1,720,988 research outputs found
Improved Green’s functions from seismic interferometry
No full textUnder certain theoretical assumptions, the theory of seismic interferometry allows the construction of artificial (or virtual) sources and
receivers at the locations of receivers in a physical experiment. This
is done by redatuming the physical sources to be at the locations of
the physical receivers. Each redatumed trace is formed by stacking
the cross-correlations of appropriate recorded traces from each physical shot. For the resulting stacked traces to be a valid approximation
certain requirements, like an adequate number of surface sources with
a small enough spacing in the acquisition geometry, must be met. If
these requirements are not met, the resulting virtual shot gather will
contain artifacts. In this paper, we analyze both the sets of correlated
traces (correlograms) and their stack. We observe that it is possible
to reduce certain artifacts in the stacked traces by novel filtering operations. These filtering operations may have broad utility in all of
seismic interferometric applications.Massachusetts Institute of Technology. Earth Resources Laborator
Fractured Reservoir Characterization using Azimuthal AVO
No full textOrdinary least squares is used to investigate the ability to detect changes in physical properties
using Amplitude Versus Offset (AVO) information collected from seismic data. In order to characterize
vertically aligned fractures within a reservoir, this method is extended to Azimuthal AVO (AVOA)
analysis. Azimuthal AVO has the potential not only to detect fractured zones, but to spatially describe the
fracture strike orientation and changes in fracture or fluid properties. Depending on the data acquisition
geometry, signal-to-noise ratio, and extent of fracturing, AVOA analysis can be marginally successful. A
study of the robustness and limitations of AVOA analysis is therefore first classified with synthetic data.
These methods are then applied to seismic data collected during an Ocean Bottom Cable (OBC) survey
over a known fractured reservoir.Massachusetts Institute of Technology. Earth Resources LaboratoryUnited States. Dept. of Energy (Grant DE-FC26-02NT15346)Eni S.p.A. (Firm
Comparison Of Scattered Energy Using Point Scatterers Versus Full 3D Finite Difference Modeling
No full textWe present results of 3D numerical modeling using a series of simple point scatterers to create synthetic seismic shot records collected over regular, discrete, vertical fracture systems. The background medium is taken to be constant velocity. The model contains a series of point scatterers delineating the top tip and bottom tip of each vertical fracture. We use these results to gain an understanding of some of the features seen in full 3D elastic modeling of vertical fractures. We compare our results to those of Willis et al (2003) and Willis et al (2004) for their 5 layer model with 50m spacing between discrete, vertical fractures. Our modeling shows that a series of back scattered events with both positive and negative moveouts are observed when the shot record is oriented normal to the direction of fracturing. When the shot record is both located in the middle of the fractured zone and is oriented normal to the direction of fracturing, a complicated series of beating is observed in the back scattered energy. When the shot record is oriented parallel to the fracturing, ringing wavetrains are observed which moveouts similar to reflections from many horizontal layers. The point scattering models are, in general, very consistent with the full 3D elastic modeling results.Eni S.p.A. (Firm)United States. Dept. of Energy (Grant number DE-FC26-02NT15346)Massachusetts Institute of Technology. Earth Resources Laborator
Interferometric correlogram-space analysis
No full textSeismic interferometry is a method of obtaining a virtual shot
gather from a collection of actual shot gathers. The set of traces
corresponding to multiple actual shots recorded at two receivers is
used to synthesize a virtual shot located at one of the receivers and
a virtual receiver at the other. An estimate of a Green’s function between
these two receivers is obtained by first cross-correlating pairs
of traces from each of the common shots and then stacking the resulting
cross-correlograms. In this paper, we study the structure of
cross-correlograms obtained from a VSP acquisition geometry using a
surface source reflected by flat or dipping layers and/or diffracted by
point inclusions. The model is purely acoustic. The shape of events
in the cross-correlogram space can be used to infer the location and
geometry of a subsurface structure. A pilot wavelet created by a curvilinear
stacking process is used as a detector of predicted events in the
cross-correlogram. Results of a semblance-based velocity scan of the
cross-correlograms using curvilinear stacks can be used to improve the
quality of the virtual gather.Massachusetts Institute of Technology. Earth Resources Laborator
Elastodynamic Redatumming for VSP Salt Dome Flank Imaging
No full textIn this paper we construct an application methodology for the theoretical elastodynamic
redatumming work by Wapenaar (2006) to be able to approximate its use in the problem of
imaging a salt dome flank using a walk away VSP (WVSP). Different from acoustic redatumming
where only one quantity, the pressure, is involved, full elastodynamic redatumming requires both
P-wave source and S-wave sources with different polarizations. We approximate these P-wave and
S-wave sources by implementing a shooting pattern for each shot position of an ordinary WVSP
acquisition. We create a synthetic WVSP dataset with modified shooting pattern for a model
composed of a simplified Gulf of Mexico vertical-velocity gradient, an embedded overhanging
salt dome together with a nearby salt canopy. To process these data, we first compile the raw
dataset due to the shooting pattern into a new dataset which approximates common receiver
gathers due to P-wave source and S-wave sources, respectively. We then apply elastodynamic
redatumming to this new dataset to mimic the response between different components for
downhole source and receiver pairs. The reconstructed traces show a good match to the traces in
the benchmark case, which we create by firing actual downhole sources and recording the
responses at all downhole receivers. Instead of a more proper, full elastic migration, we apply
simple acoustic migration on individual components of the full elastic Green’s functions to
produce pre-stack depth migrated images of the salt dome flank and the dipping sediments. By
performing four migrations of these components, we obtain very good separate images of the
subsurface using predominately either P or S wave energy. The horizontal sediments and vertical
salt flank decouple and appear on separate images.Massachusetts Institute of Technology. Earth Resources Laborator
Fracture Characterization from Scattered Energy: A Case Study
No full textWe use 3D surface seismic data to determine the presence and the preferred orientation of fracture corridors in a field. The Scattering Index method is proving to be a robust tool for detecting and mapping fracture corridors. Fracture corridors largely control permeability and fluid flow in some fractured reservoirs. To apply the Scattering Index method, we compute the scattering transfer functions from the reservoir interval using prestack migrated data collected in four azimuth sectors. By measuring the azimuthal differences in the amount of scattering, we obtain maps of density of fracture corridors and their orientation across the survey area. We use geostatistical filtering to improve the spatial correlation of scattering index maps. The distribution and orientation of the final fracture corridors are interpreted considering the structure, fault network, and stress information. In the field, we observe several regions of high fracturing near the anticline’s crest and on its steepest slopes, on the southwest flank. Around well locations, fractures are oriented to the NW and NNW, which agrees with estimates of maximum stress direction from well data.Massachusetts Institute of Technology. Earth Resources Laborator
Going Beyond Counting First Authors in Author Co-citation Analysis
Full text linkThe present study examines one of the fundamental aspects of author co-citation analysis (ACA) - the way co-citation
counts are defined. Co-citation counting provides the data on which all subsequent statistical analyses and mappings
are based, and we compare ACA results based on two different types of co-citation counting - the traditional type that
only counts the first one among a cited work's authors on the one hand and a non-traditional type that takes into
account the first 5 authors of a cited work on the other hand. Results indicate that the picture produced through this non-traditional author co-citation counting contains more coherent author groups and is therefore considerably clearer. However, this picture represents fewer specialties in the research field being studied than that produced through the traditional first-author co-citation counting when the same number of top-ranked authors is selected and analyzed. Reasons for these effects are discussed
Orientation Estimation for Multiple Large Fractures by Scattering Energy
No full textWe have done the numerical modeling of seismic response to multiple sets of vertical large fractures by using finite-difference method (FD), which can easily handle media with monoclinic anisotropy. We consider three types of fracture distributions: a set of parallel fractures, two sets of orthogonal fractures and two sets of non-orthogonal fractures intersecting at 45 degrees. We address the seismic scattering response to large fractures by using a 3-layer model and a 5-layer model, where a fractured reservoir is in the middle layer of these two models. Seismic scattered energy is analyzed by the Scattering Index (SI) method to estimate the orientation of these multiple fractures. In both models, SI indicates the correct orientation of the two orthogonal fracture sets but is ambiguous for non-orthogonal fracture sets. Information about the fracture spacing and compliance can also be extracted from the azimuthal SI in some situations. More compliant fracture sets result in higher SI values while the relationship between fracture spacing and SI depends on the source wavelength. Variations in the SI energy can be caused by fracture spacing and compliance variations, and these relationships need further investigation.United States. Dept. of Energy (Award No. DE-FC26-02NT15346)Massachusetts Institute of Technology. Earth Resources Laborator
A Novel Application of Time Reversed Acoustics: Salt Dome Flank Imaging Using Walkaway VSP surveys
No full textGEOPHYSICS, VOL. 71, NO. 2 (MARCH-APRIL 2006); P. A7–A11, 4 FIGS.
10.1190/1.2187711In this paper we present initial results of applying Time-Reversed Acoustics (TRA) technology to saltdome flank, seismic imaging. We created a set of synthetic traces representing a multilevel, walkaway VSP for a model composed of a simplified Gulf of Mexico vertical-velocity gradient and an embedded salt dome. We first applied the concepts of TRA to the synthetic traces to create a set of redatummed traces without having to perform velocity analysis, moveout corrections, or complicated processing. Each redatummed trace approximates the output of a zero-offset, downhole source and receiver pair. To produce the final salt-dome flank image, we then applied conventional, poststack, depth migration to the zero-offset section. Our results show a very good image of the salt when compared to an image derived using data from a downhole, zero-offset source and receiver pairs. The simplicity of our TRA implementation provides a virtually automated method to estimate a zero-offset, seismic section as if it had been collected from the reference frame of the borehole containing the VSP survey.Massachusetts Institute of Technology. Earth Resources LaboratoryUnited States. Air Force Research Laboratory (Contract F19628-03-C-0126)Shell Gamechange
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