Reconsidering the physics of the Chesapeake Bay estuarine turbidity maximum

A series of cruises was carried out in the estuarine turbidity maximum (ETM) region of Chesapeake Bay in 1996 to examine physical and biological variability and dynamics. A large flood event in late January shifted the salinity structure of the upper Bay towards that of a salt wedge, but most of the massive sediment load delivered by the Susquehanna River appeared to bypass the ETM zone. In contrast, suspended sediments delivered during a flood event in late October were trapped very efficiently in the ETM. The difference in sediment trapping appeared to be due to increases in particle settling speed from January to October, suggesting that the fate of sediments delivered during large events may depend on the season in which they occur. The ETM roughly tracked the limit of salt (defined as the intersection of the 1 psu isohaline with the bottom) throughout the year, but it was often separated significantly from the limit of salt with the direction of separation unrelated to the phase of the tide. This was due to a lag of ETM sediment resuspension and transport behind rapid meteorologically induced or river flow induced motion of the salt limit. Examination of detailed time series of salt, suspended sediment, and velocity collected near the limit of salt, combined with other indications, led to the conclusion that the convergence of the estuarine circulation at the limit of salt is not the primary mechanism of particle trapping in the Chesapeake Bay ETM. This convergence and its associated salinity structure contribute to strong tidal asymmetries in sediment resuspension and transport that collect and maintain a resuspendable pool of rapidly settling particles near the salt limit. Without tidal resuspension and transport, the ETM would either not exist or be greatly weakened. In spite of this repeated resuspension, sedimentation is the ultimate fate of most terrigenous material delivered to the Chesapeake Bay ETM. Sedimentation rates in the ETM channel are at least an order of magnitude greater than on the adjacent shoals, probably due to focusing mechanisms that are poorly understood.     

Oil- and gas-resource assessment in certain South American Basins—An application of ARDS (Ver. 5.0) to complex exploration and discovery histories

The modified Arps-Roberts Discovery Process Modeling System [ARDS (Ver. 4.01)] has recently been upgraded [ARDS (Ver. 5.0)] and applied to a wide variety of field discovery and wildcat drilling data with differing characteristics. ARDS is designed to forecast the number and sizes of undiscovered fields in an exploration play or basin by using historical drilling and discovery data. Fields used as input may be grown or ungrown. Two models for field growth—one offshore and the other onshore—have been implemented (Schuenemeyer and Drew, 1996). Uncertainty attributable to field growth is estimated via simulation. This upgrade of ARDS has been designed to handle situations when the data cannot be partitioned into homogeneous regions, but where estimation of the number of remaining oil and gas fields is still meaningful. In this upgrade of ARDS, many restrictions, which include those on the number of fields and wildcat wells required to forecast the size distribution of the oil and gas fields that remain to be discovered in an exploration play, a basin, or other target area, have been removed. In addition, flexibility has been gained by reforming the criteria for convergence of the model. In all, 32 basins and subbasins in South America were examined, 18 of which had sufficient data to be amenable to forecasting the field-size distribution of undiscovered oil and gas resources directly by using the Petroconsultants Inc. (1993) field discovery and wildcat drilling data. Overall, ARDS (Ver. 5.0) performed well in estimating the field-size distribution of undiscovered oil and gas resources in the 18 basins and subbasins. The aggregate volume of undiscovered petroleum resources was characterized by using histograms of the distribution of resources and the following five statistics: the mean, the 80% trimmed mean, and the 10,50 (median), and 90 quantiles. More than 38 billion barrels of oil equivalent (BOE) in fields that contain more than one million BOE individually were forecast as remaining to be discovered. The largest basin, the Campos (Brazil), is forecast to contain nearly 10 billion BOE undiscovered resources. The East Venezuela Basin (excluding the Furrial Trend) is forecast to contain about 8 billion BOE; the Austral-Magallanes Basin (Argentina and Chile), about 7 billion BOE; and the Napo (Colombia and Ecuador) and the Neuquen (Argentina) Basins, between 3 billion and 4 billion BOE. A subset of these basins that illustrate the increased flexibility of ARDS are discussed.     

A petroleum discovery-rate forecast revisited—The problem of field growth

A forecast of the future rates of discovery of crude oil and natural gas for the 123,027-km² Miocene/Pliocene trend in the Gulf of Mexico was made in 1980. This forecast was evaluated in 1988 by comparing two sets of data: (1) the actual versus the forecasted number of fields discovered, and (2) the actual versus the forecasted volumes of crude oil and natural gas discovered with the drilling of 1,820 wildcat wells along the trend between January 1, 1977, and December 31, 1985. The forecast specified that this level of drilling would result in the discovery of 217 fields containing 1.78 billion barrels of oil equivalent; however, 238 fields containing 3.57 billion barrels of oil equivalent were actually discovered. This underestimation is attributed to biases introduced by field growth and, to a lesser degree, the artificially low, pre-1970's price of natural gas that prevented many smaller gas fields from being brought into production at the time of their discovery; most of these fields contained less than 50 billion cubic feet of producible natural gas.     

Laboratory Demonstration of Hydrocarbon Migration in the Vadose Zone: Effectiveness of the U-Tube Design for Underground Storage Tank Leak Detection Monitoring

The experimental results derived from a laboratory model of gasoline migration from a simulated leaking underground storage tank (UST) demonstrate that the direction of migration through the simulated fill material is essentially straight down. Consequently, the U-tube design for leak detection monitoring of USTs has limited utility because that design relies on the assumption that the path of migration of the leaked fluids will follow the surface of the tank to the bottom and be intercepted by the U-tube below.     

Fumarole distribution,morphology, and encrustation mineralogy associated with the 1986 eruptive deposits of mount St. Augustine,Alaska

Numerous rootless fumaroles were developed on pyroclastic flows and a lava flow generated during the March 1986 eruptive cycle of Mount St. Augustine. Gases issued from fumarole vents with four different shapes: fissure, phreatic explosion crater, single/multiple ovoid opening, and diffuse, multiple opening. Fumarole distribution and morphology were controlled by preeruption drainage and topography, as well as by the thickness, compaction, and settling of the flow deposits. Fumarole temperatures measured in June and July 1986 ranged from 75°–394°C. Varying amounts of colorful and often roughly zoned encrustations are associated with all fumarole vent shapes. Only six types of crystalline phases were detected by X-ray diffraction, with gypsum the most abundant mineral, followed by anhydrite, sulfur, tridymite, halite, and soda alum. Scanning electron microscopy and energy dispersive X-ray analysis revealed a number of amorphous phases, mainly halogen-rich, as well as other minor crystalline phases. The mineral assemblages in the encrustations suggest formation conditions for these deposits within a general range of 25°–250°C in an oxidizing environment. Many of the amorphous phases are metastable and upon cooling of the fumarole lose nonstructural water and crystallize to more stable forms. The high halogen contents of the fumarole condensates and the mineralogy, chemistry, and morphology of the encrustations support leaching of the andesitic ash and lava flow by condensed acid vapors as the primary source for the chemical components contained in the encrustations. Comparison of traceelement (Sr, Ba, V, Co, Ni, and Cr) contents in unaltered and altered ash suggests that trace-element distribution follows a pattern of isomorphic substitution in the encrustation phases.     

Preliminary Studies of Speleothem in Central Asia

正Objective The Westerlies is important in linking the Northern high latitude,North Atlantic and East Asian monsoon.The location and intensity of the Westerly jet can not only control the climate of central Asia(including northwest arid China),but also significantly influence the climate of the East Asian monsoon region.However,it remains     

An evaluation of unsupervised and supervised learning algorithms for clustering landscape types in the United States

Knowledge of landscape type can inform cartographic generalization of hydrographic features, because landscape characteristics provide an important geographic context that affects variation in channel geometry, flow pattern, and network configuration. Landscape types are characterized by expansive spatial gradients, lacking abrupt changes between adjacent classes; and as having a limited number of outliers that might confound classification. The US Geological Survey (USGS) is exploring methods to automate generalization of features in the National Hydrography Data set (NHD), to associate specific sequences of processing operations and parameters with specific landscape characteristics, thus obviating manual selection of a unique processing strategy for every NHD watershed unit. A chronology of methods to delineate physiographic regions for the United States is described, including a recent maximum likelihood classification based on seven input variables. This research compares unsupervised and supervised algorithms applied to these seven input variables, to evaluate and possibly refine the recent classification. Evaluation metrics for unsupervised methods include the Davies–Bouldin index, the Silhouette index, and the Dunn index as well as quantization and topographic error metrics. Cross validation and misclassification rate analysis are used to evaluate supervised classification methods. The paper reports the comparative analysis and its impact on the selection of landscape regions. The compared solutions show problems in areas of high landscape diversity. There is some indication that additional input variables, additional classes, or more sophisticated methods can refine the existing classification.