231 research outputs found
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Shallow Seismic Data Acquisition, Processing, and Interpretation at Playa 5, Carson County, Texas
Seismic methods were used to determine the physical properties and geological development of Playa 5, a playa basin located on the U.S. Department of Defense's former Pantex Ordnance Plant, for comparison with results from other basins (Sevenmile Basin, Pantex Playa 3, and Pantex Lake) as well as with results from seismic data collected in interplaya areas. These studies have led to a better understanding of stratigraphic differences between playa basins, which serve as preferential recharge points for the Ogallala aquifer, and between playa and unaltered interplaya areas, where little Ogallala recharge is thought to occur. Playa 5 is a nearly circular playa that is 0.7 to 0.9 km across. It is enclosed by a basin that is about 2 km across and has 5 m of relief between the highest and lowest closed elevation contours. Refraction surveys show that the surface layer at Playa 5 is a few meters thick and has typical seismic velocities of 420 to 440 m/s. This layer is underlain by a layer with higher seismic velocities of 808 to 910 m/s that has similar texture but more pedogenic carbonate. Refraction methods also detected a layer at more than 60-m depth with significantly higher seismic velocities of about 2000 m/s. This layer probably represents a competent horizon above the modern Ogallala water table that has been cemented by either pedogenic or hydrologic processes.
Reflection data collected across Playa 5 show that relief on seismic horizons increases with age. Modern surface relief is 6 m, which increases to 30 m on a horizon that is interpreted to be correlative to a fine-grained zone that perches groundwater beneath parts of the Pantex Plant. Relief increases to 50 m on a horizon that is interpreted to be the top of Permian or Triassic bedrock. Internal bedrock reflectors dip toward the basin center beneath the playa, suggesting that subsidence related to dissolution of underlying Permian salt has contributed to the development of the Playa 5 basin. Playa 5 subsidence has occurred at average rates of 0.33 to 0.42 m per meter of deposition, rates that are similar to those at Playa 3 and Pantex Lake and are less than half those inferred for Sevenmile Basin.Bureau of Economic Geolog
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Identifying and Assessing Ground Water in the Lower Rio Grande Valley, Texas Using Airborne Electromagetic Induction
- Airborne EM can be used to explore to depths of 150 to 300m in fresh to moderately saline coastal plain aquifers.
- Airborne EM surveys are a realistic alternative to drilling and seismic surveys to investigate Quaternary depositional systems.
- Groundwater quality (as measured by TDS) appears to correlate to conductivity derived from airborne EM data.
- Depositional patterns can be detected and environments inferred from airborne EM if the surveys are flown at appropriate line spacing and orientation.
- Anomaly shapes and conductivity contrasts can be used to interpret likely water resource quality by combining geological and hydrological concepts.Bureau of Economic Geolog
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Seismic Reflection, Refraction, and Surface Wave Studies at the Proposed Low-Level Radioactive Waste Repository, Hudspeth County, Texas
Seismic reflection, refraction, and surface wave methods were employed to characterize the shallow subsurface at the proposed low-level radioactive waste repository site located on Faskin Ranch about 5 miles (8 km) southeast of Sierra Blanca, Texas. Reversed seismic refraction data were used to (1) determine near-surface compressional velocities for elevation datum corrections, (2) obtain preliminary velocity profiles for processing seismic reflection data, and (3) obtain depth-to-bedrock estimates. Seismic reflection data were used to determine basin geometry beneath the site, depth to bedrock, and internal basin-fill stratigraphy. Surface waves were analyzed to generate shear-wave-velocity models of the shallow subsurface.
Seismic reflection, refraction, and surface wave data were acquired in May and June of 1992 using a 500-lb (230-kg) accelerated weight drop seismic source, a 48-channel seismograph, and an acquisition crew supplied by the Bureau of Economic Geology (BEG) and The University of Texas at El Paso (UTEP). Refraction data were collected at six sites on Faskin Ranch and were processed and analyzed at BEG. Nearly 3.9 miles (6.2 km) of seismic reflection data were collected along one line oriented northwest-southeast across the site and three crossing lines oriented northeast-southwest. These data were processed and analyzed at both BEG and UTEP. Surface wave data were collected near the center of the proposed repository and were processed and analyzed at UTEP.Bureau of Economic Geolog
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Shallow Seismic Studies of an Ephemeral Lake (Playa A) Basin on the Southern High Plains, Texas Panhandle
Shallow seismic data collected at Sevenmile Basin, a large ephemeral lake (playa) basin in the Texas Panhandle, reveal that subsidence has been an important agent in the formation of the basin. Several hypotheses have been considered for the origin of thousands of playa basins on the Southern High Plains of Texas and New Mexico, including eolian deflation, evaporite or carbonate dissolution and subsidence, piping, and animal activity. Seismic methods, adapted to investigations in the shallow subsurface (200 m or less), provide data that indicate subsidence caused by evaporite dissolution is the most important of these mechanisms at Sevenmile Basin.
Sevenmile Basin is 5.5 by 3.6 km across and 14 m deep and is inset into the Quaternary Blackwater Draw Formation. The Blackwater Draw overlies the upper Tertiary Ogallala Formation, which hosts the economically important Ogallala aquifer. Shallow seismic refraction and reflection data were collected from this unconsolidated and variably saturated elastic sequence to understand the physical properties, geological history, and hydrogeological framework of playa basins, which are the principal recharge areas for the Ogallala aquifer.Bureau of Economic Geolog
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Shallow Seismic Data Acquisition, Processing, and Interpretation at Playa 3, Pantex Plant, Carson County, Texas
Shallow seismic refraction and reflection data were collected in 1993 at Pantex Playa 3, a small (0.5-kilometer diameter), nearly circular ephemeral lake near the northern boundary of the Pantex Plant, as part of a hydrogeological study of the Pantex area playa and interplaya environments. These studies will be used to help understand the hydrogeological framework of the Pantex Plant and the paths of groundwater and potential contaminants in the subsurface.
Seismic refraction data collected along two reversed spreads show that near-surface compressional velocities increase from less than 400 meters per second at the surface to 700 to 1200 meters per second a few meters below the surface. Two shallow seismic reflection lines across Playa 3, each 1.8 kilometers long, reveal the presence of four major reflecting horizons beneath the playa basin. Horizon 0, the shallowest, is interpreted to be from the Ogallala caprock and appears to be absent directly beneath Playa 3. Horizon 1 is interpreted as a fine-grained zone within the upper Ogallala Formation that may perch groundwater above the main Ogallala aquifer. Horizon 2, the strongest reflector on the seismic sections, is a lower Ogallala reflector that may be either a stratigraphic unit or a horizon related to past Ogallala water levels. Horizon 3, the deepest major reflector recognized, is interpreted to be the top of Permian or Triassic bedrock.
Each horizon visible on the reflection lines mimics surface topography. Relief increases with depth: the playa floor is 8 meters below the upland, Horizon 0 (caprock) has 16 to 24 meters of relief, Horizon 1 (upper Ogallala fine-grained zone) has 30 meters of relief, Horizon 2 (lower Ogallala reflector) has 35 meters of relief, and Horizon 3 (bedrock) has 75 meters of relief. Increasing relief with age, coupled with the presence of internal bedrock reflectors that dip toward the basin center beneath the margins of Playa 3, indicate that subsidence has been important in the formation of the basin. Subsidence is probably caused by dissolution of underlying Permian evaporites.Bureau of Economic Geolog
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Seismic Evaluation and Reconnaissance TDEM Survey of the Southeast and Playa 3 Areas of the Pantex Plant, Carson County, Texas
In response to a recommendation in the recent "Protecting the Ogallala Aquifer II" report, the Bureau of Economic Geology (the Bureau) evaluated the potential of seismic reflection and TDEM (Time-Domain Electromagnetic) methods to define the distribution and thickness variations of the Ogallala fine-grained zone (FGZ) that perches groundwater beneath the Pantex Plant. Bureau researchers briefly examined existing seismic reflection data collected at and near the Pantex Plant by the U.S. Army Corps of Engineers (COE) and by the Bureau, conducted TDEM modeling studies, and acquired reconnaissance TDEM data in the Pantex Southeast and Playa 3 areas.
The existing seismic reflection surveys had broad objectives that extended from the Ogallala caprock to the base of the Ogallala Formation. These surveys provided basic data on the stratigraphic framework of playa and interplaya areas. Although the top of the FGZ appears to be a good seismic reflector in existing data, reprocessing the limited COE and Bureau seismic data in the Pantex Southeast area might enhance that reflection and reveal other associated reflectors in the same depth range. Further improvement in the seismic characterization of the FGZ would require acquisition of new seismic data that would benefit from the restricted target depth and recent advances in seismic equipment.Bureau of Economic Geolog
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Beach and Vegetation-Line Changes at Galveston Island, Texas: Erosion, Deposition, and Recovery from Hurricane Alicia
On August 18, 1983, Hurricane Alicia crossed the Upper Texas Gulf Coast and caused extensive property damage, especially along West Beach of Galveston Island. Aerial photographs taken before and after Alicia, along with field measurements made during the first post-storm year, provide a basis for determining nearshore changes associated with a major storm and for predicting potential beach recovery. Alicia caused substantial landward retreat of both the shoreline and the vegetation line. Retreat of the vegetation line ranged from 20 to 145 ft and averaged 80 ft. Erosion was generally greatest near the Sea Isle and Bay Harbor subdivisions, where storm processes were most intense; beach erosion generally decreased away from San Luis Pass, which is near the site of storm landfall. Because erosion was so severe, surface elevations were lowered as much as 4.5 ft and many Gulf-front houses were undermined and exposed on the beach after the storm.
Alicia eroded several million cubic yards of sand from West Beach. About one-tenth of that sand was deposited on the adjacent barrier flat as a washover terrace. Washover penetration was greatest to the east of the storm's eye and along developed shoreline segments. The remaining eroded beach sand was deposited offshore as shoreface bars or as storm deposits on the inner shelf. The shoreface deposits promoted rapid forebeach accretion during the first post-storm year; at the same time, the backbeach elevation remained about 3 ft lower than before the storm, and the natural post-Alicia vegetation line remained essentially unchanged. Recovery of the vegetation line 1 year after the storm was insignificant mainly because the depth of beach erosion exceeded the depth of root penetration, thus eliminating plants from some areas that were densely vegetated before the storm.Bureau of Economic Geolog
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Historical Monitoring of Shoreline Changes in Corpus Christi, Nueces, and Oso Bays
Changes in the position and stability of shorelines in Corpus Christi, Nueces, and Oso Bays since the late 1800s were documented using historical monitoring techniques. This is accomplished by comparing shorelines from topographic charts (dated 1867 to 1882) and aerial photographs (taken in 1930 to 1937 and 1982), measuring the magnitude (distance) of shoreline movement at specific sites, and calculating the rates of change for particular time periods (late 1800s to 1930s, 1930s to 1982, and late 1800s to 1982). Geological interpretations of the maps and photographs are used in conjunction with meteorological data and historical records to explain the important trends revealed in the tabulated shoreline data.
Unprotected sediments forming the margins of Corpus Christi, Nueces, and Oso Bays are subjected to natural processes and modified by human activities that together cause shoreline movement. These unstabilized shorelines include high clay bluffs, moderate slopes composed mainly of sand, salt-water marshes, sand and shell beaches, and newly formed areas filled by dredged material. Composition of the shoreline material and orientation of the shoreline with respect to prevailing wind directions and wave fetch largely determine the response and consequent movement of the shoreline. In some areas property owners have attempted to stabilize the shoreline and prevent further movement by building seawalls and bulkheads and using riprap to dissipate wave energy.
Factors contributing to shoreline changes include (1) regional and worldwide climate, (2) local changes in relative sea-level position, (3) local alterations in sediment supply, (4) storm frequency and intensity, and (5) human activities. Historical data compiled for these various factors indicate that warming temperatures, rising sea level, decreasing sediment supply, recurring severe storms, and ongoing human activities all favor continued erosion of exposed shorelines.Bureau of Economic Geolog
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Wetland Plant Communities, Galveston Bay System
Wetlands and aquatic habitats are critical components of the biologically productive Galveston Bay estuarine system. This report is the culmination of a field investigation of wetland plant communities and is one phase of the project "Trends and Status of Wetland and Aquatic Habitats of the Galveston Bay System, Texas," sponsored by the Galveston Bay National Estuary Program (GBNEP). For purposes of this topical report, wetlands are defined and classified in terms of more classical definitions, for example, salt, brackish, and fresh marshes, in accordance with project requirements. The relationship of these wetland classes to the Cowardin and others (1979) classification system used to map wetlands is presented through various examples. Wetlands in this study were not defined in accordance with the "Federal Manual for Identifying and Delineating Jurisdictional Wetlands" (currently being revised) and thus should not be regarded as jurisdictional wetlands.
More than 150 sites were examined in the Galveston Bay system. Wetland plants were identified at selected field survey sites, principally along transects aligned perpendicular to the hydrologic gradient so that plant assemblages from the water's edge to upland areas were intercepted. Topography surveys were conducted along several transects. Measurements of elevation, distance, and plant community composition were made along the survey lines, which crossed salt marshes and brackish to fresh marshes. Elevations were measured to the nearest 0.5 cm and distances to the nearest meter. County soil surveys were used to define and characterize soils at the various field check sites. The locations of field survey sites were plotted on aerial photographs and later accurately transferred to USGS 7.5-minute quadrangle topographic maps. Universal Transverse Mercator (UTM) coordinates were determined for each site, and these data were entered into computer data management systems, including the geographic information system, ARCsINfO.Bureau of Economic Geolog
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Potential Sinks for Geologic Storage of CO2 Generated in the Carolinas
This document summarizes a scoping study of the current state of knowledge of carbon storage options for our geographic area.
The focus is on one aspect of carbon capture and storage—identification of deep saline aquifers in which carbon dioxide (CO2
) generated in the Carolinas might be stored. The study does not address other aspects of CO2 storage projects, such as capture and compression of the gas, well construction and development, or injection. Transport of CO2 is touched upon in this study but has not been fully addressed.
The information contained in this document is primarily from review of published geologic literature and unpublished data. No field data collection has been completed as part of this study. Further work will be necessary to increase confidence in the suitability of the potential CO2 storage sites identified in this report. This study does not address the regulatory, environmental, or public policy issues associated with carbon storage, which are under development at this time.Duke Energy, Progress Energy, Santee Cooper Power, South Carolina Electric and Gas, Electric Power Research Institute (EPRI), Southern States Energy Board (SSEB)Bureau of Economic Geolog
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