Wide-angle Data Research Articles

Bending-related faulting of the incoming oceanic plate and its effect on lithospheric hydration and seismicity: A passive and active seismological study offshore Maule, Chile

We have studied the dependency between incoming plate structure, bending-related faulting, lithospheric hydration, and outer rise seismic activity offshore Maule, Chile. We derived a 2D Poisson's ratio distribution from P- and S-wave seismic wide angle data collected in the trench-outer rise. High values of Poisson's ratio in the uppermost mantle suggest that the oceanic lithosphere is highly hydrated due to the water infiltration through bending-related normal faults outcropping at the seafloor. This process is presumably facilitated by the presence of a seamount in the area. We conclude that water infiltrates deep into the lithosphere, when it approaches the Chile trench, producing a reduction of crustal and upper mantle velocities, supporting serpentinization of the upper mantle. Further, we observed a mantle Vp anisotropy of 8%, with the fast velocity axis running normal to the abyssal hill fabric and hence in spreading direction, indicating that outer rise processes have yet not affected anisotropy.The first weeks following the megatrust Mw=8.8 Maule earthquake in 2010 were characterized by a sudden increase of the outer rise seismic activity, located between 34°S and 35°30′S. We concluded that this phenomenon is a result of an intensification of the water infiltration process in the outer rise, presumably triggered by the main shock, whose epicenter was located some 100km to the south east of the cluster.

Proton irradiation of Zr–1 wt.% Nb cladding material: A depth-wise assessment of inhomogeneous microstructural damage using X-ray diffraction line profile analyses

The present work summarizes the effect of irradiation on the microstructure of Zr–1wt.% Nb fuel cladding alloy using 5MeV proton beam from a Variable Energy Cyclotron. The correlation between the damage energy deposition profile and the consequent microstructural modification has been obtained for different irradiation doses employing detailed line profile analysis of wide angle and Synchroton grazing incidence X-ray diffraction data. The inhomogeneous microstructure resulting from ion irradiation as a function of depth could be analyzed by these techniques. The size of the coherent domains was found to decrease drastically upon irradiation together with an increase in the microstrain values within these domains. A progressive decrease in the domain size, with a corresponding increase in the microstrain, was noticed upon increasing the dose of irradiation and along the depth of the irradiated samples.

Sedimentary markers in the Provençal Basin (western Mediterranean): a window into deep geodynamic processes

AbstractDeep Earth dynamics impact so strongly on surface geological processes that we can use sediment palaeo‐markers as a window into the deeper Earth. Derived from climatic and tectonic erosive actions on the continents, and related to eustasy, subsidence and isostasy, the sediment in a deep basin is the main recorder of these processes. Nevertheless, defining and quantifying the relative roles of parameters that interact to give the final sedimentary architecture is not a simple task. Using a 3D‐grid of seismic and wide‐angle data, boreholes and numerical stratigraphic modelling, we propose here a quantification of post‐rift vertical movements in the Provençal Basin (West Mediterranean) involving three domains of subsidence: seaward tilting on the platform and the slope and purely vertical subsidence in the deep basin. These domains fit the deeper crustal domains highlighted by previous geophysical data. Post‐break‐up sedimentary markers may therefore be used to identify the initial hinge lines of the rifting phase and the subsidence laws.

On the Use of Fractal Surfaces to Understand Seismic Wave Propagation in Layered Basalt Sequences

The aim of this study is to better understand how a layered basalt sequence affects the propagation of a seismic wave, which has implications for sub-basalt seismic imaging. This is achieved by the construction of detailed, realistic models of basalt sequences, using data derived directly from outcrop analogues. Field data on the surface roughness of basaltic lava flows were captured using terrestrial laser scanning and satellite remote sensing. The fractal properties of the surface roughness were derived, and it can be shown that the lava flow surface is fractal over length scales up to approximately 2 km. The fractal properties were then used to construct synthetic lava flow surfaces using a von Karman power spectrum, and the resulting surfaces were then stacked to create a synthetic lava flow sequence. P-wave velocity data were then added, and the resulting model was used to generate synthetic seismic data. The resulting stacked section shows that the ability to resolve the internal structure of the lava flows is quickly lost due to scattering and attenuation by the basalt pile. A further result from generating wide-angle data is that the appearance of a lower-velocity layer below the basalt sequence may be caused by destructive interference within the basalt itself.

Probabilistic lithofacies prediction from prestack seismic data in a heterogeneous carbonate reservoir

We mapped probabilities of lithology and fluid based on seismic and well observations in a heterogeneous carbonate reservoir of the Sichuan Basin, southwest China, thus characterizing the reservoir complexity and identifying the potential sweet spot. Rock physical analysis showed that the presence of a vuggy-fracture porosity system combined with the fluids’ effect complicate the elastic responses of heterogeneous carbonates. This also gives physical insight into how the elastic properties of different lithofluid classes can be distinguished. With the application of Bayesian linearized amplitude variation with offset inversion, we found that the posterior distribution of P- and S-wave velocities were reliably extracted, whereas the inverted density tended to show high uncertainty due to a lack of wide angle data in the migrated seismic gathers. Finally, a Bayesian approach was implemented to propagate uncertainty from prestack seismic data to lithofluid classes in an integrated framework. The gas-carbonate predictions with high posterior probabilities were consistent with well observations at the well location and the present geological continuity. The probability distributions for the anhydrite, limestone, and dolostone were essential to understand the reservoir architecture and delineate the reservoir heterogeneities. We also ascertained that the uncertainty of lithofacies prediction is determined by the uncertainty of seismic inversion as well as the uncertainty of the link between lithofacies to their corresponding elastic responses.

Seismic imaging of the eastern Algerian margin off Jijel: integrating wide-angle seismic modelling and multichannel seismic pre-stack depth migration

This study presents the results of a deep seismic survey across the north Algerian margin, based on the combination of 2-D multichannel and wide-angle seismic data simultaneously recorded by 41 ocean bottom seismometers deployed along a north-south line extending 180 km off Jijel into the Algerian offshore basin, and 25 land stations deployed along a 100-km-long line, cutting through the Lesser Kabylia and the Tellian thrust-belt. The final model obtained using forward modelling of the wide-angle data and pre-stack depth migration of the seismic reflection data provides an unprecedented view of the sedimentary and crustal structure of the margin. The sedimentary layers in the Algerian basin are 3.75 km thick to the north and up to 4.5-5 km thick at the foot of the margin. They are characterized by seismic velocities from 1.9 to 3.8 km s(-1). Messinian salt formations are about 1 km thick in the study area, and are modelled and imaged using a velocity between 3.7 and 3.8 km s(-1). The crust in the deep sea basin is about 4.5 km thick and of oceanic origin, presenting two distinct layers with a high gradient upper crust (4.7-6.1 km s(-1)) and a low gradient lower crust (6.2-7.1 km s(-1)). The upper-mantle velocity is constrained to 7.9 km s(-1). The ocean-continent transition zone is very narrow between 15 and 20 km wide. The continental crust reaches 25 km thickness as imaged from the most landward station and thins to 5 km over a less than 70 km distance. The continental crust presents steep and asymmetric upper- and lower-crustal geometry, possibly due to either asymmetric rifting of the margin, an underplated body, or flow of lower crustal material towards the ocean basin. Present-time deformation, as imaged from three additional seismic profiles, is characterized by an interplay of gravity-driven mobile-salt creep and active thrusting at the foot of the tectonically inverted Algerian margin.

초고층건물의 사각조망에서 촬영된 지붕표면 열화상의 신뢰도 평가

It is usual to evaluate the performance of the cool roof by measuring in-site rooftop temperature using thermal infra-red camera. The principal advantage of rooftop thermal infrared image acquired in oblique vantage point of super high-rise building as a remote sensor is to provide, in a cost-effective manner, area-wide information required for a scattered rooftop target with different colors, utilizing wide view angle and multi-temporal data coverage. This research idea was formulated by incorporating the concept of traditional remote sensing into rooftop temperature monitoring. Correlations between infrared image of super high-rise building and in-situ data were investigated to compare rooftop surface temperature for a total of four different rooftop locations. The results of the correlations analyses indicate that the rooftop surface temperature by the infrared images of super high-rise building alone could be explained yielding values of 0.951. The visible permanent record of the oblique thermal infra-red image was quite useful in better understanding the nature and extent of rooftop color that occurs in sampling points. This thermal infrared image acquired in oblique vantage point of super high-rise made it possible to identify area wide patterns of rooftop temperature change subject to many different colors, which cannot be acquired by traditional in-site field sampling. The infrared image of super high-rise building breaks down the usual concept of field sampling established as a conventional cool roof performance evaluation technique.

Heterogeneous chemical crosslinking and high-voltage polarization in a ferroelectric copolymer of vinylidene fluoride and tetrafluoroethylene

The low-temperature chemical crosslinking of a copolymer of vinylidene fluoride and tetrafluoroethylene was studied through various physicochemical methods. The reaction was conducted in solution in the presence of diethylenetriamine as a crosslinking agent. The penetration of the mixture molecules and the crosslinking agent only in the amorphous phase of the copolymer was provided via selection of the ratio between a good solvent (dimethylformamide) and a poor solvent (ethanol). Owing to this, the crosslinking reaction occurred in the amorphous phase and hardly involved the crystals. This outcome was confirmed by wide-angle X-ray diffraction data. The structural and chemical changes in the amorphous phase during crosslinking were recorded with the use of IR spectroscopy and differential scanning calorimetry. It is shown that crosslinking is accompanied by the formation of C=C bonds in the copolymer chains. The study of high-voltage polarization and conductivity during exposure to bipolar rectangular pulses suggested that crosslinking leads to an increase in the carrier concentration. It was found that the surface potentials in the films increase with an increase in the number of high-voltage pulses applied to the sample. This circumstance is attributed to the fact that the double bonds formed in the copolymer chains can effectively trap negatively charged carriers.

Fabrication of CuO nanotubes by electrospinning, metallisation and annealing

A straightforward combined technology of electrostatic spinning and metallisation was developed to produce novel metallic nanotubes and nanofibres. The metals were deposited onto electrospun poly(vinyl alcohol) (PVA) nanofibres as a template by metallisation method. After removal of the nanofibre template by heat treatment at 400°C for 24 h, the fibrous morphologies were satisfactorily conserved, suggesting the successful deposition of metal layer on the surface of nanofibre template. Wide-angle X-ray diffraction data confirmed that the PVA nanofibres metallised with the copper layer of 200 nm clearly showed typical copper crystalline peaks, whereas after calcination of the nanofibre templates, interestingly, new crystalline peaks were observed at 2θ = 32.5°, 35.5°, 38.7°, 48.7°, 53.4° and 58.4°, corresponding to the (1 1 0), (0 0 2), (1 1 1), ( 0 2), (0 2 0) and (2 0 2) reflections, respectively. The resulting metallic nanotubes and nanofibres were found to show a typical CuO crystalline structure.

Inversion of a hyper-extended rifted margin in the southern Central Range of Taiwan

Seismic reflection and wide-angle data acquired across, south, and west of Taiwan show that extended to hyper-extended continental crust of the Chinese continental margin is present more than 200 km south of the shelf and is subducting at the Manila Trench. Furthermore, crustal-scale tomographic velocity models show that this crust is underthrusted to ∼15 km depth below the accretionary prism, where it then is structurally underplated to the base of the prism. We document an increasing volume of accreted crust from south to north, and in our northern transect high-velocity material of the accretionary prism can be directly linked to outcrops of Central Range basement rocks. In map view the Central Range of Taiwan is clearly contiguous with the Hengchun Peninsula and Hengchun submarine ridge to the south. Accordingly, we propose a new model in which the Central Range forms directly from the accretionary prism, including the basement core, which originates from subducted, and then accreted, extended to hyper-extended continental crust.

Shifted-elliptical nonstretch moveout correction of wide-angle seismic data in the τ-p domain, using an example from the Faeroe-Shetland Basin

We developed a method of moveout correction in the [Formula: see text] domain to tackle some of the problems associated with processing wide-angle seismic reflection data, including residual moveout and normal-moveout stretching. We evaluated the concept of the shifted ellipse in the [Formula: see text] domain as an alternative to the well-known concept of the shifted hyperbola in the [Formula: see text] domain. We used this shifted-ellipse concept to address the problem of residual moveout caused by vertical heterogeneity in the subsurface. We also addressed the stretching problem associated with dynamic corrections by combining selected strips from a set of constant-moveout stacks generated using a shifted-ellipse equation. Application of this method to a wide-angle data set from the Faeroe-Shetland Basin provided an enhanced image of the subbasalt structure.

Petrophysical analysis of a mid-crustal reflector in the IBERSEIS profile, SW Spain

The origin of strong crustal reflectors in vertical incidence reflection seismic data is generally attributed to either rock layering, deformation fabrics in shear zones, fluids, or igneous intrusions. The IBERSEIS normal incidence reflection and wide-angle seismic profiles in SW Spain imaged a large, high velocity, subhorizontal reflector in the middle crust (the IBERSEIS Reflective Body) whose origin has been attributed to a mafic intrusion. In order to test this hypothesis, in this paper we present laboratory measurements of Vp, Vs, and density from 17 samples of mafic igneous and metamorphic rocks, and metasediments that are thought to be equivalent to the proposed IBERSEIS Reflective Body. These measurements are then corrected to 400°C at 600MPa and used to calculate Poisson's ratio and to compare it, Vp, and Vs to values measured in situ by wide-angle data. Finally, normal incidence reflection coefficients are calculated to test if the measured lithologies could reproduce the reflectivity imaged in the vertical incidence reflection seismic data for the IBERSEIS Reflective Body. Our physical property measurements are very similar to those modeled from the wide-angle data, and our reflection coefficients are sufficiently high to cause strong mid-crustal reflectivity. Our data indicate, therefore, that previous interpretations of the IBERSEIS Reflective Body as a mafic sill are quite reasonable.

Simple and Efficient Calculation of Scattering Intensities of Proteins in Solution from Atomistically Detailed Structures

Solution scattering experiments provide signatures of the atomistic structure of proteins, nucleic acids, and biomolecular assemblies under near physiological conditions. Coarse-grained structural properties like shape and volume can be inferred from an essentially model-free analysis using information in the small-angle regime. The wide-angle regime offers higher resolution information which can be interpreted using atomistic models. A combination of time-resolved scattering experiments, molecular simulations, and ensemble refinement methods helps reveal structural changes in proteins as they perform their biological functions. To address this challenge, we developed a mathematically simple and computationally efficient method to calculate the scattering intensity of atomistically detailed structures of proteins in solution. Compared to other methods, our method, which is based on Debye's formula, has the advantage that there is no trade-off between computational efficiency and accuracy in the promising wide-angle regime. We present results for a variety of proteins and different water models and discuss some fundamental differences in interpretation of small- and wide-angle data.

Crustal Structure in the Southern Appalachians: A Comparison of Results Obtained from Broadband Data and Three-Component, Wide-Angle P and S Reflection Data

Abstract Data for two broadband stations and three-component recordings of wide-angle reflections are used to constrain the velocity structure and depth to Moho beneath a portion of the southern Appalachians. Broadband data from the U.S. National Seismic Network (USNSN) station GOGA (Carolina terrane/Inner Piedmont boundary), constrained by average crustal V P derived from wide-angle data, suggest that crustal thickness for individual back azimuths ranges from 41 to 43 km. This is consistent with wide-angle estimates of 38–40 km for the Carolina terrane and 40–45 km for the Inner Piedmont. Average crustal V P / V S is 1.72–1.76, compared with 1.74–1.76 derived from SmS/PmP travel-time ratios for the Carolina terrane. The results for station MYNC (Blue Ridge Mountains), again constrained by wide-angle data, suggest a crustal thickness of 50–52 km for individual back azimuths. Average V P / V S is 1.75–1.77. The range in average V P / V S derived from wide-angle data is broader (1.74–1.80), indicating variations in average composition across the Blue Ridge Mountains, as suggested by gravity data. Large-amplitude direct P and Ps arrivals on transverse-component receiver functions also indicate significant lateral variations in structure beneath MYNC. Comparison of migrated images underscores the differences in resolution for single-station broadband data and wide-angle data recorded with short-aperture arrays. Ps waveforms for MYNC show a double peak. The earlier peak migrates to depths of 48.5–51 km, in agreement with wide-angle events interpreted as Moho. For the later peak, migrated depths range from 60 to 65 km, compared with 59–68 km for peaks in wide-angle reflectivity observed for the Blue Ridge Mountains. The two methods yield consistent estimates for crustal thickness that lend support to models that predict a significant crustal root beneath the high elevations of the Blue Ridge.

2D waveform tomography applied to long-streamer MCS data from the Scotian Slope

Detailed velocity models of the earth’s subsurface can be obtained through waveform tomography. The accuracy of the long-wavelength component of such velocity models, which is the background velocity field, is particularly sensitive to modeling low-frequency refracted waves that have long paths through target structures. Thus, field examples primarily have focused on the analysis of long-offset wide-angle data sets collected using autonomous receivers, in which refractions arrive at significantly earlier times than reflections. Modern marine acquisition with long streamers now offers the ability to record refracted waves with high spatial density and uniform source, both in shallow and deep water. We used 2D multichannel seismic (MCS) data acquired with a 9-km-long streamer over the Scotian Slope in water depths of ∼1600 m. The refracted arrivals, although mostly restricted to far-offset receivers, provided sufficient information to successfully invert for a high-resolution background velocity field. Using a frequency-domain acoustic code over frequencies from 8 to 24 Hz on two crossing profiles, we found that the limited refracted waves can constrain the velocity field above the depth of the turning waves (∼1.5 km below seafloor). Several important features were resolved by the waveform velocity model that were not present in the initial traveltime model. In particular, a high-velocity layer at 300 m below the seafloor, interpreted as gas hydrates, was imaged even where a characteristic bottom-simulating reflector was not visible. At 750-m depth, a strong velocity increase of 300 m/s existed beneath a gently dipping reflector along which low-velocity zones, possibly related to gas, were present. Velocity models were highly consistent at the crossing point between the two profiles. The depth extent of the MCS waveform tomography constrained by refractions could be extended by even longer streamers (e.g., 15 km) or by joint inversion with data from ocean-bottom seismographs.

Revealing the deep structure and rupture plane of the 2010 Maule, Chile earthquake (Mw = 8.8) using wide angle seismic data

The 27 February, 2010 Maule earthquake (Mw = 8.8) ruptured ~ 400 km of the Nazca-South America plate boundary and caused hundreds of fatalities and billions of dollars in material losses. Here we present constraints on the fore-arc structure and subduction zone of the rupture area derived from seismic refraction and wide-angle data. The results show a wedge shaped body ~ 40 km wide with typical sedimentary velocities interpreted as a frontal accretionary prism (FAP). Landward of the imaged FAP, the velocity model shows an abrupt velocity-contrast, suggesting a lithological change which is interpreted as the contact between the FAP and the paleo accretionary prism (backstop). The backstop location is coincident with the seaward limit of the aftershocks, defining the updip limit of the co-seismic rupture and seismogenic zone. Furthermore, the seaward limit of the aftershocks coincides with the location of the shelf break in the entire earthquake rupture area (33°S–38.5°S), which is interpreted as the location of the backstop along the margin. Published seismic profiles at the northern and southern limit of the rupture area also show the presence of a strong horizontal velocity gradient seismic backstop at a distance of ~ 30 km from the deformation front. The seismic wide-angle reflections from the top of the subducting oceanic crust constrain the location of the plate boundary offshore, dipping at ~ 10°. The projection of the epicenter of the Maule earthquake onto our derived interplate boundary yielded a hypocenter around 20 km depth, this implies that this earthquake nucleated somewhere in the middle of the seismogenic zone, neither at its updip nor at its downdip limit.

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

[1] Seismic wide-angle record sections from favorably placed source and limited-aperture recording spreads, particularly across collision boundaries, reveal bright unusual high-velocity 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 scatterers. The crustal-scale south Delhi thrust fault on the western margin of the DFB, delineated as the stack of reflectors dipping 25°–30°SE, probably developed during the Mesoproterozoic collisional episode related to the Delhi orogeny, contemporaneous with the formation of Rodinia. It is concluded that complex structural features developed during collisional episodes can survive through geological periods, longer than generally expected, depending on the accretionary characteristics at the time of orogenic evolution. The 2-D velocity model further reveals significant variations of crustal seismic structure, with a relatively sharp and shallow Moho (at 39–40 km) in the MB and a deeper Moho (at 45–47 km) underlying a 12 km thick subhorizontal lamella of high-velocity (6.9–7.5 km/s) layers in the DFB regions.

Elastic full waveform inversion of multicomponent ocean-bottom cable seismic data: Application to Alba Field, U. K. North Sea

Elastic full waveform inversion of multichannel seismic data represents a data-driven form of analysis leading to direct quantification of the subsurface elastic parameters in the depth domain. Previous studies have focused on marine streamer data using acoustic or elastic inversion schemes for the inversion of P-wave data. In this paper, P- and S-wave velocities are inverted for using wide-angle multicomponent ocean-bottom cable (OBC) seismic data. Inversion is undertaken using a two-dimensional elastic algorithm operating in the time domain, which allows accurate modeling and inversion of the full elastic wavefield, including P- and mode-converted PS-waves and their respective amplitude variation with offset (AVO) responses. Results are presented from the application of this technique to an OBC seismic data set from the Alba Field, North Sea. After building an initial velocity model and extracting a seismic wavelet, the data are inverted instages. In the first stage, the intermediate wavelength P-wave velocity structure is recovered from the wide-angle data and then the short-scale detail from near-offset data using P-wave data on the [Formula: see text] (vertical geophone) component. In the second stage, intermediate wavelengths of S-wave velocity are inverted for, which exploits the information captured in the P-wave’s elastic AVO response. In the third stage, the earlier models are built on to invert mode-converted PS-wave events on the [Formula: see text] (horizontal geophone) component for S-wave velocity, targeting first shallow and then deeper structure. Inversion of [Formula: see text] alone has been able to delineate the Alba Field in P- and S-wave velocity, with the main field and outlier sands visible on the 2D results. Inversion of PS-wave data has demonstrated the potential of using converted waves to resolve shorter wavelength detail. Even at the low frequencies [Formula: see text] inverted here, improved spatial resolution was obtained by inverting S-wave data compared with P-wave data inversion results.

Improvement of image resolution of brain SPECT by use of the wide-angle offset acquisition method

Cerebral SPECT images require high spatial and contrast resolution for precise evaluation of the abnormal tracer distribution in the brain. A shorter data acquisition time is preferable so that artifacts due to patient movement are avoided. We tried to shorten data acquisition time applying larger sampling angle and offset acquisition method, in which half degree of the step angle was shifted in the opposite gamma camera of the dual-detector SPECT system. A simulation study was performed with a 3-dimensional mathematical phantom. The phantom studies were performed with a hot-rod phantom and a brain phantom. A clinical study with ⁹⁹(m)Tc-ECD SPECT was also performed on a patient who had a cerebral infarction. Reconstruction of images was done for the normal 6° and 12° onset and 12° offset. Data for the 12° offset were acquired by shifting of sampling angles of the opposite detector by half (6°) of the sampling angles of 12°. The MLEM algorithm was used for image reconstruction. Image qualities in the simulation study, the phantom studies, and the clinical study were compared for the 6° and 12° onset, and for the 12° offset by quantitative analysis with use of profile curves. Analysis of the profile curves revealed that the image quality of the 12° offset was better than that of the 12° onset and compared to that of the 6° onset in the simulation study, the phantom studies, and the clinical study. The present study indicates that wide-angle offset data acquisition improves the image resolution of brain SPECT compared to onset data acquisition with the same sampling time.

Geophysical model of the lithosphere across the Variscan Belt of SW-Iberia: Multidisciplinary assessment

A multidisciplinary geophysical study along a large seismic transect in the SW-Iberian Peninsula has been carried out. This study integrates the crustal structure, geometry and composition obtained from normal incidence and wide-angle seismic reflection data with other observables (geoid, gravity and topography). The internal architecture of the lithosphere across the Variscan Orogen of SW-Iberia is constrained by the 300 km long high resolution deep normal seismic reflection IBERSEIS Transect. The most prominent feature imaged by the seismic survey is the Iberseis Reflective Body (IRB), a 140 km long high amplitude reflective body located in the middle crust of the northern half of the transect. The seismic velocity (Vp) distribution within the crust and the upper mantle is constrained by two wide-angle seismic transects acquired in the same area. The velocity models show a complex crust, with a specially complex middle crust, which features higher velocities than the average continental crust. Also the wide-angle data revealed that the IRB is characterized by high velocities. This feature was then interpreted as sill-like structure built up by a series of mafic intrusions. Therefore, a key issue is to study if this relatively mafic crust is consistent with other geophysical observables. Based on the velocity models, two lithospheric density models have been derived along the IBERSEIS wide-angle transects. The geoid, gravity and topography response of these models have been calculated using a finite elements code that solves, simultaneously, the geopotential, lithostatic, and heat flow equations. The resulting values are then compared with the measured observables and the crustal and lithospheric mantle geometry and density is modified until the best fit is obtained. The initial density models calculated from the seismic data adjust quite well to the real potential field data. However, minor modifications have been required in order to properly fit the observables. The final density models are consistent with the existence of relatively high density bodies in the mid-crust providing further support to the seismic interpretation. In addition, they place new constraints on the location of the lithosphere–asthenosphere boundary and on the tectonic evolution of SW-Iberia.