Influence of Unsaturated Soil Properties Uncertainty on Moisture Flow Modeling

The quality of a numerical modeling solution of moisture flow through unsaturated soil, in part, depends on properly described unsaturated soil properties. The variability of the Soil Water Characteristic Curve, SWCC, is attributed to hysteresis and reproducibility of measurement. Because the unsaturated conductivity function is rarely directly measured, the variability of the unsaturated soil hydraulic conductivity function is attributed to the uncertainty associated with the estimation of this parameter with currently available fitting functions, and hence a range of reasonable variation was considered. One-dimensional modeling of expansive soil under dry initial conditions (suction of 1,500 kPa) was performed; both potential evaporation and infiltration boundary conditions were considered. It was found that small variations in the unsaturated soil hydraulic conductivity function result in significantly different modeling outputs, as expected, while substential variation in SWCC alone (assuming the same unsaturated soil hydraulic conductivity for all SWCCs) produced almost identical soil response in terms of soil suction when the slope of the SWCC is similiar. Thus, proper characterization of the slope of the SWCC is important to proper suction profile determination.     

The relative importance of buffering and brine inputs in controlling the abundance of Na and Ca in sedimentary formation waters

The concentration of Ca in the formation waters of petroleum reservoirs can play a major role in influencing the outcome of a number of processes that are of great significance to the oil industry. For example, formation water Ca concentration affects the risk of carbonate scale formation during production. In order to better understand the concentrations of Ca in formation waters, we have investigated the chemistries of formation waters from a range of onshore and offshore basins worldwide, using published sources, as well as unpublished data held by BP. Although calcium and sodium are the principal cations in almost all formation waters they vary enormously in their relative proportions. We have identified three distinct trends on a plot of X_Ca (Ca/(Na + Ca)) against Cl. Most data lie on a high-Ca trend, here termed Trend 1, and show an increase in X_Ca with salinity. We interpret this as tracking equilibration with Ca and Na-bearing minerals, with the ratio (mol Ca/mol Na²) remaining approximately constant irrespective of salinity for chloride-dominated fluids. At very high salinities, Br-enriched bittern brines that have taken part in dolomitisation lie at the Cl-rich end of this trend. Some brines remain Na-dominated up to very high salinities and define a distinct low-Ca trend, Trend 2. These are associated with dissolution of halite beds and are interpreted to arise when the amount of Na in the pore fluid greatly exceeds the amount of Ca available in minerals. We refer to such brines as mass-limited; the sparsity of Ca in the rock-fluid system constrains X_Ca to a low value. Remarkably few brines lie between these trends. Finally, dilute formation waters show very large variations in X_Ca and may have bicarbonate as the dominant anion. They define a distinct low-Cl trend, Trend 3. We conclude that the behaviour of Na and Ca in most formation waters reflects equilibration with minerals, and concentrations of Ca in solution are sensitive to pH and P_CO2 as well as to chloride concentration. For some brines however, the amount of salts in solution is sufficient to overwhelm the buffering capacity of the wallrocks.     

Foundation design considerations for a pile supported critical facility

A large reinforced concrete structure supported on piles extending to bedrock is evaluated using a seismic soil–structure interaction analysis. The physical structure, the supporting pile system, analysis model and analytical methodology used are described. Important considerations in the design of the foundation pile system using the seismic analysis loadings and deformations are discussed.     

A recharge model for high altitude,arid, Andean aquifers

Evidence for groundwater recharge in arid zones is mounting, despite early ideas that recharge was unlikely where evaporation greatly exceeded precipitation. The mechanisms and magnitude of groundwater recharge in the Andes and Atacama Desert are not well known but the subject of current research. Diffuse recharge is expected to be limited to high altitude areas with coarse‐grained soils devoid of vegetation. A recharge model for this environment is developed based on a simple soil moisture budgeting technique and the calculation of actual evaporation based on empirical studies. The model is run with data for the Linzor basins, over 4000 m elevation at 22·2°S on the west slope of the Andes. It is checked against independent estimates based on the chloride mass balance (CMB) method and flood events measured downstream in the Río Salado and found to provide robust and reliable results. The results indicate that irregular and volumetrically limited amounts of diffuse recharge occur at high elevations in half of all years, with a tendency to cluster during La Niña episodes. For the Linzor Basins, mean annual recharge is found to be equivalent to 28 mm a^?1, although no recharge occurs in years with precipitation less than 120 mm, and increases proportionately with annual rainfall amounts above this limit. Copyright © 2009 John Wiley & Sons, Ltd.     

Quantitative phosphorus measurement: A field test procedure for archaeological site analysis at Piedras Negras,Guatemala

Currently there is a wide interest in the use of chemical analyses for the evaluation of anthropogenically altered soils and other archaeological deposits. Because soil phosphorus levels increase in areas of human habitation, and leave a permanent signature that can only be removed by erosion of the soil itself, phosphorus mapping has become a popular field procedure to indicate areas of habitation where overt evidence of ancient occupance is absent. We have developed a methodology to obtain accurate acid‐extractable phosphorus concentrations (mg/kg) in calcareous soils under the primitive field conditions of Piedras Negras, Guatemala. Predicated on Mehlich‐II acid extractant and colorimetric methods, this procedure processed 36 samples per hour at very low cost per sample. Based on eight replicate measurements of a group of samples, the coefficient of variation of the procedure was 8.3%. Subsequent analysis of 35 soil samples in a controlled laboratory revealed a moderate correlation of 0.44 between the Mehlich‐extractable phosphorus and total phosphorus. The correlation was 0.91 between the Mehlich procedure and Olsen bicarbonate extractable phosphorus, indicating that Mehlich‐based results are similar to those obtainable using a traditional extractable phosphorus method on soils of neutral to alkaline pH. There was a moderate correlation between Mehlich P and ring‐test rating (r = 0.42). The wider dynamic range of the Mehlich extraction, coupled with the use of a battery‐operated colorimeter, facilitated the finding of a refuse midden within an area of phosphate enriched soils. Further tests indicated that phosphorus concentrations measured in the field deviated by only 7% from those made under controlled laboratory conditions. © 2000 John Wiley & Sons, Inc.     

The cheyenne belt: analysis of a proterozoic suture in Southern Wyoming

The Cheyenne belt of southeastern Wyoming is a major shear zone which separates Archean rocks of the Wyoming province to the north from 1800-1600 Ma old eugeoclinal gneisses to the south. Miogeoclinal rocks (2500-2000 Ma old) unconformably overlie Archean basement immediately north of the shear zone and were deposited under transgressive conditions along a rift-formed continental margin. Intrusive tholeiitic sills and dikes are interpreted as rift-related intrusions and a date of 2000 Ma on a felsic differentiate of these intrusions gives the approximate age of rifting. There are no known post-2000 Ma felsic intrusions north of the Cheyenne belt.Volcanogenic gneisses and abundant syntectonic calc-alkaline plutons of the southern terrane are interpreted as island are volcanic and plutonic rocks. The volcanics are a bimodal basalt-rhyolite assemblage. Plutons include large gabbroic complexes and quartz diorite (1780 Ma), syntectonic granitoids (1730-1630 Ma) and post-tectonic anorthosite and granite (1400 Ma). There is no evidence for Archean crust south of the Cheyenne belt.Structural data (thrusts in the miogeoclinal rocks, vertical stretching lineations, and the same fold geometries north and south of the shear zone) suggest that juxtaposition of the two terranes took place by thrusting of the southern terrane (island arc) over the northern terrane (craton and miogeocline), probably as a continuation of the south-dipping subduction which generated calc-alkaline plutons of the southern terrane. A metamorphic discontinuity across the shear zone, with greenschist facies rocks to the north and upper amphibolite facies rocks and migmatites to the south, also suggests thrusting of the southern terrane (deeper crustal levels) over the northern terrane (shallower levels).The Cheyenne belt may be a deeply-eroded master decollement, perhaps analogous to a ramp in the master decollement in the southern Appalachians. This interpretation of the Cheyenne belt as a Proterozoic suture zone provides an explanation for the geologic, geochronologic, geophysical, metallogenic, and metamorphic discontinuities across the shear zone.     

The 1986 Kermadec earthquake and its relation to plate segmentation

To evaluate the tectonic significance of the October 20, 1986 Kermadec earthquake (M _w =7.7), we performed a comprehensive analysis of source parameters using surface waves, body waves, and relocated aftershocks. Amplitude and phase spectra from up to 93 Rayleigh waves were inverted for centroid time, depth, and moment tensor in a two-step algorithm. In some of the inversions, the time function was parameterized to include information from the body-wave time function. The resulting source parameters were stable with respect to variations in the velocity and attenuation models assumed, the parameterization of the time function, and the set of Rayleigh waves included. The surface wave focal mechanism derived (=275°, =61°, =156°) is an oblique-compressional mechanism that is not easy to interpret in terms of subduction tectonics. A seismic moment of 4.5×10²⁰ N-m, a centroid depth of 45±5 km, and a centroid time of 13±3 s were obtained. Directivity was not resolvable from the surface waves. The short source duration is in significant contrast to many large earthquakes.We performed a simultaneous inversion ofP andSH body waves for focal mechanism and time function. The focal mechanism agreed roughly with the surface wave mechanism. Multiple focal mechanisms remain a possibility, but could not be resolved. The body waves indicate a short duration of slip (15 to 20 s), with secondary moment release 60s later. Seismically radiated energy was computed from the body-wave source spectrum. The stress drop computed from the seismic energy is about 30 bars. Sixty aftershocks that occurred within three months of the mainshock were relocated using the method of Joint Hypocentral Determination (JHD). Most of the aftershocks have underthrusting focal mechanisms and appear to represent triggered slip on the main thrust interface. The depth, relatively high stress drop, short duration of slip, and paucity of true aftershocks are consistent with intraplate faulting within the downgoing plate. Although it is not clear on which nodal plane slip occurred, several factors favor the roughly E-W trending plane. The event occurred near a major segmentation in the downgoing plate at depth, near a bend in the trench, and near a right-lateral offset of the volcanic are by 80 km along an E-W direction. Also, all events in the region from 1977 to 1991 with CMT focal mechanisms similar to that of the Mainshock occurred near the mainshock epicenter, rather than forming an elongate zone parallel to the trench as did the aftershock activity. We interpret this event as part of the process of segmentation or tearing of the subducting slab. This segmentation appears to be related to the subduction of the Louisville Ridge, which may act as an obstacle to subduction through its buoyancy.     

Modeling of storm-induced coastal flooding for emergency management

This paper describes a model package that simulates coastal flooding resulting from storm surge and waves generated by tropical cyclones. The package consists of four component models implemented at three levels of nested geographic regions, namely, ocean, coastal, and nearshore. The operation is automated through a preprocessor that prepares the computational grids and input atmospheric conditions and manages the data transfer between components. The third generation spectral wave model WAM and a nonlinear long-wave model calculate respectively the wave conditions and storm surge over the ocean region. The simulation results define the water levels and boundary conditions for the model SWAN to transform the storm waves in coastal regions. The storm surge and local tides define the water level in each nearshore region, where a Boussinesq model uses the wave spectra output from SWAN to simulate the surf-zone processes and runup along the coastline. The package is applied to hindcast the coastal flooding caused by Hurricanes Iwa and Iniki, which hit the Hawaiian Island of Kauai in 1982 and 1992, respectively. The model results indicate good agreement with the storm-water levels and overwash debris lines recorded during and after the events, demonstrating the capability of the model package as a forecast tool for emergency management.