Traveltime Tomography Technique Research Articles

Acoustic Travel-Time TOMography Technique to Reconstruct the Indoor Temperature: How to Improve the Field Reconstruction Quality?

Acoustic Travel-Time TOMography Technique to Reconstruct the Indoor Temperature: How to Improve the Field Reconstruction Quality?

Complex Patterns of Past and Ongoing Crustal Deformations in Southern California Revealed by Seismic Azimuthal Anisotropy

AbstractWe present a high‐resolution P‐wave azimuthally anisotropic velocity model for the upper and middle crust beneath southern California by a novel adjoint‐state traveltime tomography technique. Our model reveals significant anisotropy variations between tectonic blocks that clearly reflect both past and current plate boundary deformation. In the shallow crust, seismic anisotropy is mostly controlled by the preferred alignment of microcracks related to the present N‐S compressive stress; while at deeper depths (>∼6 km), seismic anisotropy mainly records paleofabrics formed during the long‐lived Farallon subduction and later extension that have not been fully reset by the present transform motion. Interestingly, our model demonstrates distinct fast axes beneath the western Transverse Ranges from its neighboring blocks, probably reflecting the large‐scale vertical axis clockwise rotation of the block. In addition, we identify layered structures with distinct anisotropy features beneath the Salton Trough, which could be a result of the current transtension.

Refraction traveltime tomography based on damped wave equation for irregular topographic model

Land seismic data generally have time-static issues due to irregular topography and weathered layers at shallow depths. Unless the time static is handled appropriately, interpretation of the subsurface structures can be easily distorted. Therefore, static corrections are commonly applied to land seismic data. The near-surface velocity, which is required for static corrections, can be inferred from first-arrival traveltime tomography, which must consider the irregular topography, as the land seismic data are generally obtained in irregular topography.This paper proposes a refraction traveltime tomography technique that is applicable to an irregular topographic model. This technique uses unstructured meshes to express an irregular topography, and traveltimes calculated from the frequency-domain damped wavefields using the finite element method. The diagonal elements of the approximate Hessian matrix were adopted for preconditioning, and the principle of reciprocity was introduced to efficiently calculate the Fréchet derivative. We also included regularization to resolve the ill-posed inverse problem, and used the nonlinear conjugate gradient method to solve the inverse problem.As the damped wavefields were used, there were no issues associated with artificial reflections caused by unstructured meshes. In addition, the shadow zone problem could be circumvented because this method is based on the exact wave equation, which does not require a high-frequency assumption. Furthermore, the proposed method was both robust to an initial velocity model and efficient compared to full wavefield inversions. Through synthetic and field data examples, our method was shown to successfully reconstruct shallow velocity structures. To verify our method, static corrections were roughly applied to the field data using the estimated near-surface velocity. By comparing common shot gathers and stack sections with and without static corrections, we confirmed that the proposed tomography algorithm can be used to correct the statics of land seismic data.

Geological structure analysis in Central Java using travel time tomography technique of S waves

Java is one of the islands in Indonesia that is prone to the earthquakes, in south of Java, there is the Australian Plate move to the Java island and press with perpendicular direction. This plate movement formed subduction zone and cause earthquakes.The earthquake is the release of energy due to the sudden movement of the plates. When an earthquake occurs, the energy is released and record by seismometers in the waveform. The first wave recorded is called the P waves (primary) and the next wave is called S waves (secondary). Both of these waves have different characteristics in terms of propagation and direction of movement.S wave is composed of waves of Rayleigh and Love waves, with each direction of movement of the vertical and horizontal, subsurface imaging by using S wave tomography technique can describe the type of the S wave through the medium. The variation of wave velocity under Central Java (esearch area) is ranging from -10% to 10% at the depth of 20, 30 and 40 km, the velocity decrease with the depth increase. Moho discontinuity is lies in the depth of 32 km under the crust, it is indicates there is strong heterogenity in Moho.

Integrated non-destructive assessment of concrete structures under flexure by acoustic emission and travel time tomography

Elastic stress wave methods are attracting increasing attention from researchers and practitioners for real time monitoring of structures as well as quantitative evaluation of damage. The acoustic emission (AE) and travel time tomography (TTT) techniques are promising elastic waves-based diagnosis approaches for condition assessment of structures. In this research, fiber reinforced concrete beams loaded under flexure were monitored using both the AE and TTT techniques. The primary aim was to evaluate the development of fracture in beams in a non-destructive manner, with the two adopted techniques complementing each other to provide a more reliable assessment. There was satisfactory agreement between the results of AE parametric analysis and tomographic reconstruction, suggesting the feasibility of integrating the two techniques for a more comprehensive and effective condition assessment of structures.

Characterization of a landslide geometry using 3D seismic refraction traveltime tomography: The La Valette landslide case history

Abstract The geometry of the bedrock, internal layers and shear surfaces/bands controls the deformation pattern and the mechanisms of landslides. A challenge to progress in the forecast of landslide acceleration in terms of early-warning is therefore to characterize the 3D geometry of the unstable mass at a high level of spatial resolution, both in the horizontal and vertical directions, by integrating information from different surveying techniques. For such characterization, seismic investigations are potentially of a great interest. In the case of complex structures, the measure and the processing of seismic data need to be performed in 3D. The objective of this work is to present the development of a 3D extension of a seismic refraction traveltime tomography technique based on a Simultaneous Iterative Reconstruction Technique (SIRT). First the processing algorithm is detailed and its performance is discussed, and second an application to the La Valette complex landslide is presented. Inversion of first-arrival traveltimes produces a 3D tomogram that underlines the presence of many areas characterized by low P-wave velocity of 500–1800 m.s − 1 . These low P-wave velocity structures result from the presence of reworked blocks, surficial cracks and in-depth fracture zones. These structures seem to extend to around 25 m in depth over a 80 × 130 m area. Based on borehole geotechnical data and previous geophysical investigations, an interface corresponding to an internal slip surface can be suspected near the isovalue of 1200 m.s − 1 at a depth of − 10 to − 15 m. The stable substratum is characterized by higher values of P-wave velocity of 1800–3000 m.s − 1 . The features identified in the 3D tomogram allow to better (1) delineate the boundary between the landslide and the surrounding stable slopes, and (2) understand the morphological structures within the landslide at a hectometric scale. The integration of the 3D seismic tomography interpretation to previous geophysical acquisitions using a geostatistical approach allows to construct a 3D geometrical model of the middle and lower part of the La Valette landslide, and to estimate the volume of the unstable mass.

Thickness variation of the descending Philippine Sea slab and its relationship to volcanism beneath the Kanto‐Tokai district, central Japan

We use an iterative nonlinear travel time tomography technique to determine three‐dimensional P‐wave and S‐wave seismic velocity models for the region beneath the Kanto‐Tokai district of central Japan. Salient points of the models are the following: (1) The subducting Philippine Sea slab has thickness variation with a stepwise offset east of the Izu Peninsula. The eastern (beneath the Kanto district) and western (beneath the area north of Izu Peninsula and the Tokai district) regions have respective thicknesses of 60 and 25 km. The slab clearly subducts continuously to a depth of 100 km from the Sagami Trough beneath the Kanto district, although the Tokai slab is indistinct at depths of 50–70 km. (2) The oceanic crust at the top of the descending Philippine Sea slab transforms into eclogite at a depth of about 70 km beneath the Kanto district. (3) Low‐velocity anomalies are associated with present and past volcanism. They are located on the west of a 110‐km depth contour of the Wadati‐Benioff zone, rather than only on the backarc side of the volcanic front. Deflection of the volcanic front might result from the subducting Philippine Sea slab, which shifts the dehydration reaction deeper as it subducts deeper.

Small-scale convection in the upper mantle beneath the Chinese Tian Shan Mountains

Small-scale convection of the upper mantle beneath the Chinese Tian Shan (Tien Shan) is investigated in terms of numerical modeling. The finite element method combined with the marker-in-cell technique is used to describe the flow of the heterogeneous upper mantle. The density model is derived from the P-wave velocity structure of the crust and upper mantle along the Kuche–Kuitun profile across the Chinese Tian Shan, which is obtained using the seismic travel time tomography technique. Our computational results reveal the southward-counterclockwise and northward-clockwise upper mantle convective cells underneath the Junggar-north Tian Shan and Tarim-south Tian Shan, respectively. Our results also show the convective scale reaches to ∼500 km and the convective speed at the top of the upper mantle should not be less than 20 mm/year for a normal viscosity model. The northward extrusion of the Tarim block plays a key role in the Tian Shan mountain building since the Cenozoic period, but it nearly does not influence the upper mantle convection. The present-day tectonic deformation in the Chinese Tian Shan is related to the small-scale convection of the upper mantle.

Crust and upper mantle structure of the Ailao Shan-Red River fault zone and adjacent regions

Using arrival data of the body waves recorded by seismic stations, we reconstructed the velocity structure of the crust and upper mantle beneath the southeastern edge of the Tibetan Plateau and the northwestern continental margin of the South China Sea through a travel time tomography technique. The result revealed the apparent tectonic variation along the Ailao Shan-Red River fault zone and its adjacent regions. High velocities are observed in the upper and middle crust beneath the Ailao Shan-Red River fault zone and they reflect the character of the fast uplifting and cooling of the metamorphic belt after the ductile shearing of the fault zone, while low velocities in the lower crust and near the Moho imply a relatively active crust-mantle boundary beneath the fault zone. On the west of the fault zone, the large-scale low velocities in the uppermost mantle beneath western Yunnan prove the influence of the mantle heat flow on volcano, hot spring and magma activities, however, the upper mantle on the east of the fault zone shows a relatively stable structure similar to the Yangtze block. The low velocities of the deep mantle beneath the southeastern extending segment of the fault zone are probably related to the mantle convection produced by the pull-apart of the South China Sea.

Three-dimensional seismic refraction tomography of the crustal structure at the ION site on the Ninetyeast Ridge, Indian Ocean

SUMMARY We have performed a 3-D seismic refraction tomography of a 48 × 48 km2 area surrounding ODP site 757, which is planned to host an International Ocean Network (ION) permanent seismological observatory, called the Ninetyeast Ridge Observatory (NERO). The study area is located in the southern part of the Ninetyeast Ridge, the trail left by the Kerguelen hotspot on the Indian plate. The GEOMAR Research Centre for Marine Geosciences and the Federal Institute for Geosciences and Natural Resources acquired 18 wide-angle profiles recorded by 23 ocean bottom hydrophones during cruise SO131 of R/V Sonne in spring 1998. We apply a first arrival traveltime tomography technique using regularized inversion to recover the 3-D velocity structure relative to a 1-D background model that was constructed from a priori information and averaged traveltime data. The final velocity model revealed the crustal structure down to approximately 8 km depth. Resolution tests showed that structures with approximately 6 km horizontal extent can reliably be resolved down to that depth. The survey imaged the extrusive layer of the upper crust of the Ninetyeast Ridge, which varies in thickness between 3 and 4 km. A high-velocity anomaly coinciding with a positive magnetic anomaly represents a volcanic centre from which crust in this area is thought to have formed. A pronounced low-velocity anomaly is located underneath a thick sedimentary cover in a bathymetric depression. However, poor ray coverage of the uppermost kilometre of the crust in this area resulted in smearing of the shallow structure to a larger depth. Tests explicitly including the shallow low-velocity layer confirmed the existence of the deeper structure. The heterogeneity of the upper crust as observed by our study will have consequences for the waveforms of earthquake signals to be recorded by the future seismic observatory.

散乱理論の応用とトモグラフィー

Tomographic inverse schemes, in particular travel-time tomography techniques, havebecome standard ones to image subsurface structures with respect to anomalies in seismicvelocities. In contrast to these ray-theoretical approaches, there exist wave-theoretical schemeswhere seismic waveforms are analyzed in terms of their relation to structural anomalies. In tomographic application of wave theories, Devaney (1984) proposed a method calleddiffraction tomography in which seismic wave-forms are composed of incident and scatteredwave-fields. Since several tens to hundred times much information might be included inseismic wave-forms, it is natural to think that there exists a strong necessity to utilize ofwave-forms in structural analyses.A theoretical jump from ray-theoretical travel-time to wave-theoretical diffraction tomographycan be examined by introducing a simple seismic scattering theory. Since thesetechniques are utilizing seismic signals, results obtained by these methods on the same datamight be considered as the derivation of the same physical properties of unknown targetmaterial in the subsurface. The resolution can also be compared between these two kindsapproaches in seismic tomography through the seismic scattering theory. The theory wasintroduced by a solution to the seismic scattering problem of a compressional plane wavetraveling in a homogeneous medium including a point scatterer, and three types of scatteredwaves were found generated depending on the contrasts in its lithological properties (the Lame's constants and the density) to the surrounding medium. They are equivalent towaves generated by three different kinds of seismic sources. Contrasts between the point scattererand the surrounding medium in bulk modulus, in density, and in rigidity work as a pole, asingle force, and a single couple, respectively. The relation of these parameters to observedscattered waves was found linear and thus invertible. The results obtained through this relationin the analysis of subsurface structure of Izu-Oshima volcano have shown the good resolutionwhich can only be achieved by wave-theoretical methods. It is concluded that seismic scatteringphenomena can be used for the following purposes:(1) to clarify the effects of inhomogeneitieson the observed seismic signals, (2) to find the inverse relation to the scatteringproblems, (3) to understand a link between the seismic methods, i. e., traveltime tomography, waveform tomography and reflection analyses.

Multiple phase analysis of the shear velocity structure in the D″ region beneath Alaska

A large data set (more than 990 total waveforms) comprised of North American station recordings of a variety of long‐period S phases (S, ScS, Sdiff, sS, sScS, SKS, SKKS) from 36 earthquakes in western Pacific subduction zones is analyzed to determine characteristics of the shear velocity structure in the D″ region beneath Alaska. Three hypotheses that have been proposed to explain observed ScS precursors and S waveform distortions for this region are appraised with the multiple phase data set, and the radial and lateral variations of the shear velocity structure in the 1500‐km broad portion of D″ sampled are also assessed. The observed travel time and waveform variations may result from diffraction by near‐source slab structure, scattering by low‐velocity heterogeneity in the lower mantle, or interaction with complex velocity gradients in D″ associated with a deep mantle velocity discontinuity. We utilize waveform modeling and differential time analysis to resolve greater detail of the structure than can presently be obtained by standard travel time tomography techniques, and find that the discontinuity model is most consistent with the data. Subdivision of the data by source region and source to receiver azimuth establishes that the observations exhibit fairly stable behavior, which is consistent with extensive local stratification of D″. The data show sufficient coherence over the 1500‐km broad region that we find it useful to present a local average radial model. The preferred model, SYLO, has a 2.75% velocity increase 243 km above the core mantle boundary and a negative velocity gradient within the D″ layer. Analysis of SKS‐S, SKS‐ScS, and SKS‐SKKS differential times also indicates that the outermost 100 km of the core below the fast D″ region underlying Alaska must be slower than the PREM structure. The observations exhibit significant scatter, most of which can be attributed to unmodeled heterogeneity in the midmantle and near‐source regions, as well as lateral variations in the D″ region. The complex structure of D″ that we prefer may be associated with large‐scale downwelling in the lower mantle, but it is not certain whether it is a purely thermal or a combined chemical and thermal effect.