Refraction Tomography Research Articles

Imaging and Quantitative Analysis on the Etching of Diatom Frustules via Digital Holographic Microscopy.

Frustules, whose length spans from a few micrometers to more than a hundred micrometers, have been the subject of various modifications to improve their physical properties because of their complex porous silica structure. However, three-dimensional measurements of these changes can be challenging because of the complex 3D architecture and limitations of known methods. In this study, we present a new method that applies digital holographic microscopy (DHM) to analyze controlled etched frustules and observe real-time degradation of frustules at the single-cell level. Frustules obtained from Craspedostauros sp. diatoms were etched in 1 N NaOH for 5 min at 25 and 60 °C, respectively, and the frustule's valve was analyzed using DHM. DHM uses a combination of holography and tomography to reconstruct a 3D refractive index image of the frustule. Measurements of the width, volume, and surface area are achieved. Results showed that at 60 °C of etching, a significant difference with the unetched frustule was observed for all measurements but with high fluctuation values. Finally, real-time observation of the degradation of the frustule is observed when immersed in a high concentration of NaOH. This is the first time the real-time etching of the frustule is observed at the single-cell level. This research provides an easy estimation of the 3D measurements of frustules that may provide new fundamental information and applications.

Vision Measurement Method Based on Plate Glass Window Refraction Model in Tunnel Construction.

Due to the harsh environment of high humidity and dust in tunnel construction, the vision measurement system needs to be equipped with an explosion-proof glass protective cover. The refractive effect of the plate glass window invalidates the pinhole model. This paper proposes a comprehensive solution for addressing the issue of plane refraction. First, the imaging model for non-parallel plane refraction is established based on dynamic virtual focal length and the Rodriguez formula. Further, due to the failure of the epipolar constraint principle in binocular vision systems caused by plane refraction, this paper proposes the epipolar constraint model for independent refractive plane imaging. Finally, an independent refraction plane triangulation model is proposed to address the issue of triangulation failure caused by plane refraction. The RMSE of the depth of field errors in the independent refraction plane triangulation model is 2.9902 mm before correction and 0.3187 mm after correction. The RMSE of the positioning errors before and after correction are 3.5661 mm and 0.3465 mm, respectively.

Improving Specificity and Axial Spatial Resolution of Refractive Index Imaging by Exploiting Uncorrelated Subcellular Dynamics

Improving Specificity and Axial Spatial Resolution of Refractive Index Imaging by Exploiting Uncorrelated Subcellular Dynamics

Investigation of fractured rock beneath a uranium-tailing storage dam through UAV digital photogrammetry and seismic refraction tomography

Failure events in dams can be associated with processes in the dam body and in the foundation of the structure. If they are properly identified in early stages, corrective actions can take place, leading to a reduction in the risk of collapse and/or rupture of the dam. Most studies on dams are carried out on the body of the dam; however, problems associated with the foundation of the structure can also lead to loss of stability and subsequent ruptures. This study presents an analysis of the advantages and limitations of the use of seismic refraction in hydrogeological studies of fractured aquifers under pressure from large loads, specifically a dam in this case. Seismic refraction data were collected on an outcrop of fractured rock near a uranium storage dam foundation in southeastern Brazil. The results and interpretations were supported by a structural analysis performed through manual strike measurements collected with a Clark compass and an uncrewed aerial vehicle digital photogrammetry survey in an outcrop. The digital photogrammetric survey mapped the spatial distribution and orientation of the geological structures of the rock mass. Although the structural measurements performed through digital photogrammetry presented greater variability than the measurements collected from the compass, the maximum density of the fracture measurements obtained from both methods were similar and were corroborated by the regional and local fracture patterning. The integration of seismic refraction data with geotechnical and geological investigations allowed us to identify the positioning of structural lineaments in the rock mass and zones with a higher degree of rock alteration. The identification of highly fractured zones in the rock mass from such non-invasive investigations could be used to assist in decision making for structural reinforcements in the foundation of the dam to avoid the loss of stability at the foot of the dam from possible leaks or water flows from the reservoir.

Multi-harmonic structured illumination-based optical diffraction tomography

Imaging speed and spatial resolution are key factors in optical diffraction tomography (ODT), while they are mutually exclusive in 3D refractive index imaging. This paper presents a multi-harmonic structured illumination-based optical diffraction tomography (MHSI-ODT) to acquire 3D refractive index (RI) maps of transparent samples. MHSI-ODT utilizes a digital micromirror device (DMD) to generate structured illumination containing multiple harmonics. For each structured illumination orientation, four spherical spectral crowns are solved from five phase-shifted holograms, meaning that the acquisition of each spectral crown costs 1.25 raw images. Compared to conventional SI-ODT, which retrieves two spectral crowns from three phase-shifted raw images, MHSI-ODT enhances the imaging speed by 16.7% in 3D RI imaging. Meanwhile, MHSI-ODT exploits both the 1st-order and the 2nd-order harmonics; therefore, it has a better intensity utilization of structured illumination. We demonstrated the performance of MHSI-ODT by rendering the 3D RI distributions of 5 µm polystyrene (PS) microspheres and biological samples.

Landslide ground model development through integrated geoelectrical and seismic imaging in Thungsong district, Nakhon Si Thammarat, Thailand

A ground model of a shallow landslide in rainfall-induced slope failure of Thungsong, Nakhon Si Thammarat, southern Thailand is developed through an integrated geophysical approach, utilising electrical resistivity tomography and P-wave seismic refraction tomography (SRT) methods. Those two methods were applied to assess landslide structure and study deformation mechanisms along four profiles. Beside the four profiles there is another profile, which was acquired near an borehole and used for the calibration with geological data. Our results show subsurface structures in terms of the ground model used to determine stratigraphic layers, zones of saturation or groundwater table, and significant differences between the landslide slip material and the underlying bedrock. The clay-rich zones (resistivity less than 500 Ωm) in the colluvium on the relatively steep slope, show enhanced potential for landslides. This silty clay plays an important role for landslide activation in this site. Moreover, a combination of steep slopes, shallow basement rocks overlain by clay-rich colluvium, and seasonally high rain fall leads to landslides in the region. The ground model produced by geophysical imaging for this region achieves a comprehensive understanding of the structure and lithology of a complex landslide system and overcomes the limitations of remote-sensing data or isolated intrusive sampling techniques alone.

Using Electrical Resistivity Tomography Method to Determine the Inner 3D Geometry and the Main Runoff Directions of the Large Active Landslide of Pie de Cuesta in the Vítor Valley (Peru)

Pie de Cuesta is a large landslide with a planar area of 1 km2 located in the Vítor district, in the Arequipa department (Peru), and constitutes an active phenomenon. It belongs to the rotational/translational type, which concerns cases that are very susceptible to reactivation because any change in the water content or removal of the lower part can lead to a new instability. In this context, a previous geological study has been decisive in recognizing the lithologies present and understanding their behavior when they are saturated. But it is also necessary to know the inner “landslide geometry” in order to gusset a geotechnical diagnosis. The present study shows how the deep electrical profiles (ERT, electrical resistivity tomography method), supported by two Vp seismic refraction tomography lines (SVP), have been used to create a 3D cognitive model that would allow the identification of the inner landslide structure: the 3D rupture surface, the volume of the sliding mass infiltration sectors or fractures, and the preferred runoff directions. Moreover, on large landsides, placing the geophysical profiles is a crucial aspect because it greatly depends on the accessibility of the area and the availability of the physical space required. In our case, we need to extend profiles up to 1100 m long in order to obtain data at greater depths since this landslide is approximately 200 m tall. Based on the geophysical results and geologic information, the 3D final model of the inner structure of this landslide is presented. Additionally, the main runoff water directions and the volume of 90.5 Hm3 of the sliding mass are also estimated.

Refractive index tomography with a physics-based optical neural network.

The non-interference three-dimensional refractive index (RI) tomography has attracted extensive attention in the life science field for its simple system implementation and robust imaging performance. However, the complexity inherent in the physical propagation process poses significant challenges when the sample under study deviates from the weak scattering approximation. Such conditions complicate the task of achieving global optimization with conventional algorithms, rendering the reconstruction process both time-consuming and potentially ineffective. To address such limitations, this paper proposes an untrained multi-slice neural network (MSNN) with an optical structure, in which each layer has a clear corresponding physical meaning according to the beam propagation model. The network does not require pre-training and performs good generalization and can be recovered through the optimization of a set of intensity images. Concurrently, MSNN can calibrate the intensity of different illumination by learnable parameters, and the multiple backscattering effects have also been taken into consideration by integrating a "scattering attenuation layer" between adjacent "RI" layers in the MSNN. Both simulations and experiments have been conducted carefully to demonstrate the effectiveness and feasibility of the proposed method. Experimental results reveal that MSNN can enhance clarity with increased efficiency in RI tomography. The implementation of MSNN introduces a novel paradigm for RI tomography.

Lenticule Intrastromal Keratoplasty for the Correction of Iatrogenic High Hyperopia.

The aim of this study was to evaluate the safety, efficacy, and predictability of lenticule intrastromal keratoplasty (LIKE) for the correction of iatrogenic high hyperopia. Three patients (4 eyes) were referred to our department because of overcorrection of myopia induced by femtosecond laser-assisted in situ keratomileusis. All eyes exhibited hyperopia (between +4.00 and +8.00 D) and thin corneas ranging from 307 to 378 μm. Because of the regression of the use of laser ablation to correct high hyperopia and thin corneas, we initially adopted LIKE to correct iatrogenic high hyperopia in all 4 eyes. The uncorrected distance visual acuity (UDVA), corrected distance visual acuity (CDVA), manifest refraction, corneal topography, and anterior segment optical coherence tomography findings were recorded during each follow-up evaluation. No postoperative complications, such as interface haze and opacification, were observed in the 4 eyes during each follow-up evaluation, with an average follow-up of 9.50 months. All eyes had significantly improved UDVA postoperatively. A total of 3 eyes achieved an UDVA of 20/25 or better while the other 1 eye had an UDVA of 20/40 postoperatively. In addition, 2 of the 4 eyes had a postoperative UDVA equal to or better than preoperative CDVA. No eyes lost any CDVA lines. All 4 eyes were within ±0.50 D of the spherical power (intended target of 0). The central corneal thickness and curvature of the anterior corneal surface in all 4 eyes increased postoperatively. Anterior segment optical coherence tomography revealed that the lenticule was transparent, with no wrinkles or offsets, during each follow-up evaluation. LIKE for the correction of iatrogenic hyperopia has good efficacy and safety. Although an extremely thin cornea after overcorrection may not be suitable for hyperopic laser enhancement, LIKE is a good choice because of its good predictability and ability to restore the normal corneal structure.

Dynamic droplet morphology measurement method based on digital image correlation and refraction effect

The measurement and analysis of the droplet morphology on a solid surface play a crucial role in investigating solid–liquid interactions and characterizing the properties of surfaces. The contact angle is a widely used parameter in characterizing the droplet, but it cannot well describe the irregular shapes, which can be easily found in the investigations of the spreading and evaporation of the droplet. In this study, a novel method for three-dimensional measurement of the transparent droplet morphology is developed by combining droplet-induced image displacement with the principles of refraction imaging. First, a full-field optical method is employed to measure the image displacement of the solid surface caused by the refraction of the droplet. Second, a physical model is established based on refraction imaging, so the relationship between image displacement and the morphology of the droplet is formulated. Finally, the three-dimensional surface of the droplet is reconstructed by resolving the physical formulations. Experimental results demonstrate that the proposed method can accurately measure the three-dimensional morphology of droplets on solid surfaces and is applicable for irregular liquid surfaces. This method provides important technical support for studying droplet spreading, evaporation, and other related behaviors.

Fucino Basin structure revealed by the tomography and the reusing of the CROP11 seismic data

The refraction reprocessing of the CROP11-1999 seismic reflection data, which were acquired for deep seismic exploration using a split-spread long offset geometry, provides valuable new insights into the main geological structures of the central Apennines. In this study, we present the geophysical interpretation of a sub-transect of the CROP11 seismic profile, crossing the Piani Palentini and Fucino basins carried out using an integrated approach based on the refraction tomography, the stacked refraction convolution section, and the seismic reflection. The reprocessing allowed us to obtain, for the first time, a high resolution (about 15 m in distance and depth) P-waves seismic velocity 2D model and the imaging of the refractor interface of the basin up to the depth of the Meso-Cenozoic carbonate substratum. The outcomes, combined with the interpretation of a CROP11 seismic reflection sub-transect and a subparallel commercial seismic reflection profile, allowed us to highlight a complex basin-fill architecture and stratigraphy. Four seismo-facies, characterized by different Vp velocity values, were recognized above the Meso-Cenozoic carbonate substratum. In particular, a low-velocity zone (LVZ) was evidenced in both basins. The geophysical interpretation and the comparison with the outcropping sequences allowed us to associate it with an upper Messinian thrust-top deposit.The obtained model constitutes an essential geophysical-geological informative base for future investigations on seismic wave propagation and site response studies at the large scale of the Fucino Basin, one of the areas of the Italian territory with a high seismic hazard.

Assessing Geotechnical Property for Construction Purposes: A Study on the Efficacy of Shallow Seismic Refraction Tomography Method

Assessing Geotechnical Property for Construction Purposes: A Study on the Efficacy of Shallow Seismic Refraction Tomography Method

Alteration of perivascular reflectivity on optical coherence tomography of branched retinal vein obstruction

This study aimed to evaluate perivascular reflectivity in patients with branched retinal vascular obstruction (BRVO) using en-face optical coherence tomography (OCT). The study retrospectively analyzed 45 patients with recurrent BRVO, 30 with indolent BRVO, and 45 age- and sex-matched controls. Using a 3.0 × 3.0-mm deep capillary plexus slab on macular scans, OCT angiography (OCTA) and structural en-face OCT scans were divided into four quadrants. Obstructive quadrants of OCTA scans were binarized using a threshold value of mean + 2 standard deviation. The selected area of high signal strength (HSS) was applied to the structural en-face OCT scans, and the corrected mean perivascular reflectivity was calculated as the mean reflectivity on the HSS area/overall en-face OCT mean reflectivity. The same procedure was performed in the quadrants of the matched controls. Regression analysis was conducted on several factors possibly associated with corrected perivascular reflectivity. The perivascular reflectivity in the obstructive BRVO quadrant was significantly higher than in the indolent BRVO and control quadrants (P = 0.009, P = 0.003). Both univariate and multivariate regression analyses showed a significant correlation between the average number of intravitreal injections (anti-vascular endothelial growth factor or dexamethasone implant) per year and refractive errors and image binarization threshold and perivascular reflectivity (P = 0.011, 0.013, < 0.001/univariate; 0.007, 0.041, 0.005/multivariate, respectively). En-face OCT scans of the deep capillary plexus slab revealed higher perivascular reflectivity in recurrent BRVO eyes than in indolent BRVO and control eyes. The results also indicate a remarkable correlation between perivascular reflectivity and the average number of intravitreal injections, suggesting a link to recurrence rates.

Refraction correction for deep-water three-dimensional visual measurement based on multi-objective optimization.

Refraction-induced errors affect the accuracy of three-dimensional visual measurements in deepwater environments. In this study, a binocular camera refractive imaging model was established, and a calibration method for the refraction parameters was proposed for high-accuracy shape and deformation measurements in deep-water environments. First, an initial estimate of the refractive axis was obtained using a three-dimensional calibration target. Then, the errors in the distance between the spatial point pairs and the reprojection errors are taken as the dual optimization objectives, and the Non-dominated Sorting Genetic Algorithm II is applied to optimize the refraction parameters. To efficiently calculate the reprojection error, an improved numerical computation method is proposed to accelerate the calculation of the analytical forward projection. Underwater experiments were conducted to verify the method's effectiveness. The results showed that the average error of the absolute position of the reconstructed points was less than 1.1 mm and the average error of the displacement was less than 0.04 mm. This study provides a sound solution for accurate three-dimensional visual measurement in deep-water environments.

Landslide susceptibility mapping of Penang Island, Malaysia, using remote sensing and multi-geophysical methods

Malaysia is one of the countries in the world experiencing landslides yearly due to natural events and human activities. Penang Island is Malaysia’s second most developed state and the largest by population. It is prone to landslides with devastating environmental impacts. Hence, the need to characterize its near-surface soil-rock conditions. This study uses remotely sensed data via frequency ratio (FR) techniques to identify landslide-prone areas based on different categories of landslide causative factors. To further understand the conditions and hydrodynamics of the soil-rock profiles causing landslides, electrical resistivity tomography and seismic refraction tomography were carried out at a landslide-suspected section in the study area. Also, the satellite-derived Bouguer gravity anomaly modeling was performed to map the varied gravity anomalies associated with landslide-triggering factors in lithologic units. The multi-geophysical models offer strongly correlated results with the causative remote sensed maps and the landslide susceptibility index (LSI) map. The likelihood of landslides occurring in the area, as suggested by the area under curve modeling of LSI data, yielded a high predicted success rate of 83.47%. Hence, prospective landslides were identified at the hilly and elevated sections, while the less susceptible sections were identified on flat reliefs. Landslides may also be triggered, for instance, at steep sections with varied contractive soil bodies and shallow structures. Most importantly, leveraging the LSI map would help the necessary agencies to forestall and mitigate future landslide occurrences in the area.

LED array microscopy system correction method with comprehensive error parameters optimized by phase smoothing criterion.

LED array microscopy is a novel computational imaging technique that can achieve two-dimensional (2D) phase imaging and three-dimensional (3D) refractive index imaging with both high resolution and a large field of view. Although its experimental setup is simple, the errors caused by LED array position and light source central wavelength obviously decrease the quality of reconstructed results. To solve this problem, comprehensive error parameters optimized by the phase smoothing criterion are put forward in this paper. The central wavelength error and 3D misalignment model with six freedom degree errors of LED array are considered as the comprehensive error parameters when the spatial positional and optical features of arbitrarily placed LED array are unknown. Phase smoothing criterion is also introduced to the cost function for optimizing comprehensive error parameters to improve the convergence results. Compared with current system correction methods, the simulation and experimental results show that the proposed method in this paper has the best reconstruction accuracy, which can be well applied to an LED array microscope system with unknown positional and optical features of the LED array.

Imaging model and vision measurement method considering refraction effect of flat glass windows in tunnel construction

Vision measurement has become an indispensable technology in tunnel construction. However, the harsh construction environment necessitates the provision of a certain thickness of flat glass windows to protect the vision system. This, in turn, leads to an obvious refraction effect, rendering the pinhole model and traditional vision measurement methods inadequate. As such, this paper offers a comprehensive solution to this problem. Firstly, the imaging model under the refraction of flat glass is shown based on the virtual camera and virtual space point, respectively. Secondly, a three-parameter calibration method for flat glass was proposed, which could determine the refractive index, unit normal vector, and light path normal distance. A practical dynamic interval constraint was introduced to address the failure of the epipolar constraint in refraction image pairs. Lastly, a new binocular disparity method is presented to avoid the impact of refraction distortion on the solution accuracy of three-dimensional coordinates. Effectiveness experiments are conducted, showing a refractive index relative error within ±1%, a miniaturized and efficient dynamic interval constraint, a maximum reconstruction error of 0.120 mm, and a maximum measurement error of only 0.093 mm. A series of stability experiments are also performed under different target attitudes, glass attitudes, working distances, and pollution conditions. The results demonstrate the robustness of the proposed solution. Although the scheme is developed for the vision system in harsh tunnel construction environments, it can serve as a reference for any vision measurement applications under glass protection windows.

Groundwater Circulation in the Shallow Crystalline Aquifer of Tharisa Mine, South Africa: Evidence from Environmental Isotopes and Near-Surface Geophysics

For underground mining, efficient groundwater management is one of the critical mining economics components. The region of interest, known as Tharisa Mine, is situated on the western limb of the Bushveld Igneous Complex, which is home to South Africa’s premier platinum-group metal resources. This work aimed to provide the findings from the investigation and imaging of the near-subsurface hydrogeological architecture in a shallow profile using stable isotopes of water (18O and 2H) and radioactive water isotopes (3H). Regarding isotope data, 18O varied from −3.5 to 1.5‰; 2H from −24 to 4.7‰; and 3H from 2.0 to 3.4 T.U. Utilizing combined geophysical techniques, the results were verified. Additionally, the geophysical methods, including seismic refraction tomography, multichannel analysis of surface waves, electrical resistivity tomography, and magnetics, helped identify the fluid’s pathways and lineaments during migration to verify the isotope results. The groundwater inflow volumes into the open pit were initially determined by integrating the following findings: the delineation of fracture systems/zones and fluid migration pathways; mining activities enhance the storage and transmission ability of the aquifer; and the main sources of water in the mine include mixing of surface and deep water sources, recycling of water possibly via lineaments, and tailings dam seepages.

Main water pathways in cultivated clayey calcisols in molassic hills in southwestern France: Toward spatialization of soil waterlogging

AbstractLocal waterlogging often occurs on the steep slopes of clayey–calcareous soils in southwestern France, causing nutrients and pollutants transfer to the river bodies and reduced ecosystems services. These soils developed in the Miocene molassic hill formation and are generally impermeable with abundant traces of hydromorphy and heterogenous spatial distribution. This article aims to describe the hydrological functioning of these soils, based on a cross analysis of pedological, hydrological, and geophysical characterizations. Our experimental site is the catchment area located in Auradé (southwestern France). Here, we analyze the flows at the outlet of the studied watershed together with piezometric and climatic monitoring from September 2020 to September 2021. We show that the hydrological year is divided into three phases: first, a soil recharge phase with an effective rainfall of about 100 mm; second, a saturation phase, when 80% of the effective precipitation is drained mostly by runoff and hypodermic flows; third, a drying phase. Soil waterlogging events usually occur during the saturation phase. They are due to several forms of flow: surface runoff associated with return flow, hypodermic flow caused by the presence of soil layers with lower hydraulic conductivity in the subsurface (swelling clays and plowing sole) and groundwater flow with intermittent connection of the soil water table in the hillside to the alluvial groundwater table. We also conducted independent seismic refraction tomography analyses that validate localized waterlogging patterns along the catchment and open the way to spatializing areas with high waterlogging potential at the scale of the study plot.

Corneal astigmatic outcomes after femtosecond laser-assisted cataract surgery combined with surface penetrating arcuate keratotomies.

To evaluate corneal astigmatic outcomes of femtosecond laser-assisted arcuate keratotomies (FAKs) combined with femtosecond-laser assisted cataract surgery (FLACS) over 12mo follow-up. Totally 145 patients with bilateral cataracts and no ocular co-morbidities were recruited to a single-centre, single-masked, prospective randomized controlled trial (RCT) comparing two monofocal hydrophobic acrylic intraocular lenses. Eyes with corneal astigmatism (CA) of >0.8 dioptres (D) received unpaired, unopened, surface penetrating FAKs at the time of FLACS. Visual acuity, subjective refraction and Scheimpflug tomography were recorded at 1, 6, and 12mo. Alpins vectoral analyses were performed. Fifty-one patients (61 eyes), mean age 68.2±9.6y [standard deviation (SD)], received FAKs. Sixty eyes were available for analysis, except at 12mo when 59 attended. There were no complications due to FAKs. Mean pre-operative CA was 1.13±0.20 D. There was a reduction of astigmatism at all post-operative visits (residual CA 1mo: 0.85±0.42 D, P=0.0001; 6mo: 0.86±0.35 D, P=0001; and 12mo: 0.90±0.39, P=0.0001). Alpins indices remained stable over 12mo. Overall, the cohort was under-corrected at all time points. At 12mo, 61% of eyes were within ±15 degrees of pre-operative astigmatic meridian. Unpaired unopened penetrating FAKs combined with on-axis phacoemulsification are safe but minimally effective. CA is largely under-corrected in this cohort using an existing unmodified nomogram. The effect of arcuate keratotomies on CA remained stable over 12mo.