Numerical simulation of hydro-mechanical constraints on the geometry of a critically tapered accretionary wedge

송인선,고희재 한국지질과학협의회 2020 Geosciences Journal Vol.24 No.3

A critically tapered active accretionary wedge was simulated using a numerical analysis of plastic slip-line theory to understand the mechanics of morphologic evolution. The concept of critical state soil mechanics was applied to describe the entire wedge area overlying a basal décollement fault. Presuming a condition of two-dimensional plane strain along the compressional direction, we obtained the numerical solution of conjugate plastic slip lines at a critical state of stress defined by the Coulomb yield criterion. The velocity vectors were obtained by applying the associate flow rule with the boundary conditions at the upper surface of the wedge. Finally, the detachment was determined from the effective stress condition inside the wedge and the sliding friction coefficient along the fault. Our numerical simulations demonstrate that the morphology of a critically tapered wedge is dependent on the frictional strengths of both the wedge materials and the basal fault. The critical taper angle decreases with increasing internal friction angle and decreasing basal friction coefficient. The results also revealed that the pore pressure controls the morphology of the accretionary wedge for cohesive sediments but not for non-cohesive materials. The effect of pore pressure on the morphology of a critically tapered accretionary wedge becomes more significant as the cohesion increases. Assuming that the cohesion is very low, we could infer the ranges of strengths that most observed wedge geometry data have 0.3–0.6 for the basal friction coefficient and ~35–45° for the internal friction angle of the wedge materials.

Groundwater supply under land subsidence constrains in the Nobi Plain

Adrian H. Gallardo,Atsunao Marui,Shinji Takeda,Fumio Okuda 한국지질과학협의회 2009 Geosciences Journal Vol.13 No.2

Groundwater overdraft resulted in land subsidence throughout the Nobi Plain, central Japan. To cope with the growing water demand in the region, a numerical model was used to determine the maximum withdrawal capacity of two confined aquifers without causing undesirable consequences. Results were validated against field data and by analytical solutions. The analysis focused in Aburashima, a site expected to experience a rapid development in the forthcoming years. Calculations showed the water availability in the upper aquifer is limited. Moreover, seasonal fluctuations in heads reduce its extraction capacity up to 44%. In contrast, storage is substantially higher in the deep aquifer. Larger quantities and lower extraction costs make this layer a more reliable source for water supply. Findings from this study will be used by authorities to update the current legislation on groundwater abstraction. Nevertheless, it is argued that to achieve a long-term sustainability, policies should not limit solely to control regulations but also to economical strategies and the expansion of the infrastructure system.

Seismic noise level variation in South Korea

신동훈,Jin Soo Shin,강태섭 한국지질과학협의회 2009 Geosciences Journal Vol.13 No.2

Variations of seismic background noise in South Korea have been investigated using power spectral analysis. We have estimated the power spectral density of seismic noise for 30 broadband stations from 2005 to 2007. In the frequency range 1–5 Hz, diurnal variations of noise level and daily differences in daytime noise level are observed at most stations, suggesting that cultural activities contribute to the noise level of a station. The variation in the number of stations detecting an event, however, suggests that cultural noise has little influence on the detection capability of events over magnitude 2.0. In the frequency range 0.1–0.5 Hz, a double-frequency (DF) peak is dominant at all stations. Clear seasonal variations of peaks are observed, while much less shown in the frequency range 1–5 Hz. DF noise levels are higher in winter than in summer. Strong DF peaks occur in summer when Pacific typhoons near the Korean Peninsula. The discrepancy in time between DF peaks at seismic stations and peaks of significant wave height at buoys, as well as the decrease of DF peaks with increased latitude, indicate that the likely source region of DF peaks is located in the Southern Sea of Korea.

Characteristics of discontinuity spacing of Yeongdeok granite

Krishna C. Devkota,Jung-Eun Ham,김교원 한국지질과학협의회 2009 Geosciences Journal Vol.13 No.2

Discontinuities are major geological features in the rock mass. They can be distributed evenly, randomly, clustered or in combinations of all these. Discontinuity spacing is one of the important parameters in describing the quality of a complete rock mass. Discontinuity surveys are carried out in the field by examining recovered drill-core to estimate possible distributions for discontinuity geometry parameters of rock masses. Discontinuity spacing and frequency data obtained at a field site in eastern Korea were examined by plotting histograms for the two parameters. The examination clearly showed that discontinuity spacing follows negative exponential distribution. A relationship has been established between mean discontinuity frequency and Rock Quality Designation (RQD) as RQDt =100e-λt(1+λt), where t refers threshold value, e is a mathematical constant and λ is an average number of discontinuities per meter. This equation enables the utilization of different threshold values of t in addition to the conventional value of 0.10 m. It helps to characterize a rock mass in terms of RQD. Therefore this study showed that the conventional threshold value is not always sensitive to study discontinuity spacing in rock masses.

Modeling of water flow and heat transport in the vadose zone: Numerical demonstration of variability of local groundwater recharge in response to monsoon rainfall in Korea

구민호,김용제 한국지질과학협의회 2008 Geosciences Journal Vol.12 No.2

The rainfall of Korea in the summer monsoon period occupies more than 50% of the annual precipitation in most areas, and thus groundwater recharge to shallow aquifers is dominantly controlled by the amount and the pattern of monsoon precipitation. This paper presents two numerical models that demonstrate linear relationships between precipitation and recharge. First, a simple heat transport model employing a lumped parameter approach is presented for estimating two lumped parameters related to water flux and thermal diffusivity in the vadose zone. The model determines the parameters by a simple optimization process that minimizes the root-mean-square error between simulated and measured temperatures. The model is applied to 22-year time series data of soil temperatures measured at a synoptic station of Korea. The impact of monsoon precipitation on the thermal regime is clearly reflected in the simulated results by illustrating a linear relationship between precipitation and the water flux in the vadose zone. Secondly, an infiltration model is presented for analyzing variability of precipitation recharge in relation to the monsoon rainfall. The model simulates the unsaturated flow from time series data of precipitation and pan evaporation, assuming immediate removal of surface ponding, a linear relationship between the evaporation rate and the soil water content, and a static water table. Numerical simulations were performed for three soil textural groups by using 20-year meteorological data. The results demonstrate that the annual recharge is linearly proportional to the annual precipitation with varying degrees of the correlation coefficient depending on soil types. Sensitivity analyses show that the uncertainties in evaporation-related model parameters significantly affect the model results with controlling tradeoff between recharge and evaporation estimates.

Estimation of river stage effect on groundwater level, discharge, and bank storage and its field application

하규철,고동찬,염병우,이강근 한국지질과학협의회 2008 Geosciences Journal Vol.12 No.2

This paper presents the hydraulic interaction between aquifer and river related with bank storage. A convolution equation was used to analyze the aquifer response, discharge, and bank storage due to river stage fluctuation. River stages were generated to describe flood peak, asymmetry, and duration. The simulation results suggest that the geometry of flood hydrograph can play an important role in affecting the discharge and bank storage. The presented model was applied to a field data, and the site is a floodplain aquifer of the Mangyeong River. Aquifer responses for the various flood duration and flood peak were observed at the site, which is well matched with the simulated results. A model parameter, the riverbed leakance was determined from calibration with the simulated and observed groundwater levels. After the determination of each parameter, the discharge and bank storage can be calculated. The presented method is a good tool to assess the hydraulic interaction between the aquifer and the river, as arbitrary river stage fluctuations are used. The limitation of the method is that the actual discharge and bank storage can be different from the calculated ones, when significant regional groundwater gradient exists. It also can be a useful tool before applying more complex numerical models.

Assessments of natural and anthropogenic controls on the spatial distribution of stream water quality in Southeastern Korea

Kyung-Seok Ko,Jin-Soo Lee,Jae-Gon Kim,Jeonghoon Lee 한국지질과학협의회 2009 Geosciences Journal Vol.13 No.2

Hydrogeochemical and geostatistical analyses were conducted to evaluate natural and anthropogenic effects using stream water data of Southeastern Korea acquired through the Hydrogeochemical Survey Program of the Korea Institute of Geoscience & Mineral Resources. The results of a variographic analysis of the spatial structure of stream water quality show that the semivarigrams of Ca, Mg, and HCO₃have similar patterns and regional distributions, reflecting the same geogenic source of Paleozoic, Tertiary, and Cretaceous sedimentary rocks. Although Ca and Mg originated from Paleozoic sedimentary rocks, the distribution of Mg is significantly different from that of Ca due to the presence of dolomite in northeastern area of Korea, which enhances the Mg concentration in stream water. Studies of mineralization and acid mine drainage resulting from previous mining activities reveal a high Ba in sedimentary rock from of Kyongsang basin, as well as anomalous concentration of Al, Fe, and SO₄in the Ogcheon and Tabaeksan mineralization areas. Saturation indices of barite determined using average Ba and SO₄concentrations in stream water suggest the existence of barite mineralization deposits in sedimentary rock of Kyongsang Group. Na, Cl, K, NO₃, and Cl, exhibit effects of anthropogenic and marine sources in the stream water, with increasing values near residential, agricultural, and coastal areas. Cokriging analyses employing Ca-HCO₃, Ca-Sr, Ba-SO₄, and Na-Cl also revealed the nature of bedrock geology, mineralization, and anthropogenic and marine sources.

Lessons from practice in the assessment and remediation of contaminated ground water

Franklin W. Schwartz,Eung Seok Lee,Yongje Kim 한국지질과학협의회 2008 Geosciences Journal Vol.12 No.2

The famous American humorist Mark Twain once wrote “don’t let school interfere with your education”. This paper builds on this theme by examining important lessons that come from work on practical problems of ground-water contamination and remediation in Canada and the United States. We draw lessons from the interesting features of the studies, and mistakes in execution, which have had important consequences. The first case study from Edmonton, Alberta Canada examines a problem of ground-water contamination due to leakage of water contaminated by 2,4-D and other contaminants from a small storage pond. This study highlights the problems related to an inadequate geologic understanding of a site, misunderstandings concerning the advantages and limitations of key tools for site investigation, and how projects can benefit from early and ongoing peer reviews. The second case study examines a problem of sewage contamination related to a deep tunnel system in Milwaukee Wisconsin. This system is designed to store surface-water overflows from an old, combined sewer system. This case study highlights the difficulties in working on unique problems without an effective conceptual hydrogeologic model, the need to always be concerned about the quality of chemical data, and the necessity of being alert to behaviors beyond typical experience. The lessons coming from these case studies have important implications for remedial work being undertaken in Korea and regulatory agencies with oversight of the projects.

Salinity effects on chlorpyrifos degradation and phosphorus fractionation in reclaimed coastal tideland soils

Eui-Yong Yun,노희명,Goon-Taek Lee,최우정 한국지질과학협의회 2010 Geosciences Journal Vol.14 No.4

We examined the effect of soil salinity on the degradation of chlorpyrifos and the residual effect of chlorpyrifos and its metabolites on soil P fractionation during 60-day aerobic incubation. A sandy loam soil (Typic Psammaquents) was collected from the Daeho reclaimed tideland and two-thirds of the soil was applied with Na salt to get three different soil salinity levels: 4.6 (low, EL), 9.7 (medium, EM), and 14.4 (high, EH) dS m–1. Estimated half-lives for chlorpyrifos degradation were 7.1 in EL, 10.0 in EM and 16.9 days in EH soils. During the degradation of chlorpyrifos in soil, microbial activity decreased by increasing soil salinity and its inhibitory effect increased with time. In contrast, the addition of chlorpyrifos did not inhibit soil alkaline phosphatase (SAP) activity, which was higher in EH than in control soils. Chlorpyrifos added at a rate of 5.0 mg a.i. kg–1 dry soil did not affect the distribution pattern of P fractions in control soils. Both an increase in soil salinity and soil sterilization increased the Ca-bound P fraction and decreased the occluded Fe + Al-bound P fraction with a significant interaction between soil salinity and sterilization. With time, the Ca-bound P fraction increased and organic- and occluded Fe + Al-bound P fractions decreased, while total-P, available-P, and non-occluded + adsorbed P fraction remained unchanged. Particularly, organic-P was mineralized more in EH than in control soils and the Ca-bound P fraction contained the highest inorganic P released. Mineralization of organic P and partitioning of released P in the recalcitrant Ca-bound P fraction increased by increasing soil salinity, while available P fraction remained unchanged, suggesting that the addition of chlorpyrifos at the currently recommended dosage level did not seem to considerably affect the available P fraction with low P leaching potential to waterways.

Towards a new potential field theory of fractal objects

Mostafa E. Mostafa 한국지질과학협의회 2010 Geosciences Journal Vol.14 No.4

The Potential Field Anomaly (PFA) data of the self similar Fractal Objects (FOs) include gravity and magnetic fields and potentials along with the related derivatives. These elements are calculated on grids due to buried FOs at different fractal orders. The objects have variable physical property distributions; while in magnetic, the orientation and magnitude of polarization or earth magnetic field is arbitrary. Using the structural index as Universal Fractal Order Invariant Measure, one of the contributions of this work is expressing the elements of the PFA data at any measuring point on a grid as geometric sequences in terms of the fractal order. We found that the common ratio of the sequences is equal to the Fractal Mass Ratio (FMR), a physical quantity characterizing the object. Therefore, we can interpolate the PFA data backward or forward from one fractal order to the other. This in turn allows us to directly calculate PFA data of FOs from the zero order objects equivalent to the solid sources or initiators. We conclude that the patterns of PFA data due to a self similar FO are scale-invariant and reflect the nature of this object. We express the FMR of a FO in a new equation describing the difference between the topological and fractal dimensions in terms of a linear scale.