Subsurface Shift Research Articles

An appraisal of geohydrological status and assessment of groundwater quality of Indpur Block, Bankura District, West Bengal, India

The present work aimed to evaluate the overall hydro-geological status of Indpur block, Bankura district, West Bengal, India. Despite of having adequate annual precipitation, south-western districts of the state of West Bengal, India, are considered to be a significantly water-stressed area of the state. This is because of unfavorable geological setting near to subsurface occurrence of impervious lithology and inundated nature of surface drainage pattern. The study was carried out both in pre- and post-monsoon seasons of 2019 to obtain an updated current status on concentration and spatiotemporal fluctuations of controlling ions of the subsurface water. Estimation of major physicochemical parameters and specific qualitative chemical characterization of groundwater were rated through field and laboratory studies. Water samples were collected from twenty-two equidistantly scattered tube wells in the block. Seasonal variations of water table elevation heads and subsurface shift of predominant recharge zones of the block were also demarked. Drinking, domestic and irrigation suitability of the block water were measured by the estimation of parameters such as Sodium Adsorption Ratio (SAR), Magnesium Adsorption Ratio (MAR), Soluble Sodium Percentage (SSP), Residual Sodium Carbonate (RSC), Permeability Index (PI), Total Hardness (TH) and Kelly’s ratio (KR) and piper trilinear plots. Sustainable non-availability of groundwater seems to be the major problem of the studied area, which intern resulted in overexploitation, mostly for cultivation practices causing considerable depletion of its suitability as drinking and irrigation. Further, results show that suitability of the water both for domestic and irrigation of the studied area may be termed as ‘good’ to ‘moderate’ with a few exceptions on a local scale. Judging by every parameter, it can be stated that groundwater of Indpur block is not much suitable for drinking purposes.

Hydrogeochemistry and Overall Appraisal of Groundwater Status of Taldangra Block, Bankura District, West Bengal, India

South-western districts of the state of West Bengal, India are considered to be a significantly water-stressed area of the state because of unfavourable geological setting, subsurface lithology, soil cover and surface drainage pattern. Updated geohydrological data on micro-scale i.e., at subdivision or block-level are scanty for Bankura District, West Bengal. For the present study, a geo-hydrological survey was conducted in Taldangra block of Bankura District, West Bengal, during post and pre-monsoon session of 2017–18, to obtain an intense status on present groundwater quality of this water-stressed block. Variation of concentration of major affecting ions and spatial-temporal seasonal variations of water table elevation head and subsurface shift of predominant recharge and discharge zones of the block were demarked explaining the possible reasons. Suitability status of the groundwater for drinking, domestic and irrigation usages was also rated by the estimation of sodium adsorption ratio (SAR), soluble sodium percentage (SSP), permeability index (PI), Piper trilinear diagram.

Nature of monovacancies on quasi-hexagonal structure of reconstructed Au(100) surface

Density functional theory and the scanning tunneling microscopy were applied to investigate the nature of the defects on an unrotated reconstructed hexagonal Au(100) surface. We show the correlation between the nature of monovacancies and the geometrical arrangement of the atoms from the first (topmost) layer with respect to the second layer. The vacancies created directly above the atoms from the second layer (on-top position) move spontaneously into the subsurface upon relaxation. For the vacancies created in other places, the subsurface shift is hindered by the energy barrier and the lifted subsurface atom is in the interlayer position. Charge density calculations were compared with results from the scanning tunneling microscopy images which show a good agreement between the theoretical and experimental data.

Subsurface offset behaviour in velocity analysis with extended reflectivity images

ABSTRACTMigration velocity analysis with the constant‐density acoustic wave equation can be accomplished by the focusing of extended migration images, obtained by introducing a subsurface shift in the imaging condition. A reflector in a wrong velocity model will show up as a curve in the extended image. In the correct model, it should collapse to a point. The usual approach to obtain a focused image involves a cost functional that penalizes energy in the extended image at non‐zero shift. Its minimization by a gradient‐based method should then produce the correct velocity model. Here, asymptotic analysis and numerical examples show that this method may be too sensitive to amplitude peaks at large shifts at the wrong depth and to artefacts. A more robust alternative is proposed that can be interpreted as a generalization of stack power and maximizes the energy at zero‐subsurface shift. A real‐data example is included.

Intraoperative Brain Shift Compensation: Accounting for Dural Septa

Biomechanical models that describe soft tissue deformation provide a relatively inexpensive way to correct registration errors in image-guided neurosurgical systems caused by nonrigid brain shift. Quantifying the factors that cause this deformation to sufficient precision is a challenging task. To circumvent this difficulty, atlas-based methods have been developed recently that allow for uncertainty, yet still capture the first-order effects associated with deformation. The inverse solution is driven by sparse intraoperative surface measurements, which could bias the reconstruction and affect the subsurface accuracy of the model prediction. Studies using intraoperative MR have shown that the deformation in the midline, tentorium, and contralateral hemisphere is relatively small. The dural septa act as rigid membranes supporting the brain parenchyma and compartmentalizing the brain. Accounting for these structures in models may be an important key to improving subsurface shift accuracy. A novel method to segment the tentorium cerebelli will be described, along with the procedure for modeling the dural septa. Results in seven clinical cases show a qualitative improvement in subsurface shift accuracy making the predicted deformation more congruous with previous observations in the literature. The results also suggest a considerably more important role for hyperosmotic drug modeling for the intraoperative shift correction environment.

A fast and efficient method to compensate for brain shift for tumor resection therapies measured between preoperative and postoperative tomograms.

In this paper, an efficient paradigm is presented to correct for brain shift during tumor resection therapies. For this study, high resolution preoperative (pre-op) and postoperative (post-op) MR images were acquired for eight in vivo patients, and surface/subsurface shift was identified by manual identification of homologous points between the pre-op and immediate post-op tomograms. Cortical surface deformation data were then used to drive an inverse problem framework. The manually identified subsurface deformations served as a comparison toward validation. The proposed framework recaptured 85% of the mean subsurface shift. This translated to a subsurface shift error of 0.4 +/- 0.4 mm for a measured shift of 3.1 +/- 0.6 mm. The patient's pre-op tomograms were also deformed volumetrically using displacements predicted by the model. Results presented allow a preliminary evaluation of correction both quantitatively and visually. While intraoperative (intra-op) MR imaging data would be optimal, the extent of shift measured from pre- to post-op MR was comparable to clinical conditions. This study demonstrates the accuracy of the proposed framework in predicting full-volume displacements from sparse shift measurements. It also shows that the proposed framework can be extended and used to update pre-op images on a time scale that is compatible with surgery.

An atlas-based method to compensate for brain shift: Preliminary results

Compensating for intraoperative brain shift using computational models has shown promising results. Since computational time is an important factor during neurosurgery, a priori knowledge of the possible sources of deformation can increase the accuracy of model-updated image-guided systems. In this paper, a strategy to compensate for distributed loading conditions in the brain such as brain sag, volume changes due to drug reactions, and brain swelling due to edema is presented. An atlas of model deformations based on these complex loading conditions is computed preoperatively and used with a constrained linear inverse model to predict the intraoperative distributed brain shift. This relatively simple inverse finite-element approach is investigated within the context of a series of phantom experiments, two in vivo cases, and a simulation study. Preliminary results indicate that the approach recaptured on average 93% of surface shift for the simulation, phantom, and in vivo experiments. With respect to subsurface shift, comparisons were only made with simulation and phantom experiments and demonstrated an ability to recapture 85% of the shift. This translates to a remaining surface and subsurface shift error of 0.7+/-0.3 mm, and 1.0+/-0.4 mm, respectively, for deformations on the order of 1cm.

Serial Intraoperative Magnetic Resonance Imaging of Brain Shift

A major shortcoming of image-guided navigational systems is the use of preoperatively acquired image data, which does not account for intraoperative changes in brain morphology. The occurrence of these surgically induced volumetric deformations ("brain shift") has been well established. Maximal measurements for surface and midline shifts have been reported. There has been no detailed analysis, however, of the changes that occur during surgery. The use of intraoperative magnetic resonance imaging provides a unique opportunity to obtain serial image data and characterize the time course of brain deformations during surgery. The vertically open intraoperative magnetic resonance imaging system (SignaSP, 0.5 T; GE Medical Systems, Milwaukee, WI) permits access to the surgical field and allows multiple intraoperative image updates without the need to move the patient. We developed volumetric display software (the 3D Slicer) that allows quantitative analysis of the degree and direction of brain shift. For 25 patients, four or more intraoperative volumetric image acquisitions were extensively evaluated. Serial acquisitions allow comprehensive sequential descriptions of the direction and magnitude of intraoperative deformations. Brain shift occurs at various surgical stages and in different regions. Surface shift occurs throughout surgery and is mainly attributable to gravity. Subsurface shift occurs during resection and involves collapse of the resection cavity and intraparenchymal changes that are difficult to model. Brain shift is a continuous dynamic process that evolves differently in distinct brain regions. Therefore, only serial imaging or continuous data acquisition can provide consistently accurate image guidance. Furthermore, only serial intraoperative magnetic resonance imaging provides an accurate basis for the computational analysis of brain deformations, which might lead to an understanding and eventual simulation of brain shift for intraoperative guidance.

Registration of 3-D intraoperative MR images of the brain using a finite-element biomechanical model.

We present a new algorithm for the nonrigid registration of three-dimensional magnetic resonance (MR) intraoperative image sequences showing brain shift. The algorithm tracks key surfaces of objects (cortical surface and the lateral ventricles) in the image sequence using a deformable surface matching algorithm. The volumetric deformation field of the objects is then inferred from the displacements at the boundary surfaces using a linear elastic biomechanical finite-element model. Two experiments on synthetic image sequences are presented, as well as an initial experiment on intraoperative MR images showing brain shift. The results of the registration algorithm show a good correlation of the internal brain structures after deformation, and a good capability of measuring surface as well as subsurface shift. We measured distances between landmarks in the deformed initial image and the corresponding landmarks in the target scan. Cortical surface shifts of up to 10 mm and subsurface shifts of up to 6 mm were recovered with an accuracy of 1 mm or less and 3 mm or less respectively.

Three-Dimensional Optical Flow Method for Measurement of Volumetric Brain Deformation from Intraoperative MR Images

A three-dimensional optical flow method to measure volumetric brain deformation from sequential intraoperative MR images and preliminary clinical results from five cases are reported. Intraoperative MR images were scanned before and after dura opening, twice during tumor resection, and immediately after dura closure. The maximum cortical surface shift measured was 11 mm and subsurface shift was 4 mm. The computed deformation field was most satisfactory when the skin was segmented and removed from the images before the optical flow computation.

Oxygen and carbon isotopic composition of marine carbonate concretions; an overview; discussion and reply

In their review of isotopic data from marine carbonate concretions, Mozley and Burns (1993, p. 8 i) incorrectly associate our work (Dix and Mullins 1987) with their interpretation of the effects of recrystallization on isotopic compositions in concretions. Mozley and Burns (1993, p. 81) state, Late-stage calcite and siderite concretions are characterized by a combination of 'normal marine' 6~3C values and depleted 6~sO values. Such isotopic values have been interpreted by some authors as indicating recrystallization (i.e., that the concretion initially had normal marine carbon and oxygen isotopic compositions and that the oxygen isotopic compositions were reset at higher temperatures; Dix and Mullins 1987 . . . . ). The association of our work with this statement is incorrect because we illustrated that: (1) the bulk of the concretions in the Middle Devonian Hamilton Group appear to have formed during very early stages of burial within the upper 10 m (p. 147); (2) the very negative 5~3C values of most concretion samples and systematic shifts with concretion growth illustrate the role of organic diagenesis within the zone of sulfate reduction, and transitional into the upper zone of methanogenesis (our Fig. II); and (3) the only late-stage concretions (containing ferroan calcite) have 6'3C values ranging between 0 and 2%, and this due to a subsurface shift in porewater 6'3C related to organic diagenesis (our Fig. 11), not recrystallization (p. 150). In their discussion of causes for consistently depleted 5'80 values in shallow-burial calcite concretions, Mozley and Bums (1993, p. 77) appear to dismiss the effect of recrystallization partly through their assumption that the original composition of concretions must be stable low-magnesium calcite. They cite, however, only three studies of concretions that unequivocally remove any possibility for recrystallization. Are these the exceptions or the rule? Combined petrographic and isotopic observations still lead us to the interpretation that recrystallization of Hamilton Group concretions (excluding late-stage concretions) was very important. 6180 data fall within the middle of an isotopic group defined by late-stage septarian calcites, and our samples of septarian calcites also fall within this group with ~laO values that are not too dissimilar to the bulk of concretion calcite (our Fig. 12). We presented petrographic evidence illustrating that two stages of septafian calcite have been replaced by subsequent calcite, including ferroan calcite (our Fig. 7), and that replacement calcite is associated with silica diagenesis in the concretion bodies and migration of petroleum-bearing fluids. If septarian calcite can be replaced, why not the concretion-body calcite too? With so many calcite concretions appearing to form under near-surface burial conditions, could the mineral-water interaction that Mozley and Burns (1993) call upon to account for net shifts in ~aO include very shallow recrystallization ofa metastable magnesium-beating carbonate? Given the diverse mechanisms for causing a shift in 6'80 (Mozley and Burns 1993), recrystallization seems to be a plausible factor (see Desrochers and AI-Aasm 1993) that should not be discarded so readily.