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on a map. This set of information includes not only the coordinate data required to draw a graphic product but also information about the various features shown on the map (for example, the number of lanes of a highway) and their relationships to each other (for example, that the railroad tracks lie between the stream and the highway). This collection of information forms the heart of a digital cartographic data base. Once the information is in the data base, computers provide the capabilities to handle geographic data in ways that were previously impossible.
Mathematically, a map can be treated as a graph and the elements of that graph
can be defined using graph theory. The various elements of a map are subdivided into nodes, lines, and areas. Nodes represent the beginning or ending of every line in the graph and occur at the intersection of linear features. Lines are ordered sets of points that describe the location and shape of a linear feature or boundary on the map. Areas are portions of the map that are bounded by lines. Figure 10 illustrates these components, and table 1 lists the types of information that are encoded in a computer to represent these data. This example illustrates how information is stored in the present digital line graph data structure.
Recently some advancements have been made to the data structure. The changes involve removing the attribute information from the area, line, and node components and placing it with the feature. The features then describe the various components of which they are composed. Table 2 illustrates this.
The new design allows users of the data to more readily access the descriptions of the features contained in a cartographic data base. For example, the various parts of the feature "Potomac River," such as the Georgetown Channel, Tidal Basin, and Washington Channel, would all be retrieved when a user requested the Potomac River.
Thematic Mapping Using Automated Processes
By Donald C. Orr
Information needs within the U.S. Geological Survey and the Department of the Interior require the ability to handle a wide variety of disparate data. Several cooperative projects are underway that make use of digital data bases, spatial data analysis techniques, and sophisticated automated systems to create thematic displays and custom products for a wide range of applications (fig. 11).
The Survey's National Mapping Division and Water Resources Division have
been cooperating in the development of automated processing procedures for deriving hydrologieally related terrain characteristics from digital elevation model data for an area in the James River basin in North Dakota. The high-resolution data were processed to identify surface depressions, depth-area-volume relationships, drainage basin boundaries, drainage lines, and land surface elevations in glaciated pothole terrain for use in a rainfall runoff model being developed by the Bureau of Reclamation.
The USGS and the Soil Conservation Service are conducting a cooperative research project to incorporate the Soil Conservation Service's State General Soil Geographic data base (STATSGO) into a geographic information system to develop techniques and procedures to archive, reproduce, and distribute 1:250,000-scale digital soils data. Other types of data being incorporated into the geographic information system data base include land use and land cover data, hydrographic data, political boundaries, transportation, and digital topographic data. The Chesapeake Bay watershed was selected as the study area to demonstrate the soils data base concept and to produce a variety of thematic displays and products.
During the past 5 years, the Survey's office in Anchorage, Alaska, has worked cooperatively with Federal and State resource management agencies to produce land cover and terrain maps for 245 million acres of Alaska. This land cover mapping effort integrates digital Landsat data, terrain data, aerial photographs, and field data by using advanced spatial data processing techniques. The land cover and terrain maps and associated data bases produced from this effort are used for resource assessment, management, and planning by the U.S. Fish and Wildlife Service, the U.S. Forest Service, the Bureau of Land Management, and the Alaska Department of Natural Resources. The digital data
bases are used in a variety of applications from comprehensive refuge planning to multiphase sampling procedures for statewide vegetation inventories.
Development of Playa Lake Basins in the Southern High Plains
By Warren W. Wood
Most of the more than 25,000 shallow playa lake basins on the southern High Plains of Texas and New Mexico probably have developed by subtle hydrologic processes of dissolution and micropiping (erosion below the land surface that is caused by water moving through the aquifer). Recognition that these basins were developed by, and are related to, hydrologic processes illustrates the dynamic role of ground water in geomorphic processes and provides insight into mechanisms of recharge and potential contamination pathways of a major aquifer system in the United States.
Hypotheses about the origin of these playa lake basins include buffalo wallows, meteorite impacts, dissolution of deeply buried salt, and wind blowouts. Hydrologists of the USGS studying geochemistry of gases in the unsaturated zone in the southern High Plains observed a carbon-dioxide-concentration distribution, a carbon-isotope composition, and carbondioxide outflow that could only be explained by particulate organic materials entering the unsaturated zone along with recharging surface water.
This conclusion ultimately led to a hypothesis that playa basins developed by
movement of clay and silt-sized organic and inorganic material from the surface into the unsaturated zone, thus enlarging the volume of the basin. The organic material introduced into the unsaturated zone oxidized to form carbon dioxide, which reacted with water to form carbonic acid. The acid in turn dissolved calcium carbonate cement in the aquifer. Dissolution of carbonate minerals provided additional void space for material transported from the surface and also caused the destabilization of the skeletal framework of the aquifer that promotes subsidence and micropiping, which further enlarged the catchment area of the basin.
The proposed hydrologic model of the development of the basins explains their distinct alignment along lineaments, their presence in an active drainage system, their propensity to form a circular shape, and the lack of saline-mineral buildup in the lacustrine sediment. The model also explains their development anywhere on a hillslope, the absence of leeward dunes at most of the basins, the dissolution of caliche below many of the basins, and other features that have been difficult to explain with other hypotheses.
The measurement of seemingly unrelated gases in the unsaturated zone has provided a solution to a vexing geomorphic problem and useful insight into the hydrology of a major aquifer system. The recognition that as much as 80 percent of the water that collects in the basin is recharged to the aquifer, rather than evaporating as had been previously assumed, provides insight on how the Ogallala aquifer receives recharge in the Texas-New Mexico area and calls for care in using playa lake basins for the disposal of waste products.
Earthquakes and Ground Water: Water Wells as Strainmeters
By John D. Bredehoeft
As part of an extensive earthquake prediction experiment that the U.S. Geological Survey is conducting in the Parkfield, California, area, scientists drilled a network of water wells at seven sites in the vicinity of Parkfield to monitor water levels. These wells, which are being continuously monitored, have turned out to be very sensitive volume strainmeters.
At all of these sites, water levels are monitored at depths ranging from approximately 289 to 820 feet. Six of the deeper wells show clearly identifiable tidal signals that range from one to several inches in amplitude. A shallower water level, less than 164 feet in depth, is also measured at five of the sites. At these seven sites, barometric pressure, rainfall, and water levels are measured every 15 minutes. Data are accumulated for 4 hours and then transmitted, via GOES satellite, over a USGS data network to the Survey offices in Menlo Park, California, where the data are analyzed with the intent of using this information as an indicator for predicting the next Parkfield earthquake, which has been forecast to occur between 1985 and 1993 and most probably in 1988.
The clearest tectonic event observed to date, using water-level data, was an earthquake at Kettleman Hills near Coalinga, California, that occurred in August 1985. This earthquake epicenter was about 23 miles to the east of the four Parkfield wells that were in operation at the time. A coseismic drop in water level was observed in each of the four wells. Using a simple computer model, USGS hydrologists calculated what the expected water-level change would have been in the four wells. The simple-model calculations computed a response within a factor of 2 for all of the wells. This close correlation was a pleasant surprise, since the geology between Kettleman Hills and the four wells is quite complex; in fact, one of the wells is situated across the active trace of the San Andreas fault from Kettleman Hills.
In addition to the Kettleman Hills coseismic water-level changes, a number of water-level changes have been observed that correlate with observed surface-creep
events. One of these events, in February 1987, was followed in the next 12 hours by a sequence of small earthquakes in the vicinity of the wells. These correlated events, along with similar well-documented experience of the Chinese and calculations from a number of fault-mechanics computer models, suggest strains may well be precursors to earthquakes. Interestingly, the
water-well strain information from several creep events suggests that the strains may be larger at depth than the surface creepmeters can register.
The water-well strain network at Parkfield is gradually being expanded. Four or five more wells, in addition to the seven currently being monitored, are planned for the network. One well, a 5,250foot-deep exploratory oil well (no significant oil found), is being reopened by the USGS. It is located less than a mile east of the fault near Parkfield. This well has a substantial well-head pressure of approximately 125 bars (1,800 pounds per square inch).
The water well is proving to be an interesting and sensitive volume strainmeter. Wells drilled in any number of geologic settings can have good Earth-tide fluctuations, indicating good sensitivity to strain. The only requirement is a confined aquifer and enough permeability so that the well will fluctuate at tidal frequencies twice daily. The success of water wells as strainmeters is an encouraging and exciting development for earthquake prediction, as well as for other aspects of engineering geology and rock mechanics.
By Edwin P. Weeks
Topographically influenced air circulation may occur through the fractures in unsaturated rocks that form Yucca Mountain in Nevada. These highly fractured rocks are under consideration by the U.S. Department of Energy as the host medium for a potential high-level radioactive waste repository, and this circulation phenomenon may affect their suitability for such usage.
During winter, cold, dry air enters the highly fractured outcrops along hillsides, picks up heat and moisture, and then exits through the hillcrests because the warm, moist air in the rocks is less dense than the air in the surrounding atmosphere. During summer, the flow reverses to some extent, with hot, dry air entering the rocks at the hillcrest, losing heat to the cooler rocks, and exiting through the hillside outcrops. In each case, the air entering the mountain is generally drier than the air exiting, resulting in a net drying effect. This effect may be sufficient to dry the rocks to below the moisture content at which water drains by gravity, reducing the potential for deeply percolating water to leach radionuclides from the wastes and transport them to the ground-water reservoir. A negative aspect of this air circulation process is that gaseous radionuclides would be transported to the atmosphere more quickly than if gaseous diffusion were the only transport mechanism for such releases.
The potential for this effect was first recognized from dramatic air exhaust observed in two boreholes drilled in unsaturated rock at the crest of Yucca Mountain. During winter, these wells typically discharge warm, moist air into the atmosphere at a velocity of about 10 feet per second. These discharges varied with time but were continuous from the two wells to the atmosphere over a 10-day observation period in February 1986. Net water-vapor discharge from the formation through the wells was calculated to be about 66 gallons of water per day. Although summertime circulation was much less, oscillating between air intake to the wells and exhaust from the wells at velocities no greater than 5 feet per second, summer drying effects are substantial.
Research Related to
By Peter B. Davies
Current USGS research on radioactive-waste disposal in bedded-salt environments is focusing on understanding the ground-water flow system in the vicinity of a waste-repository site in southeastern New Mexico. The Waste Isolation Pilot
Plant (WIPP) is a U.S. Department of Energy project that is intended to provide a facility both for the permanent disposal of approximately 6 million cubic feet of transuranic waste from defense-related activities and for research on the interaction of high-level waste with a bedded-salt environment. In the event of a repository breach, ground-water flow would provide the primary method for transporting radionuclides beyond the immediate site area.
Ground-water flow in bedded-salt environments is complex. Dissolution of highly soluble evaporite minerals creates substantial spatial variation in fluid density. Under these conditions, ground-water flow is driven not only by fluid-pressure differentials but also by gravitational forces. Standard techniques for analyzing and simulating ground-water flow are based on the assumption that the effects of density-related gravity are insignificant.
Recent Survey research has produced an analytic technique for assessing the relative magnitude of such gravity effects in variable-density flow systems. Application of this new technique to the bedded-salt environment in the WIPP area, coupled with variable-density computer simulations, has revealed significant errors in the flow patterns predicted by standard techniques. In a critical area along the flow paths leaving the site, flow directions predicted in previous studies may be in error by as much as 170 degrees and flow magnitudes may be underestimated by as much as a factor of 10.
The primary conduit for ground-water flow in the WIPP area is a 26-foot-thick dolomite unit. Until recently, this dolomite unit was thought to be vertically isolated from other components of the flow system. Transient cross-sectional flow simulations have demonstrated the importance of small but significant vertical fluxes in the dynamics of flow within both the dolomite unit and the total flow system. In addition to simulating ground-water flow, USGS personnel are currently assessing the geochemical characteristics of the ground-water flow system using a recently developed computer program specifically designed to characterize chemical variations in the highly saline waters that are characteristic of bedded-salt environments.
As a result of the WIPP project research, techniques have been developed that have broader application to the under