Adriana Araujo Castro Lopes, Márcia Abrahão Moura, Anna Lorena Gomes da Costa Testa
et al.
Abstract The Novo Mundo granitic Complex lies in the southernmost margin of the Amazonian Craton, Brazil. New petrological, geochemical, U–Pb and Sm–Nd isotopic data allow us to propose a new petrogenetic model for granite genesis in the southernmost portion of the Craton. Earlier studies attribute the complex to the Tapajós–Parima Tectonic Province. Four granitic facies have been identified: quartz monzonite, monzogranite, biotite granodiorite, and syenogranite, all of which are crosscut by diabase dikes. The monzogranite has primary muscovite. The biotite from the granites shares chemical signatures with that of volcanic arc settings. Petrographic data, mineral chemical, lithogeochemical, and isotopic data allow conclude that the Novo Mundo rocks into I-type, meta– to peraluminous, and calc–alkaline to alkali-calcic granites which are compatible with volcanic arc settings and varying degrees of crustal contribution. U–Pb LA–ICP–MS ages of magmatic zircons (quartz monzonite 2032 ± 6 Ma; monzogranite 2029 ± 4 Ma; biotite granodiorite 1989 ± 6.2 Ma; syenogranite 1987 ± 7.4 Ma) and TDM Nd model ages (2.52-2.15 Ga; ɛNd(t) = -2.0 to +1.81) indicate that the granites originated from either mantle-derived or juvenile Orosirian crustal melts, with additional continental crust input. The weak positive ɛNd(t) anomalies suggest an upwelling asthenospheric mantle that provided the thermal input to remelt the lower crust, as well as the isotopic data emphasize varying levels of crustal reworking during arc evolution. These ages are coeval with the evolution of the Paleoproterozoic Cuiú-Cuiú Magmatic Arc emplaced during the main stage of arc subduction in the region (2.05-1.99 Ga). The proposed evolutionary model suggests that quartz monzonite and monzogranite are associated with the early stages of arc evolution. The granodiorite records the contribution of juvenile sources, correlated with episodes of mafic magma influx, while the syenogranite represents the final stages of the arc formation, indicating a predominance of continental crustal contribution. The new data, combined with recent studies, allow us to suggest the existence of a Paleoproterozoic continuous magmatic arc into the south part of the Amazonian craton instead of the previously defined tectonic provinces previously defined in the literature.
Understanding the fluid storage and production mechanisms in sedimentary rocks is vital for optimising natural gas extraction and subsurface resource management. This study applies high-resolution X-ray computed tomography (≈15 μm) to digitise rock samples from onshore Cyprus, producing digital rock models from DICOM images. The workflow, including digitisation, numerical simulation of natural gas flow, and experimental validation, demonstrates strong agreement between digital and laboratory-measured porosity, confirming the methods’ reliability. Synthetic sand packs generated via particle-based modelling provide further insight into the gas storage mechanisms. A linear porosity–permeability relationship was observed, with porosity increasing from 0 to 35% and permeability from 0 to 3.34 mD. Permeability proved critical for production, as a rise from 1.5 to 3 mD nearly doubled the gas flow rate (14 to 30 fm<sup>3</sup>/s). Grain morphology also influenced gas storage. Increasing roundness enhanced porosity from 0.30 to 0.41, boosting stored gas volume by 47.6% to 42 fm<sup>3</sup>. Although based on Cyprus retrieved samples, the methodology is applicable to sedimentary formations elsewhere. The findings have implications for enhanced oil recovery, CO<sub>2</sub> sequestration, hydrogen storage, and groundwater extraction. This work highlights <i>digital rock physics</i> as a scalable technology for investigating transport behaviour in porous media and improving characterisation of complex sedimentary reservoirs.
Abstract The Longdong region is a recently discovered area for exploring natural gas in the Upper Paleozoic era within the Ordos Basin. The primary layer that produces gas in this area is the Shan1 member of the Permian Shanxi Formation. The research focused on analyzing the petrological characteristics, physical properties, pore structure, and diagenesis characteristics of the reservoir in the 13th member of the Permian Shanxi Formation in the Longdong area of the southwest Ordos Basin. The research was conducted by combining core observation, cast-thin members, physical properties, and other relevant data. Additionally, the study also discussed the primary factors that influence reservoir performance. The analysis indicates that the dominant sandstone types found in the 13th member of the mountain reservoir are primarily quartz sandstone and lithic quartz sandstone. These sandstone types originate from high-energy environments and are located far from the source. The predominant types of pores are intragranular pores and cutting-karst pores, with the pore size primarily falling within the microporous to mesoporous range. The sedimentary facies and diagenesis exert control over the physical qualities of the reservoir. Specifically, the sedimentary facies determines the fundamental physical conditions of the reservoir, while the structure of the sand body significantly influences its physical properties. Compaction is the primary cause of the increased density of the layer's physical properties. The presence of illite is the main factor contributing to the densification of the reservoir through cementation. The limited extent of reservoir dissolution has a minor effect on enhancing reservoir quality.
Zuhair AlYousef, Ayrat Gizzatov, Hana AlMatouq
et al.
Abstract The use of foams is a promising technique to overcome gas mobility challenges in petroleum reservoirs. Foam reduces the gas mobility by increasing the gas apparent viscosity and reducing its relative permeability. A major challenge facing foam application in reservoirs is its long-term stability. Foam effectiveness and stability depends on several factors and will typically diminish over time due to degradation as well as the foam-rock-oil interactions. In this study, the effect of crude oil on CO2-foam stability and mobility will be investigated using in-house build microfluidics system developed for rapid prescreening of chemical formulations. Two-phase flow emulsification test (oil-surfactant solutions) and dynamic foam tests (in the absence and presence of crude oil) were conducted to perform a comparative assessment for different surfactant solutions. A microfluidics device was used to evaluate the foam strength in the presence and absence of crude oil. The assessment was conducted using five surfactant formulations and different oil fractions. The role of foam quality (volume of gas/total volume) on foam stability was also addressed in this study. The mobility reduction factor (MRF) for CO2-foam was measured in the absence and presence of crude oil using high salinity water and at elevated temperatures. The results indicated that foam stability has an inverse relationship with the amount of crude oil. Crude oil has a detrimental effect on foams, and foam stability decreased as the amount of crude oil was increased. Depending on the surfactant type, the existence of crude oil in porous media, even at very low concentrations of 5% can significantly impact the foam stability and strength. The oil can act as an antifoaming agent. It enters the thin aqueous film and destabilizes it. This resulted in a lower foam viscosity and less stable foams. Thus, the CO2 MRF dropped significantly in the presence of higher oil fractions. This study also demonstrated that in-house assembled microfluidics system allows for a rapid and cost-efficient screening of formulations.
Abstract The Weiyuan shale gas field faces problems of long drilling cycles and high development costs. Improving the drilling efficiency of polycrystalline diamond compact bits in shale formations will significantly reduce the overall well cost and duration. Previous applications have demonstrated that conventional PDC bits on the market cannot meet the demand for drilling acceleration. In this work, a new three-dimensional concave-shaped PDC cutter was proposed to improve drilling efficiency. The special 3D concave-shaped cutter has two symmetrical curved ridges on the concave surface and a circular plane at the center. The cutting mechanism of the new 3D concave-shaped cutter has been studied by laboratory experiments and numerical simulations. The research data revealed that, compared with a flat cutter, the tangential force of the original 3D concave-shaped cutter was reduced by 1.4%–35.0%, the axial force was reduced by 6.7%–37.6%, and the mechanical specific energy (MSE) was reduced by 1.6%–35.59%. Simulations showed that the shear action of the 3D concave-shaped cutter was divided into two continuous parts, with the sides and the center surface being stressed successively, which is helpful for extending shear cracks, forming trilobal cuttings, and improving cutting efficiency. With the special 3D concave-shaped cutter, an 8½-inch drill bit was designed and manufactured and tested on the Longmaxi shale in the Weiyuan block. Through field tests, we further compared the performance of the 3D concave-shaped cutter PDC bit with that of the flat cutter PDC bit. The 3D concave-shaped PDC bit had a 41.8% better footage and 22.6% better rate of penetration (ROP) in field test.
Research on the characteristics and distribution of natural fractures is of great importance for the exploration and development of low-permeability sandstone gas reservoirs. In this study, fracture identification and characterization were carried out using cores and imaging logging. Then, comprehensive fracture development indicators were constructed to predict the distribution of fractures in wells by conventional logging. The main factors that affect the development of natural fractures and the implications of fractures on hydrocarbon exploration and development were discussed. The results showed that the natural fractures were mainly low-angle tectonic fractures in sandstone reservoirs. Most of fractures are unfilled, but the distribution of the fractures in the thin sections has a discrete fracture structure, indicating that the connectivity of the fracture system is poor. The development of natural fractures is mainly influenced by rock strength, petrographic composition, and petrology, and the fractures are more developed in sandstones with a higher content of brittle minerals. The fracture densities are mainly distributed below 0.05 m/m and up to 0.1 m/m. In the present in situ stress state, all of the natural fractures in the LD-A gas field are invalid fractures. The critical pressure of the natural fracture is approximately 16.5–25.4 MP/km; when the pore pressure exceeds this value, the fractures become effective fractures. These results provide new geological knowledge and guidance for the exploration and development of LD-A gas fields and other low-permeability tight sandstone reservoirs.
Sakine Amraei, Mehrdad Behzadi, Mohammad Yazdi
et al.
The Ti- Fe mineralization in Bafq 15 anomaly located 35 Km NW of Bafq city, and is a part of Poshte Badam Block in the Central Iran. Mineralization including magnetite, titanomagnetite, ilmenite, and minor pyrite which hosted by gabbro and pyroxenite intrusions syngeneticly. Based on the whole rock chemistry, FeOt, TiO2, CaO, Ni, Cr and V show a positive correlation with MgO, whereas Al2O3, Na2O+K2O, and SiO2 display a negative correlation. These correlations are in agreement with the crystallization of clinopyroxene, amphibole, plagioclase, and oxide minerals in the intrusion. The positive correlation of V, Cr, and Ni with Fe indicates the concentration of these elements in Fe minerals. The chemical composition of ore minerals mostly plots in the solid solution of magnetite- ulvospinel (titanomagnetite) and magnetite- ilmenite fields. In QFM+1> conditions, the high Fe-Ti contents along high H2O content (>2 Wt. %) of parental magma are the most prominent factors controlling Fe- Ti mineralization. According to the proposed model for mineralization, as a new pulse of magma enters the chamber, the high H2O content sufficiently depressed the crystallization temperature of silicates in analogy to oxides, giving rise to early crystallization of Fe-Ti minerals. Increasing of H2O content and magmatic volatiles during magma fractionation consequently may induce immiscibility and separation of oxides from residual melt in late magmatic stage whereas this dense oxides melt flow through pre-crystalized silicates and solidified as intercumulus phase.
Nowadays we are commonly used that important books, containing new data, new theories, new karst and cave descriptions, etc are published in English. This is why a publication in a less familiar language may be less important from the global point of view but may be much more important; of a key importance even, in the community of its language. Hence we are very glad to highlight the importance of two books, one in the (Brazilian) Portuguese which in fact is not at all less spoken language, and another one in the Croatian language; both authors are former students of the Karstology doctoral programme at the University of Nova Gorica (Slovenia).
Mulugeta C. Fenta, Zelalem L. Anteneh, János Szanyi
et al.
The groundwater of volcanic aquifers, dissected by various structures and affected by several volcanic eruption events, varies in quality. A large number of rural people depend on shallow aquifers tapped by shallow hand wells and springs. On the other hand, the urban population is dependant on deep aquifers using drilled boreholes. The location of springs, shallow hand-dug wells and boreholes inside or close to farmlands, and the advancement of irrigation water use from groundwater by the government entail the assessment of groundwater quality. Therefore, the focus of the present study is to determine the quality and suitability of groundwater around Dangila Town, Northwest Ethiopia, for drinking and irrigation uses. The water quality assessment was conducted by collecting groundwater samples from 14 shallow hand-dug wells, 4 springs, and 7 deep boreholes then analysing for different physical and chemical parameters.A total of 25 selected groundwater samples from shallow and deep aquifers were analysed in a laboratory for physical and chemical parameters. The physical parameters measured both in the field and the laboratory included pH, electrical conductivity (EC) and total dissolved solids (TDS). The chemical parameters analysed in the laboratory comprised cations of calcium (Ca2+), magnesium (Mg2+), sodium (Na+), potassium (K+), Iron (Fe), manganese (Mn2+) and anions of bicarbonate (HCO3-), sulfate (SO42-), carbonate (CO32-), chlorine (Cl-), nitrate (NO3-), fluoride (F-), and boron (B). Based on the laboratory results, the variation in groundwater facies, and major cation and anion sources were determined. Furthermore, the groundwater quality for human consumption was assessed and sodium adsorption ratio (SAR), Na%, and the residual sodium carbonate (RSC) values, which are crucial to determine the overall groundwater quality for irrigational uses, were calculated. Detailed interpretations of the data have been presented in the paper entitled “Hydrogeological framework of the volcanic aquifers and groundwater quality in Dangila Town and the surrounding area, Northwest Ethiopia” [1]. The presented dataset demonstrates the necessity of water quality assessments that would be helpful to water sectors, government, and policymakers for sustainable groundwater management.
Computer applications to medicine. Medical informatics, Science (General)
Arthur Santos da Silva, Gilmara Regina Lima Feio, João Paulo Silva Alves
et al.
Abstract The Pedra Branca suite (2.75 Ga) is located in the Canãa dos Carajás domain in the southeastern Amazonia Craton. It diverges from typical TTG in mineralogical and geochemical terms, by the presence of hornblende and clinopyroxene and because it has a high content of HFSE (Zr, Y, Ti, and Nb). It belongs to the low-K subalkaline series, which varies from metaluminous to peraluminous, and are mostly calc and ferroan granitoids. Amphibole is calcic and classified as ferroan-edenite, and hastingsite. Plagioclase is mainly oligoclase. The Fe/(Fe + Mg) ratios found in the amphiboles indicate that these granitoids were formed under high to moderate fO2 conditions. Geobarometric calculations suggest pressures between 9.3 and 71 Kbar for the origin and 4.8-53.4 Kbar for the emplacement. Geothermometric calculations suggest initial crystallization temperatures between 945 and 862°C, and the water content in the magma is estimated to be higher than 4 wt%. The magma source was defined as tholeiitic continental gabbro melted in an extensional setting (Carajás Rift) with geochemical features similar to diabase from Nova Canadá (PA). The Pedra Branca magma was originated by partial melting (~28%), leaving a residue with plagioclase (An40), hornblende, clinopyroxene, and may or may not have magnetite.
Editorial is a privilege of the editors to share their opinions and news with the readers. It’s been a while since the last editorial was published in Acta Carsologica. In the following, I present my opinion on some issues related to our work and publishing. It was provoked by the Opinion, written by Wolfgang Dreybrodt and published in this issue. We are all aware that the way we do and publish scientific work has been changing fast and dramatically. Rapid progress, broad and productive collaborations, ever better access to sophisticated research equipment, the need for publishing and the search for funds sometimes distract us from deep thinking on the research questions that should be the key element of scientific work. The somehow provoking Opinion opens up many concerns that we all face. It does not reflect the opinion of the editorial board, although I agree with most of them. At some point one might say that times have changed, we need to collaborate and promote our work in order to be seen. However, basic scientific ethics as stressed in the Opinion, should not suffer from the changes. One ethical question facing the editors is whether we should publish in our own journals. A principle answer would be no. Acta Carsologica has an inherent interest in the research at Classical Karst, where most of our editors work. Therefore, we publish in our journal, but we do not interfere into the editorial process of our manuscripts. In times when you have to pay either to read and/or publish a manuscript, new opportunities arise for journals like ours. These journals can attract authors who, for whatever reason, cannot or do not want to spend money on publication and want to keep their work open. However, authors and readers should be aware that publishing is not our main task and that it is difficult to offer such good services as in journals belonging to the large publishing groups. In addition to managing reviews and making decisions, the editors’ tasks often include providing funds for layout and printing, reporting, website maintenance, proofreading, translation, transport, packaging and dispatch of issues. Surely this should not be an excuse for the delays in publication or the long editorial process, but I hope for some understanding of those waiting. We can of course improve. The whole system of scientific publishing is dependent on the review process. The everlasting problem of this and other journals is to get reviewers and then the timely reviews. Looking at the number of publications processed at any time, even in a “narrow” field like ours, we can imagine how many reviewers are needed. Most potential reviewers are busy, active researchers who are aware of the importance of the reviews, but are unable to respond to the requests. If a review takes one day, how many reviews per year can one accept? And, when it comes to selection, why not do a review for a high-ranking journal in which you want to publish your own work. Journals like ours therefore depend on a strong community interested in ensuring that their research field vivid and recognised. Small focused journals provide such a platform. I am asking you all to consider your own role in the karst community and accept the review from our and other karst science journals. This will keep us well and alive, and your work published and seen as well.
Polgári Márta, Gyollai Ildikó, Bérczi Szaniszló
et al.
Evolution of chondritic parent body is influenced by thermal, impact metamorphism and aqueous alteration, studied in Mező-Madaras, Knyahinya, Mócs and Nyírábrány in aspect of high resolution in situ textural, mineralogical and organic geochemical characteristics, using optical microscopy, FTIR-ATR and Raman spectroscopy. Our observations focused on Fe-containing opaque grains, glass, olivines and pyroxenes, which were well populated by micrometer-sized microbial filamentous elements in their boundary region within matrix and inside the minerals resembling mineralized microbially produced textures (MMPT), affecting 70-80 vol% of samples. In MMPT iron oxides (ferrihydrite, goethite), olivine, montmorillonite, kandite minerals and various hydrocarbon compounds were identified.
Abstract The formation of dominant channel has restricted the improvement in oil recovery in heterogeneous thick reservoirs greatly. How to effectively solve the problem has become a key issue in the development process. In this paper, firstly, the plugging effect of polymer flooding on the dominant channel is studied by visual microscopic-model displacement experiment. And then, we established a flow resistance model and simulated the effect of polymer flooding on improving dominant channel. The result shows that the present model matches the oilfield testing data greatly. And the higher the concentration, the more effective the plugging. When the polymer concentration is 2.0 g/L, the water content can reduce by 21.24% and the enhanced oil recovery can reach 13.69%. Besides, the efficiency of the reverse rhythm is higher than the positive rhythm under the same condition. This study has provided a quick and accurate assessment of the polymer flooding for the dominant channel in the heterogeneous thick reservoir.
Abstract The successful application of enhance oil recovery (EOR) technology requires an accurate understanding of the internal architecture of reservoirs and complex distribution of flow unit characteristics within different lithofacies. However, flow unit characteristics in the reservoir lithofacies of West Baram Delta Offshore, Sarawak, remain unexplored. Therefore, this paper investigates potential hydraulic flow units (HFUs) present within the five (5) identified sandstone lithofacies using thin sections, scanning electron microscopy (SEM), two-dimensional (2D) petrographic image analysis and helium porosimetery. The identified lithofacies include massive coarse-grained sandstone (MCGS), massive medium-grained sandstone (MMGS), massive fine-grained sandstone (MFGS), massive friable fine sandstone (MFFS) and massive very fine grained sandstone lithofacies (MVFGS). A plot of reservoir quality index (RQI) against normalized porosity (фz) showed five main hydraulic flow units (HFUs) within the identified lithofacies. The MFGS, MFFS and MVFGS lithofacies exhibited moderate to high matrix fractions and moderate to poor sorting, suggesting high tortuosity and hence characterized by low flow zone indicator (FZI). On the other hand, the MCGS and MMGS lithofacies exhibit very low to low matrix fractions, medium to coarse grain sizes and moderate to well sorting, signifying lower tortuosity, and hence characterized by relatively higher flow zone indicator (FZI). Thus, a hydraulic flow unit (HFU) can be formed by more than one lithofacies that exhibit similar fluid characteristic attributes, regardless of the difference in depositional environment and diagenetic processes undergone. This study concludes that HFU scattered plot can serve as a qualitative model for the characterization of flow unit characteristics in a well-comprising different lithofacies, particularly for field development plans in the West Baram Delta.
Yan-Fei Wang, Liang-Liang Rong, Long-Qing Qiu
et al.
Abstract Full tensor magnetic gradient measurements are available nowadays. These are essential for determining magnetization parameters in deep layers. Using full or partial tensor magnetic gradient measurements to determine the subsurface properties, e.g., magnetic susceptibility, is an inverse problem. Inversion using total magnetic intensity data is a traditional way. Because of difficulty in obtaining the practical full tensor magnetic gradient data, the corresponding inversion results are not so widely reported. With the development of superconducting quantum interference devices (SQUIDs), we can acquire the full tensor magnetic gradient data through field measurements. In this paper, we study the inverse problem of retrieving magnetic susceptibility with the field data using our designed low-temperature SQUIDs. The solving methodology based on sparse regularization and an alternating directions method of multipliers is established. Numerical and field data experiments are performed to show the feasibility of our algorithm.
Abstract As conventional hydrocarbon reserves have gone into depletion state, oil companies around the world have turned their attention to heavy oil reserves, which were previously overlooked due to their less prolific capability compared to conventional hydrocarbon reservoirs. Bituminous heavy oil resources are known to be plentiful in quantity and size, but not without disadvantages, in which the astronomical viscosity is a troublesome aspect to be considered in exploiting the reservoir. It is not seldom that the viscosity itself is so high that bituminous oil would appear as solid-like substance under reservoir pressure and temperature. Electromagnetic heating has long been touted as the solution to overcome viscosity barrier in exploiting bituminous heavy oil reservoirs. The introduction of heat from electromagnetic wave propagation enables more efficient well stimulation technique compared to resistive heating. However, as sophisticated as the models are, they seem to be lacking a techno-economic model to consider feasibility of the project. This mathematical presentation incorporates technical aspects of heating and EM propagation model to properly model unsteady-state temperature and heat propagation as a function of time. The model is then tested on a sample bituminous heavy oil reservoir with thinly layered production zone and it has been highly reliable to swiftly predict project feasibility of nanoparticle-assisted EM heating.
The main oil source in western Qaidam Basin is the Paleogene saline lacustrine source rocks. Traditionally, it was considered that these source rocks were characterized by low abundance and poor type of organic matter as well as limited oil generation potential, but this poor source rock condition was very inconsistent with good hydrocarbon exploration results. A previous study has just confirmed that this area develops high-quality source rocks, which is of great significance for recognizing potential of hydrocarbon resource and consolidating confidence to search large and medium-sized oil and gas fields in western Qaidam Basin. Based on fine geochemical analysis of source rocks, petrology, element geochemistry and organic geochemistry are applied to discuss development conditions of high-quality source rocks in this area. The results showed that the saline lacustrine sediments in western Qaidam Basin were deposited in relatively shallow and quiet water with relatively high salinity and low nutrient content; influenced by the drought and cold climate, the terrigenous supply of fresh water was limited, and few aquatic organisms were developed. Good preservation condition is favorable for rapid preservation of oil generating biomass (such as aquatic organisms), to form source rocks rich in hydrogen element with high soluble organic matter content; but relatively lower paleo-productivity makes organic matter abundance lower than that in the eastern saline lacustrine basins. The semi-deep lake, with relatively low salinity and abundant nutrients, is the most favorable are for development of high-quality source rocks; the source rocks have large thickness in the sedimentary depression areas, but due to high water salinity and far away from the provenance area, the water stratification was obvious, so it have moderate to high organic matter abundance. Keywords: Saline lacustrine source rocks, TOC, Major and trace elements, Biomarker, Paleo-productivity, Preservation condition, Qaidam basin
Oils, fats, and waxes, Petroleum refining. Petroleum products
So far reported unstable correlations between environmental settings and doline geometry require a revision of doline delineating methods and morphometric analyses that follows from such input data. We examine the reliability of the most accessible data sources for doline delineation, namely basic topographic maps at a scale of 1:5,000 and ortho-rectified aerial photographs (digital orthophotos) of 0.5 m resolution. Their reliability has been quantified in the Dinaric karst (Krk Island, NE Adriatic Sea). The adequacy of most commonly used morphometric parameters (viz. elongation and circularity) and their variations applied in previous studies are mathematically verified and practically tested. The simulation of an individual method behaviour performed on polygons with increasing circularity and elongation demonstrated their ranges of validity. The efficiency of a particular method was verified by the application to the real dataset of dolines. The optimal variation of an individual parameter (circularity = Pcc/Pd; elongation = ε of the best fitting ellipse) is identified and suggested for use in order to provide uniform results and increase the comparability of future morphometric analyses. Asymmetry (the distance between the centroid of doline ground plan and the centroid of the largest inscribed circle) as an additional parameter for describing doline ground plans has been developed and is proposed for further use.
Key words: karst, doline delineating, morphometric analyses, measurement errors, Krk Island.
Morfometrične analize vrtač – problem določanja oboda in računanja osnovnih parametrov
Nezanesljivost korelacij med okoljskimi dejavniki in geometrijo vrtač je spodbudila pregled do sedaj uporabljenih metod določanja oboda vrtač in primernosti morfometričnih analiz, ki sledijo iz teh podatkov. V prispevku preverjamo zanesljivost najširše dostopnih podatkovnih virov za določanje oboda vrtač, in sicer topografskih kart v merilu 1 : 5000 in ortofotov z ločljivostjo 0,5 m. Zanesljivost smo določali na primeru Dinarskega krasa (otok Krk, SV Jadransko morje). Primernost do sedaj najpogosteje uporabljenih parametrov (tj. sploščenost in krožnost) in njunih različic, ki se pojavljajo v literaturi, smo matematično preverili in preizkusili pri realnem naboru vrtač. Simulacija delovanja posamezne metode na poligonih z naraščajočo krožnostjo in sploščenostjo je pokazala njihova območja veljavnosti. Učinkovitost metod smo nato preverili še pri realnem vzorcu vrtač, pri katerem smo ugotovili, katera je najprimernejša različica posameznega parametra (krožnost = Pcc/Pd; sploščenost = ε prilegajoče elipse), in jo, da bi zagotovili večjo usklajenost in primerljivost rezultatov, predlagali za nadaljnjo rabo v prihodnjih morfometričnih analizah vrtač. Kot dodaten parameter za analizo tlorisa vrtač smo razvili in preizkusili indeks asimetrije oboda vrtač, tj.razdaljo med težiščem tlorisa vrtače in težiščem največjega včrtanega kroga.
Ključne besede: kras, določanje oboda vrtač, morfometrične analize, merske napake, otok Krk.