Book reviews

Natural Resources Research -     

Strontium isotopic equilibration during metamorphism of tillites from the Ogcheon Belt,South Korea

The NE/SW trending Ogcheon Belt formed during the Mesozoic by deformation and metamorphism of a sedimentary sequence which includes tillite formations of late Precambrian or Lower Palaeozoic age. In this study Rb-Sr isotopic data are reported from cm-scale sampling of clasts and matrix in these tillites, which underwent metamorphism at grades ranging from the biotite zone to the kyanite zone. Sr-isotopic disequilibrium between clasts and matrix persists up to the highest grade but in four out of five rocks the matrix samples approximate to isochrons (MSWD's<12). The mechanism for this selective isotopic equilibration is considered to be related to the metamorphic reactions in the matrix, which were probably accompanied by a substantial fluid flow. The matrix isochrons for three of the rocks, together with separated white micas from associated schists, suggest that metamorphism took place in the Trias, approximately 200 Myr ago. Metamorphism was followed by slow cooling and biotite closure temperatures were not reached until 190-150 Myr. — These results suggest that where high fluid/rock ratios can be demonstrated on independent petrological grounds, the analysis of small-scale whole-rock samples may allow effective Rb/Sr dating of metamorphism.     

Thermo-mechanical Stability Analysis for a Multi-level Radioactive Waste Disposal Concept

To dispose of the spent fuels generated from the Korean nuclear power plants in an underground repository, a large area of about 4 km² is required. This could be a constraint for selecting an adequate repository site and it is required to investigate the possibility of a multi-level repository design. In this study different parameters related to the multi-level repository design such as the level distance, waste type disposed of at each level, and the time interval between the operations at the levels, were investigated using the three-dimensional code, FLAC3D. For obtaining more reliable results, rock properties measured from deep boreholes were used. From the analysis, it was possible to conclude that a multi-level repository concept could be an attractive alternative to reduce the underground area as well as to dispose of the spent fuels from Pressurized Water Reactors (PWR) and Canadian Deuterium Uranium (CANDU) reactors at different levels with different time schedules.     

Sequential tracer tests for determining water seepage paths in a large rockfill dam, Nakdong River basin, Korea

The rockfill dam of this study has a clay core, filter zone, and sandy gravel shell, and it is located in the Nakdong River basin in Korea. Filling the reservoir began in August 1994 and it was full by April 1998. When the reservoir level was approximately 150 m, three settlement holes were found in the dam crest. To determine possible seepage paths and potential damaged areas within the rockfill dam, 15 tracer tests were performed during two periods. During the two test periods, six and nine tracer tests were conducted when the average water levels of the reservoir were 145 m and 142 m, respectively. Rhodamine WT (RWT) and bromide ion were used as tracers. For each test, 1800 to 4000 L of tracer solution were injected into the riprap for about 5 h. The 15 injection points were uniformly distributed from the right to left sides of the embankment and the two tracers were injected alternately. Tracer concentrations were monitored at 26 observation wells, which were divided into four groups by their geographical positions: right side of the crest, left side of the crest, right side of the dam toe, and left side of the dam toe. For each tracer test, more than 30 water samples were taken during a period of 96 h at each observation well. Based the breakthrough curves obtained from the sequential tracer tests, it was inferred that the most probable seepage paths and potential damaged area were in the left side of the crest (tongue wing zone) and, to a lesser extent, in the right side of the dam abutment.     

Efficient spoofing identification using baseline vector information of multiple receivers

In a spoofing environment, a Global Navigation Satellite System (GNSS) receiver must employ anti-spoofing techniques for obtaining a normal navigation solution from the GNSS signal. We propose a new method for identifying spoofing signals using the norm of the difference of baseline vectors (NDB) obtained from multiple receivers. The main focuses of this research are to reduce the initial time required to identify the spoofing signal and to mitigate the physical constraints on multiple antennas placement. First, the multi-correlators of each receiver track both GNSS and spoofing signals simultaneously and classify them into two signal groups. Then, the baseline vectors are generated from the double-differenced carrier phase measurements of the classified signal groups, and the NDB is calculated. If the target positions of the spoofing signal groups are almost the same, the NDB has a fairly small value when the base position of the selected baseline vectors is calculated from one of the GNSS groups and the rover positions of the baseline vectors are calculated from each spoofing group of the multiple receivers. Using the NDB, a hypothesis is established, and a hypothesis test is conducted for identifying the spoofing signal. The performance of the proposed test statistics is analyzed with respect to the distance between the GPS antennas and the tuning parameter. Our experimental results show that the proposed method effectively performs spoofing identification with a short baseline. Additionally, the method exhibits a very low probability of fault detection and fast response time. This means that the immediate anti-spoofing can work properly in spoofing environments.     

Predicting future urban impervious surface distribution using cellular automata and regression analysis

Urban impervious surfaces are considered as key indicator of urbanization intensity and environmental quality. Due to their significant impact on surface runoff, flood frequency, and water quality, impervious surfaces have been identified as an important indicator for examining the hydrological impact of urbanization. The amount and distribution of impervious surfaces have been estimated using remote sensing and geographic information system (GIS) techniques. Little research, however, has been conducted to predict future impervious surface distributions. To address this problem, we developed an integrated residential/commercial growth and impervious surface distribution model to predict urban impervious surface distribution. Taking Milwaukee River Basin, Wisconsin as a case study, we simulated future residential and commercial developments using a CA model. Further, we developed a linear regression model to predict impervious surface distributions in residential and commercial land uses. Analysis of results suggests that the proposed model performs significantly better than the traditional approaches.     

Influence of hydraulic characteristics on stability of unsaturated slope under transient seepage conditions

This paper examines the current procedure for determining the soil-water characteristic curve (SWCC) model with a particular focus on its application to slope stability analysis under transient unsaturated seepage conditions. A series of laboratory experiments was performed to determine the SWCC of different soils, ranging from high plasticity clay to silty sand, found across the Korean Peninsula. The experimental results were utilized to identify the suitable SWCC model for each soil type based on the fitting criterion. Also, this paper developed a numerical framework for infinite slope stability analysis under transient unsaturated seepage conditions. The significant advantage of the proposed framework, from the practical viewpoint, is to directly predict the timing of failure and potential failure plane based on rainfall recording. The effect of choice of SWCC models on predictability in stability analysis was evaluated by adopting the present framework along with the identified SWCC models. Furthermore, a case study of landslides after a 3-month rainfall in Pohang, Korea, was revisited to assess the performance of the proposed framework. The obtained results demonstrate the significant role of SWCC model on the results of slope stability analysis. The analysis using the SWCC model satisfying the fitting criterion could still not capture the real behavior of unsaturated soil. The comprehensive transient analysis is strongly suggested as a complementary means to the current fitting criterion for determining the suitable SWCC model for stability analysis under transient seepage conditions.     

The characteristics of the Mocoa compound disaster event,Colombia

A rainfall-induced compound disaster happened in Mocoa in the pre-dawn hours of 1 April 2017. More than 300 people were killed, and a large number of houses and roads were destroyed in the worst catastrophe in the history of Mocoa. To investigate this disaster, a detailed interpretation was carried out using high-resolution images. Analysis of disaster characteristics based on satellite image revealed that the disaster could be identified as a consequence of compound mountain hazards including landslides, debris flows, and mountain torrents. The mountain hazards converged in the mountain watershed, which amplified the disaster’s effects. Analysis considers that this disaster is the result of heavy rainfalls. Moreover, in-depth interpretation of rainfall data and satellite images spanning over 16 years reveals that the previous El Niño event (2014–2016) also played an important role, which caused reduced rainfall and vegetation coverage. The long period of drought brought by El Niño affected the growth of vegetation and reduced the ability of vegetation to cope with heavy rainfalls. The results reveal that both antecedent rainfalls and climate impact need to be taken into consideration for mountain hazard analysis.