scholarly journals Can near-surface velocity structure be improved via dispersion analysis of conventional reflection data?

2015 ◽  
Vol 2015 (1) ◽  
pp. 1-5
Author(s):  
Shaun Strong ◽  
Steve Hearn
Keyword(s):  
Velocity Structure ◽  
Dispersion Analysis ◽  
Surface Velocity ◽  
Near Surface ◽  
Reflection Data

Refraction wavefield migration

Geophysics ◽  
2020 ◽  
Vol 85 (6) ◽  
pp. Q27-Q37
Author(s):  
Yang Shen ◽  
Jie Zhang
Keyword(s):  
Signal To Noise Ratio ◽  
Surface Velocity ◽  
Data Set ◽  
Near Surface ◽  
Depth Migration ◽  
Reflection Data ◽  
Imaging Tool ◽  
Imaging Results ◽  
Migration Method ◽  
Single Method

Refraction methods are often applied to model and image near-surface velocity structures. However, near-surface imaging is very challenging, and no single method can resolve all of the land seismic problems across the world. In addition, deep interfaces are difficult to image from land reflection data due to the associated low signal-to-noise ratio. Following previous research, we have developed a refraction wavefield migration method for imaging shallow and deep interfaces via interferometry. Our method includes two steps: converting refractions into virtual reflection gathers and then applying a prestack depth migration method to produce interface images from the virtual reflection gathers. With a regular recording offset of approximately 3 km, this approach produces an image of a shallow interface within the top 1 km. If the recording offset is very long, the refractions may follow a deep path, and the result may reveal a deep interface. We determine several factors that affect the imaging results using synthetics. We also apply the novel method to one data set with regular recording offsets and another with far offsets; both cases produce sharp images, which are further verified by conventional reflection imaging. This method can be applied as a promising imaging tool when handling practical cases involving data with excessively weak or missing reflections but available refractions.

Estimation of the near-surface velocity structure of the Yasur-Yenkahe volcanic complex, Vanuatu

2012 ◽  
Vol 227-228 ◽  
pp. 50-60 ◽  
Author(s):  
Laurence Perrier ◽  
Jean-Philippe Métaxian ◽  
Jean Battaglia ◽  
Esline Garaebiti
Keyword(s):  
Velocity Structure ◽  
Surface Velocity ◽  
Volcanic Complex ◽  
Near Surface

Turning‐ray tomography

Geophysics ◽  
1995 ◽  
Vol 60 (6) ◽  
pp. 1917-1929 ◽  
Author(s):  
Joseph P. Stefani
Keyword(s):  
Velocity Structure ◽  
Initial Point ◽  
Real Data ◽  
Point Sources ◽  
Surface Velocity ◽  
Inversion Algorithm ◽  
Low Velocity ◽  
Linear Inversion ◽  
Near Surface ◽  
Velocity Inversion

Turning‐ray tomography is useful for estimating near‐surface velocity structure in areas where conventional refraction statics techniques fail because of poor data or lack of smooth refractor/velocity structure. This paper explores the accuracy and inherent smoothing of turning‐ray tomography in its capacity to estimate absolute near‐surface velocity and the statics times derived from these velocities, and the fidelity with which wavefields collapse to point diffractors when migrated through these estimated velocities. The method comprises nonlinear iterations of forward ray tracing through triangular cells linear in slowness squared, coupled with the LSQR linear inversion algorithm. It is applied to two synthetic finite‐ difference data sets of types that usually foil conventional refraction statics techniques. These models represent a complex hard‐rock overthrust structure with a low‐velocity zone and pinchouts, and a contemporaneous near‐shore marine trench filled with low‐ velocity unconsolidated deposits exhibiting no seismically apparent internal structure. In both cases velocities are estimated accurately to a depth of one‐ fifth the maximum offset, as are the associated statics times. Of equal importance, the velocities are sufficiently accurate to correctly focus synthetic wavefields back to their initial point sources, so migration/datuming applications can also use these velocities. The method is applied to a real data example from the Timbalier Trench in the Gulf of Mexico, which exhibits the same essential features as the marine trench synthetic model. The Timbalier velocity inversion is geologically reasonable and yields long and short wavelength statics that improve the CMP gathers and stack and that correctly align reflections to known well markers. Turning‐ray tomography estimates near‐surface velocities accurately enough for the three purposes of lithology interpretation, statics calculations, and wavefield focusing for shallow migration and datuming.

scholarly journals Crustal velocity structure of the Superior and Grenville provinces of the southeastern Canadian Shield

1997 ◽  
Vol 34 (8) ◽  
pp. 1167-1184 ◽  
Author(s):  
S. Winardhi ◽  
R. F. Mereu
Keyword(s):  
Greenstone Belt ◽  
Velocity Structure ◽  
Canadian Shield ◽  
Crustal Thickening ◽  
Surface Velocity ◽  
Data Set ◽  
Tectonic Zone ◽  
Near Surface ◽  
Abitibi Greenstone Belt ◽  
Sudbury Structure

The 1992 Lithoprobe Abitibi–Grenville Seismic Refraction Experiment was conducted using four profiles across the Grenville and Superior provinces of the southeastern Canadian Shield. Delay-time analysis and tomographic inversion of the data set demonstrate significant lateral and vertical variations in crustal velocities from one terrane to another, with the largest velocity values occurring underneath the Central Gneiss and the Central Metasedimentary belts south of the Grenville Front. The Grenville Front Tectonic Zone is imaged as a southeast-dipping region of anomalous velocity gradients extending to the Moho. The velocity-anomaly maps suggest an Archean crust may extend, horizontally, 140 km beneath the northern Grenville Province. Near-surface velocity anomalies correlate well with the known geology. The most prominent of these is the Sudbury Structure, which is well mapped as a low-velocity basinal structure. The tomography images also suggest underthrusting of the Pontiac and Quetico subprovinces beneath the Abitibi Greenstone Belt. Wide-angle PmP signals, indicate that the Moho varies from a sharp discontinuity south of the Grenville Front to a rather diffuse and flat boundary under the Abitibi Greenstone Belt north of the Grenville Front. A significant crustal thinning near the Grenville Front may indicate post-Grenvillian rebound and (or) the extensional structure of the Ottawa–Bonnechere graben. Crustal thickening resulting from continental collision may explain the tomographic images showing the Moho is 4–5 km deeper south of the Grenville Front.

Seismic Microzonation using 6C Measurements

2020 ◽  
Author(s):  
Sabrina Keil ◽  
Joachim Wassermann ◽  
Heiner Igel
Keyword(s):  
Velocity Structure ◽  
Building Design ◽  
Wave Dispersion ◽  
Velocity Profiles ◽  
Surface Velocity ◽  
Intelligent Building ◽  
Near Surface ◽  
Velocity Models ◽  
Single Station ◽  
Risk Areas

<p>Microzonation is one of the essential tools in seismology to mitigate earthquake damage by estimating the near surface velocity structure and developing land usage plans and intelligent building design. The number of microzonation studies increased in the last few years as induced seismicity becomes more relevant, even in low risk areas. While of vital importance, especially in densely populated cities, most of the traditional techniques suffer from different short comings. The microzonation technique presented here tries to reduce the existing ambiguity of the inversion results by the combination of single-station six-component (6C) measurements, including three translational and three rotational motions, and more traditional H/V techniques. By applying this new technique to a microzonation study in Munichs (Germany) inner city using an iXblue blueSeis-3A rotational motion sensor together with a Nanometrics Trillium Compact seismometer we were able to estimate Love and Rayleigh wave dispersion curves. These curves together with H/V spectral ratios are then inverted to obtain shear wave velocity profiles of the upper 100 m. The resulting 1D velocity profiles are used to estimate the local shaking characteristics in Munich. In addition, the comparison between the estimated velocity models and the borehole-derived lithology gives a positive correlation, indicating the applicability of our method.</p>

3D seismic traveltime tomography imaging of the shallow subsurface at the C O2 SINK project site, Ketzin, Germany

Geophysics ◽  
2009 ◽  
Vol 74 (1) ◽  
pp. G1-G15 ◽  
Author(s):  
Sawasdee Yordkayhun ◽  
Ari Tryggvason ◽  
Ben Norden ◽  
Christopher Juhlin ◽  
Björn Bergman
Keyword(s):  
Velocity Structure ◽  
Velocity Model ◽  
Geological Structure ◽  
Seismic Survey ◽  
Storage Site ◽  
Near Surface ◽  
Shallow Subsurface ◽  
Traveltime Tomography ◽  
Reflection Seismic ◽  
Reflection Data

A 3D reflection seismic survey was performed in 2005 at the Ketzin carbon dioxide [Formula: see text] pilot geological-storage site (the [Formula: see text] project) near Berlin, Germany, to image the geological structure of the site to depths of about [Formula: see text]. Because of the acquisition geometry, frequency limitations of the source, and artefacts of the data processing, detailed structures shallower than about [Formula: see text] were unclear. To obtain structural images of the shallow subsurface, we applied 3D traveltime tomography to data near the top of the Ketzin anticline, where faulting is present. Understanding the shallow subsurface structure is important for long-term monitoring aspects of the project after [Formula: see text] has been injected into a saline aquifer at about [Formula: see text] depth. We used a 3D traveltime tomography algorithm based on a combination ofsolving for 3D velocity structure and static corrections in the inversion process to account for artefacts in the velocity structure because of smearing effects from the unconsolidated cover. The resulting velocity model shows low velocities of [Formula: see text] in the uppermost shallow subsurface of the study area. The velocity reaches about [Formula: see text] at a depth of [Formula: see text]. This coincides approximately with the boundary between Quaternary units, which contain the near-surface freshwater reservoir and the Tertiary clay aquitard. Correlation of tomographic images with a similarity attribute slice at [Formula: see text] (about [Formula: see text] depth) indicates that at least one east-west striking fault zone observed in the reflection data might extend into the Tertiary unit. The more detailed images of the shallow subsurface from this study provided valuable information on this potentially risky area.

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