scholarly journals Depth characterization of shallow aquifers with seismic reflection, Part II—Prestack depth migration and field examples

Geophysics ◽  
2002 ◽  
Vol 67 (1) ◽  
pp. 98-109 ◽  
Author(s):  
John H. Bradford ◽  
D. S. Sawyer
Keyword(s):  
Seismic Reflection ◽  
Water Saturation ◽  
Compressional Wave ◽  
Unconsolidated Sediments ◽  
Prestack Depth Migration ◽  
Improve Image Quality ◽  
Depth Migration ◽  
Shallow Seismic ◽  
Partial Water

It is common in shallow seismic studies for the compressional‐wave velocity in unconsolidated sediments to increase by a factor of four or more at the transition from dry or partial water saturation to full saturation. Under these conditions, conventional NMO velocity analysis fails and leads to large depth and layer thickness estimates if the Dix equation is assumed valid. Prestack depth migration (PSDM) is a means of improving image accuracy. A comparison of PSDM with conventional NMO processing for three field examples from differing hydrogeologic environments illustrates that PSDM can significantly improve image quality and accuracy.

scholarly journals Imaging complex structure in shallow seismic-reflection data using prestack depth migration

Geophysics ◽  
2006 ◽  
Vol 71 (6) ◽  
pp. B175-B181 ◽  
Author(s):  
John H. Bradford ◽  
Lee M. Liberty ◽  
Mitch W. Lyle ◽  
William P. Clement ◽  
Scott Hess
Keyword(s):  
Seismic Reflection ◽  
Complex Structure ◽  
Volcanic Rocks ◽  
Depth Range ◽  
Prestack Depth Migration ◽  
Measured Velocity ◽  
Seismic Reflection Data ◽  
Depth Migration ◽  
Time Migration ◽  
Shallow Seismic

Prestack depth migration (PSDM) analysis has the potential to significantly improve the accuracy of both shallow seismic reflection images and the measured velocity distributions. In a study designed to image faults in the Alvord Basin, Oregon, at depths from [Formula: see text], PSDM produced a detailed reflection image over the full target depth range. In contrast, poststack time migration produced significant migration artifacts in the upper [Formula: see text] that obscured reflection events and limited the structural interpretation in the shallow section. Additionally, an abrupt increase from [Formula: see text] to [Formula: see text] in the PSDM velocity model constrained the interpretation of the transition from sedimentary basin fill to basement volcanic rocks. PSDM analysis revealed a complex extensional history with at least two distinct phases of basin growth and a midbasin basement high that forms the division between two major basin compartments.

Coal seismic surveying over near-surface basalts: Experience from Central Queensland, Australia

Geophysics ◽  
2014 ◽  
Vol 79 (2) ◽  
pp. B109-B122 ◽  
Author(s):  
Binzhong Zhou ◽  
Peter Hatherly ◽  
Troy Peters ◽  
Weijia Sun
Keyword(s):  
Surface Waves ◽  
Seismic Reflection ◽  
Unconsolidated Sediments ◽  
Successful Outcome ◽  
Low Velocity ◽  
Prestack Depth Migration ◽  
Seismic Line ◽  
Near Surface ◽  
Depth Migration ◽  
Seismic Surveying

Seismic reflection surveying in basalt-covered areas often fails to image underlying reflectors. To gain insights into the nature of the problem and obtain potential solutions, we have conducted experimental 2D seismic reflection and offset VSP surveys at two coal mines in the Bowen Basin of Australia. At the first mine, the basalt is relatively deep (114 m) and relatively thin (20 m). Conventional seismic acquisition and processing of a 2D seismic line provide poor results. However, upgoing reflections from layers below the basalt are clearly evident in the VSP survey and prestack depth migration is able to improve the continuity of the reflectors beneath the basalt. At the second mine, the 360 m wide basalt is at a depth of 40 m and has a thickness of about 40 m. It is fresh and unweathered and consists of multiple flows which are interlayered with unconsolidated sediments. Long-offset data acquisition combined with prestack depth migration was expected to produce satisfactory results but this is not the case. The associated VSP survey suggests that the problems at this mine are due to (1) the generation of complex downgoing and upgoing wave-fields within the basalt and (2) significant scattering of surface waves from outside the basalt at the margins of the basalt. Another problem is that the target coal seams are at about 300 m depth and the muting required to remove refraction events limited the effectiveness of the prestack depth migration. Reducing the strength of the surface waves through selection of an appropriate source and placement of shots at the base of the low-velocity zone (as had been the case at the first mine) will therefore improve the chances for a successful outcome. A Vibroseis survey subsequently undertaken at the second mine, which produced shot records with reduced surface waves, shows this to be the case.

scholarly journals Depth characterization of shallow aquifers with seismic reflection, Part I—The failure of NMO velocity analysis and quantitative error prediction

Geophysics ◽  
2002 ◽  
Vol 67 (1) ◽  
pp. 89-97 ◽  
Author(s):  
John H. Bradford
Keyword(s):  
Layer Thickness ◽  
Water Table ◽  
Seismic Reflection ◽  
Negative Impact ◽  
Compressional Wave ◽  
Unconsolidated Sediments ◽  
Velocity Analysis ◽  
Seismic Reflection Data ◽  
Reflection Data ◽  
Saturated Layer

As seismic reflection data become more prevalent as input for quantitative environmental and engineering studies, there is a growing need to assess and improve the accuracy of reflection processing methodologies. It is common for compressional‐wave velocities to increase by a factor of four or more where shallow, unconsolidated sediments change from a dry or partially water‐saturated regime to full saturation. While this degree of velocity contrast is rare in conventional seismology, it is a common scenario in shallow environments and leads to significant problems when trying to record and interpret reflections within about the first 30 m below the water table. The problem is compounded in shallow reflection studies where problems primarily associated with surface‐related noise limit the range of offsets we can use to record reflected energy. For offset‐to‐depth ratios typically required to record reflections originating in this zone, the assumptions of NMO velocity analysis are violated, leading to very large errors in depth and layer thickness estimates if the Dix equation is assumed valid. For a broad range of velocity profiles, saturated layer thickness will be overestimated by a minimum of 10% if the boundary of interest is <30 m below the water table. The error increases rapidly as the boundary shallows and can be very large (>100%) if the saturated layer is <10 m thick. This degree of error has a significant and negative impact if quantitative interpretations of aquifer geometry are used in aquifer evaluation such as predictive groundwater flow modeling or total resource estimates.

Specular ray parameter extraction and stationary‐phase migration

Geophysics ◽  
2004 ◽  
Vol 69 (1) ◽  
pp. 249-256 ◽  
Author(s):  
Jing Chen
Keyword(s):  
Stationary Phase ◽  
Seismic Reflection ◽  
Parameter Extraction ◽  
Phase Approximation ◽  
Full Data ◽  
Prestack Depth Migration ◽  
Depth Migration ◽  
Kirchhoff Type ◽  
Migration Operator ◽  
Ray Parameter

The Kirchhoff‐type prestack depth migration operator is a diffraction stack of seismic reflection energies over seismic traces. Typically, the diffraction stack is carried out over the full data aperture, producing not only images of reflectors but also aliasing artifacts. Aliasing artifacts often break reflector continuity in images. The stationary‐phase approximation to the migration operator shows that, for image points on reflectors, traces within the neighborhood of specular rays contribute most to the diffraction stack. Traces outside this vicinity introduce aliasing artifacts into the image, especially in the case of coarse trace spacing and aperture truncation. A new migration algorithm, denoted as stationary‐phase migration, is proposed to find the specular ray parameters and then to migrate the specular‐ray energies and reject nonspecular ray energies to yield images with less aliasing.

scholarly journals Shallow seismic AVO variations related to partial water saturation during a pumping test

2007 ◽  
Vol 34 (22) ◽  
Author(s):  
Steven D. Sloan ◽  
Georgios P. Tsoflias ◽  
Don W. Steeples
Keyword(s):  
Water Saturation ◽  
Pumping Test ◽  
Shallow Seismic ◽  
Partial Water

Estimation of anisotropy and anisotropic 3-D prestack depth migration, offshore Zaire

Geophysics ◽  
1995 ◽  
Vol 60 (5) ◽  
pp. 1495-1513 ◽  
Author(s):  
Greg Ball
Keyword(s):  
Rock Physics ◽  
Transversely Isotropic ◽  
Compressional Wave ◽  
Seismic Profile ◽  
Well Log ◽  
Velocity Measurements ◽  
Low Velocity ◽  
Vertical Seismic Profile ◽  
Prestack Depth Migration ◽  
Depth Migration

Chevron (operator), Teikoku Oil, and Unocal hold the offshore Zaire concession, West Africa. Recently, Unocal performed an anisotropic 3-D prestack depth migration in an attempt to optimally image tilted fault blocks in the presalt section. Depth migration was required because large‐displacement faults juxtaposed a high‐velocity massive carbonate and a low‐velocity marl. Large compressional‐wave anisotropy caused by thin layering was inferred for most of the depth section. Evidence on the nature and magnitude of the anisotropy came from comparing stacking velocities with well velocities, analyzing a multioffset vertical seismic profile (VSP), thin‐layer modeling in the massive carbonate using well log data, and rock physics laboratory velocity measurements on core from the marl. The subsurface was characterized as being locally transversely isotropic and the Thomsen parameters were used to describe the anisotropy. This required that five parameters be specified at each subsurface location to represent the compressional‐wave 3-D velocity field. The observed anisotropy deviated far from the elliptical condition, and modeling indicated that isotropic migration would exhibit misfocusing and mispositioning of events, and its quality would be dip‐dependent. The anisotropic migration was performed using Kirchhoff summation for which the traveltimes were determined using a finite‐difference scheme. Interpreted horizons in the migrated depth section accurately tied well depths.

On: “Pulse distortion in depth migration,” M. Tygel, J. Schleicher, and P. Hubral (October 1994 GEOPHYSICS, 59, p. 1561–1569).

Geophysics ◽  
1995 ◽  
Vol 60 (6) ◽  
pp. 1942-1944 ◽  
Author(s):  
Arthur E. Barnes
Keyword(s):  
Seismic Reflection ◽  
Prestack Depth Migration ◽  
Seismic Reflection Data ◽  
Depth Migration ◽  
Reflection Data ◽  
Time Migration ◽  
Pulse Distortion ◽  
Nmo Stretch ◽  
Frequency Shifting

Tygel et al. have written an excellent and rigorous discussion of pulse distortion in seismic reflection data caused by prestack depth migration. Such distortion is easily understood by recognizing that it is more or less the same effect as normal moveout (NMO) stretch combined with frequency shifting due to poststack time migration.

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