Carlos Ortiz-Aleman

Gravity and magnetic field modeling and structure of the Chicxulub Crater, Mexico

The Chicxulub crater is an ∼180-km-diameter peak-ring crater based on drill hole logs and samples, potential fields, seismic reflection profiles, and surface fracture patterns. A structural cross section produced based on these constraints has the features expected for a large complex impact crater. The Bouguer-gravity anomaly consists of a broad ∼90-km radius, ∼30-mGal low with a central ∼20-km radius, ∼20-mGal high and two <5-mGal concentric lows at ∼35- and ∼60-km radius. The gravity anomaly is disrupted by large-scale basement anomalies and possibly by large-scale slumping and backwash erosion effects. The magnetic field anomaly over the crater consists of three zones, an outer zone from ∼45- to ∼90-km radius of low-amplitude, short-wavelength anomalies with an irregular perimeter, a middle zone from ∼20- to ∼45-km radius of high-amplitude, short-wavelength anomalies slightly elongated NNW-SSE, and an inner ∼20-km-radius single large-amplitude anomaly. Magnetic field modeling indicates that the melt pool averages ∼90 km in diameter and the melt volume in the crater is estimated at ∼20,000 km3. The melt pool size constrains the collapsed transient cavity diameter to ∼90 km. Gravity and magnetic field modeling indicate that the structural uplift is irregular in shape but ∼40 km in diameter and underlies or protrudes into the melt pool. The preliminary structural cross section indicates that the inferred peak-ring is decoupled from the structural uplift. The geometry and Bouguer gravity signature of the crater indicate that no significant uplift of the Moho or relaxation of the crater has occurred.

Mapping Chicxulub crater structure with gravity and seismic reflection data

Abstract Aside from its significance in establishing the impact-mass extinction paradigm, the Chicxulub crater will probably come to exemplify the structure of large complex craters. Much of Chicxulub’s structure may be ‘mapped’ by tying its gravity expression to seismic-reflection profiles revealing an ∼180 km diameter for the now-buried crater. The distribution of karst topography aids in outlining the peripheral crater structure as also revealed by the horizontal gradient of the gravity anomaly. The fracturing inferred to control groundwater flow is apparently related to subsidence of the crater fill. Modelling the crater’s gravity expression based on a schematic structural model reveals that the crater fill is also responsible for the majority of the negative anomaly. The crater’s melt sheet and central structural uplift are the other significant contributors to its gravity expression. The Chicxulub impact released ∼1.2 × 1031 ergs based on the observed collapsed disruption cavity of ∼86 km diameter reconstructed to an apparent disruption cavity (Dad) of ∼94 km diameter (equivalent to the excavation cavity) and an apparent transient cavity (Dat) of ∼80 km diameter. This impact energy, together with the observed ∼2 × 1011 g global Ir fluence in the Cretaceous-Tertiary (K-T) fireball layer indicates that the impactor was a comet estimated as massing ∼1.8 × 1018 g of ∼16.5 km diameter assuming a 0.6 gcm−3 density. Dust-induced darkness and cold, wind, giant waves, thermal pulses from the impact fireball and re-entering ejecta, acid rain, ozone-layer depletion, cooling from stratospheric aerosols, H2O greenhouse, CO2 greenhouse, poisons and mutagens, and oscillatory climate have been proposed as deleterious environmental effects of the Chicxulub impact with durations ranging from a few minutes to a million years. This succession of effects defines a temperature curve that is characteristic of large impacts. Although some patterns may be recognized in the K-T extinctions, and the survivorship rules changed across the boundary, relating specific environmental effects to species’ extinctions is not yet possible. Geochemical records across the boundary support the occurrence a prompt thermal pulse, acid rain and a ∼5000 year-long greenhouse. The period of extinctions seems to extend into the earliest Tertiary.

Reconstruction of permittivity images from capacitance tomography data by using very fast simulated annealing

In this paper we introduce an image reconstruction technique for imaging permittivity distributions using electrical capacitance tomography, based on global optimization by very fast simulated annealing. Electrical capacitance measurement data are obtained between electrodes placed around the outer wall of an electrically insulating pipe. Such data are used to infer material distributions inside the pipe. The data are processed in order to reconstruct an image of the spatial distribution of the relative electrical permittivity (also known as dielectric constant) inside the pipe, which reflects a material distribution. In the very fast simulated annealing method, the permittivity image is reconstructed by minimizing iteratively a cost function related to the difference between the measured data and those calculated for an estimated permittivity distribution that is repeatedly updated, in a semi-random search process that mimics the thermodynamic phenomena of annealing (as metals slowly cool down) or crystallization (as liquids freeze). The images are refined until their calculated capacitance data match the measured data, in which case it is considered that such images properly resemble the permittivity distribution that produced the measured capacitance data.

Rayleigh-wave scattering by shallow cracks using the indirect boundary element method

The scattering and diffraction of Rayleigh waves by shallow cracks using the indirect boundary element method (IBEM) are investigated. The detection of cracks is of interest because their presence may compromise structural elements, put technological devices at risk or represent economical potential in reservoir engineering. Shallow cracks may give rise to scattered body and surface waves. These waves are sensitive to the cracks geometry, size and orientation. Under certain conditions, amplitude spectra clearly show conspicuous resonances that are associated with trapped waves. Several applications based on the scattering of surface waves (e.g. Rayleigh and Stoneley waves), such as non-destructive testing or oil well exploration, have shown that the scattered fields may provide useful information to detect cracks and other heterogeneities. The subject is not new and several analytical and numerical techniques have been applied for the last 50 years to understand the basis of multiple scattering phenomena. In this work, we use the IBEM to calculate the scattered fields produced by single or multiple cracks near a free surface. This method is based upon an integral representation of the scattered displacement fields, which is derived from Somiglianas identity. Results are given in both frequency and time domains. The analyses of the displacement field using synthetic seismograms and snapshots reveal some important effects from various configurations of cracks. The study of these simple cases may provide an archetype to geoscientists and engineers to understand the fundamental aspects of multiple scattering and diffraction by cracks.

Two-phase oil–gas pipe flow imaging by simulated annealing

We introduce a numerically improved image reconstruction technique for imaging two-phase oil–gas pipe flows using electrical capacitance tomography, based on simulated annealing and iterative linear forward modelling. In the simulated annealing method, a permittivity image is reconstructed by minimizing iteratively an energy function related to the difference between the measured ECT data and those calculated for an estimated permittivity distribution. The permittivity model is repeatedly updated, in a semi-random process that mimics the thermodynamic phenomenon of crystallization in a liquid that is being cooled. In this work, the forward problem is calculated by using a numerically optimized linear approach that makes use of the sensitivity matrix which is computed in the beginning of the process and is not subsequently updated. The images are refined as they go through the processes of cooling and random perturbation of model parameters until their calculated capacitance data match the measured data in a least-squares sense. As each new model is roughly the same as the previous one except for one perturbed parameter, the forward problem computation can be accelerated by avoiding redundant operations. We found this technique to be faster and more accurate than traditional linear methods commonly used in the context of this application.

Fuzzy logic and image processing techniques for the interpretation of seismic data

Since interpretation of seismic data is usually a tedious and repetitive task, the ability to do so automatically or semi-automatically has become an important objective of recent research. We believe that the vagueness and uncertainty in the interpretation process makes fuzzy logic an appropriate tool to deal with seismic data. In this work we developed a semi-automated fuzzy inference system to detect the internal architecture of a mass transport complex (MTC) in seismic images. We propose that the observed characteristics of a MTC can be expressed as fuzzy if-then rules consisting of linguistic values associated with fuzzy membership functions. The constructions of the fuzzy inference system and various image processing techniques are presented. We conclude that this is a well-suited problem for fuzzy logic since the application of the proposed methodology yields a semi-automatically interpreted MTC which closely resembles the MTC from expert manual interpretation.

A genetic algorithm for filter design to enhance features in seismic images

ABSTRACT We present a novel method to enhance seismic data for manual and automatic interpretation. We use a genetic algorithm to optimize a kernel that, when convolved with the seismic image, appears to enhance the internal characteristics of salt bodies and the sub‐salt stratigraphy. The performance of the genetic algorithm was validated by the use of test images prior to its application on the seismic data. We present the evolution of the resulting kernel and its convolved image. This image was analysed by a seismic interpreter, highlighting possible advantages over the original one. The effects of the kernel were also subject to an automatic interpretation technique based on principal component analysis. Statistical comparison of these results with those from the original image, by means of the Mann‐Whitney U‐test, proved the convolved image to be more appropriate for automatic interpretation.

TESLA GPUs versus MPI with OpenMP for the Forward Modeling of Gravity and Gravity Gradient of Large Prisms Ensemble

An implementation with the CUDA technology in a single and in several graphics processing units (GPUs) is presented for the calculation of the forward modeling of gravitational fields from a tridimensional volumetric ensemble composed by unitary prisms of constant density. We compared the performance results obtained with the GPUs against a previous version coded in OpenMP with MPI, and we analyzed the results on both platforms. Today, the use of GPUs represents a breakthrough in parallel computing, which has led to the development of several applications with various applications. Nevertheless, in some applications the decomposition of the tasks is not trivial, as can be appreciated in this paper. Unlike a trivial decomposition of the domain, we proposed to decompose the problem by sets of prisms and use different memory spaces per processing CUDA core, avoiding the performance decay as a result of the constant calls to kernels functions which would be needed in a parallelization by observations points. The design and implementation created are the main contributions of this work, because the parallelization scheme implemented is not trivial. The performance results obtained are comparable to those of a small processing cluster.

Seismic data interpretation using the Hough transform and principal component analysis

In this work two novel image processing techniques are applied to detect and delineate complex salt bodies from seismic exploration profiles: Hough transform and principal component analysis (PCA). It is well recognized by the geophysical community that the lack of resolution and poor structural identification in seismic data recorded at sub-salt plays represent severe technical and economical problems. Under such circumstances, seismic interpretation based only on the human-eye is inaccurate. Additionally, petroleum field development decisions and production planning depend on good-quality seismic images that generally are not feasible in salt tectonics areas. In spite of this, morphological erosion, region growing and, especially, a generalization of the Hough transform (closely related to the Radon transform) are applied to build parabolic shapes that are useful in the idealization and recognition of salt domes from 2D seismic profiles. In a similar way, PCA is also used to identify shapes associated with complex salt bodies in seismic profiles extracted from 3D seismic data. To show the validity of the new set of seismic results, comparisons between both image processing techniques are exhibited. It is remarkable that the main contribution of this work is oriented in providing the seismic interpreters with new semi-automatic computational tools. The novel image processing approaches presented here may be helpful in the identification of diapirs and other complex geological features from seismic images. Conceivably, in the near future, a new branch of seismic attributes could be recognized by geoscientists and engineers based on the encouraging results reported here.

A texture‐based region growing algorithm for volume extraction in seismic data

ABSTRACT We present a novel approach to automated volume extraction in seismic data and apply it to the detection of allochthonous salt bodies. Using a genetic algorithm, we determine the optimal size of volume elements that statistically, according to the U‐test, best characterize the contrast between the textures inside and outside of the salt bodies through a principal component analysis approach. This information was used to implement a seeded region growing algorithm to directly extract the bodies from the cube of seismic amplitudes. We present the resulting three‐dimensional bodies and compare our final results to those of an interpreter, showing encouraging results.