Saul E. Guevara

Imaging over complex structures ‐ Colombian Andes case

Subsurface imaging in overthrust areas poses a major challenge to exploration operations today. The topographic and velocity variations in these areas affect the quality of seismic sections and hinder the interpreters from defining both structure and lithology . This paper is focused on the image quality around a poorly defined target horizons on a 2d seismic section from a Colombian overthrust area We carried out a comprehensive approach of 2-D prestack depth migration over a seismic line acquired in Colombia over a complex area using Imaging technology from different contractors.

Tomography techniques and a near surface model in the Andes

In this work we apply 2-dimensional seismic tomography techniques to determine the velocity model of the near surface strata in data from the Catatumbo area in Colombia, a zone of complex subsurface. Seismic tomography includes linear and non-linear techniques. Within the linear ones there are the Algorithmic Reconstruction Technique and the Simultaneous Iterative Reconstruction Technique, which require initial velocity models very near to the true velocity to ensure exactness and convergence. On the other side, the non-linear techniques have better theoretical principles and are less based on the initial model, but perhaps require more computational resources. The techniques used are applied to only refraction data. The ray tracing needed by the tomographic algorithms is performed using a hybrid technique based in the minimum travel time problem of graph theory and the ray bending technique. With these tomographic techniques it is also possible to obtain a velocity model of deep strata.

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.

3-C geophone orientation and wave modes polarization

Summary A test of the relation between the horizontal components of 3-C geophone orientation and seismic wave polarization is carried out on land data. We use several different events, Polarization analysis methods are applied to three events: first arrival, identified as P-wave refraction, an event selected in the horizontal component, identified as S wave refraction, and ground roll. As a result a strong correlation between polarization and source receiver direction is observed in S-waves and ground roll and a weak correlation is observed in P-waves. This property of horizontal polarization shows promise for use in an improved multicomponent acquisition procedure.

A method for P-P and P-S mode separation in the presence of statics

A method to separate P-P and P-S wave modes in the presence of near-surface P and S statics is applied to synthetic and real 2-D land data. The method uses the free surface response and information about statics to separate the wavefields in the ω-p domain. More continuity can be observed in stacked sections of real data after mode separation. We believe that the method could contribute to velocity analysis and stacking, and hence to better seismic sections.