Traveltime Tomography Method Research Articles

Assessment of Two Refraction Traveltime Tomography Methods Applied for Statics Corrections of Land Data

ABSTRACT. Many refraction tomography algorithms for statics corrections have model parameterization, forward modeling, and inversion limitations. For instance, most traditional methods use a regular grid to parameterize the model and only invert one parameter, either velocity or thickness of the layer. Generally, these conventional techniques do not allow the simultaneous inversion of traveltimes of turning-rays and head-waves either. In this study, for the first time, we applied and evaluated the well-known 2D refraction tomography algorithm Rayinvr, which is widely used to study deep crustal models, to estimate near-surface layered velocity models and static corrections from real seismic reflection data collected for oil exploration. Here, the Rayinvr and its results are compared with another refraction tomography algorithm widely used in the petroleum industry. It employs the concept of generalized linear inversion and uses a simplified earth model composed of a layer over a half-space, which is parameterized by vertical rectangles of constant velocity. The results of both methods were applied and evaluated on two real datasets of seismic lines located in Brazilian onshore basins. The Rayinvr tomography algorithm provides precise measurement statics corrections, better velocity models, and more reliable geological structures than the standard tomography algorithm.Keywords: refraction, seismic tomography, traveltimes, near-surface geophysics. Avaliação de dois métodos de tomografia de tempos de trânsito de refração aplicados para correções estáticas de dados terrestresRESUMO. Muitos algoritmos de tomografia de refração para correções estáticas têm limitações em re- lação à parametrização do modelo, da modelagem direta e da inversão. Por exemplo, a maioria dos méto- dos tradicionais usa uma malha regular para parametrizar o modelo e apenas invertem um parâmetro, seja a velocidade ou a espessura da camada. Geralmente essas técnicas convencionais também não permitem a inversão simultânea dos tempos de trânsito de turning-rays e head-waves. Neste estudo, pela primeira vez, aplicamos e avaliamos o conhecido algoritmo de tomografia de refração 2D Rayinvr, que é amplamente usado no estudo de modelos crustais profundos, na estimativa de modelos de veloci- dade em camadas próximas à superfície e nas correções estáticas de dados reais de reflexão sísmica coletados para exploração de petróleo. Aqui, o Rayinvr e os resultados são comparados com outro al- goritmo de tomografia de refração, amplamente utilizado na indústria petrolífera. Ele emprega o conceito de inversão linear generalizada e usa um modelo terrestre simplificado composto por uma camada sobre um meio-espaço, que é parametrizado por retângulos verticais de velocidade constante. Os resultados de ambos os métodos foram aplicados e avaliados em dois conjuntos de dados reais de linhas sísmicas localizadas em bacias terrestres brasileiras. O algoritmo de tomografia Rayinvr forneceu correções es- táticas mais precisas, melhores modelos de velocidade e estruturas geológicas mais confiáveis do que o algoritmo padrão de tomografia.Palavras-chave: refração sísmica, tomografia sísmica, tempos de trânsito, geofísica rasa.

The correlation of earthquake swarms and local velocity heterogeneities in the Brawley seismic zone, southern California

Earthquake swarms with the possible largest magnitude greater than 5.0 frequently strike the Brawley seismic zone in southern California. The latest swarm of earthquakes occurred on 5 June 2021, including a magnitude 5.2 quake. The Salton Sea Geothermal Field lies at the northern end of the Brawley seismic zone. A good understanding of the detailed subsurface structure is necessary for seismic hazard assessment and the monitoring of geothermal resources exploration in this region. We apply the novel adjoint-state traveltime tomography method to image the crustal P- and S-wave velocity structures of the Brawley seismic zone and the adjacent area to the west. This new method requires no ray tracing but maintains a high level of accuracy in complex media. High-resolution P- and S-wave velocity models are constructed by inverting 85,321 first P-wave arrival times and 43,912 first S-wave arrival times, respectively. The velocity models reveal strong crustal heterogeneities. At 5–12 km depth the Brawley seismic zone is occupied by earthquake-prone high-velocity rocks; while the near surface is dominated by low-velocity sedimentary rocks. The source zone of the 1987 Superstition Hills earthquake sequence is filled with low-velocity rocks, possibly suggesting the presence of fluids. Fluids may have also played a critical role in the generation of earthquake swarms in the Brawley seismic zone.

Adjoint‐State Traveltime Tomography for Azimuthally Anisotropic Media and Insight Into the Crustal Structure of Central California Near Parkfield

AbstractSeismic anisotropy provides crucial information on the stress state and geodynamic processes inside the Earth. We develop a novel adjoint‐state traveltime tomography method using P‐wave traveltime data to simultaneously determine velocity heterogeneity and azimuthal anisotropy of the subsurface. First, an anisotropic eikonal equation is derived to model first‐arrival traveltimes in azimuthally anisotropic media. Traveltime tomography is then formulated as an optimization problem constrained by the anisotropic eikonal equation, which is subsequently solved by the adjoint‐state method. Ray tracing is not required. Its high accuracy is achieved by solving the anisotropic eikonal equation and the associated adjoint equation with efficient numerical solvers. In addition, an eikonal equation‐based earthquake location method for azimuthally anisotropic media is developed to solve the coupled hypocenter‐velocity problem. The tomography and earthquake location methods are applied to central California near Parkfield to test their performance in practice. A total of 1,068,850 first P‐wave traveltimes clearly maps the velocity heterogeneity and azimuthal anisotropy in the upper and middle crust. The average P‐wave velocity model shows a striking velocity contrast across the San Andreas Fault (SAF). In the upper crust, we find structural anisotropy in the SAF zone and stress‐induced anisotropy off the SAF zone. In the middle crust, the fast P‐wave velocity directions are generally fault‐parallel due to the decreased effect of the maximum horizontal compressive stress. In all, the real‐data application suggests that the new adjoint‐state traveltime tomography method can be reliably used to investigate anisotropic seismic structures.

Adjoint‐State Traveltime Tomography: Eikonal Equation‐Based Methods and Application to the Anza Area in Southern California

AbstractRay tracing is avoidable in seismic traveltime tomography. In this study, seismic traveltime tomography is reformulated as an eikonal equation‐constrained optimization problem solved by the ray‐free adjoint‐state method. The resultant approach is called adjoint‐state traveltime tomography. For completeness, an eikonal equation‐based earthquake location method is developed to locate the hypocenters of earthquakes when necessary. The multiple‐grid model parameterization is adopted to discretize the relative slowness perturbation . The step‐size‐controlled gradient descent method, which has an effect of damping regularization, is used to find optimal earthquake hypocenters and velocity models. The performances of the earthquake location and adjoint‐state traveltime tomography methods are tested in the source area of the 2020 Mw 4.9 Anza earthquake, a seismologically active place in southern California. The obtained high‐resolution P‐wave velocity model demonstrates that the 2020 Mw 4.9 Anza earthquake and other historic moderate‐sized Anza earthquakes occurred near or at the boundaries of low VP rocks, a typical seismogenic environment for moderate to large crustal earthquakes. Meanwhile, the source zone of the Anza earthquakes is characterized by high VP/VS, indicating the possible existence of crustal fluids as well as the critical role of fluids in the occurrence of those moderated‐sized Anza earthquakes. On the whole, the well‐performed eikonal equation‐based earthquake location method and adjoint‐state traveltime tomography method provide competent and attractive tools for hypocenter and tomographic inversions.

Slab Models Beneath Central Myanmar Revealed by a Joint Inversion of Regional and Teleseismic Traveltime Data

AbstractThe intermediate‐depth (50–180 km) seismicity beneath Myanmar provides direct evidence of the subducting Indian slab. However, the historic lack of regional seismic observations leads to previous low‐resolution models that show large variations in slab geometry beneath Myanmar. The depth extent and morphology of the slab are still poorly known. In this study, we conduct a joint inversion of regional and teleseismic P‐wave traveltimes from recently installed networks to image seismic velocity structures beneath central Myanmar by adopting an eikonal equation‐based traveltime tomography method. The observations contain a total of 6,069 regional first P‐wave arrivals and 29,787 teleseismic P‐wave differential traveltimes. We find a high P‐wave velocity anomaly beneath central Myanmar, which starts from ∼50 km depth and extends continuously to the mantle transition zone (MTZ) and is interpreted as the subducting Indian slab. Below 100 km depth, the dip angle of the slab in the south is ∼15° larger than that of the slab in the north, suggesting a possible slab tearing. Based on our tomographic results and previous studies, the slab in the north is inferred to have a deep stagnant segment lying above the 660‐km discontinuity in the MTZ, but whether it is connected with the shallow dipping slab cannot be confirmed. Meanwhile, the slab in the south may just stay in the upper mantle (above 410 km), but it may also have penetrated the 410‐km discontinuity. Taking into account all the scenarios, we propose four possible models of the Indian Plate subduction system beneath central Myanmar.

Determination of a stress-dependent rock-physics model using anisotropic time-lapse tomographic inversion

In the petroleum industry, time-lapse (4D) studies are commonly used for reservoir monitoring, but they are also useful to perform risk assessment for potential overburden deformations (e.g., well shearing, cap-rock integrity). Although complex anisotropic velocity changes are predicted in the overburden by geomechanical studies, conventional time-lapse inversion workflows only deal with vertical velocity changes. To retrieve the geomechanically induced anisotropy, we have adopted a reflection traveltime tomography method coupled with a time-shift estimation algorithm of prestack data of the baseline and monitor simultaneously. For the 2D approach, we parameterize the anisotropy using five coefficients, enough to cover any type of anisotropy. Before applying the workflow to a real data set, we first study a synthetic data set based on the real data set and include velocity variations between baseline and monitor found in the literature (vertical P-wave velocity decrease in the cap rock and isotropic P-wave velocity change in the reservoir). On the synthetics, we measure the angular ray coverage necessary to retrieve the target anisotropy and observe that the retrieved anisotropies depend on the offset range. Based on a synthetic experiment, we believe that the acquisition of the real case study is suitable for performing tomographic inversion. The anisotropic velocity changes obtained on three inlines separated by 375 m are consistent and show a strong positive anomaly in the cap rock along the 45° direction (the [Formula: see text] parameter in Thomsen notation), whereas the vertical velocity change is surprisingly almost negligible. We adopt a rock-physics explanation compatible with these observations and geologic considerations: a reactivation of water-filled subvertical cracks.

Rytov-approximation-based wave-equation traveltime tomography

Most finite-frequency traveltime tomography methods are based on the Born approximation, which requires that the scale of the velocity heterogeneity and the magnitude of the velocity perturbation should be small enough to satisfy the Born approximation. On the contrary, the Rytov approximation works well for large-scale velocity heterogeneity. Typically, the Rytov-approximation-based finite-frequency traveltime sensitivity kernel (Rytov-FFTSK) can be obtained by integrating the phase-delay sensitivity kernels with a normalized weighting function, in which the calculation of sensitivity kernels requires the numerical solution of Green’s function. However, solving the Green’s function explicitly is quite computationally demanding, especially for 3D problems. To avoid explicit calculation of the Green’s function, we show that the Rytov-FFTSK can be obtained by crosscorrelating a forward-propagated incident wavefield and reverse-propagated adjoint wavefield in the time domain. In addition, we find that the action of the Rytov-FFTSK on a model-space vector, e.g., the product of the sensitivity kernel and a vector, can be computed by calculating the inner product of two time-domain fields. Consequently, the Hessian-vector product can be computed in a matrix-free fashion (i.e., first calculate the product of the sensitivity kernel and a model-space vector and then calculate the product of the transposed sensitivity kernel and a data-space vector), without forming the Hessian matrix or the sensitivity kernels explicitly. We solve the traveltime inverse problem with the Gauss-Newton method, in which the Gauss-Newton equation is approximately solved by the conjugate gradient using our matrix-free Hessian-vector product method. An example with a perfect acquisition geometry found that our Rytov-approximation-based traveltime inversion method can produce a high-quality inversion result with a very fast convergence rate. An overthrust synthetic data test demonstrates that large- to intermediate-scale model perturbations can be recovered by diving waves if long-offset acquisition is available.

Eikonal Equation-Based Seismic Tomography of the Source Areas of the 2008 Mw 7.9 Wenchuan Earthquake and the 2013 Mw 6.6 Lushan Earthquake

ABSTRACT We use the eikonal equation-based seismic travel-time tomography method to image the source areas of the 2008 Wenchuan earthquake and the 2013 Lushan earthquake in the Longmenshan fault zone. High-resolution VP and VS models are obtained by inverting 75,686 P-wave and 74,552 S-wave travel times of local earthquakes during the period from 2009 to 2018. The tomographic models reveal strong crustal velocity heterogeneities in the study area. A significant velocity contrast exists across the Longmenshan fault zone: The western Songpan–Ganzi block is a high-velocity body, whereas the eastern Sichuan basin is a low-velocity anomaly. The hypocenter of the 2008 Wenchuan earthquake is between a high-velocity and a low-velocity anomaly. Beneath the Wenchuan mainshock, there is a significant low-velocity structure in the lower crust. The 2013 Lushan earthquake occurred in rocks associated with a high-velocity anomaly. A distinct low-velocity zone with low seismicity is imaged between the 2008 Wenchuan earthquake and the 2013 Lushan earthquake, where the crustal ductile deformation is likely to occur. The Baoxing complex to the northwest of the Lushan hypocenter exhibits as a high-velocity anomaly, which may be a carrier of stress accumulation and more prone to seismic activities in the future.

A B-Spline Method With AIS Optimization for 2-D IoT-Based Overpressure Reconstruction

In blast wave monitoring, a traditional travel time tomography method is encountered with local minimum travel time and low coverage density of rays. In this article, a novel B-spline fitting method with the knot-optimization artificial immune system (AIS) is proposed for 2-D overpressure reconstruction. It possesses the advantages of handling point sets of large sizes and adjusts the knot vector flexibly. Based on the overpressure value in the explosion from the travel time tomography method, the proposed method combining the advantages of B-splines and knot point optimization AIS is able to achieve the optimal sensor distribution and raise the reconstruction precision. The detailed experimental results about the comparison of linear fitting interpolation, cubic fitting interpolation, natural neighbor fitting interpolation, v4 fitting interpolation, Delaunay triangulation fitting, and B-spline method are also given. Furthermore, for the knot optimization issue in B-spline, the proposed adaptive fitting method with knot-optimization AIS has a smaller root-mean-square (RMS) error with eight knot nodes in comparison with the classic B-spline fitting method. This article is conducted to provide new insights to reconstructing 2-D Internet-of-Things-based (IoT-based) overpressure in blast wave monitoring more precisely under limited sensor deployment and further give a new approach to overpressure reconstruction scenarios.

Joint refraction traveltime tomography and migration for multilayer near-surface imaging

In near-surface velocity structure estimation, first-arrival traveltime tomography tends to produce a smooth velocity model. If the shallow structures include a weathering layer over high-velocity bedrock, first-arrival traveltime tomography may fail to recover the sharp interface. However, with the same traveltime data, refraction traveltime migration proves to be an effective tool for accurately mapping the refractor. The approach downward continues the refraction traveltime curves and produces an image (position) of the refractor for a given overburden velocity model. We first assess the validity of the refraction traveltime migration method and analyze its uncertainties with a simple model. We then develop a multilayer refraction traveltime migration method and apply the migration image to constrain traveltime tomographic inversion by imposing discontinuities at the refraction interfaces in model regularization. In each subsequent iteration, the shape of the migrated refractors and the velocity model are simultaneously updated. The synthetic tests indicate that the joint inversion method performs better than the conventional first-arrival traveltime tomography method with Tikhonov regularization and the delay-time method in reconstructing near-surface models with high-velocity contrasts. In application to field data, this method produces a more accurately resolved velocity model, which improves the quality of common midpoint stacking by making long-wavelength static corrections.

Ray‐based reflection traveltime tomography using approximate stationary points

ABSTRACTReflection traveltime tomography has been used to describe subsurface velocity structures which, in practice, can be used as a background or initial model for pre‐stack depth migration or full waveform inversion. Conventional reflection traveltime tomography is performed by solving an optimization problem based on a ray‐tracing method. As a result, reflection traveltime tomography requires heavy computational efforts to carry out ray tracing and solve a large matrix equation. In addition, like most data‐domain tomography methods, reflection traveltime tomography depends on initial guesses and suffers from non‐uniqueness and uncertainty of solutions. In this research, we propose a deterministic ray‐based reflection traveltime tomography method by applying seismic interferometry. This method does not suffer from the non‐uniqueness problem and does not require a priori information on subsurface media. By adding a virtual layer (whose properties are known) on top of the real surface and applying convolution‐type interferometry, we approximately determine the stationary points (i.e., incident raypaths in the virtual layer). Then, we generate reflection points for a range of assumed velocities and estimate the velocity by considering the number of reflection points and the traveltime difference between the observed and calculated data. The reflection surface can then be recovered by using the estimated velocity. Once the first target layer is resolved, we can recover the whole media by recursively applying the same method to the lower layers. Numerical examples using surface seismic profile data for homogeneous‐layer (with a low‐velocity layer) and inhomogeneous‐layer models and real field data experiments on the Congo data set demonstrate that our method can successfully recover the velocities and depths of subsurface media without initial guesses. However, our method has some limitations for multi‐layer models because the method does not have sufficient reflection points for the deeper layers.

Travel Time Tomography With Adaptive Dictionaries

We develop a two-dimensional travel time tomography method, which regularizes the inversion by modeling groups of slowness pixels from discrete slowness maps, called patches, as sparse linear combinations of atoms from a dictionary. We propose to use dictionary learning during the inversion to adapt dictionaries to specific slowness maps. This patch regularization, called the local model, is integrated into the overall slowness map, called the global model. The local model considers small-scale variations using a sparsity constraint, and the global model considers larger-scale features constrained using $\ell _2$ regularization. This strategy in a locally sparse travel time tomography (LST) approach enables simultaneous modeling of smooth and discontinuous slowness features. This is in contrast to conventional tomography methods, which constrain models to be exclusively smooth or discontinuous. We develop a maximum a posteriori formulation for LST and exploit the sparsity of slowness patches using dictionary learning. The LST approach compares favorably with smoothness and total variation regularization methods on densely, but irregularly sampled synthetic slowness maps.

Travel time tomography with local sparse modeling and dictionary learning

In this talk, we present a machine learning-based approach to 2D travel time tomography. Travel time tomography methods image slowness structures (e.g., Earth geology) from acoustic and seismic wave travel times across sensor arrays. Typically, slowness is obtained via an ill-posed linear inverse problem, which requires regularization to obtain physically plausible solutions. We propose to regularize this inversion by modeling rectangular groups of slowness pixels from the image, called patches, as sparse linear combinations of atoms from a dictionary. In this locally-sparse travel time tomography (LST) method, the dictionary, which represents elemental slowness features, is initially unknown and is learned from the travel time data using an unsupervised machine learning task called dictionary learning. This local model constrains small-scale slowness features, and is combined with a global model, which constrains larger-scale features with L2 regularization. In contrast to conventional regularization, which allows only for smoothness or discontinuous slowness, LST permits increased resolution where warranted by data. A maximum a posteriori formulation ofLST is derived, which is solved as an iterative algorithm. LST performance is evaluated on both synthetic and real data in the context of ambient noise tomography.

Time-lapse seismic tomography of an underground mining zone

In this study, we determined P-wave tomographic images of the Yongshaba deposit, an underground mining zone in Guizhou Province, China, by inverting arrival-time data of micro-seismic events and blasts recorded by a passive seismic array consisting of 28 sensors during January to April 2014 using an event location technique and a travel-time tomography method that can handle complex seismic discontinuities. The damping parameter used in the damped least-squares method is determined with the help of trade-off curve. To reveal internal P-wave velocity changes of the study area under severe mining activities, a time-lapse data partition scheme is used. To assess the human influence on the underground structure, we introduce a parameter to measure the relative velocity changes in different periods. The resolution of the tomographic images and the robustness of the obtained features are examined by conducting a series of checkerboard resolution tests and a restoring resolution test. Our tomographic results obtained from the entire data set reveal a prominent low-velocity (low-V) zone and many obvious high-velocity (high-V) zones. These features match well with the geological setting and the excavation plan carried out in the mine, according to in-site surveys. The low-V zone may reflect empty volumes, stress releases and rock breaking and cracking caused by mining activities, whereas the high-V zones are probably the consequences of local stress concentrations caused by regional stress redistribution. The time-lapse tomographic images may reveal the process of stress concentrations caused by rock breaking due to the continuing excavations in the entire observation period, indicating that the mining activities influenced the rock property and caused complex changes of the underground structure.

3D first-arrival traveltime tomography with modified total variation regularization

3D seismic surveys have become a major tool in the exploration and exploitation of hydrocarbons. 3D seismic first-arrival traveltime tomography is a robust method for near-surface velocity estimation. A common approach for stabilizing the ill-posed inverse problem is to apply Tikhonov regularization to the inversion. However, the Tikhonov regularization method recovers smooth local structures while blurring the sharp features in the model solution. We present a 3D first-arrival traveltime tomography method with Modified Total-Variation (MTV) regularization to preserve sharp velocity contrasts and improve the accuracy of velocity inversion. To solve the minimization problem of the new traveltime tomography method, we decouple the original optimization problem into two following subproblems: a standard traveltime tomography problem with the traditional Tikhonov regularization and a L2 total-variation problem. We apply the conjugate gradient method and split-Bregman iterative method to solve these two subproblems, respectively. Our synthetic examples show that the new method produces higher resolution models than the conventional traveltime tomography with Tikhonov regularization. We apply the technique to field data. Stacking section shows significant improvements with statics corrections from the MTV traveltime tomography.

Crust and upper mantle velocity structure beneath the Ordos Block and its tectonic significance

The Ordos Block in the western North China Craton (NCC) occupies an important position in the tectonic evolution and deep earthquake environment research in China continent. In previous seismic tomography studies, the sparse distribution of seismic stations resulted in a relatively low lateral resolution of 3D structure of crust and upper mantle in the Ordos Block. In this paper, a dense broadband seismograph network was established, including the permanent seismic stations of the regional networks and the temporary seismic stations funded by the ″Destruction of the North China Craton″ project and the North China Scientific Seismic Array. Based on the waveform data recorded by 338 seismic stations in the Ordos Block and surrounding regions, we used FMTOMO traveltime tomography method to determine the high-resolution P-wave velocity structures of the crust and upper mantle down to 400 km depth, and analyzed the relationship between deep structure and a range of geodynamic issues, such as the volcanic and strong earthquake activities and the destruction of the NCC. The tectonic implications of the lateral and the vertical variations of P-wave velocity in the crust and upper mantle structures are described as follow. The transverse variation and vertical difference of velocity structure of crust and uppermost mantle indicate significant correlations between deep tectonics and surface geological setting. The thick lithosphere with positive velocity anomaly beneath the Ordos block suggests that the Ordos block did not suffer extensive destruction. The velocity anomaly distribution of the Taihang orogenic belt in the central NCC is characterized by discontinuous strip anomalies, which is significantly different from that of the North China Plain and Ordos block. The low-velocity anomalies deeper than 400 km might be related to the lithosphere destruction in the eastern NCC. A low-velocity columnar anomaly down to 400 km depth beneath the Datong volcanic groups indicates that the magmatism of the Datong volcanic group might originate from the mantle transitional zone, due to the upwelling and expulsion of basaltic magma caused by the subduction of the Pacific plate. The low-velocity zone in approximately EW direction in the upper mantle between the Ordos Block and the Sichuan basin suggests the possible existence of a narrow outflow channel of the asthenosphere. In summary, the low-velocity anomaly down to the upper mantle in the Ordos Block and surrounding regions indicates that the north-eastward compressive stress of the Tibetan Plateau and the westward subduction of the Pacific plate may induce the migration and upwelling of mantle materials, causing the overall uplift of the Ordos Block and the formation of extensional fault belt in surrounding regions. The vertical upwelling and lateral movement of the deep materials provide the stress conditions for the preparation and occurrence of strong earthquakes resulting in frequent occurrence of strong earthquakes in the regions around the Ordos Block.

Velocity structure of the western North China basement from high-resolution wide-angle reflection/refraction profiles

Two 1420 km-long, high-resolution wide-angle reflection/refraction profiles were produced for the western part of the North China Craton. Twenty-one shots were fired along the two profiles and seismic data was collected by 700 portable three-component digital seismographers. The crust-mantle structure was investigated in different regions, including the Ordos Block, Haiyuan arcuate tectonic region, Alashan Block, and Yinchuan Basin. First-arrival Pg waves were analyzed using the finite-difference method of first-arrival traveltime tomography to construct the 2D interface morphology and velocity structure of the crystalline basement. The study identified the location and shape of major faults, and notable variations in basement depth and velocity structure in different regions. The crystalline basement in the Ordos Block is typically stable with a depth of 4.5–5.5 km and exhibits only minor fluctuations in the lateral velocity gradient. This is evidenced by Pg wave features, including stable travel time, long tracing distance and high velocity. Between the Helan Mountains and the Yinchuan Basin, notable variations in topography, basement depth, and velocity structure were identified, indicating strong divergence in vertical movement and deformation. In the Yinchuan Basin, the Pg wave traveltime curve revealed the complex structure of the crystalline basement and variations in the velocity gradient. Since the Cenozoic Era, the western North China Craton has experienced strong and frequent tectonic activity causing strong deformation on the periphery of the Alashan Block. The presence of small, low-velocity, near-surface basins, and variations in the crystalline basement interface and velocity isolines provides evidence of strong tectonic deformation of the basement near Bayan Ura Hill, on the eastern margin of the Alashan Block. The Haiyuan arcuate tectonic region is complex and interacts with the Qilian Mountains, Alashan Block, and Ordos Block. Basement depth extends to 6.0 km and varies considerably from the adjacent uplift zone, which has a depth of 2.0 km. The highly deformed and unstable basement structure at the northeastern margin of the Tibetan Plateau is probably related to the collision and squeezing of the Indian Plate and the Eurasian Plate. Seismology data indicates differences between the crystalline basement interface and the velocity structure, as well as strong transverse inhomogeneity. Differences in basement depth in the Ordos Block, together with significant lateral heterogeneity, compressional fold deformation, and the presence of fault zones in the marginal basement, block the continuous compressive movement of the Tibetan Plateau in a northeast direction.

Application of Frequency-Dependent Traveltime Tomography to 2D Crosswell Seismic Field Data

We applied Zelt's new frequency-dependent traveltime tomography (FDTT) method to 2D crosswell seismic field data from an eastern oil field in China. The FDTT uses the frequency content in the seismic waves in both the forward and inverse modeling steps. Although FDTT only uses a 300 Hz frequency to invert the dataset, the degree of matching between the inverted layers from FDTT and that of a sonic well logging curve is high, which shows that FDTT provides a high resolution reconstruction of subsurface structure through the simple use of the first-arrival traveltime data. The case study demonstrates that the FDTT algorithm is practical and can stand up to the complexities of a real 2D crosswell dataset. Additionally, we show that the FDTT method can create a high resolution long wavelength velocity model.

Upper crustal P-wave velocity structure beneath two volcanic areas in northern Iran

Detailed information about the crustal structure is essential for better understanding the occurrence and mechanisms of earthquakes and volcanoes. Here we present a study of the upper crustal P-wave velocity structure of two seismically and volcanically active areas in northern Iran using the two-dimensional Pg travel time tomography method. The imaging results suggest low velocities in the upper crust beneath the Damavand and Sahand-Sabalan volcanic areas in the central and western parts of northern Iran, respectively. The upper crustal low velocities in these two areas roughly coincide with previously imaged low Pn velocity anomalies, suggesting that the Late Cenozoic volcanic activity was probably caused by the upwelling of hot materials from the mantle. The image feature of the Pg velocity structure beneath the Sahand-Sabalan volcanic area further indicates that the hot materials stored in the upper crust beneath Sahand may be larger in size than those stored beneath Sabalan. Comparison of the Pg velocity images with the earthquake distribution in north Iran suggests that earthquakes mainly occur at moderately low velocity or low to high velocity boundary areas instead of significantly low or high velocity regions. The anisotropy results show that the Pg wave fast direction is consistent with the GPS direction at high Pg velocity areas and the fast direction is inconsistent with the GPS direction but consistent with the strike direction of faults at low velocity areas. Our new upper crustal structural images provide the basic observation for better understanding of the regional seismicity and volcanism, and link the surface geological phenomena to deep crustal and mantle processes associated with the active tectonics in northern Iran.

Attenuated traveltime tomography method for estimation of seismic attenuation

Due to the viscous-elastic properties of subsurface media, seismic waves experience a loss of amplitude and distortion of phase during propagation. Compared with frequency domain methods, tomography methods are designed to calculate a set of Q values simultaneously. But we have noticed that most existing tomography inversion methods may result in inaccurate Q estimation because of many adverse factors. In addition, these methods can only calculate effective Q between layers. Improved attenuated traveltime (t⁎) tomography is introduced here as a more accurate and efficient method for estimation of true Q distribution, instead of effective Q values. The method updates an initial Q guess to the true Q distribution through inversion. The accuracy of t⁎ is essential for t⁎ tomography. Minute errors of t⁎ can cause unreasonable Q inversion results. A stabilized method is designed to improve the accuracy of t⁎ by statistically selecting the optimum frequency band for regression. Synthetic and field data examples demonstrate the feasibility and effectiveness of this method. Depending on the more accurate Q distribution, seismic resolution can be improved after Q compensation migration.