R-3 Repository :: Browsing by Author "Talwani, Manik"

Browsing by Author "Talwani, Manik"

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A new approach to traveltime selection for Kirchoff migration
(2001) Stieglitz, Theodore Charles; Talwani, Manik
Prestack migration of seismic data is a dominant tool for imaging hydrocarbon bearing structures in the Earth. The standard prestack depth migration is a Kirchhoff approach using single-valued traveltimes computed from eikonal solvers or ray tracing. In simple structures with only one ray path between a surface location and a subsurface point, the single-valued assumption is valid, and the migration yields an acceptable image. However, in complex structures more than one ray path may exist between any image point and any surface location. Although it would be ideal to use all the ray paths, standard practice is to choose a single ray from the multiple arrivals to represent the wave propagation between two points. Standard choices include the ray having the minimum arrival time, the ray with the greatest amplitude or the ray traveling the shortest distance. We are critical of these standard selection rules as these are local comparisons of individual rays of the multi-valued traveltime operator. Single-valued migration operators based upon these selection rules can lead to uncontrolled jumps between the branches of the multi-valued traveltime operator, resulting in poor image quality. In this work we improve the Kirchhoff approach using single-valued traveltimes. We expect that better images can result from selection rules based on analysis of the full multi-valued operator, selecting rays yielding more continuous, yet energetic single-valued operators. We examine a particular selection rule of this type: choosing rays associated with branches of the multi-valued operator having the maximum angular aperture (MAA) about the image point. We provide several examples in 2D and 3D on synthetic and real data comparing the MAA selection rule with standard selection choices. Our results demonstrate that the MAA single-valued operator performs consistently as well as standard single-valued operators.
Application of seismic tools and techniques to ground-penetrating radar (GPR) studies
(1998) Loughridge, James Robert; Talwani, Manik
Seismic techniques and tools have not routinely been transferred to ground-penetrating radar (GPR) studies due to computational, financial, and logistical limitations. A simple processing package was developed to aid in the processing of GPR data for those without access to commercial seismic processing packages. An idealized processing flow was derived and utilized to aid in the processing of GPR datasets. Several three-dimensional GPR datasets were collected, processed, and analyzed. Many limitations were overcome by utilizing a two-pass method of migration adopted from the seismic community. Multi-offset three-dimensional data were acquired to improve imaging by means of derivation of improved subsurface velocity information and increased signal to noise ratio. Many targets with varying shapes, orientations, and electromagnetic properties were buried and successfully imaged. Three-dimensional imaging was much improved and the resolving of features and objects buried in the subsurface became possible with modest computational capabilities.
Characterizing shallow aquifers with wave-propagation based geophysical methods: Imaging and attribute analysis
(1999) Bradford, John; Sawyer, Dale S.; Talwani, Manik
As the results of seismic reflection and ground penetrating radar (GPR) studies become more prevalent as input for quantitative groundwater and engineering studies, it is important to evaluate traditional approaches to data processing and analysis. Where conventional methods fail it is necessary to investigate and/or develop non-traditional approaches to data analysis. I present five stand-alone studies that are focused on characterizing shallow aquifers using seismic reflection and GPR data processing and analysis. Each of the projects involves a new approach to data analysis either through alternative processing strategies that are not widely applied in environmental studies or development of new processing methods and/or algorithms. The first two studies are focused on seismic reflection imaging problems that arise in the shallow environment. I first present a detailed discussion of the errors that can result from conventional normal-moveout (NMO) processing, and the application of pre-stack depth migration (PSDM) to improve image accuracy. Second, I apply dip-moveout (DMO) processing to a data set from the Puget Sound. DMO is rarely applied in environmental studies, but can improve velocity analysis and image quality where there are dipping layers or scattering events. The final three studies are focused on attribute analysis of GPR and seismic reflection data in direct detection studies. Over the past 10--15 years, direct detection has been used successfully in the energy industry to identify oil and gas reserves from exploration scale seismic reflection data, but is a new approach to the analysis of GPR and shallow seismic reflection data. Direct detection studies use reflected wave attributes such as amplitude, frequency content, and phase to estimate material properties. I first present a detailed discussion of GPR amplitude vs. offset (AVO) analysis for direct detection of free phase non-aqueous phase liquid contaminants (NAPLs). The analysis is not straightforward and requires careful consideration of electromagnetic dynamic properties. Second, I present a shallow seismic case study where a predictable AVO response was detected. Finally, I present a new wavelet decomposition and time-frequency representation, and illustrate applications in GPR attenuation analysis for NAPL detection and lithology characterization.
Crossline migration of three-dimensional prestack data
(1996) Rampersad, Tara Rookmin; Talwani, Manik
A new technique for imaging 3-D prestack seismic reflection data has been developed. This method approximates the Kirchhoff one-pass 3-D prestack depth migration and is based on directly splitting the complex one-pass process into a simpler 2-D x 2-D operation. The first step is 2-D migration in the crossline direction. This is performed as a prestack time migration and creates a "true" 2-D line, provided that the crossline structure is not very complex. The second step, inline migration, is used to determine the velocity function and to obtain the depth image. The kinematics and amplitude behavior of this new scheme closely approximate that of the one-pass process. It is a significantly faster process, and offers a practical alternative for adjusting migration velocities, and building a velocity model. When applied to a field data example, the image created by our new technique compares quite favorably to other available 3-D migration schemes.
Finite-difference seismic wave modeling including surface topography
(1999) Hestholm, Stig Ottar; Talwani, Manik
I present synthetics of seismic wave propagation near free surface topography. The velocity-stress formulations of both the full elastic and viscoelastic wave equations are used, and I have derived exact boundary conditions for any arbitrary, smooth topography in terms of the particle velocities. Program codes are developed for 2 and 3 dimensions (2-D and 3-D) using finite-difference (F-D) methods for both spatial and temporal numerical discretizations. An 8th order F-D method is used inside the physical model space, and the spatial F-D order decreases gradually towards the free surface topography. The discretization of the medium equations along the side and bottom boundaries, the free surface topography boundary conditions, and the forward time stepping, are all by 2nd order F-D methods. The leap-frog technique is used for time stepping everywhere except for the memory variable equations in the viscoelastic cases, where an explicit version of the unconditionally stable Crank-Nicholson method is used. I show synthetics applying the schemes to isotropic 2-D and 3-D media covered by topographies that are either described by analytic expressions or by real elevation data. These data are taken from an area in South-Eastern Norway that contains the NORESS seismic receiver array. Domains up to 60 x 60 kilometers are used in 3-D simulations, and the applied sources are plane waves generated by a plane of Ricker type point sources. These sources represent earthquakes or teleseismic explosions. For 2-D simulations I have used both plane waves and point sources, since the larger models permissible in 2-D allow for point sources to represent earthquakes or teleseismic explosions quite well. For 2-D simulations I have also included examples using layered media with randomization by a 2-D von Karman function with and without apparent anisotropy. Synthetic snapshots and seismograms show Rayleigh (Rg)-waves emanating from areas of prominent topography as well as strong surface wave directivity from some topographic features. Full viscoelastic modeling with relatively low Q-values, describing near-surface sedimentary layers, exhibit intrinsic attenuation and physical dispersion of the wavefield. Results coincide with numerous observations. 3-D simulations are performed using domain decomposition parallelization implemented by Message Passing Interface (MPI).
Full volume AVO analysis of seismic reflection data after dip moveout and prestack imaging
(1998) John, Riju; Talwani, Manik
Seismic amplitude-versus-offset (AVO) analysis is a powerful geophysical technique for direct detection of gas on seismic records, and has increased the drilling success ratio in many areas. Due to the shifting of focus from 2-D migration to 3-D migration, to image correctly in regions with complex geological structures, full-volume AVO analysis of seismic reflection data is useful in detecting areas of anomalous AVO attributes (intercept and gradient), in a 3-D seismic data volume. Gardner et al. (1986) introduced 3-D dip moveout (DMO) and 3-D pre-stack imaging (PSI) as an alternative approach to 3-D time migration. One objective of this thesis is to study the amplitude behavior of the DMO and PSI methods, and show that PSI can be used to preserve the amplitude information. This thesis presents a new implementation of the PSI method. The kinematic correctness of this new PSI implementation, and the existing DMO implementation (Canning, 1993) is illustrated using examples from their application to three-dimensional synthetic data. The amplitude behavior of the both these implementations are illustrated using several examples. Based on the amplitude behavior, two different schemes to estimate the AVO attributes (intercept and gradient) are presented. Both these schemes provide reliable estimates of the AVO attributes, for reflection events with dip angles less than 30 degrees. (Abstract shortened by UMI.)
Geology and Petroleum Potential of Central Asia
(James A. Baker III Institute for Public Policy, 1998) Talwani, Manik; Belopolsky, Andrei; Berry, Dianne L.; James A. Baker III Institute for Public Policy
Geophysical study of deep-crustal structure: Mid-Atlantic passive continental margin
(1997) Lang, Zhongmin; Talwani, Manik
Migration results of two multi-channel vertical incidence reflection seismic dataset offshore Virginia are presented. A new velocity model derived from multi-channel and wide-angle reflection/refraction data is developed and fit into regional geology context. The validity and limitation of the new model is discussed by means of ray-tracing travel time forward modeling and 2-D acoustic wave-equation modeling methods. Our new results show that there is a sharp boundary separating normal continental crust from crust produced by the sea-floor spreading process in this region and that the Moho appears to be continuous across the boundary. The zone of so-called "magmatic underplating" proposed previously (Sheridan et al., 1993) may well be magmas produced during the initial stage of sea-floor spreading. There may be some fragments of continental crust entrained in the region immediately seaward of the boundary, but there is no evidence of the existence of large volume of continental crust and we believe the crust in this area is also primarily product of the Mesozoic sea-floor spreading. (Abstract shortened by UMI.)
Oil and Gas in Mexico: Geology, Production Rates and Reserves
(James A. Baker III Institute for Public Policy, 2011) Talwani, Manik; James A. Baker III Institute for Public Policy
Historically, Mexico has been one of the largest producers of oil in the world. At one time its production rate was second only to Saudi Arabia. But existing reserves are low, and at the present rate of production, will last only about nine more years. The source rocks, for the most part, were deposited in the early Mesozoic. Differing geological episodes in different parts of the country, as well as differences in the type of kerogen, have resulted in the production of differing varieties of hydrocarbons. In the north, the Laramide orogeny resulted in a mountain chain, the Sierra Madre Oriental, that in geological terms is known as a fold and thrust belt. Paralleling this chain is a deep valley, known geologically as the foredeep, in which thick layers of sediment were deposited. The deep burial of source rocks led to their overmaturation and consequent generation of gas, which is produced in the Sabinas basin and in the western parts of the Burgos and Veracruz basins. The reservoirs that contain the gas are often Tertiary sands. Almost all the production of oil at the present time comes from the Sureste (southeast) basin. This area was not subject to an orogeny that produced a fold and thrust belt and a parallel foredeep. In the offshore parts of this basin, salt tectonics are most important in giving rise to structural and stratigraphic traps. The source beds containing oil-prone kerogens did not overmature and much oil was generated. However, production in the Sureste basin is declining, which is a source of much concern. As far as the future is concerned, the Chicontopec basin with its large deposits of “in-place heavy oil” is an important target. The geological nature of the reservoir makes it difficult to apply conventional enhanced oil recovery (EOR) techniques. Barring sudden technological breakthroughs in the development of new EOR techniques, a process that is generally incremental, the recovery rates will only increase slowly, taking many years. Exploration in the deep sea with the possibility of large subsalt reservoirs also holds much promise. Known technology is applicable, but even discoveries based on ongoing geophysical surveys will lead to production of oil in no less than three to five years in the most optimistic case. In any event, large investments will be necessary, and risks associated with these efforts will be large.
Small Venusian coronae
(1993) Rupert, Suzanne Tatin; Talwani, Manik
The common defining factor of coronae is a distinct annulus of tectonic origin. Current coronae classification is based on an intensive study of coronae primarily with maximum widths in excess of 100 km. Topographically coronae with maximum widths in excess of 250 km tend to exhibit raised interiors with exterior rim and moat structures, which may or may not correspond to coronae annuli. These coronae are believed to be the result of gravitational relaxation of a diapir or hot-spot generated uplift. This mode of formation however, does not account for small coronae, coronae with diameters less than 200 km, that tend to exhibit depressed interiors with down-dropped or topographically level annuli. These small coronae represent caldera-like activity on Venus.
The Orinoco Heavy Oil Belt in Venezuela (Or Heavy Oil to the Rescue?)
(James A. Baker III Institute for Public Policy, 2002) Talwani, Manik; James A. Baker III Institute for Public Policy
Three-dimensional AVO analysis: A modeling approach
(1996) Lodh, Amit; Talwani, Manik
3D AVO (Amplitude Variation with Offset) analysis has evolved as an important interpretation method of 3D seismic data, in the last decade. A new approach is proposed in the present work for quantitative AVO analysis of 3D seismic data. The method is based on fitting the observed seismic data, in a least-squares approach, to a function of average angle (average of angle of incidence and angle of transmittance) or offset. The aim of the method is to obtain the estimates of changes in the elastic parameters (P-wave modulus, S-wave modulus and density) at a particular output location, using a combination of pre-stack Kirchhoff migration and matrix inversion. The method includes ways to do migration velocity analysis and estimation of dip. As a further extension, the method is modified to find out the orientation of vertical fractures and the extent of anisotropy, for the case when an isotropic medium overlies a vertically fractured medium.

Browsing by Author "Talwani, Manik"

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