Presence Of High-density Material Research Articles

Investigating subduction orogeny dynamics of Manabhum high in the Upper Assam foreland basin, NE India by geophysical data-based modelling

The Manabhum structural high, situated on the southern bank of the Brahmaputra River in the foreland part of the Upper Assam basin, is characterised by an elongated NNW-SSE direction of the fold axis. This study utilises seismic and satellite gravity data to model subsurface geological cross-sections across the Manabhum structure, providing insights into the presence of high-density material at the basement level, supported by the distribution of ophiolites at varying depths. The Manabhum, a compressional structure controlled by the complex folding involved in the Mishmi Hills and Naga Schuppen belt, subjected to compression at different stages, presents an excellent opportunity to understand subduction orogeny involving the collision between the Indian plate and the Tethys oceanic lithosphere subducting beneath the Myanmar continental lithosphere.

Automatic exposure compensation of digital radiographic technologies does not affect alveolar bone-level measurement.

To evaluate whether the automatic exposure compensation in the presence of high-density materials can affect the measurement of alveolar bone level. Thirty regions of seven dry skulls and six mandibles were radiographed with and without a high-density material, using two digital radiographic technologies: photostimulable phosphor plate (PSP, Digora Optime) and sensor (CMOS, Digora Toto), totaling 120 images. The distances from the cement-enamel junction to the alveolar bone crest were measured using cone-beam computed tomography (CBCT) images to represent the reference standard. The same measurements of alveolar bone level and the average of the pixel values of the image were evaluated on the radiographs. Paired t test compared the average pixel values and alveolar bone-level measurements between images with and without high-density material. One-way analysis of variance compared the difference between radiographic and CBCT measurements (α = 0.05). The high-density material reduced the pixel values in PSP (p = 0.002) and CMOS (p < 0.001) technologies, demonstrating the AEC functioning in both technologies. There was no difference in bone-level measurements between the images without and with the high-density material for both technologies (p ≥ 0.091), or between the tomographic and radiographic measurements (p ≥ 0.319). In the presence of high-density material, the automatic exposure compensation reduces the average pixel values of the images (i.e., images get darker), but does not influence the radiographic measurements of alveolar bone level.

Radiographic diagnosis of proximal caries is not affected by exposure protocols and presence of high-density material on systems with automatic exposure compensation.

To investigate the performance of radiographic systems with automatic exposure compensation (AEC) on the caries diagnosis in images acquired with different exposure parameters and in the presence of high-density material. Also, the image quality was assessed. Forty posterior teeth (80 proximal surfaces) were radiographed using a phosphor plate and a CMOS system. Images were acquired with different exposure times (0.06, 0.10 and 0.16s) and kilovoltages (60 and 70kVp), in the absence and presence of high-density material in the X-rayed region (control and high-density groups). Five radiologists assessed the caries using a 5-point scale. Diagnostic values were compared using two-way ANOVA. For both radiographic systems, there were no significant differences in the area under the ROC curve (0.60-0.73), sensitivity (0.79-0.87) and specificity (0.29-0.48) between the control and high-density groups, exposure times or kilovoltages (p > 0.05). For image quality, scores assigned to the control and high-density groups were similar in each exposure protocol in both systems. The presence of high-density material, exposure time and kilovoltage did not affect the caries diagnosis in any of the systems tested. It is recommended to use protocols with lower doses to reduce the patient's exposure.

Evaluation of the use of paired modified Wright's and periodic acid Schiff stains to identify microbial aggregates on cytological smears of dogs with microbial otitis externa and suspected biofilm.

Micro-organisms associated with canine otitis externa (OE) may cause biofilm-associated infections (BAI). A key component of biofilm is microbial aggregate and extracellular polymeric substance (EPS). Periodic acid Schiff (PAS) can stain polysaccharide EPS in human otitis media with effusion, but this has not been tested in canine OE. There is no cytological definition for microbial aggregate, and definitive methods for identifying BAI in a clinical setting in canine OE have not been defined. To establish whether PAS stain can identify polysaccharide matrix on cytological smears; and to determine the reproducibility of identification of microbial aggregates within a discrete area of stained matrix, using paired modified Wright's and PAS-stained smears. Forty privately-owned dogs presenting to a dermatological referral practice. In this prospective, cross-sectional study, three investigators independently and blindly classified 40 paired modified Wright's-PAS slide sets into groups: aggregate-associated infection (AAI) and non-AAI (n=27); and control (n=13). Agreement between investigators for presence of AAI was measured using Fleiss' kappa statistic (FK). Agreement between investigators and dermatologists for presence of AAI upon cytological evaluation, and suspected BAI based on clinical examination, was measured using Cohen's kappa statistic. The matrix was confirmed to stain PAS-positive. Interinvestigator agreement for AAI was very good using PAS (0.82 FK) and fair using modified-Wright's (MW) (0.33 FK). Reproducible cytological features associated with AAI were the presence of: three or more distinct aggregates (0.76 FK); discrete areas of PAS-positive matrix (0.70 FK); and the presence of high-density material (0.70 FK) using PAS stain. PAS can stain the extracellular matrix on otic smears, and a novel protocol for reproducible identification of cytological features such as microbial aggregates has been established.

PO-1618 Validation of the Monte Carlo calculation in the presence of high-density materials using gafchromic

PO-1618 Validation of the Monte Carlo calculation in the presence of high-density materials using gafchromic

Combination of dual-energy computed tomography and iterative metal artefact reduction to increase general quality of imaging for radiotherapy patients with high dense materials. Phantom study.

To evaluate the use of pseudo-monoenergetic reconstructions (PMR) from dual-energy computed tomography, combined with the iterative metal artefact reduction (iMAR) method. Pseudo-monoenergetic CT images were obtained using the dual-energy mode on the Siemens Somatom Definition AS scanner. A range of PMR combinations (70-130keV) were used with and without iMAR. A Virtual Water™ phantom was used for quantitative assessment of error in the presence of high density materials: titanium, alloys 330 and 600. The absolute values of CT number differences (AD) and normalised standard deviations (NSD) were calculated for different phantom positions. Image quality was assessed using an anthropomorphic pelvic phantom with an embedded hip prosthesis. Image quality was scored blindly by five observers. AD and NSD values revealed differences in CT number errors between tested sets. AD and NSD were reduced in the vicinity of metal for images with iMAR (p<0.001 for AD/NSD). For ROIs away from metal, with and without iMAR, 70keV PMR and pCT AD values were lower than for the other reconstructions (p=0.039). Similarly, iMAR NSD values measured away from metal were lower for 130keV and 70keV PMR (p=0.002). Image quality scores were higher for 70keV and 130keV PMR with iMAR (p=0.034). The use of 70keV PMR with iMAR allows for significant metal artefact reduction and low CT number errors observed in the vicinity of dense materials. It is therefore an attractive alternative to high keV imaging when imaging patients with metallic implants, especially in the context of radiotherapy planning.

Automatic exposure compensation and subjective image enhancement in the radiographic diagnosis of caries.

This study aims to assess the influence of high-density material on the radiographic diagnosis of proximal caries in digital systems with automatic exposure compensation, and to evaluate the effect of subjective adjustment of brightness and contrast to undertake this diagnostic task. Twenty bitewing radiographs of forty posterior human teeth with non-cavitated carious lesions, confirmed by micro-CT, were obtained with two digital systems. A porcelain-fused-to-metal crown attached to a titanium implant was inserted into the exposed area, and all the radiographs were repeated. Five radiologists assessed the radiographs and diagnosed proximal carious lesions. Afterwards, the observers were asked to adjust image brightness and contrast, based on their subjective perception, and to reassess the images. Thirty percent of each experimental group was reassessed to test intraobserver reproducibility, totaling 208 images per observer. Intraobserver and interobserver agreements ranged from fair to substantial. Sensitivity, specificity, predictive values, and area under the ROC curve were calculated and compared for each radiographic system, using ANOVA (α = 0.05). Overall, presence of high-density material and adjustment of brightness and contrast did not significantly influence the radiographic diagnosis of proximal caries (p ≥ 0.05). Regarding Digora Optime, adjustment of brightness and contrast significantly increased (p < 0.05) the diagnostic accuracy of proximal carious lesions in the presence of high-density material. In conclusion, the presence of high-density material in the X-rayed region does not influence radiographic diagnosis of proximal caries. However, when it is present in the X-rayed area, subjective adjustment of brightness and contrast is recommended for use with the Digora Optime digital system.

Back-projection Filtration Image Reconstruction Approach for Reducing Out-of-plane Artifacts in Laminography

Because of incomplete depth information in computed laminography, 3D reconstructed images are contaminated by out-of-plane artifacts, or ripple artifacts along the depth direction. Furthermore, these issues become more severe whenever printed circuit boards (PCB) are imaged due to the presence of high density materials, such as ball grid array, chips, etc. In this reasearch, in order to remove these artifacts, we proposed the use of the voting strategy. This method is a weighted back-projection which gives less weight to outliers, or odd values. However, standard analytical reconstruction methods, such as filtered back-projection (FBP), cannot fully judge and remove the outliers. Thus, we proposed to combine this strategy with the back-projection filtration (BPF) reconstruction method, which first reconstructs the derivative of the projection image, and then filters the reconstructed image by the Hilbert transform. Ripple artifacts were shown to have been dramatically reduced by using the voting strategy in the BPF algorithm.

Dosimetric evaluation of scattered and attenuated radiation due to dental restorations in head and neck radiotherapy

In radiotherapy of head and neck cancer, the presence of high density materials modifies photon dose distribution near these high density materials during treatment. The aim of this study is to calculate the backscatter and attenuation effects of a healthy tooth, Amalgam, Ni-Cr alloy and Ceramco on the normal tissues before and after these materials irradiated by 6 and 15 MV photon beams, respectively. All measurements were carried out in a water phantom with dimension of 50 × 50 × 50 cm3with an ionization chamber detector. Two points before and four points after the dental sample were considered to score the photon dose. The depth dose on the central beam axis was explored in a water phantom for source to surface distance (SSD) of 100 cm in a 10 × 10 cm2 field size. The percentage dose change was obtained relative to the dose in water versus depth of water, tooth, Amalgam, Ni-Cr alloy and Ceramco for the photon beams. The absolute dose (cGy) was measured by prescription of 100 cGy dose in the water phantom at depth of 2.0 and 3.1 cm for 6 and 15 MV photons, respectively. At depth of 0.6 cm, the maximum percentage dose increase was observed with values of 6.99% and 9.43%for Ni-Cr and lowest percentage dose increase of 1.49% and 2.63% are related to the healthy tooth in 6 and 15 MV photon beams, respectively. The maximum absolute dose of 95.58 cGy and 93.64 cGy were observed at depth of 0.6 cm in presence of Ni-Cr alloy for 6 and 15 MV photon beams, respectively. The presence of dental restorations can cause backscattering dose during head and neck radiation therapy. Introduction of compositions and electron density of high density materials can improve the accuracy of dosimetric calculations in treatment planning systems to deliver the relevant dose to target organ and reduce the backscattering dose in healthy tissues in the surrounding of tooth.

Influence of Artifact Reduction Tools in Micro–computed Tomography Images for Endodontic Research

IntroductionMicro–computed tomography (μCT) is an imaging modality of growing application in endodontic research because of its nondestructive technology that enables visualization at the micrometer level. In the presence of high-density material, images acquired with μCT may present artifacts. Thus, the aim of this study was to evaluate subjectively and objectively the influence of artifact reduction tools available for μCT image reconstruction to determine whether their use can influence endodontic research outcomes. MethodsTen teeth were scanned in a μCT device, and these images were reconstructed by applying 13 protocols of artifact reduction tools, combining ring artifact reduction (RAR) and beam-hardening artifact reduction (BAR). Images were assessed subjectively (observer's preference) and objectively (root canal surface area and volume). Observer's preference for RAR and BAR protocols was tested by χ2. Analysis of variance was used to compare volumes and surface area of root canals on different protocols. Intraobserver and interobserver reproducibility was calculated by the weighted kappa test. ResultsThere was no preference for a particular protocol for BAR (P = .91) or RAR (P = .80). There were no statistically significant differences for volume (P = .999) or surface area (P = .972) of root canals for all protocols. ConclusionsTo evaluate root canal volume and root canal surface area, artifact reduction tools applied to μCT images can be used according to the observer's visual preference without influence on objective image analysis.

The impacts of dental filling materials on RapidArc treatment planning and dose delivery: Challenges and solution

The presence of high-density material in the oral cavity creates dose perturbation in both downstream and upstream directions at the surfaces of dental filling materials (DFM). In this study, the authors have investigated the effect of DFM on head and neck RapidArc treatment plans and delivery. Solutions are proposed to address (1) the issue of downstream dose perturbation, which might cause target under dosage, and (2) to reduce the upstream dose from DFM which may be the primary source of mucositis. In addition, an investigation of the clinical role of a custom-made plastic dental mold∕gutter (PDM) in sparing the oral mucosa and tongue reaction is outlined. The influence of the dental filling artifacts on dose distribution was investigated using a geometrically well-defined head and neck intensity modulated radiation therapy (IMRT) verification phantom (PTW, Freiberg, Germany) with DFM inserts called amalgam, which contained 50% mercury, 25% silver, 14% tin, 8% copper, and 3% other trace metals. Three RapidArc plans were generated in the Varian Eclipse System to treat the oral cavity using the same computer tomography (CT) dataset, including (1) a raw CT image, (2) a streaking artifacts region, which was replaced with a mask of 10 HU, and (3) a 2 cm-thick 6000 HU virtual filter [a volume created in treatment planning system to compensate for beam attenuation, where the thickness of this virtual filter is based on the measured percent depth dose (PDD) data and Eclipse calculation]. The dose delivery for the three plans was verified using Gafchromic-EBT2 film measurements. The custom-made PDM technique to reduce backscatter dose was clinically tested on four head and neck cancer patients (T3, N1, M0) with DFM, two patients with PDM and the other two patients without PDM. The thickness calculation of the PDM toward the mucosa and tongue was purely based on the measured upstream dose. Patients' with oral mucosal reaction was clinically examined initially and weekly during the course of radiotherapy. For a RapidArc treatment technique, the backscatter dose from the DFM insert was measured to be 9.25±2.17 in the IMRT-verification-phantom. The measured backscatter upstream dose from DFM for a single-field was 22% higher than without the DFM, whereas the downstream dose was lower by 14%. The values of homogeneity index for the plans with and without the application of mask were 0.09 and 0.14, respectively. The calculated mean treatment planning volume (PTV) dose differed from the delivered dose by 13% and was reduced to 2% when using the mask and virtual filter together. A grade 3 mucosa reaction was observed in the control group after 22-24 fractions (44-48 Gy). In contrast, no grade 3 mucositis was observed in the patients wearing the PDM after 25-26 fractions (50-52 Gy). The backscatter from the DFM for a single, parallel-opposed fields, and RapidArc treatment technique was found significant. The application of mask in replacing streaking artifacts can be useful in improving dose homogeneity in the PTV. The use of a virtual filter around the teeth during the planning phase reduces the target underdosage issue in the phantom. Furthermore, a reduction in mucositis is observed in the head and neck patients with the use of PDM.

SU-E-T-524: Effects On Dose and RBE From High Density Materials in Proton Therapy

Purpose: To evaluate the maco and micro-dosimetric impact of metal inhomogeneities (as seen in patients who have undergone reconstructive surgery) in proton therapy Methods: Monte Carlo simulations were performed of 3.1mm CP Grade 4 titanium and 3.4mm thick 316 stainless steel plates in a water phantom. Analogous experimental proton dose data was collected in a water phantom using GAFCHROMIC EBT2 radiochromic film (Ashland, Wayne, NJ) and a PTW Markus Ionization Chamber. Beam energy (157 and 225 MeV) and plate placement (eg, at center of modulation) were chosen to be clinically relevant to para spinal and prostate irradiations in the presence of spinal and hip implants, respectively. Film was scanned on an Epson 10000XL scanner and dose maps were calculated from three channel data using FilmQAPro software (Wayne, NJ). Macroscopic dose around the implant was evaluated and impact on proton range was determined and compared to clinical treatment planning system parameters. Dose equivalent and average quality factor distributions around the implant were also studied using Monte Carlo and microdosimetric methods. Results: Ion chamber data revealed range shifts due to titanium (316 stainless steel) plates of 6.4 to 6.9mm (15.0 to 15.5mm) corresponding to an effective relative density of 3.03 to 3.21 (5.46 to 5.59). Ion chamber data did not show small scale dose redistribution upstream of plates. However, dose data collection and analysis using film (which has higher spatial resolution) is ongoing. In addition, analysis of Monte Carlo simulations will investigate microdosimetric changes and corresponding fluctuations of RBE in plate vicinity. Conclusion: The presence of high-density materials can introduce changes in the spatial distribution of dose in the vicinity of an implant. This can introduce uncertainty in the dose distribution predicted by proton treatment planning software directly, or due to more subtle consequences of changes in RBE near the material. Department of Defense (DOD# W81XWH-BAA-10-1)

SU-E-T-734: Evaluation of a New, Grid-Based Linear Boltzmann Transport Equation Solver for Dose Calculation in the Presence of High-Density Materials

Purpose: To evaluate the accuracy of a new, deterministic linear Boltzmann transport equation solver, AcurosXB, for radiotherapydose calculations in the presence of high‐density materials by comparison with Monte Carlo.Methods: AcurosXB, EGSnrc‐based Monte Carlo and Analytic Anisotropic Algorithm (AAA) were used to model 6MV and 18MV photon beams incident on phantom and patient geometries containing stainless steel and titanium high‐density objects. Monte Carlo was used as the evaluation benchmark while AAA was included as a representative of the convolution‐based clinical modeling algorithms currently used for radiotherapy treatment planning. Calculation geometries include a virtual cubic phantom created in MatLab, a cubic water phantom containing stainless steel rods and prosthetic hips and patient CT images with associated clinical treatment plans. Dose distributions were processed and analyzed in MatLab.Results: Agreement between AcurosXB and Monte Carlo is within 5% and 2mm. There is a backscatter peak position offset of up to one pixel (2mm) between AcurosXB and Monte Carlo which can be accounted for by differences in voxel coordinate definition, contour definition and interpolation effects. Additional differences between AcurosXB and Monte Carlo fall within statistical variance associated with Monte Carlo. Disagreement between AAA and Monte Carlo was far more dramatic; AAA was unable to model the sharp backscatter peak associated with photon transport across low‐ to high‐density boundaries resulting in inaccurate dose distribution through and past high density regions Conclusions: AcurosXB has shown to be an improvement on existing algorithms for calculating dose distribution in the presence of high‐density regions, nearly matching the current gold standard Monte Carlo method. Discrepancies between AcurosXB and Monte Carlo do not translate into clinically significant dose differences, while AAA has shown to under‐ or over‐estimate dose by as much as 20%. Varian Medical Systems has provided AcurosXB hardware and software for research activities.

Shallow Crustal Configuration of the Narmada–Son Lineament Transition Zone Near the Sahdol–Katni Area of Central India Using Simultaneous Gravity and Magnetic Observations

Gravity and magnetic studies have been carried out to map the different depth formations of alluvium, Gondwana, Vindhyan, Mahakoshal, and the crystalline basement in the Narmada–Son lineament (NSL) near the Sahdol–Katni area, India. Higher elevations in the northern part of the study area have lower gravity; the southern part of the study area, however, is moderately elevated and also has a higher gravity anomaly, which justifies the isostatic adjustment. This indicates the presence of high-density material, for example Mahakoshal rocks, in the upper crust which causes the higher anomaly value. The Mahakoshal rocks are widely exposed at Mau, Chanaura, Nadawar, Khamaria, Umria, and near the Tala–Barhi area and also extend from Sidhi to Agoni village further east of the area. The Mahakoshal rocks are thinner between the river Son and Tikwa village which causes a different gravity anomaly pattern changing from the NNE–SSW direction to the N–S direction. However, the trend of magnetic anomaly follows the same pattern toward the NNE–SSW direction, mainly because of the effect of the crystalline basement. Two magnetic highs are prominent in the Tikwa and Amarpur regions, 800 and 400 nTesla, respectively, because of the presence of the crystalline basement. The depth of the crystalline basement obtained by 2½ D gravity–magnetic modelling varies from 2.7 to 2.9 km. From spectral analysis the average depth of the crystalline basement varies from 2.83 to 3.05 km. The different crustal depth sections obtained by 2½ D simultaneous gravity–magnetic modelling correlate well with other constrained data.

Submillimeter line emission from LMC N159W: a dense, clumpy PDR in a low metallicity environment

Context. Star formation at earlier cosmological times takes place in an interstellar medium with low metallicity. The Large Magellanic Cloud (LMC) is ideally suited to study star formation in such an environment. Aims. The physical and chemical state of the ISM in a star forming environment can be constrained by observations of submm and FIR spectral lines of the main carbon carrying species, CO, Ci and Cii, which originate in the surface layers of molecular clouds i lluminated by the UV radiation of the newly formed, young stars. Methods. We present high-angular resolution sub-millimeter observations in the N159W region in the LMC obtained with the NANTEN2 telescope of the 12 CO J = 4→ 3, J = 7→ 6, and 13 CO J = 4→ 3 rotational and [Ci] 3 P1− 3 P0 and 3 P2− 3 P1 fine-structure transitions. The 13 CO J = 4→ 3 and [Ci] 3 P 2− 3 P 1 transitions are detected for the first time in the LMC. We deri ve the physical and chemical properties of the low-metallicity molecular gas using an escape probability code and a self-consistent solution of the chemistry and thermal balance of the gas in the framework of a clumpy cloud PDR model. Results. The separate excitation analysis of the submm CO lines and the carbon fine structure lines shows that the emitting gas in th e N159W region has temperatures of about 80 K and densities of about 10 4 cm −3 . The estimated C to CO abundance ratio close to unity is substantially higher than in dense massive star-forming regions in the Milky Way. The analysis of all observed lines together, including the [Cii] line intensity reported in the literature, in the context of a clu mpy cloud PDR model constrains the UV intensity to about �≈ 220 and an average density of the clump ensemble of about 10 5 cm −3 , thus confirming the presence of high density material in the LMC N159W region.

Heterogeneity in crustal structure across the Southern Granulite Terrain (SGT): Inferences from an analysis of gravity and magnetic fields in the Periyar plateau and adjoining areas

The study region forms the western part of the Madurai block (southern block) and shares several lithological characteristics of the Proterozoic exhumed South Indian Granulite Terrain (SGT). The crustal structure of the area has been derived from gravity data, constrained partly by aeromagnetic data. The Bouguer anomaly map of the region prepared based on detailed gravity observations shows a number of features (i) the Periyar lineament separates two distinctly different gravity fields, one, a high gravity gradient tending to be positive towards the coast in south west and significant gravity lows ranging from − 85 to as low as − 150 mGal in the NE covering a large part of the Periyar plateau (ii) within the broad gravity low, three localised circular anomalies of considerable amplitude occur in the region of Munnar granite. A magnetic low region in the central part coincides with the area of retrogressed charnockites and the major lineaments suggestive of a genetic link and considerable downward extent. The crustal models indicate that the upper layer containing exhumed lower crustal rocks (2.76 gm/cc) is almost homogeneous, most part of the gravity field resulting from variations in intracrustal layers of decharnockitised hornblendic gneisses and granite bodies. Below it, a denser layer (2.85 gm/cc) of unknown composition exists with Moho depth ranging from 36 to 41 km. The structure below the region is compared with that of two other segments of the SGT from which it differs markedly. The Wynad plateau forming the western part of the Northern Block of the SGT is characterised by a heterogeneity due to the presence of contrasting crustal blocks on either side of the Bavali shear zone, possibly a westward extension of the Moyar shear zone and presence of high density material in the mid-to-lower crustal portions. The crust below the Kuppam–Palani transect has a distinctive four-layer structure with a mid-crustal low density layer. The differences in crustal structure are consistent with the different tectonic settings of the three regions discussed in the paper. It is suggested that the crustal structure below the Kuppam–Palani transect corridor is not representative of the SGT as a whole, an aspect of great relevance to intra-continental comparisons and trans-continental reconstructions of continent configurations of the Gondwanaland.

A Cold Suboceanic Mantle Belt at the Earth's Equator

An exceptionally low degree of melting of the upper mantle in the equatorial part of the mid-Atlantic Ridge is indicated by the chemical composition of mantle-derived mid-ocean ridge peridotites and basalts. These data imply that mantle temperatures below the equatorial Atlantic are at least approximately 150 degrees C cooler than those below the normal mid-Atlantic Ridge, suggesting that isotherms are depressed and the mantle is downwelling in the equatorial Atlantic. An equatorial minimum of the zero-age crustal elevation of the East Pacific Rise suggests a similar situation in the Pacific. If so, an oceanic upper mantle cold equatorial belt separates hotter mantle regimes and perhaps distinct chemical and isotopic domains in the Northern and Southern hemispheres. Gravity data suggest the presence of high density material in the oceanic equatorial upper mantle, which is consistent with its inferred low temperature and undepleted composition. The equatorial distribution of cold, dense upper mantle may be ultimately an effect of the Earth's rotation.

Pn velocity study using a temporary seismographic network in the Shillong Plateau, Northeast India

Abstract Seismic P -wave velocity ( Pn ) of the uppermost mantle beneath the Shillong Plateau has been studied using a temporary seismographic network. Refracted Pn arrivals of shallow distant earthquakes ( M > 3.0) recorded at pairs of stations, lying within 2° azimuthal differences with respect to a given epicenter, are used in this study. Since the method is based on relative travel-time differences, the measurement is independent of origin time and location errors. The average Pn velocity is found to be 8.57 ± 0.02 km/sec, which is higher than the average mantle velocity 8.13 km/sec under other areas of the Indian subcontinent. The observed high Pn velocity beneath the plateau may be attributed to the presence of high density material.

Thermal and tectonic evolution of the Kashmir basin vis-à-vis petroleum prospects

Investigation of the formation of peri-Atlantic basins and evaluation of the mechanism of lithospheric break-up provide a guide for understanding the pre-collision history of the Himalayan region. As an example the Kashmir basin is discussed. The present form of the Kashmir basin is that of a graben bounded by two horsts. The stratigraphic succession shows a complete record of shallow marine sedimentation from the Cambrian to Middle Carboniferous. In the Upper Carboniferous the depositional environment changed to continental type, accompanied by large-scale erosion (especially on the western side) and the deposition of Agglomeratic Slate and then, in the Lower Permian, by the Panjal Trap volcanism. Whilst the eastern side, after this continental phase, again subsided and received marine sediments until the end of the Triassic, the western side experienced repeated uplift and volcanism. Gravity and seismic data indicate the presence of high-density material and a shallow depth of the Moho beneath the basin, and also the eruption of the Panjal Trap lava along two deep-seated faults ( F 1 and F 2) on the flanking horsts with dips towards the present Indus-Tsangpo suture. After the upper Carboniferous a continental phase is noted all over the Himalaya. This is thought to mark the initiation of the break-up of the Indian plate from the Siberian-Cathysian block which finally occurred at the end of the Triassic. Based on the stratigraphie and tectonic set-up of Kashmir in particular and the whole Himalaya in general, a model is presented to explain the formation of the Kashmir basin. Good compatibility of the model with observed tectonic and stratigraphie aspects of Kashmir and the whole Himalaya is found. The geothermal history of different strata in this basin indicates promising conditions for hydrocarbon generation; a few of the more promising strata are identified for intensive investigations.