Scales of Heterogeneities in the Continental Crust and Upper Mantle

Tittgemeyer, Marc; Wenzel, Friedemann; Ryberg, Trond; Fuchs, Karl

doi:10.1007/s000240050288

Cited by 33 publications

(3 citation statements)

References 83 publications

(43 reference statements)

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“…Excitation of scattered waves from distributed heterogeneities has been extensively investigated to unravel the structural characteristics of random heterogeneity in the Earth (e.g., Sato, 1984Sato, , 1989Benites et al, 1997;Margerin et al, 2000). The random heterogeneities with scale lengths less than 1 km are observed in logging data of deep wells (e.g., Holliger, 1996;Shiomi et al, 1997), and those with characteristic scale lengths of a few kilometers are found in tomography with artificial seismic sources (e.g., Hammer et al, 1994) and by reflection surveys (e.g., Tittgemeyer et al, 1999).…”

Section: Introductionmentioning

confidence: 99%

Laboratory Study on Scattering Characteristics of Shear Waves in Rock Samples

Fukushima¹,

Nishizawa

Sato³

et al. 2003

Bulletin of the Seismological Society of America

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To understand the scattering characteristics of seismic shear waves in the Earth, a laboratory physical model experiment in ultrasonic frequency range was performed. By analyzing waveform envelopes and particle motions of shear waves in media with different correlation distance and fluctuation intensity, we studied the strength of scattered wave excitation and the distortion of shear-wave polarization as a function of ka (wavenumber times the correlation distance). We used gabbro and granite as media having different small-scale random heterogeneities. The granite we used contained preferred-oriented thin microcracks. The correlation distance a and the standard deviation of the fractional fluctuation of the shear-wave velocity e were estimated by fitting exponential-type autocorrelation functions. The estimated values were a ‫ס‬ 0.84 mm and e ‫ס‬ 8.1% for the gabbro, and a ‫ס‬ 0.39 mm and e ‫ס‬ 17.0% for the granite. Waveform measurements were performed in the frequency range 0.25-1 MHz, which roughly corresponds to ka as 0.25-1. Envelope broadening appeared when ka exceeds 0.4 for the granite and 1.4 for the gabbro. Shear-wave polarizations were distorted by scattering when ka exceeds 0.2 for the granite and 0.7 for the gabbro. The results on envelope broadening suggest the importance of large-angle scattering due to small-scale random heterogeneities and cracks in the envelope broadening, in addition to diffraction effects due to large-scale heterogeneities. The effect of aligned microcracks on scattering was also examined. It was found that scattering was prominent when shear wave propagates perpendicular to the aligned crack plane.

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Section: Introductionmentioning

confidence: 99%

Laboratory Study on Scattering Characteristics of Shear Waves in Rock Samples

Fukushima¹,

Nishizawa

Sato³

et al. 2003

Bulletin of the Seismological Society of America

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“…The correlation lengths for the fractals were made much larger than the seismic wavelengths, and Mereu [2003] found that the resulting model bears a strong resemblance to many images observed in the near-offset reflection experiments. Tittgemeyer et al [1999] characterized the crust and upper mantle by fluctuations in media properties (elastic parameters) rather than by their average velocities. They suggested that different scales of heterogeneities prevail in different layers of the crust and upper mantle, and they carry significant information on the tectonic processes.…”

Section: Introductionmentioning

confidence: 99%

Velocity modeling of a complex deep crustal structure across the Mesoproterozoic south Delhi Fold Belt, NW India, from joint interpretation of coincident seismic wide-angle and near-offset reflection data: An approach using unusual reflections in wide-angle records

Krishna

Rao

2011

J. Geophys. Res.

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[1] Seismic wide-angle record sections from favorably placed source and limited-aperture recording spreads, particularly across collision boundaries, reveal bright unusual highvelocity contrast reflections of limited lateral coherence. The arrival times of the unusual reflections for recording updip of the reflecting plane align as high apparent velocity phases at wide-angle offsets. Coincident near-offset reflection images from similar structural features provide additional control for modeling the unusual phases on the wide-angle record sections. We propose an approach for joint interpretation of these coincident seismic data sets and modeling the unusual phases on the processed wide-angle records to delineate both the velocity stratification and the geometry of the deep crustal structures consistent with the near-offset reflection images. Processed and migrated near-offset reflection images reveal several steeply dipping isolated reflections on the western margin of the south Delhi Fold Belt (DFB), NW India, interpreted as the south Delhi thrust fault. Coincident limited-aperture, wide-angle records reveal bright unusual reflections, which are also found to be associated with the same thrust fault. Observations of similar dipping reflection patterns in various other regions lend support to consider them as genuine in-plane reflections resulting from a structural fabric imprinted on horizontally oriented geological structures. We use the reflectivity structural model from the near-offset data set that provides necessary constraints for modeling and simulating the travel times and amplitudes of the unusual reflections recognized in the wide-angle records. The unusual reflections are effectively used as part of the wide-angle data set for Gaussian beam synthetic seismogram modeling, thus delineating for the first time a complex 2-D model of the seismic velocity structure of the deep crust and uppermost mantle across the Marwar Basin (MB) and the DFB. The dipping events revealed in the wide-angle data set are successfully modeled and interpreted as reflections from the thrust fault, consistent with the near-offset reflection image. The 2-D velocity model, parameterized as a stack of isolated dipping reflectors embedded in deep crustal layers, is successively refined by including the structural features recognizable both in the near-offset reflection image and in the wide-angle records. The unusual reflections are modeled as a set of discrete reflectors representing local velocity variations imprinted on the background of a deep crustal velocity structure. The lamellar structure revealed by short, isolated dipping and the subhorizontal middle to lower crustal reflections suggests heterogeneities at scale lengths on the order of at least the Fresnel zone diameter as resolved here. The local velocity variations in the deep crust can be considered analogous, on a different scale, to the models in which velocity heterogeneities on scale lengths of the seismic wavelength constitute a medium of random distribution of sc...

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“…From the seismic perspective, a frequently cited model for the Moho is characterized by a variable-thickness transition zone composed of anastomosing layers progressing with depth from mafi c to ultramafi c rocks (Jarchow and Thompson, 1989;Braile and Chiang, 1986). Alternatively, the Moho transition may represent a change in scale of vertical layering and horizontal extent of inhomogeneities (Tittgemeyer et al, 1999;Hurich, 2003;Carpentier and Roy-Chowdhury, 2007).…”

Section: Introductionmentioning

confidence: 99%

Nature of the Moho transition in NW Canada from combined near-vertical and wide-angle seismic-reflection studies

Oueity

Clowes

2010

Lithosphere

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Although the year 2009 marked one century since the discovery of the Moho, or crust-mantle boundary, the exact nature of that boundary and the manner in which it formed remain major uncertainties in lithospheric studies. In northwestern Canada, sharp Moho refl ections at both near-vertical and wide-angle incidence have been imaged beneath the Great Bear magmatic arc. They show a remarkably fl at Moho and do not refl ect the complex tectonic history of the Wopmay orogen, of which the Great Bear arc is one part. In order to understand the origin of these refl ections and the nature of the Moho, we calculated near-vertical and wide-angle synthetic seismograms for a number of crust-mantle transition models using one-and two-dimensional wave propagation algorithms. Only laterally and vertically heterogeneous models can properly simulate the observed seismic signature recorded on both near-vertical and wide-angle refl ection data. The heterogeneity is achieved by either laterally discontinuous layering or a lamellae structure with randomly distributed ellipses. These models suggest that the Moho represents a thermal/metamorphic front, a regional décollement, or both.

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Scales of Heterogeneities in the Continental Crust and Upper Mantle

Cited by 33 publications

References 83 publications

Laboratory Study on Scattering Characteristics of Shear Waves in Rock Samples

Laboratory Study on Scattering Characteristics of Shear Waves in Rock Samples

Velocity modeling of a complex deep crustal structure across the Mesoproterozoic south Delhi Fold Belt, NW India, from joint interpretation of coincident seismic wide-angle and near-offset reflection data: An approach using unusual reflections in wide-angle records

Nature of the Moho transition in NW Canada from combined near-vertical and wide-angle seismic-reflection studies

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