8. Wavefront Methods

Rockwell, Donald W.; Schenck, Frederick L.; Musgrave, A. W.; Woolley, W. C.; Gray, H.

doi:10.1190/1.9781560802679.ch5d

Cited by 24 publications

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“…Wavefront construction methods (Rockwell 1967) are usually regarded as the most appropriate because they do not restrict the interpretation model in any way. However, they do require complete definition of all layers above the target refractor.…”

Section: Can Narrow Low-velocity Zones Be Resolved ?mentioning

confidence: 99%

THE RESOLUTION OF NARROW LOW‐VELOCITY ZONES WITH THE GENERALIZED RECIPROCAL METHOD¹

Palmer

1991

Geophysical Prospecting

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PALMER, D. 1991. The resolution of narrow low-velocity zones with the generalized reciprocal method. Geophysical Prospecting 39,1031-1060.The depth to the surface of a refractor and the seismic velocity within the refractor are very often intimately related. In the shallow environment, increased thicknesses of weathering occur in areas of jointing, shearing or lithological variations, and these zones of deeper weathering can have lower subweathering refractor velocities. This association is important in geotechnical investigations and in the measurement of weathering thicknesses and subweathering velocities for statics corrections for reflection seismic surveys.Algorithms, which employ forward and reverse traveltime data and which explicitly accommodate the offset distance through the process known as refraction migration, are necessary if detailed structure on a refractor and rapid lateral variations of the seismic velocity within it are to be resolved. These requirements are satisfied with wavefront construction techniques, Hales' method and the generalized reciprocal method (GRM).However, these methods employ refraction migration in fundamentally different manners. Most methods compute an offset distance with an often imprecise knowledge of the seismic velocities of the overlying layers. In contrast, the GRM uses a range of offset distances from less than to greater than the optimum value, with the optimum value being selected with a minimum-varjance criterion.The approach of the GRM is essential where there are undetected layers and where there are rapid variations in the depth to a refractor and the seismic velocity within it. In the latter situations the offset distance necessary to define the seismic velocities can differ considerably from the value required to define depths.The etlicacy of the GRM in resolving structure and seismic velocity is demonstrated with three model Studies and two field examples.

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Section: Can Narrow Low-velocity Zones Be Resolved ?mentioning

confidence: 99%

THE RESOLUTION OF NARROW LOW‐VELOCITY ZONES WITH THE GENERALIZED RECIPROCAL METHOD¹

Palmer

1991

Geophysical Prospecting

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“…The basal 500 m penetrated by the borehole consisted of rhyodacite porphyry whose sonic velocity is approximately 5.2 km/s. Inversion into a velocity model was done using interactive ray-tracing programs [Ackerrnann, 1979], which adapt wave front techniques of Rockwell [1967], Schenck [1967], and Thornburgh [1930]. This method requires t-x plots between reversed shot point pairs which overlap in the subsurface for each layer being interpreted.…”

Section: Data Acquisitionmentioning

confidence: 99%

Structure along the northwest edge of the Snake River Plain interpreted from seismic refraction

Pankratz¹,

Ackermann²

1982

J. Geophys. Res.

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The results of a seismic refraction survey at the Idaho National Engineering Laboratory along the northwest boundary of the Snake River Plain show that velocities of the volcanic rocks beneath the Plain increase from 1.5 km/s at the surface to 5.2–5.5 km/s at depths between approximately 2200 and 2500 m. An exploration well in the area (INEL‐1) encountered Cenozoic volcanic rock with some interbedded sediments to a depth of 2460 m underlain by 700 m of a rhyodacite porphyry. The refraction data demonstrate a nearly vertical faultlike discontinuity of about 1000‐m displacement beneath the plain, about 1.8 km from the southeast flank of the Arco Hills (Lost River Range). Whether this discontinuity represents the northwest flank of the Snake River Plain graben, the east flank of the Lost River Range fault, Or a caldera wall is not known. The results further indicate that Paleozoic rocks may extend beneath the plain as far as 5.6 km southeastward from the Arco Hills.

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“…For more than 50 years, the major application of the seismic refraction method has been the delineation of weathered layers for environmental, geotechnical and groundwater studies, and for statics corrections for seismic reflection surveys. Since the near-surface region can often exhibit considerable variability in both depth and seismic velocity, detailed 2D refraction inversion methods, such as wavefront reconstruction (Thornburg 1930;Rockwell 1967;Aldridge and Oldenburg 1992), Hales' method (Hales 1958;Sjogren 1979Sjogren , 1984, the generalized reciprocal method (GRM) (Palmer 1980(Palmer , 1981(Palmer , 1986(Palmer , 1991 and the refraction convolution section (Palmer 2001b(Palmer , 2001cPalmer and Jones 2005;de Franco 2005), can be required.…”

Section: Introductionmentioning

confidence: 99%

Refraction traveltime and amplitude corrections for very near‐ surface inhomogeneities

Palmer

2006

Geophysical Prospecting

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A B S T R A C TThe performance of refraction inversion methods that employ the principle of refraction migration, whereby traveltimes are laterally migrated by the offset distance (which is the horizontal separation between the point of refraction and the point of detection on the surface), can be adversely affected by very near-surface inhomogeneities. Even inhomogeneities at single receivers can limit the lateral resolution of detailed seismic velocities in the refractor.The generalized reciprocal method 'statics' smoothing method (GRM SSM) is a smoothing rather than a deterministic method for correcting very near-surface inhomogeneities of limited lateral extent. It is based on the observation that there are only relatively minor differences in the time-depths to the target refractor computed for a range of XY distances, which is the separation between the reverse and forward traveltimes used to compute the time-depth. However, any traveltime anomalies, which originate in the near-surface, migrate laterally with increasing XY distance. Therefore, an average of the time-depths over a range of XY values preserves the architecture of the refractor, but significantly minimizes the traveltime anomalies originating in the near-surface. The GRM statics smoothing corrections are obtained by subtracting the average time-depth values from those computed with a zero XY value. In turn, the corrections are subtracted from the traveltimes, and the GRM algorithms are then re-applied to the corrected data. Although a single application is generally adequate for most sets of field data, model studies have indicated that several applications of the GRM SSM can be required with severe topographic features, such as escarpments.In addition, very near-surface inhomogeneities produce anomalous head-wave amplitudes. An analogous process, using geometric means, can largely correct amplitude anomalies. Furthermore, the coincidence of traveltime and amplitude anomalies indicates that variations in the near-surface geology, rather than variations in the coupling of the receivers, are a more likely source of the anomalies.The application of the GRM SSM, together with the averaging of the refractor velocity analysis function over a range of XY values, significantly minimizes the *

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8. Wavefront Methods

Cited by 24 publications

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THE RESOLUTION OF NARROW LOW‐VELOCITY ZONES WITH THE GENERALIZED RECIPROCAL METHOD¹

THE RESOLUTION OF NARROW LOW‐VELOCITY ZONES WITH THE GENERALIZED RECIPROCAL METHOD¹

Structure along the northwest edge of the Snake River Plain interpreted from seismic refraction

Refraction traveltime and amplitude corrections for very near‐ surface inhomogeneities

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8. Wavefront Methods

Cited by 24 publications

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THE RESOLUTION OF NARROW LOW‐VELOCITY ZONES WITH THE GENERALIZED RECIPROCAL METHOD1

THE RESOLUTION OF NARROW LOW‐VELOCITY ZONES WITH THE GENERALIZED RECIPROCAL METHOD1

Structure along the northwest edge of the Snake River Plain interpreted from seismic refraction

Refraction traveltime and amplitude corrections for very near‐ surface inhomogeneities

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THE RESOLUTION OF NARROW LOW‐VELOCITY ZONES WITH THE GENERALIZED RECIPROCAL METHOD¹

THE RESOLUTION OF NARROW LOW‐VELOCITY ZONES WITH THE GENERALIZED RECIPROCAL METHOD¹