Discrete Element Modeling of Stick-Slip Instability and Induced Microseismicity

Using Particle Flow Code, a discrete element model is presented in this paper that allows direct modeling of stick-slip behavior in pre-existing weak planes such as joints, beddings, and faults. The model is used to simulate a biaxial sliding experiment from literature on a saw-cut specimen of Sierra granite with a single fault. The fault is represented by the smooth-joint contact model. Also, an algorithm is developed to record the stick-slip induced microseismic events along the fault. Once the results compared well with laboratory data, a parametric study was conducted to investigate the evolution of the model’s behavior due to varying factors such as resolution of the model, particle elasticity, fault coefficient of friction, fault stiffness, and normal stress. The results show a decrease in shear strength of the fault in the models with smaller particles, smaller coefficient of friction of the fault, harder fault surroundings, softer faults, and smaller normal stress on the fault. Also, a higher rate of displacement was observed for conditions resulting in smaller shear strength. An increase in b-values was observed by increasing the resolution or decreasing the normal stress on the fault, while b-values were not sensitive to changes in elasticity of the fault or its surrounding region. A larger number of recorded events were observed for the models with finer particles, smaller coefficient of friction of the fault, harder fault surroundings, harder fault, and smaller normal stress on the fault. The results suggest that it is possible for the two ends of a fault to be still while there are patches along the fault undergoing stick-slips. Such local stick-slips seem to provide a softer surrounding for their neighbor patches facilitating their subsequent stick-slips.     

A semi-analytical solution based on a numerical solution of the solute transport of a conservative and nonreactive tracer

In the evaluation of potential risk from ingestion of groundwater near an impacted site, numerical simulation of fate and transport processes of chemicals of concern is often required. If there is potential concern about multiple chemicals, numerical simulation of each chemical separately is often needed. In this paper, a semi-analytical solution is presented based on a numerical solution of the transport of a conservative and nonreactive tracer. When multiple chemicals undergoing sorption and first-order degradation need to be modeled, we can avoid performing individual numerical simulations for each chemical by applying the semi-analytical solution. Numerical test runs were conducted to verify the semi-analytical solution; simulation results reveal that the concentrations derived from the semi-analytical solution are identical to those derived from the individual numerical fate and transport model simulations. The semi-analytical solution requires steady-state flow conditions, no continuing contaminant source, and similar initial source concentration distributions.     

Suspended sediment transport and deposition over a dune: Río Paraná, Argentina

Field data from the Rio Paraná, Argentina, are used to examine patterns of suspended sediment transport over a sand dune. Measurements of three‐dimensional velocity are made with an acoustic Doppler current profiler whilst suspended sediment concentration and particle size have been quantified using a laser in situ sediment scattering transmissometer. Suspended sediment concentration and streamwise and vertical sediment flux are highest close to the bed, with an upward vertical flux over the stoss side of the dune and downward flux over the lee side. Suspended sediment concentrations are higher over the crest compared with the trough and suspended sediment is coarsest near the bed. About 17% of the suspended‐load transported over the crest is deposited in the lee side before it reaches the trough. Most of this deposited sand is coarser sediment that originates close to the bed over the crest, a result consistent with simulations based on the model of Mohrig and Smith (Water Resources Research 1996; 32: 3207–3217) for the excursion lengths of sediment dispersed in the lee side of a dune. Copyright © 2009 John Wiley & Sons, Ltd.     

The value of high-resolution nutrient monitoring: A case study of the River Frome, Dorset, UK

The River Frome was sampled at sub-daily sampling interval, with additional storm sampling, through an annual cycle. Samples were analysed for total phosphorus (TP), soluble reactive phosphorus (SRP), total oxidisable nitrogen (TON) and dissolved reactive silicon (Si). The resulting data set was artificially decimated to mimic sampling frequencies from 12 h to monthly time interval. Monthly sampling interval resulted in significant errors in the estimated annual TP and SRP load of up to 35% and 28% respectively, and the resulting data sets were insufficient to observe peaks in P concentration in response to storm events. Weekly sampling reduced the maximum percentage errors in annual load estimate to 15.4% and 6.5%. TON and silicon concentrations were less variable with changing river flow, and monthly sampling was sufficient to predict annual load estimates to within 10%. However, to investigate within-river nutrient dynamics and behaviour, it is suggested that a weekly sampling interval would be the minimum frequency required for TON and Si studies, and daily sampling would be a minimum requirement to adequately investigate phosphorus dynamics. The loss in nutrient-concentration signal due to reduced sampling interval is presented. Hysteresis in the nutrient concentration/flow relationships for all 32 storm events during the study period were modelled and seasonal patterns discussed to infer nutrient sources and behaviour. The high-resolution monitoring in this study identified, for the first time, major peaks in phosphorus concentration in winter that coincide with sudden falls in air temperature, and was associated with biofilm breakdown. This study has shown that to understand complex catchment nutrient processes, accurately quantify nutrient exports from catchments, and observe changes in water quality as a result of nutrient mitigation efforts over time, it is vital that the newly emerging field-based automated sampler/analyzer technologies begin to be deployed, to allow for routine high-resolution monitoring of our rivers in the future.     

High-resolution vertical profiles of groundwater electrical conductivity (EC) and chloride from direct-push EC logs

Elevated groundwater salinity associated with produced water, leaching from landfills or secondary salinity can degrade arable soils and potable water resources. Direct-push electrical conductivity (EC) profiling enables rapid, relatively inexpensive, high-resolution in-situ measurements of subsurface salinity, without requiring core collection or installation of groundwater wells. However, because the direct-push tool measures the bulk EC of both solid and liquid phases (EC_a), incorporation of EC_a data into regional or historical groundwater data sets requires the prediction of pore water EC (EC_w) or chloride (Cl^?) concentrations from measured EC_a. Statistical linear regression and physically based models for predicting EC_w and Cl^? from EC_a profiles were tested on a brine plume in central Saskatchewan, Canada. A linear relationship between EC_a/EC_w and porosity was more accurate for predicting EC_w and Cl^? concentrations than a power-law relationship (Archie’s Law). Despite clay contents of up to 96%, the addition of terms to account for electrical conductance in the solid phase did not improve model predictions. In the absence of porosity data, statistical linear regression models adequately predicted EC_w and Cl^? concentrations from direct-push EC_a profiles (EC_w = 5.48 EC_a + 0.78, R ² = 0.87; Cl^? = 1,978 EC_a – 1,398, R ² = 0.73). These statistical models can be used to predict EC_w in the absence of lithologic data and will be particularly useful for initial site assessments. The more accurate linear physically based model can be used to predict EC_w and Cl^? as porosity data become available and the site-specific EC_w–Cl^? relationship is determined.     

Permutation Methods for Determining the Significance of Spatial Dependence

A class of multivariate nonparametric tests for spatial dependence, Multivariate Sequential Permutation Analyses (MSPA), is developed and applied to the analysis of spatial data. These tests allow the significance level (P value) of the spatial correlation to be computed for each lag class. MSPA is shown to be related to the variogram and other measures of spatial correlation. The interrelationships of these measures of spatial dependence are discussed and the measures are applied to synthetic and real data. The resulting plot of significance level vs. lag spacing, or P-gram, provides insight into the modeling of the semivariogram and the semimADogram. Although the test clearly rejects some models of correlation, the chief value of the test is to quantify the strength of spatial correlation, and to provide evidence that spatial correlation exists     

Interfacial tension studies between Fe-Cu-Ni sulfide and halo-norilsk basalt slag system

There are a lot of mafic and ultra-mafic bodies in the earth,but only a few of them include Cu-Ni-PGE deposits.Of interest has been the formation of such deposits as Cu-Ni-PGE sulfide;especially the separa-tion of sulfide liquid from silicate melts.A lot of re-searcher[1—11]have investigated into the tectonic back-ground of Cu-Ni-PGE deposits,the mechanism of the origin and evolution of magma,and the metal element partition coefficient in sulfide liquid and silicate magma.However,the da…     

Estimation of hydraulic conductivity in an alluvial system using temperatures

Well water temperatures are often collected simultaneously with water levels; however, temperature data are generally considered only as a water quality parameter and are not utilized as an environmental tracer. In this paper, water levels and seasonal temperatures are used to estimate hydraulic conductivities in a stream-aquifer system. To demonstrate this method, temperatures and water levels are analyzed from six observation wells along an example study site, the Russian River in Sonoma County, California. The range in seasonal ground water temperatures in these wells varied from <0.2 degrees C in two wells to approximately 8 degrees C in the other four wells from June to October 2000. The temperature probes in the six wells are located at depths between 3.5 and 7.1 m relative to the river channel. Hydraulic conductivities are estimated by matching simulated ground water temperatures to the observed ground water temperatures. An anisotropy of 5 (horizontal to vertical hydraulic conductivity) generally gives the best fit to the observed temperatures. Estimated conductivities vary over an order of magnitude in the six locations analyzed. In some locations, a change in the observed temperature profile occurred during the study, most likely due to deposition of fine-grained sediment and organic matter plugging the streambed. A reasonable fit to this change in the temperature profile is obtained by decreasing the hydraulic conductivity in the simulations. This study demonstrates that seasonal ground water temperatures monitored in observation wells provide an effective means of estimating hydraulic conductivities in alluvial aquifers.     

The Khyex River landslide of November 28, 2003, Prince Rupert British Columbia Canada

On November 28, 2003, at about 00:30 PST, 35 km east of Prince Rupert in northwestern British Columbia, an extremely rapid, retrogressive liquefaction earth flow, or a clay flow-slide, severed the natural gas pipeline. As a result, Prince Rupert residents were without natural gas heat for 10 days. The landslide has a steep main scarp that is 45 m high by 345 m wide. It consists of glaciomarine sediments mantled by rubbly colluvium lying on, and against smooth bedrock of the valley wall. It covers an area of 32 ha, and displaced about 4.7 M m³ of material. This displaced material flowed up and down river over a distance of 1.7 km, blocked the river, and caused flooding upstream for a distance of 10 km. This landslide is the most recent of four large landslides that have occurred over the last four decades in glaciomarine sediments in northwestern British Columbia.