied on a tomographical inversion of real near surface data, giving a measurement of the associated error. These tecnhiques are computationally expensive, because they explore intensively the model space. Eventhough, with the use of current high-performing computers, their practical use have been made possible.