Nonlinear Tomography Research Articles

Impact of refractive index mismatches on coherent anti-Stokes Raman scattering and multiphoton autofluorescence tomography of human skin in vivo

Optical non-linear multimodal tomography is a powerful diagnostic imaging tool to analyse human skin based on its autofluorescence and second-harmonic generation signals. Recently, the field of clinical non-linear imaging has been extended by adding coherent anti-Stokes Raman scattering (CARS)—a further optical sectioning method for the detection of non-fluorescent molecules. However, the heterogeneity of refractive indices of different substances in complex tissues like human skin can have a strong influence on CARS image formation and requires careful clinical interpretation of the detected signals. Interestingly, very regular patterns are present in the CARS images, which have no correspondence to the morphology revealed by autofluorescence at the same depth. The purpose of this paper is to clarify this phenomenon and to sensitize users for possible artefacts. A further part of this paper is the detailed comparison of CARS and autofluorescence images of healthy human skin in vivo covering the complete epidermis and part of the upper dermis by employing the flexible medical non-linear tomograph MPTflex CARS.

Nonlinear elastic wave tomography for the imaging of corrosion damage

This paper presents a nonlinear elastic wave tomography method, based on ultrasonic guided waves, for the image of nonlinear signatures in the dynamic response of a damaged isotropic structure. The proposed technique relies on a combination of high order statistics and a radial basis function approach. The bicoherence of ultrasonic waveforms originated by a harmonic excitation was used to characterise the second order nonlinear signature contained in the measured signals due to the presence of surface corrosion. Then, a radial basis function interpolation was employed to achieve an effective visualisation of the damage over the panel using only a limited number of receiver sensors. The robustness of the proposed nonlinear imaging method was experimentally demonstrated on a damaged 2024 aluminium panel, and the nonlinear source location was detected with a high level of accuracy, even with few receiving elements. Compared to five standard ultrasonic imaging methods, this nonlinear tomography technique does not require any baseline with the undamaged structure for the evaluation of the corrosion damage, nor a priori knowledge of the mechanical properties of the specimen.

Upper mantle structure around the Trans-European Suture Zone obtained by teleseismic tomography

Abstract. The presented study aims to resolve the upper mantle structure around the Trans-European Suture Zone (TESZ), which is the major tectonic boundary in Europe. The data of 183 temporary and permanent seismic stations operated during the period of the PASsive Seismic Experiment (PASSEQ) 2006–2008 within the study area from Germany to Lithuania was used to compile the data set of manually picked 6008 top-quality arrivals of P waves from teleseismic earthquakes. We used the TELINV nonlinear teleseismic tomography algorithm to perform the inversions. As a result, we obtain a model of P wave velocity variations up to about ±3% with respect to the IASP91 velocity model in the upper mantle around the TESZ. The higher velocities to the east of the TESZ correspond to the older East European Craton (EEC), while the lower velocities to the west of the TESZ correspond to younger western Europe. We find that the seismic lithosphere–asthenosphere boundary (LAB) is more distinct beneath the Phanerozoic part of Europe than beneath the Precambrian part. To the west of the TESZ beneath the eastern part of the Bohemian Massif, the Sudetes Mountains and the Eger Rift, the negative anomalies are observed from a depth of at least 70 km, while under the Variscides the average depth of the seismic LAB is about 100 km. We do not observe the seismic LAB beneath the EEC, but beneath Lithuania we find the thickest lithosphere of about 300 km or more. Beneath the TESZ, the asthenosphere is at a depth of 150–180 km, which is an intermediate value between that of the EEC and western Europe. The results imply that the seismic LAB in the northern part of the TESZ is in the shape of a ramp dipping to the northeasterly direction. In the southern part of the TESZ, the LAB is shallower, most probably due to younger tectonic settings. In the northern part of the TESZ we do not recognize any clear contact between Phanerozoic and Proterozoic Europe, but further to the south we may refer to a sharp and steep contact on the eastern edge of the TESZ. Moreover, beneath Lithuania at depths of 120–150 km, we observe the lower velocity area following the boundary of the proposed paleosubduction zone.

Reconstruction of the complex refractive index in nonlinear phase contrast tomography

A new nonlinear method to reconstruct the complex refractive index distribution with in-line phase tomography measurements is presented. The inverse problem is regularized with the Tikhonov smoothing norm of the index. The original nonlinear iterative approach is based on the Fréchet derivative of the intensity recorded at a single propagation distance and on a Simultaneous Algebraic Reconstruction technique. The reconstruction method requires no a priori knowledge about the materials. The algorithm is successfully applied to some simulated data with noise.

Traces of the crustal units and the upper-mantle structure in the southwestern part of the East European Craton

Abstract. The presented study is a part of the passive seismic experiment PASSEQ 2006–2008, which took place around the Trans-European Suture Zone (TESZ) from May 2006 to June 2008. The data set of 4195 manually picked arrivals of teleseismic P waves of 101 earthquakes (EQs) recorded in the seismic stations deployed to the east of the TESZ was inverted using the non-linear teleseismic tomography algorithm TELINV. Two 3-D crustal models were used to estimate the crustal travel time (TT) corrections. As a result, we obtain a model of P-wave velocity variations in the upper mantle beneath the TESZ and the East European Craton (EEC). In the study area beneath the craton, we observe up to 3% higher and beneath the TESZ about 2–3% lower seismic velocities compared to the IASP91 velocity model. We find the seismic lithosphere–asthenosphere boundary (LAB) beneath the TESZ at a depth of about 180 km, while we observe no seismic LAB beneath the EEC. The inversion results obtained with the real and the synthetic data sets indicate a ramp shape of the LAB in the northern TESZ, where we observe values of seismic velocities close to those of the craton down to about 150 km. The lithosphere thickness in the EEC increases going from the TESZ to the NE from about 180 km beneath Poland to 300 km or more beneath Lithuania. Moreover, in western Lithuania we find an indication of an upper-mantle dome. In our results, the crustal units are not well resolved. There are no clear indications of the features in the upper mantle which could be related to the crustal units in the study area. On the other hand, at a depth of 120–150 km we indicate a trace of a boundary of proposed palaeosubduction zone between the East Lithuanian Domain (EL) and the West Lithuanian Granulite Domain (WLG). Also, in our results, we may have identified two anorogenic granitoid plutons.

A Nonlinear Microwave Breast Cancer Imaging Approach Through Realistic Body–Breast Modeling

A nonlinear microwave tomography approach which suggests monitoring of differences inside the breast with respect to a reference breast model that adequately represents the healthy breast is presented. This approach simplifies the breast cancer imaging problem through realistic modeling of the body-breast configuration and does not require the imaging of the entire breast tissues. The tumor is considered as a buried object inside the breast which is assumed to be located on the interface of two-half spaces media composed of air or a coupling liquid and the chest wall or the base of the bed on which patient lies in a prone position. Then, the tumor, as well as the deviations of the actual breast from the breast model, is imaged by using the contrast source inversion method at a single frequency. The Green's function of the inhomogeneous background required in the application of the inversion algorithm is obtained through the buried object approach and accelerated by an adaptation of the discrete complex images method. Through the simulations it has been shown that, the proposed modeling and nonlinear inversion algorithm is capable of reconstructing tumors as small as 4 mm in radius for 3-D scenarios.

Non-Linear Phase Tomography Based on Fréchet Derivative

Phase imaging coupled to micro-tomography acquisition has emerged as a powerful tool to investigate specimens in a non-destructive manner. While the intensity data can be acquired and recorded, the phase information of the signal has to be “retrieved” from the data modulus only. Phase retrieval is an ill-posed non-linear problem and regularization techniques including a priori knowledge are necessary to obtain stable solutions. Several linear phase recovery methods have been proposed and it is expected that some limitations resulting from the linearization of the direct problem will be overcome by taking into account the non-linearity of the phase problem. To achieve this goal, we propose and evaluate a non-linear algorithm for in-line phase micro-tomography based on an iterative Landweber method with an analytic calculation of the Fréchet derivative of the phase-intensity relationship and of its adjoint. The algorithm was applied in the projection space using as initialization the linear mixed solution. The efficacy of the regularization scheme was evaluated on simulated objects with a slowly and a strongly varying phase. Experimental data were also acquired at ESRF using a propagation-based X-ray imaging technique for the given pixel size 0.68 μm. Two regularization scheme were considered: first the initialization was obtained without any prior on the ratio of the real and imaginary parts of the complex refractive index and secondly a constant a priori value was assumed on . The tomographic central slices of the refractive index decrement were compared and numerical evaluation was performed. The non-linear method globally decreases the reconstruction errors compared to the linear algorithm and is achieving better reconstruction results if no prior is introduced in the initialization solution. For in-line phase micro-tomography, this non-linear approach is a new and interesting method in biomedical studies where the exact value of the a priori ratio is not known.

Crustal velocity structure associated with the eastern Tennessee seismic zone: Vp and Vs images based upon local earthquake tomography

AbstractWe present three‐dimensional P and S wave velocity models for the active eastern Tennessee seismic zone (ETSZ) using arrival time data from more than 1000 local earthquakes. A nonlinear tomography method is used that involves sequential inversion for model and hypocenter parameters. We image several velocity anomalies that persist through most of the inversion volume. Some anomalies support the presence of known features such as an ancient rift zone in northern Tennessee. Other anomalies reveal the presence of basement features that can be correlated with regional gravity and magnetic anomalies. We image a narrow, NE‐SW trending, steeply dipping zone of low velocities that extends to a depth of at least 24 km and is associated with the vertical projection of the prominent New York‐Alabama magnetic lineament. The low‐velocity zone may have an apparent dip to the SE at depths exceeding 15 km. The low‐velocity zone is interpreted as a major basement fault juxtaposing Granite‐Rhyolite basement to the NW from Grenville southern Appalachian basement to the SE. Relocated hypocenters align in near‐vertical segments suggesting reactivation of a distributed zone of deformation associated with a major strike‐slip fault. We suggest that the ETSZ represents reactivation of an ancient shear zone established during formation of the super continent Rodinia.

Possibilities of tomography system prototype using third-order acoustic nonlinear effects

Third-order nonlinear acoustical tomography is very important for medical diagnostics, because it will provide information on the new and unexplored quantity—third-order nonlinear acoustical parameter. A prototype of the tomography system for reconstructing the distributions of the acoustic nonlinear parameters is developed in our group on the basis of effect of nonlinear noncollinear interaction of three primary waves. Application of coded primary signals with further correlation processing of a detected signal at combination frequencies makes it possible to reconstruct the complete image of an object using three transmitters and one receiver. Third-order effects are born from two competing processes—the pure third order interaction (informational for diagnostic purposes) and the twofold interaction of the second order (interfering). To explore the application boundaries of the third-order nonlinear acoustical tomography, these two competing processes are considered in details. Two cases are compared. In the first case, the interaction takes place between two broadband coded signals and one monochromatic signal. In the second case, all three primary signals are broadband coded; then, amplitude of the interfering part of the signal is smaller. Moreover, in this case, the possibility of reconstructing the spatial distribution of the second and third-order nonlinear acoustical parameters arises due to the reciprocity principle generalized on the nonlinear scattering processes.

Bayesian inversion with total variation prior for discrete geologic structure identification

The characterization of geologic heterogeneity that affects flow and transport phenomena in the subsurface is essential for cost‐effective and reliable decision‐making in applications such as groundwater supply and contaminant cleanup. In the last decades, geostatistical inversion approaches have been widely used to tackle subsurface characterization problems and quantify their corresponding uncertainty. Some well‐established geostatistical methods use models that assume gradually varying parameters. However, in many cases, the subsurface can often be better represented as consisting of a few relatively uniform geologic facies or zones with abrupt changes at their boundaries. We advance a Bayesian inversion approach with the gradient represented not through a Gaussian but a Laplace prior, also known as total variation prior, for the case that there are reasons to believe that discrete geologic structures with relatively homogeneous properties predominate in the subsurface but their number, locations, and shapes are unknown a priori. Structural parameters (or hyperparameters of the inversion scheme) are determined in a Bayesian framework by maximizing the marginal distribution of these parameters using an expectation‐maximization approach; this allows proper weighting of prior versus data information and produces results with realistic uncertainty quantification. We present here three applications of the method: a time‐varying extraction rate estimation at a well, a linear cross‐well seismic tomography, and a nonlinear hydraulic tomography. These results are compared with those achieved in the classical geostatistical method and it is shown that the Bayesian inversion approach with total variation prior can be a useful tool to identify discrete geologic structures.

Study on two-phase flow regime visualization and identification using 3D electrical capacitance tomography and fuzzy-logic classification

From variety of industry-oriented imaging solutions the electrical capacitance tomography applied to the two-phase gas–liquid mixtures visualization and the phase distribution calculation is getting popular especially when flow key parameters are required. Industry demands particularly include efficient non-invasive automatic phase fraction calculation and flow structure identification in the vertical and horizontal pipelines. This can be solved by using non-deterministic fuzzy-logic based techniques for analysis of volumetric images. This paper presents a preliminary study on automated two-phase gas–liquid flow pattern identification based on a fuzzy evaluation of series of reconstructed 3D ECT volumetric images. The set of volume data is obtained by using nonlinear electrical capacitance tomography reconstruction algorithms. Finally a set of fuzzy-based features is calculated for flow substructure classification. As a result of this analysis obtained features will be used to classify given volumetric image into one of known flow regime structures.

New constraints on the 3D shear wave velocity structure of the upper mantle underneath Southern Scandinavia revealed from non-linear tomography

We analyse travel times of shear waves, which were recorded at the MAGNUS network, to determine the 3D shear wave velocity ( v S ) structure underneath Southern Scandinavia. The travel time residuals are corrected for the known crustal structure of Southern Norway and weighted to account for data quality and pick uncertainties. The resulting residual pattern of subvertically incident waves is very uniform and simple. It shows delayed arrivals underneath Southern Norway compared to fast arrivals underneath the Oslo Graben and the Baltic Shield. The 3D upper mantle v S structure underneath the station network is determined by performing non-linear travel time tomography. As expected from the residual pattern the resulting tomographic model shows a simple and continuous v S perturbation pattern: a negative v S anomaly is visible underneath Southern Norway relative to the Baltic Shield in the east with a contrast of up to 4% v S and a sharp W–E dipping transition zone. Reconstruction tests reveal besides vertical smearing a good lateral reconstruction of the dipping v S transition zone and suggest that a deep-seated anomaly at 330–410 km depth is real and not an inversion artefact. The upper part of the reduced v S anomaly underneath Southern Norway (down to 250 km depth) might be due to an increase in lithospheric thickness from the Caledonian Southern Scandes in the west towards the Proterozoic Baltic Shield in Sweden in the east. The deeper-seated negative v S anomaly (330–410 km depth) could be caused by a temperature anomaly possibly combined with effects due to fluids or hydrous minerals. The determined simple 3D v S structure underneath Southern Scandinavia indicates that mantle processes might influence and contribute to a Neogene uplift of Southern Norway. ► New 3D upper mantle shear wave velocity ( v S ) model for Southern Scandinavia. ► A low v S anomaly underneath Southern Norway relative to the Baltic Shield. ► Lateral variations in lithospheric thickness as cause of the upper v S anomaly. ► Temperature anomaly beneath Southern Norway as possible cause of a deeper anomaly. ► Mantle processes might influence the Neogene uplift of the Scandes.

Seismic monitoring of radioactive waste repositories

Safe isolation of high-level radioactive waste (HLW) and spent nuclear fuel (SF) from the biosphere is an important concern for humanity. Many countries consider monitoring the evolving conditions of a HLW/SF repository over a long period noninvasively, in such a way as to not compromise the sealing of the repository. Remote active seismic methods are possible options. We performed extensive seismic measurements at the Mont Terri Rock Laboratory, located within a clay formation that is a potential host rock in the Swiss HLW/SF program. A scaled-down HLW/SF repository was simulated as a 13-m-long microtunnel of 1 m diameter. The dry sand-filled microtunnel was progressively water saturated and slightly overpressured. The monitoring system consisted of a pressure seismic source and a hydrophone cable located in two boreholes drilled perpendicular to the microtunnel and eight geophones equally distributed around the periphery of the microtunnel. Nonlinear anisotropic tomography of crosshole traveltimes allowed the gross velocity structure of the host rock to be determined, but due to the experiment geometry, the hydrophone traveltimes were inadequate for monitoring changes within the repository. In contrast, analysis of the geophone seismic traces enabled the saturation process to be characterized. An integrated interpretation of the geophone data in combination with extensive numerical modeling suggested that injection of the water resulted in significant self sealing of the microtunnel’s excavation damage/disturbed zone (EDZ) and substantial increases in the P-wave velocity of the microtunnel sand and EDZ. On the basis of our investigations, we concluded that seismic monitoring of a HLW/SF repository is feasible, as long as the sensors are installed as close to the repository as regulations allow.

Difference-frequency generation in nonlinear scattering of acoustic waves by a rigid sphere

In this paper, the partial-wave expansion method is applied to describe the difference-frequency pressure generated in a nonlinear scattering of two acoustic waves with an arbitrary wavefront by means of a rigid sphere. Particularly, the difference-frequency generation is analyzed in the nonlinear scattering with a spherical scatterer involving two intersecting plane waves in the following configurations: collinear, crossing at right angles, and counter-propagating. For the sake of simplicity, the plane waves are assumed to be spatially located in a spherical region which diameter is smaller than the difference-frequency wavelength. Such arrangements can be experimentally accomplished in vibro-acoustography and nonlinear acoustic tomography techniques. It turns out to be that when the sphere radius is of the order of the primary wavelengths, and the downshift ratio (i.e. the ratio between the fundamental frequency and the difference-frequency) is larger than five, difference-frequency generation is mostly due to a nonlinear interaction between the primary scattered waves. The exception to this is the collinear scattering for which the nonlinear interaction of the primary incident waves is also relevant. In addition, the difference-frequency scattered pressure in all scattering configurations decays as r-1lnr and 1/r, where r is the radial distance from the scatterer to the observation point.

Seismic wavespeed images across the Iapetus and Tornquist suture zones

Closures of the Iapetus Ocean and the Tornquist Sea lead to the collision of the paleocontinents of Laurentia, Baltica and Eastern Avalonia during the Caledonian orogeny. It has been speculated that relicts of these two closures may be preserved within the crust or upper mantle. Over the past decades, numerous wide‐angle seismic profiles were gathered in northwestern Europe to search for related subsurface features. Although active source studies revealed detailed crustal structures across the Iapetus and Tornquist suture zones, there are relatively few clear three‐dimensional upper mantle images beneath this region. We use a new European crust and upper mantle model, EU30, determined based on continental scale, nonlinear adjoint tomography, to explore upper mantle structures across these two suture zones. Model EU30 reveals two fast anomalies within the upper mantle: one dips in a northwesterly direction down to approximately 400 km beneath the North Sea, and the other dips in a southwesterly direction down to nearly 250 km across the Tornquist Suture Zone. In addition, we observe a “gap” between the lithospheres of Laurentia and Eastern Avalonia across the Iapetus Suture Zone beneath the central British Isles. These seismic images suggest that heterogeneity related to the closures of the Iapetus Ocean and the Tornquist Sea have been preserved within the upper mantle over hundreds of millions of years.

Role of entropy perturbations in third-order nonlinear acoustic tomography problems

The effect of the entropy perturbations on the acoustic pressure, which arises due to third-order nonlinear interaction, has been considered. The contribution to entropy perturbation due to the presence of viscosity and thermal conductivity (by means of the general equation of heat transfer) and also due to the nonlinearity of the considered medium has been estimated. It has been shown that when all possible factors responsible for the change in entropy are taken into account, the third-order contribution to the acoustic pressure due to density perturbations in primary waves strongly exceeds that due to entropy perturbations. It has been concluded that an isoentropic mathematical description of third-order nonlinear effects in nonlinear acoustic tomography problems is appropriate.

Acoustic properties of organic powders as ultrasonic contrast agents

The results of experiments on measuring attenuation and the effective acoustic nonlinear parameter of the second order are given for a suspension of cocoa-powder in water at different concentrations of the suspension. In the process of evaluating the value of the nonlinear parameter the attenuation in the suspension and generation of the second harmonic not only in the suspension but also in water are taken into account. The obtained results are evidence of the possibility of using a suspension of cocoa-powder in water as a technical substitute for ultrasonic contrast agents. The values of attenuation (up to 60 m−1 at the concentration of 1 g of the powder per 1 l of water) and the nonlinear parameter (up to 120 m−1 at the same concentration) mean that the suspension of cocoa-powder in water has smaller attenuation and the nonlinear parameter than ultrasonic contrast agents at the same concentration. However, these values for the suspension differ considerably from corresponding values for water or blood and, therefore, a suspension of cocoa-powder in water is a promising “substitute” for ultrasonic contrast agents in the case of technical testing of systems for nonlinear tomography of a blood flow, but cannot replace them in medical studies.

Seismic rays and traveltime tomography of strongly heterogeneous mantle structure: application to the Central Mediterranean

SUMMARY The structure of the Earth’s upper mantle near convergent plate margins, such as along the Nubia‐Eurasia collision zone in the Mediterranean, involves strong seismic wave speed contrasts associated with subducting lithospheric slabs and opening backarc basins. In this environment, seismic wave propagation is strongly influenced by heterogeneity, and requires appropriate modelling practice. Although accurate numerical methods are often used to model seismic traveltimes in the crust, only approximate techniques have been used for the mantle, on the assumption that speed contrasts are weaker. We devise, optimize, and test a method aimed at recovering strongly heterogeneous mantle structures using a finite-difference scheme to calculate first-arrival traveltimes and trace seismic rays with high accuracy even in the presence of strong gradients. We adapt this forward scheme—successfully used in local-scale tomography—to spherical geometry through source-specific Earth flattening approximations, and we split calculations in meshes with different step size to model optimally the crust (with a 2 km step) and the mantle (6 km step). We then use an iterative non-linear inversion approach, starting from a simple 1-D prior model. We test the ability of this procedure to reconstruct sample structures, devised to be illustrative for the Mediterranean region, using synthetic data calculated on the real distribution of sources and stations reported by the Bulletins of the International Seismological Centre (ISC). Besides regular checkerboard patterns, we also reconstruct a more representative model. Different strategies are used and compared in linear and non-linear inversion. We find that a linear approach, by which rays are only traced once in the background model, may result in an illusory fit to data. Realistic upper-mantle structures strongly deflect seismic rays, and correct paths can only be found after a few iterations. Although linear inversion seems able to identify the main features quite well, we verify how non-linear inversion and 3-D ray tracing significantly improve the results, especially when we attempt to reconstruct a realistic structure. We also apply the finite-difference, non-linear, traveltime tomography to data from the ISC to retrieve upper-mantle structure in the Central Mediterranean. We verify that the non-linear inversion is able to reveal sharpened velocity contrasts and thinner bodies than linear inversion. Clear differentiation found in the nonlinear result, between signatures of northern and southern Dinarides—showing lithosphere subducting only beneath the southern sector—is more coherent with the regional geodynamic framework. Such improvements due to non-linear mantle tomography may contribute to the general picture of slab detachment and small-scale mantle convection in the Mediterranean region, and therefore, significantly impact on geodynamic implications of resulting models.

Lithospheric structure beneath the Zagros collision zone resolved by non-linear teleseismic tomography

The upper-mantle structure across the Zagros collision zone, in southwest Iran, is investigated using a non-linear weighted damped least-squares teleseismic tomography approach. The resolution of the structures/transitions in the upper mantle is enhanced significantly by correcting the teleseismic relative arrival time residuals for an a priori crustal velocity model and then performing the inversion with fixed crustal blocks. To investigate whether or not the lithospheric blocks and major transitions in the resulting model are required by the data or are artefacts of the inversion, the data were inverted using two different inverse methods (singular value decomposition and a quadratic programming method). New high-quality seismic velocity models show apparent correlation between surface geological features and seismic velocity structures at lithospheric depth across the Zagros collision zone. The image shows a sharp lithospheric boundary at the Main Zagros Thrust between 100 km and 250 km depth with P-wave velocity about 3 per cent faster within the Arabian Shield to the south. A step-like increase in lithospheric thickness across the Zagros collision zone is assumed to separate two different mantle structures namely the Arabian (to the south) and the Eurasian (to the north) domains. The most striking feature resolved is a north-dipping slab-like positive velocity anomaly.

Upper-mantle S-velocity structure across the Zagros collision zone resolved by nonlinear teleseismic tomography

Non-linear teleseismic S-phase tomography across the Zagros collision zone in southwestern Iran is used to determine a high-resolution image of the upper-mantle structure. The inversion was done using 41 high-quality earthquakes recorded by 19 broad-band and medium-band stations along a 620 km long profile across the collision zone. Smearing from strong crustal velocity anomalies into the upper-mantle is suppressed by travel-time corrections calculated based on a 3-D crustal model for the study area. Our results show that the relatively old and cold Arabian shield has a higher velocity (up to 6% faster, at depths between 70 and 300 km) than the younger lithosphere farther north in Central Iran. These two upper-mantle domains are separated by a sharp near-vertical transition whose surface expression coinciding with the Main Zagros Thrust.