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.

Mode misidentification in Rayleigh waves: Ellipticity as a cause and a cure

Geophysics ◽  
2013 ◽  
Vol 78 (4) ◽  
pp. EN17-EN28 ◽  
Author(s):  
Jacopo Boaga ◽  
Giorgio Cassiani ◽  
Claudio L. Strobbia ◽  
Giulio Vignoli
Keyword(s):  
Rayleigh Wave ◽  
Dispersion Curve ◽  
Velocity Structure ◽  
Vertical Component ◽  
Real Data ◽  
Smooth Transition ◽  
Correct Identification ◽  
Low Velocity ◽  
Low Frequencies ◽  
Near Surface

The surface wave method is a popular tool for geotechnical characterization because it supplies a cost-effective testing procedure capable of retrieving the shear wave velocity structure of the near-surface. Several acquisition and processing approaches have been developed to infer the Rayleigh wave dispersion curve which is then inverted. Typically, in active testing, single-component vertical receivers are used. In most cases, the inversion is carried out assuming that the experimental dispersion curve corresponds to a single mode, mostly the fundamental Rayleigh mode, unless clear evidence dictates the existence of a more complex response, e.g., in presence of low-velocity layers and inversely dispersive sites. A correct identification of the modes is essential to avoid serious errors. Here we consider the typical case of higher-mode misidentification known as “osculation” (“kissing”), where the energy peak shifts at low frequencies from the fundamental to the first higher mode. This jump occurs, with a continuous smooth transition, around a well-defined frequency where the two modes get very close to each other. Osculation happens generally in presence of strong velocity contrasts, typically with a fast bedrock underlying loose sediments. The practical limitations of the acquired active data affect the spectral and modal resolution, making it often impossible to identify the presence of two modes. In some cases, modes have a very close root and cannot be separated at the osculation point. In such cases, mode misidentification can create a large overestimation of the bedrock velocity and a large error on its depth. We examine the subsoil conditions that can generate this unwanted condition, and the common field acquisition procedures that can contribute to producing data having such deceptive Rayleigh dispersion characteristics. This mode misidentification depends strongly on the usual approach of measuring only the vertical component of ground motion, as the mode osculation is linked to the Rayleigh wave ellipticity polarization, and therefore we conclude that multicomponent data, using also horizontal receivers, can help discern the multimodal nature of surface waves. Finally, we introduce a priori detectors of subsoil conditions, based on passive microtremor measurements, that can act as warnings against the presence of mode osculation, and relate these detectors to the frequencies at which dispersion curves can be misidentified. Theoretical results are confirmed by real data acquisition tests.

Tomographic velocity model building of the near surface with velocity-inversion interfaces: A test using the Yilmaz model

Geophysics ◽  
2010 ◽  
Vol 75 (6) ◽  
pp. U39-U47 ◽  
Author(s):  
Hui Liu ◽  
Hua-wei Zhou ◽  
Wenge Liu ◽  
Peiming Li ◽  
Zhihui Zou
Keyword(s):  
Model Building ◽  
Velocity Model ◽  
Surface Velocity ◽  
Low Velocity ◽  
Near Surface ◽  
Velocity Inversion ◽  
Velocity Models ◽  
Velocity Model Building ◽  
First Arrival ◽  
The Impact

First-arrival traveltime tomography is a popular approach to building the near-surface velocity models for oil and gas exploration, mining, geoengineering, and environmental studies. However, the presence of velocity-inversion interfaces (VIIs), across which the overlying velocity is higher than the underlying velocity, might corrupt the tomographic solutions. This is because most first-arrival raypaths will not traverse along any VII, such as the top of a low-velocity zone. We have examined the impact of VIIs on first-arrival tomographic velocity model building of the near surface using a synthetic near-surface velocity model. This examination confirms the severe impact of VIIs on first-arrival tomography. When the source-to-receiver offset is greater than the lateral extent of the VIIs, good near-surface velocity models can still be established using a multiscale deformable-layer tomography (DLT), which uses a layer-based model parameterization and a multiscale scheme as regularization. Compared with the results from a commercial grid-based tomography, the DLT delivers much better near-surface statics solutions and less error in the images of deep reflectors.

FULL-WAVEFORM INVERSION WITH SOURCE AND RECEIVER COUPLING EFFECTS CORRECTION

Geophysics ◽  
2021 ◽  
pp. 1-37
Author(s):  
Xinhai Hu ◽  
Wei Guoqi ◽  
Jianyong Song ◽  
Zhifang Yang ◽  
Minghui Lu ◽  
...  
Keyword(s):  
Waveform Inversion ◽  
Nonlinear Least Squares ◽  
Real Data ◽  
Model Parameters ◽  
Full Waveform Inversion ◽  
Linear Inversion ◽  
Near Surface ◽  
Model Parameter ◽  
Full Waveform ◽  
Coupling Factors

Coupling factors of sources and receivers vary dramatically due to the strong heterogeneity of near surface, which are as important as the model parameters for the inversion success. We propose a full waveform inversion (FWI) scheme that corrects for variable coupling factors while updating the model parameter. A linear inversion is embedded into the scheme to estimate the source and receiver factors and compute the amplitude weights according to the acquisition geometry. After the weights are introduced in the objective function, the inversion falls into the category of separable nonlinear least-squares problems. Hence, we could use the variable projection technique widely used in source estimation problem to invert the model parameter without the knowledge of source and receiver factors. The efficacy of the inversion scheme is demonstrated with two synthetic examples and one real data test.

scholarly journals 3-D reflection seismic imaging of the Hontomín structure in the Basque-Cantabrian Basin (Spain)

2013 ◽  
Vol 5 (2) ◽  
pp. 1575-1614
Author(s):  
J. Alcalde ◽  
D. Martí ◽  
C. Juhlin ◽  
A. Malehmir ◽  
D. Sopher ◽  
...  
Keyword(s):  
Co2 Storage ◽  
Processing Parameters ◽  
Surface Velocity ◽  
Storage Site ◽  
Near Surface ◽  
Velocity Inversion ◽  
Geological Co2 Storage ◽  
Reflection Seismic ◽  
Carbonate Formations ◽  
Basque Cantabrian Basin

Abstract. The Basque-Cantabrian Basin of the Northern Iberia peninsula constitutes a unique example of a major deformation system, featuring a dome structure developed by extensional tectonics followed by compressional reactivation. The occurrence of natural resources in the area and the possibility of establishing a geological storage site for carbon dioxide motivated the acquisition of a 3-D seismic reflection survey in 2010, centered on the Jurassic Hontomín dome. The objectives of this survey were to obtain a geological model of the overall structure and to establish a baseline model for a possible geological CO2 storage site. The 36 km2 survey included approximately 5000 mixed (Vibroseis and explosives) source points recorded with a 25 m inline source and receiver spacing. The target reservoir is a saline aquifer, at approximately 1450 m depth, encased and sealed by carbonate formations. Acquisition and processing parameters were influenced by the rough topography and relatively complex geology. A strong near surface velocity inversion is evident in the data, affecting the quality of the data. The resulting 3-D image provides constraints on the key features of the geologic model. The Hontomín structure is interpreted to consist of an approximately 107 m2 large elongated dome with two major W–E and NW–SE striking faults bounding it.

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

Numerical modeling of refraction arrivals in complex areas

Geophysics ◽  
1985 ◽  
Vol 50 (1) ◽  
pp. 90-98 ◽  
Author(s):  
N. R. Hill ◽  
P. C. Wuenschel
Keyword(s):  
Numerical Modeling ◽  
Plane Wave ◽  
Complex Structure ◽  
Real Data ◽  
Synthetic Seismograms ◽  
Weak Signal ◽  
Surface Complex ◽  
Low Velocity ◽  
Near Surface ◽  
Plane Wave Field

Use of refracted arrivals to delineate near‐surface complex structure can sometimes be difficult because of rapid lateral changes in the refraction event along the line of control. The interpreter must correlate over zones of interference and zones of weak signal. During correlation it is often difficult to stay on the correct cycle of the waveform. We present a method to model refracted arrivals numerically in an area where these problems occur. The computation combines plane‐wave field decomposition to calculate propagation in complex regions with a WKBJ method to calculate propagation in simple regions. To illustrate the method, we study a case where the near‐surface complex structure is caused by the presence of low‐velocity gaseous mud. The modeling produces synthetic seismograms showing the interference patterns and changes in intensity that are seen in real data. This modeling shows how correlations may be done over difficult areas, particularly where cycle skips can occur.

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>

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