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USGS2007

Upper-crustal structure beneath the strait of Georgia, Southwest British Columbia

We present a new three-dimensional (3-D) P -wave velocity model for the upper-crustal structure beneath the Strait of Georgia, southwestern British Columbia based on non-linear tomographic inversion of wide-angle seismic refraction data. Our study, part of the Georgia Basin Geohazards Initiative (GBGI) is primarily aimed at mapping the depth of the Cenozoic sedimentary basin and delineating the near-surface crustal faults associated with recent seismic activities (e.g. M = 4.6 in 1997 and M = 5.0 in 1975) in the region. Joint inversion of first-arrival traveltimes from the 1998 Seismic Hazards Investigation in Puget Sound (SHIPS) and the 2002 Georgia Basin experiment provides a high-resolution velocity model of the subsurface to a depth of ∼7 km. In the southcentral Georgia Basin, sedimentary rocks of the Cretaceous Nanaimo Group and early Tertiary rocks have seismic velocities between 3.0 and 5.5 km s −1 . The basin thickness increases from north to south with a maximum thickness of 7 (±1) km (depth to velocities of 5.5 km s −1 ) at the southeast end of the strait. The underlying basement rocks, probably representing the Wrangellia terrane, have velocities of 5.5–6.5 km s −1 with considerable lateral variation. Our tomographic model reveals that the Strait of Georgia is underlain by a fault-bounded block within the central Georgia Basin. It also shows a correlation between microearthquakes and areas of rapid change in basin thickness. The 1997/1975 earthquakes are located near a northeast-trending hinge line where the thicknesses of sedimentary rocks increase rapidly to the southeast. Given its association with instrumentally recorded, moderate sized earthquakes, we infer that the hinge region is cored by an active fault that we informally name the Gabriola Island fault. A northwest-trending, southwest dipping velocity discontinuity along the eastern side of Vancouver Island correlates spatially with the surface expression of the Outer Island fault. The Outer Island fault as mapped in our seismic tomography model is a thrust fault that projects directly into the Lummi Island fault, suggesting that they are related structures forming a fault system that is continuous for nearly 90 km. Together, these inferred thrust faults may account for at least a portion of the basement uplift at the San Juan Islands.

British Columbia↗
USGS2019

Characterization of Big Chino subbasin hydrogeology near Paulden, Arizona, using controlled source audio-frequency magnetotelluric surveys

The Big Chino subbasin is located in central-northwest Arizona in the transition zone between the Colorado Plateau and the Basin and Range Province. The controlled source audio-frequency magnetotelluric (CSAMT) geophysical method, a low-impact, non-intrusive, electrical resistance sounding technique, was used to evaluate the subsurface hydrogeology of the southern third of the Big Chino subbasin. The Big Chino subbasin is a northwest-trending, late Tertiary graben bordered by the Big Chino Fault along its northeast flank where there is as much as 1,100 meters of displacement. The main water-bearing stratigraphic unit of the basin is Tertiary alluvial-fill sediment. The Devonian Martin Formation provides water to wells near Drake and the Mississippian Redwall Limestone provides water to wells east of the basin and in the Paulden area. The purpose of the CSAMT surveys was to improve the conceptual model of the aquifer by constraining the basin geometry and identifying stratigraphic units and their subsurface extents. CSAMT methods were used to map the subsurface along 100 kilometers (62 miles) of survey lines across the southern third of the subbasin. Of 21 survey lines, 14 were west of the town of Paulden and another 7 were east of Paulden. Data were cleaned and prepared for entry into Zonge SCS2D software and then inverted to provide a two-dimensional resistivity profile for each survey line. Final inversion models representing the best fit to measured data were compared to driller’s logs or borehole data where present. Data from the CSAMT lines west and north of Paulden are consistent with thicker alluvial basin deposits that range from 100 meters thick to a few hundred meters thick. Data from the CSAMT lines east of Paulden are consistent with thinner alluvial and basalt deposits overlying Paleozoic Martin Formation and Redwall Limestone, Tapeats Sandstone, and Precambrian granite and schist.

Arizona↗
USGS2009

Geologic Cross Section D-D' Through the Appalachian Basin from the Findlay Arch, Sandusky County, Ohio, to the Valley and Ridge Province, Hardy County, West Virginia

Geologic cross section D-D' is the second in a series of cross sections constructed by the U.S. Geological Survey to document and improve understanding of the geologic framework and petroleum systems of the Appalachian basin. Cross section D-D' provides a regional view of the structural and stratigraphic framework of the Appalachian basin from the Findlay arch in northwestern Ohio to the Valley and Ridge province in eastern West Virginia, a distance of approximately 290 miles. The information shown on the cross section is based on geological and geophysical data from 13 deep drill holes, several of which penetrate the Paleozoic sedimentary rocks of the basin and bottom in Mesoproterozoic (Grenville-age) crystalline basement rocks. This cross section is a companion to cross section E-E' (Ryder and others, 2008) that is located about 25 to 50 mi to the southwest. Although specific petroleum systems in the Appalachian basin are not identified on the cross section, many of their key elements (such as source rocks, reservoir rocks, seals, and traps) can be inferred from lithologic units, unconformities, and geologic structures shown on the cross section. Other aspects of petroleum systems (such as the timing of petroleum generation and preferred migration pathways) may be evaluated by burial history, thermal history, and fluid flow models based on information shown on the cross section. Cross section D-D' lacks the detail to illustrate key elements of coal systems (such as paleoclimate, coal quality, and coal rank), but it does provide a general geologic framework (stratigraphic units and general rock types) for the coal-bearing section. Also, cross section D-D' may be used as a reconnaissance tool to identify plausible geologic structures and strata for the subsurface storage of liquid waste or for the sequestration of carbon dioxide.

Scientific Investigations Map↗
USGS2006

Proceedings of the U.S. Geological Survey Sixth Biennial Geographic Information Science Workshop, Denver, Colorado, April 24-28, 2006

The U.S. Geological Survey's (USGS) Sixth Biennial Geographic Information Science Workshop April 24 - 28, 2006, at the Denver Federal Center in Denver, Colorado, provides a unique opportunity for multi-disciplinary Geographic Information Systems (GIS) and associated scientific professionals to share, learn, present, and discuss a wide variety of geospatial-related topics. Information is exchanged through a series of plenary sessions, hands-on technical workshops, user and commercial vendor demonstrations, lecture and poster sessions, and specialty meetings. Workshop attendance is limited to Department of Interior (DOI) and USGS employees and contractors; however, plenary, hands-on, and lecture sessions have presenters from other Federal agencies, numerous commercial vendors, universities, and several consortia. Over 175 participants are expected to attend the Workshop. Several prominent speakers are featured at this Workshop. Monday evening Star Guest Speaker and National Aeronautics and Space Administration (NASA) Astronaut Captain Dominic Gorie will talk about his experiences as a veteran of three space flights and over 32 days in space, including the NASA Space Shuttle Radar Topography Mission that mapped more than 47 million miles of the Earth's land surface. Selected as an astronaut candidate by NASA in December 1994, Captain Gorie is currently Chief of the Astronaut Shuttle Branch. Monday evening also features a town hall meeting with Geographic Information Office (GIO) leaders Karen Siderelis, Kevin Gallagher, Bob Pierce, Steve Guptill, Mark DeMulder, John Mahoney, and Mark Negri, who will discuss changes and activities within the GIO in an open discussion format. Tuesday plenary sessions feature keynote speaker Dr. P. Patrick Leahy, Acting USGS Director. Dr. Leahy holds undergraduate and graduate degrees in geology (1968) and geophysics (1970) from Boston College. In 1979, he received his doctorate in geology from Rensselaer Polytechnic Institute, where he specialized in regional ground-water studies and hydraulics. Dr. Leahy has been with the USGS since 1974, and has held various technical and managerial positions, including Associate Director for Geology and Chief of the National Water-Quality Assessment (NAWQA) Program. Dr. Leahy will be discussing a broad range of topics including Bureau accomplishments, initiatives, and budgets. The purpose of this proceedings volume is to serve as an activity reference for Workshop attendees as well as an archive of technical abstracts submitted, presented, and discussed at the Workshop. Author, co-author, and presenter names, affiliations, and contact information are listed with presentation titles along with submitted abstracts. Some hands-on sessions are offered twice. In these instances, abstracts submitted for publication are presented in the proceedings on both days they are offered. All acronyms used in these proceedings are explained in the text of each abstract. The term 'ArcGIS' refers to an integrated collection of GIS software products produced by Environmental Systems Research Institute, Inc. (ESRI).

Scientific Investigations Report↗
USGS2018

Delineation of the hydrogeologic framework of the Big Sioux aquifer near Sioux Falls, South Dakota, using airborne electromagnetic data

The U.S. Geological Survey, in cooperation with the City of Sioux Falls, South Dakota, began developing a groundwater-flow model of the Big Sioux aquifer in 2014 that will enable the City to make more informed water management decisions, such as delineation of areas of the greatest specific yield, which is crucial for locating municipal wells. Innovative tools are being evaluated as part of this study that can improve the delineation of the hydrogeologic framework of the aquifer for use in development of a groundwater-flow model, and the approach could have transfer value for similar hydrogeologic settings. The first step in developing a groundwater-flow model is determining the hydrogeologic framework (vertical and horizontal extents of the aquifer), which typically is determined by interpreting geologic information from drillers’ logs and surficial geology maps. However, well and borehole data only provide hydrogeologic information for a single location; conversely, nearly continuous geophysical data are collected along flight lines using airborne electromagnetic (AEM) surveys. These electromagnetic data are collected every 3 meters along a flight line (on average) and subsequently can be related to hydrogeologic properties. AEM data, coupled with and constrained by well and borehole data, can substantially improve the accuracy of aquifer hydrogeologic framework delineations and result in better groundwater-flow models. AEM data were acquired using the Resolve frequency-domain AEM system to map the Big Sioux aquifer in the region of the city of Sioux Falls. The survey acquired more than 870 line-kilometers of AEM data over a total area of about 145 square kilometers, primarily over the flood plain of the Big Sioux River between the cities of Dell Rapids and Sioux Falls. The U.S. Geological Survey inverted the survey data to generate resistivity-depth sections that were used in two-dimensional maps and in three-dimensional volumetric visualizations of the Earth resistivity distribution. Contact lines were drawn using a geographic information system to delineate interpreted geologic stratigraphy. The contact lines were converted to points and then interpolated into a raster surface. The methods used to develop elevation and depth maps of the hydrogeologic framework of the Big Sioux aquifer are described herein. The final AEM interpreted aquifer thickness ranged from 0 to 31 meters with an average thickness of 12.8 meters. The estimated total volume of the aquifer was 1,060,000,000 cubic meters based on the assumption that the top of the aquifer is the land-surface elevation. A simple calculation of the volume (length times width times height) of a previous delineation of the aquifer estimated the aquifer volume at 378,000,000 cubic meters; thus, the estimation based on AEM data is more than twice the previous estimate. The depth to top of Sioux Quartzite, which ranged in depth from 0 to 90 meters, also was delineated from the AEM data.

South Dakota↗
USGS2001

Io in the near infrared: Near-Infrared Mapping Spectrometer (NIMS) results from the Galileo flybys in 1999 and 2000

Galileo's Near-Infrared Mapping Spectrometer (NIMS) observed Io during the spacecraft's three flybys in October 1999, November 1999, and February 2000. The observations, which are summarized here, were used to map the detailed thermal structure of active volcanic regions and the surface distribution of SO 2 and to investigate the origin of a yet unidentified compound showing an absorption feature at ∼1 μm. We present a summary of the observations and results, focusing on the distribution of thermal emission and of SO 2 deposits. We find high eruption temperatures, consistent with ultramafic volcanism, at Pele. Such temperatures may be present at other hot spots, but the hottest areas may be too small for those temperatures to be detected at the spatial resolution of our observations. Loki is the site of frequent eruptions, and the low thermal emission may represent lavas cooling on the caldera's surface or the cooling crust of a lava lake. High-resolution spectral observations of Emakong caldera show thermal emission and SO 2 within the same pixels, implying that patches of SO 2 frost and patches of cooling lavas or sulfur flows are present within a few kilometers from one another. Thermal maps of Prometheus and Amirani show that these two hot spots are characterized by long lava flows. The thermal profiles of flows at both locations are consistent with insulated flows, with the Amirani flow field having more breakouts of fresh lava along its length. Prometheus and Amirani each show a white ring at visible wavelengths, while SO 2 distribution maps show that the highest concentration of SO 2 in both ring deposits lies outside the white portion. Visible measurements at high phase angles show that the white deposit around Prometheus extends into the SO 2 ring. This suggests that the deposits are thin and that compositional or grain size variations may occur in the radial direction. SO 2 mapping of the Chaac region shows that the interior of a caldera adjacent to Chaac has almost pure SO 2 . The deposit appears to be topographically controlled, suggesting a possible origin by liquid flow.

Journal of Geophysical Research E: Planets↗
USGS1995

Distribution of carbonate-rock aquifers and the potential for their development, southern Nevada and adjacent parts of California, Arizona, and Utah

In 1985, the State of Nevada entered into a cooperative effort with the U.S. Department of the Interior to study and test the State's carbonate- rock aquifers. The studies were focused on southern Nevada and were intended to address the following concerns: Where is water potentially available in the aquifers?; How much water potentially can bewithdrawn from aquifers?; and What effects might result from development of the aquifers? The studies included basic-data collection, geologic mapping, geophysical and geochemical analyses, well drilling, and aquifer testing. The studies showed that the carbonate rocks are continuous and extensive enough to form regional aquifer systems only beneath thecentral third of the region. About 130,000 acre-feet per year of ground water flows through all the aquifers in this corridor (carbonate and noncarbonate), and about 77,000 acre-feet per year discharges directly from the carbonate-rock aquifers at regional springs in southern Nevada or at discharge areas in Death Valley, California. A larger volume of water -as much as 6 million acre-feet in the upper 100 feet alone-is stored in the rocks. Once depleted, however, that resource would be replenished by natural processes only very slowly. Ultimately, long-term development of the carbonate-rock aquifers would result in depletion of stored water, or in the capture of water that otherwise would discharge from the aquifers of southern Nevada and vicinity, or both. In manyplaces, development might extract water from both carbonate-rock and basin-fill aquifers. Possible effects of developing the carbonate-rock aquifers include declining water levels, decreasing springflow rates, drying up of some streams, playas, and meadows, and changing water quality. Specific impacts would depend upon the magnitude and length of development and site-specific conditions around the areas where the water is withdrawn. Confidence in predictions of the potential effects ofdevelopment of the carbonate-rock aquifers will remain limited until observations become available that document changes as the aquifers respond locally to long-term pumping stresses. However, if staged development were undertaken together with adequate monitoring, effects of continued or increased development could be estimated with progressively higher degrees of confidence.

Water-Resources Investigations Report↗
USGS1960

The chief oxide-burgin area discoveries, East Tintic district, Utah; A case history

In 1955 exploration for base and precious metals was undertaken by Bear Creek Mining Company immediately north of the Main Tintic district , Utah . During the course of this work Bear Creek became interested in the East Tintic district , primarily as a result of the activities of the U.S. Geological Survey in that area . Data published on the East Tintic district by the Survey and others were studied and map data made available from various mining companies were compiled. Preliminary economic studies were made to determine the present day value of the type of ore body discovered previously in the district . Encouraging results from these investigations led to the selection of specific targets for exploration. Recommendations for a project program were made and approved. Negotiations were successfully concluded in mid-1956 for a unit lease agreement on lands in the East Tintic district owned by the Tintic Standard and Chief Consolidated mining companies and their subsidiaries. Of the targets selected for exploration, the Chief Oxide area seemed to be one of the most prominent. Our preliminary work in the Chief Oxide area corroborated the findings of the U.S. Geological Survey described in Part I of this paper. After careful consideration it was decided to gamble the cost of an exploration shaft in this area for the purpose of providing an underground drilling platform. We also hoped by means of underground workings to establish the existence and nature of the postulated fault. A limited amount of surface drilling was done prior to shaft sinking in order to locate a shaft site and also to obtain additional information of subvolcanic structure and alteration. Underground exploration in the Chief Oxide area was started in January, 1957. The Burgin shaft was sunk to a depth of 1,100 feet and by August, 1959, lateral development on the 1050 level totaled 4,721 feet and underground diamond drilling totaled 15,480 feet. Results of the work done to date are as follows: ( a ) Identification of the sedimentary rock section and an interpretation of the structure in the Burgin mine area . (b) Discovery by penetration of the previously postulated East Tintic thrust fault. (c) Discovery of large zones of manganese oxides and carbonates, which were found to be closely related to silver-lead-zinc mineralization. (d) Discovery of ore-grade lead-zinc mineralization within the footwall rocks of the East Tintic thrust. (e) Discovery of high-grade silver-lead ore within the thrust zone. Insofar as ore localization is concerned the most important structural feature in the Burgin mine area is the East Tintic thrust fault- a fact that opens up new ore potential over a large part of the East Tintic district not previously explored. Although the discoveries to date must be attributed essentially to the application of geology to exploration, the tools of geochemical prospecting and geophysics were also used, and the geochemical work, in particular, was found to be a definite aid in the selection of areas for further exploration.

Utah↗
USGS2007

Abstracts of the Annual Meeting of Planetary Geologic Mappers, Tucson, AZ 2007

Introduction Report of the Annual Mappers Meeting Planetary Science Institute Tucson, Arizona June 28 and 29, 2007 Approximately 22 people attended this year's mappers meeting, and many more submitted abstracts and maps in absentia. The 2007 meeting was convened by Tracy Gregg, Les Bleamaster, Steve Saunders, and Ken Tanaka and was hosted by David Crown and Les Bleamaster of the Planetary Science Institute (PSI) in Tucson, Arizona. Oral presentations and poster discussions took place on Thursday, June 28 and Friday, June 29. This year's meeting also included a unique opportunity to visit the operations centers of two active Mars missions; field trips to the University of Arizona took place on Thursday and Friday afternoons. Outgoing Geologic Mapping Subcommittee (GEMS) chairperson, Tracy Gregg, commenced the meeting with an introduction and David Crown followed with a discussion of logistics and the PSI facility; Steve Saunders (Planetary Geology and Geophysics Discipline Scientist) then provided a brief program update. Science presentations kicked off with Venus mapper Vicki Hansen and graduate students Eric Tharalson and Bhairavi Shankar of the University of Minnesota, Duluth, showing a 3-D animation of the global distribution of tesserae and discussing the implications, a progress report for V-45 quadrangle mapping, and a brief discussion of circular lows. Les Bleamaster (PSI) followed with a progress report on mapping of the V-50 quadrangle and the 1:10M Helen Planitia quadrangle. David Crown (PSI) concluded the Venus presentations with a discussion of progress made on the V-30 quadrangle. The remainder of Thursday's presentations jumped around the Solar System including Mars, Io, and Earth. Ken Tanaka of the U.S. Geological Survey (USGS) began the afternoon with a general discussion of the status of the planetary mapping program at USGS. Buck Janes (University of Arizona) provided background information about the Mars Odyssey Gamma Ray Spectrometer (GRS) and presented some new element maps, which may be useful for geologic mapping. Dave Williams of Arizona State University reported on the progress of his global Io map and James Dohm (University of Arizona) discussed results of terrestrial remote mapping studies. Thursday afternoon, the mappers were given a tour of the High Resolution Imaging Science Experiment (HiRISE) operations facility and were given some basic information about how the images are obtained, processed, and publicly released. With official GEMS transition completed at lunch on Thursday, incoming GEMS chair Leslie Bleamaster took the reigns of Friday's meeting. Science presentations began with Ken Tanaka discussing 1:20M-scale global and 1:2M-scale polar mapping of Mars. Jim Zimbelman (Smithsonian Institution) described his 1:1M Medusae Fossae map (MC-8 SE), which is nearing completion, and new mapping (MC-16 NW and MC-23 NW) to further evaluate the Medusae Fossae. Brent Garry, also of the Smithsonian Institution, presented work on Ascraeus Mons. Peter Mouginis-Mark (University of Hawai`i) reported progress on his 1:200K and larger maps of Tooting crater and of the Olympus Mons summit caldera. Laszlo Keszthelyi (USGS) presented mapping of Athabasca Valles, with much of the credit going to Windy Jaeger. Jim Skinner (USGS) introduced a new mapping project including nine MTM quadrangles in the Utopia Planitia region. Tracy Gregg finished off the day's science presentations with discussion of Hesperia Planum. After discussion was complete, the group once again traveled to the University of Arizona - this time for a tour of the Mars Phoenix operations center. Principal Investigator Peter Smith beamed as he led mappers through the multi-million dollar facility. A main topic of discussion throughout the entire meeting was that of nomenclature, specifically how to classify the individual depressions at the tops of volcanoes. Paterae, as has been used for Mars, Venus, and Io, was suggested, but i

Open-File Report↗
USGS2022

Mapping the altitude of the top of the Dockum Group and paleochannel analysis using surface geophysical methods on and near Cannon Air Force Base in Curry County, New Mexico, 2020

The hydrogeology on and near Cannon Air Force Base (AFB) in eastern New Mexico was assessed to gain a better understanding of preferential groundwater flow paths through paleochannels. In and near the study area, paleochannels incised the top surface of the Dockum Group (Chinle Formation) and were subsequently filled in with electrically resistive coarse-grained sediments of the overlying Ogallala Formation, resulting in a preferential groundwater flow path in the form of a paleochannel network. A better understanding of the spatial characteristics of this preferential groundwater flow path is needed to support ongoing efforts to remediate groundwater contamination at Cannon AFB. Therefore, the U.S. Geological Survey, in cooperation with the U.S. Air Force Civil Engineer Center, used surface geophysical resistivity methods and data compiled from previous studies to better understand the spatial distribution and characteristics of the paleochannel network incised into the top of the Dockum Group. Previous studies have shown these paleochannels incised into the top of the Dockum Group with increasing resolution, but limited borehole data on and near Cannon AFB continued to make accurately mapping the top of Dockum Group challenging. For this study, surface geophysical resistivity measurements in the form of time-domain electromagnetic soundings made by the U.S. Geological Survey were used in conjunction with data previously published by Architecture, Engineering, Construction, Operations, and Management and borehole data compiled from the New Mexico Water Rights Reporting System database to prepare an updated map of the top of the Dockum Group that includes the location and characteristics of paleochannels incised into the top of the Dockum Group (Chinle Formation). A total of 149 borehole picks (determinations of the tops and bases of geologic units and their hydrogeologic-unit equivalents) were obtained from previous studies, along with 72 additional borehole picks from the New Mexico Water Rights Reporting System database and 43 picks from newly collected time-domain electromagnetic soundings. The data were gridded and contoured using Oasis Montaj v. 9.8.1. The updated map of the top of Dockum Group has many areas of uncertainty greater than 20 feet, because there are not enough data for the gridding process to reliably determine a value. However, this interpretation of the altitude of the top of the Dockum Group represents a substantial improvement in data resolution compared to previous studies. Two methodologies were used to evaluate paleochannels incised in the top of the Dockum Group across the study area: (1) trend-removal grid analysis and (2) analysis with Esri’s ArcMap Hydrology toolset. These two paleochannel analysis techniques show groundwater flow direction as well as areas having the deepest saturated thickness. Hydrologically, these techniques show where aquifer storage is highest (in the deepest parts of the paleochannel network), as well as the spatial distribution of preferential groundwater flow paths (the paleochannels). The analyses indicate a large paleochannel trending to the southeast, with smaller channels feeding in from the west. Areas where groundwater management could be more beneficial are indicated by locations where these flow lines intersect the deeper parts of the paleochannel.

New Mexico↗
USGS2007

Hydrology of Polk County, Florida

Local water managers usually rely on information produced at the State and regional scale to make water-resource management decisions. Current assessments of hydrologic and water-quality conditions in Polk County, Florida, commonly end at the boundaries of two water management districts (South Florida Water Management District and the Southwest Florida Water Management District), which makes it difficult for managers to determine conditions throughout the county. The last comprehensive water-resources assessment of Polk County was published almost 40 years ago. To address the need for current countywide information, the U.S. Geological Survey began a 3?-year study in 2002 to update information about hydrologic and water-quality conditions in Polk County and identify changes that have occurred. Ground-water use in Polk County has decreased substantially since 1965. In 1965, total ground-water withdrawals in the county were about 350 million gallons per day. In 2002, withdrawals totaled about 285 million gallons per day, of which nearly 95 percent was from the Floridan aquifer system. Water-conservation practices mainly related to the phosphate-mining industry as well as the decrease in the number of mines in operation in Polk County have reduced total water use by about 65 million gallons per day since 1965. Polk County is underlain by three principal hydrogeologic units. The uppermost water-bearing unit is the surficial aquifer system, which is unconfined and composed primarily of clastic deposits. The surficial aquifer system is underlain by the intermediate confining unit, which grades into the intermediate aquifer system and consists of up to two water-bearing zones composed of interbedded clastic and carbonate rocks. The lowermost hydrogeologic unit is the Floridan aquifer system. The Floridan aquifer system, a thick sequence of permeable limestone and dolostone, consists of the Upper Floridan aquifer, a middle semiconfining unit, a middle confining unit, and the Lower Floridan aquifer. The Upper Floridan aquifer provides most of the water required to meet demand in Polk County. Data from about 300 geophysical and geologic logs were used to construct hydrogeologic maps showing the tops and thicknesses of the aquifers and confining units within Polk County. Thickness of the surficial aquifer system ranges from several feet thick or less in the extreme northwestern part of the county and along parts of the Peace River south of Bartow to more than 200 feet along the southern part of the Lake Wales Ridge in eastern Polk County. Thickness of the intermediate aquifer system/intermediate confining unit is highly variable throughout the county because of past erosional processes and sinkhole formation. Thickness of the unit ranges from less than 25 feet in the extreme northwestern part of the county to more than 300 feet in southwestern Polk County. The altitude of the top of the Upper Floridan aquifer in the county ranges from about 50 feet above National Geodetic Vertical Datum of 1929 (NGVD 29) in the northwestern part to more than 250 feet below NGVD 29 in the southern part. Water levels in the Upper Floridan aquifer fluctuate seasonally, increasing during the wet season (June through September) and decreasing during the rest of the year. Water levels in the Upper Floridan aquifer also can change from year to year, depending on such factors as pumpage and climatic variations. In the southwestern part of the county, fluctuations in water use related to phosphate mining have had a major impact on ground-water levels. Hydrographs of selected wells in southwestern Polk County show a general decline in water levels that ended in the mid-1970s. This water-level decline coincides with an increase in water use associated with phosphate mining. A substantial increase in water levels that began in the mid-1970s coincides with a period of decreasing water use in the county. Despite reductions in water use since 1970, howev

Scientific Investigations Report↗
USGS2006

Distribution of stress drop, stiffness, and fracture energy over earthquake rupture zones

Using information provided by slip models and the methodology of McGarr and Fletcher (2002), we map static stress drop, stiffness (k = ????/u, where ???? is static stress drop and u is slip), and fracture energy over the slip surface to investigate the earthquake rupture process and energy budget. For the 1994 M6.7 Northridge, 1992 M7.3 Landers, and 1995 M6.9 Kobe earthquakes, the distributions of static stress drop show strong heterogeneity, emphasizing the importance of asperities in the rupture process. Average values of static stress drop are 17, 11, and 4 Mpa for Northridge, Landers, and Kobe, respectively. These values are substantially higher than estimates based on simple crack models, suggesting that the failure process involves the rupture of asperities within the larger fault zone. Stress drop as a function of depth for the Northridge and Landers earthquakes suggests that stress drops are limited by crustal strength. For these two earthquakes, regions of high slip are surrounded by high values of stiffness. Particularly for the Northridge earthquake, the prominent patch of high slip in the central part of the fault is bordered by a ring of high stiffness and is consistent with expectations based on the failure of an asperity loaded at its edge due to exterior slip. Stiffness within an asperity is inversely related to its dimensions. Estimates of fracture energy, based on static stress drop, slip, and rupture speed, were used to investigate the nature of slip weakening at four locations near the hypocenter of the Kobe earthquake for comparison with independent results based on a dynamic model of this earthquake. One subfault updip and to the NE of the hypocenter has a fracture energy of 1.1 MJ/m2 and a slip-weakening distance, Dc, of 0.66 m. Right triangles, whose base and height are Dc and the dynamic stress drop, respectively, approximately overlie the slip-dependent stress given by Ide and Takeo (1997) for the same locations near the hypocenter. The total fracture energy for the Kobe earthquake, 3.7 ?? 1014 J, is about the same as the seismic energy (Ea = 3.2 ?? 1014 J.

Journal of Geophysical Research B: Solid Earth↗
USGS2007

Hydrostratigraphic Framework and Selection and Correlation of Geophysical Log Markers in the Surficial Aquifer System, Palm Beach County, Florida

The surficial aquifer system is the major source of freshwater for public water supply in Palm Beach County, Florida, yet many previous studies of the hydrogeology of this aquifer system have focused only on the eastern one-half to one-third of the county in the more densely populated coastal area (Land and others, 1973; Swayze and others, 1980; Swayze and Miller, 1984; Shine and others, 1989). Population growth in the county has resulted in the westward expansion of urbanized areas into agricultural areas and has created new demands on the water resources of the county. Additionally, interest in surface-water resources of central and western areas of the county has increased. In these areas, plans for additional surface-water storage reservoirs are being made under the Comprehensive Everglades Restoration Plan originally proposed by the U.S. Army Corps of Engineers and the South Florida Water Management District (1999), and stormwater treatment areas have been constructed by the South Florida Water Management District. Surface-water and ground-water interactions in the Everglades are thought to be important to water budgets, water quality, and ecology (Harvey and others, 2002). Most of the previous hydrogeologic and ground-water flow simulation studies of the surficial aquifer system have not utilized a hydrostratigraphic framework, in which stratigraphic or sequence stratigraphic units, such as those proposed in Cunningham and others (2001), are delineated in this stratigraphically complex aquifer system. A thick zone of secondary permeability mapped by Swayze and Miller (1984) was not subdivided and was identified as only being within the Anastasia Formation of Pleistocene age. Miller (1987) published 11 geologic sections of the surficial aquifer system, but did not delineate any named stratigraphic units in these sections. This limited interpretation has resulted, in part, from the complex facies changes within rocks and sediments of the surficial aquifer system and the seemingly indistinct and repetitious nature of the most common lithologies, which include sand, shell, sandstone, and limestone. Model construction and layer definition in a simulation of ground-water flow within the surficial aquifer system of Palm Beach County utilized only the boundaries of one or two major hydrogeologic zones, such as the Biscayne aquifer and surficial aquifer system; otherwise layers were defined by average elevations rather than geologic structure or stratigraphy (Shine and others, 1989). Additionally, each major permeable zone layer in the model was assumed to have constant hydraulic conductivity with no allowance for the possibility of discrete (thin) flow zones within the zone. The key to understanding the spatial distribution and hydraulic connectivity of permeable zones in the surficial aquifer system beneath Palm Beach County is the development of a stratigraphic framework based on a consistent method of county-wide correlation. Variability in hydraulic properties in the system needs to be linked to the stratigraphic units delineated in this framework, and proper delineation of the hydrostratigraphic framework should provide a better understanding and simulation of the ground-water flow system. In 2004, the U.S. Geological Survey, in cooperation with the South Florida Water Management District, initiated an investigation to develop a hydrostratigraphic framework for the surficial aquifer system in Palm Beach County.

Scientific Investigations Map↗
USGS2009

Geophysical Characterization of the Quaternary-Cretaceous Contact Using Surface Resistivity Methods in Franklin and Webster Counties, South-Central Nebraska

To help manage and understand the Platte River system in Nebraska, the Platte River Cooperative Hydrology Study (COHYST), a group of state and local governmental agencies, developed a regional ground-water model. The southern boundary of this model lies along the Republican River, where an area with insufficient geologic data immediately north of the Republican River led to problems in the conceptualization of the simulated flow system and to potential problems with calibration of the simulation. Geologic descriptions from a group of test holes drilled in south-central Nebraska during 2001 and 2002 indicated a possible hydrologic disconnection between the Quaternary-age alluvial deposits in the uplands and those in the Republican River lowland. This disconnection was observed near a topographic high in the Cretaceous-age Niobrara Formation, which is the local bedrock. In 2003, the U.S. Geological Survey, in cooperation with the COHYST, collected surface geophysical data near these test holes to better define this discontinuity. Two-dimensional imaging methods for direct-current resistivity and capacitively coupled resistivity were used to define the subsurface distribution of resistivity along several county roads near Riverton and Inavale, Nebraska. The relation between the subsurface distribution of resistivity and geology was defined by comparing existing geologic descriptions of test holes to surface-geophysical resistivity data along two profiles and using the information gained from these comparisons to interpret the remaining four profiles. In all of the resistivity profile sections, there was generally a three-layer subsurface interpretation, with a resistor located between two conductors. Further comparison of geologic data with the geophysical data and with surficial features was used to identify a topographic high in the Niobrara Formation near the Franklin Canal which was coincident with a resistivity high. Electrical properties of the Niobrara Formation made accurate interpretation of the resistivity profile sections difficult and less confident because of similar resistivity of this formation and that of the coarser-grained sediment of the Quaternary-age deposits. However, distinct conductive features were identified within the resistivity profile sections that aided in delineating the contact between the resistive Quaternary-age deposits and the resistive Niobrara Formation. Using this information, an interpretive boundary was drawn on the resistivity profile sections to represent the contact between the Quaternary-age alluvial deposits and the Cretaceous-age Niobrara Formation. A digital elevation model (DEM) of the top of the Niobrara Formation was constructed using the altitudes from the interpreted contact lines. This DEM showed that the general trend of top of the Niobrara Formation dips to the southeast. At the north edge of the study site, the Niobrara Formation topographic high trends east-west with an altitude range of 559 meters in the west to 543 meters in the east. Based on the land-surface elevation and the Niobrara Formation DEM, the estimated thickness of the Quaternary-age alluvial deposits throughout the study area was mapped and showed a thinning of the Quaternary-age alluvial deposits to the north, approximately where the topographic high of the Niobrara Formation is located. This topographic high in the Niobrara Formation has the potential to act as a barrier to ground-water flow from the uplands alluvial aquifer to the Republican River alluvial aquifer as shown in the resistivity profile sections. The Quaternary-age alluvial deposits in the uplands and those in the Republican River Valley are not fully represented as disconnected because it is possible that there are ground-water flow paths that were not mapped during this study.

Scientific Investigations Report↗
USGS2008

Synthesis of the Hydrogeologic Framework of the Floridan Aquifer System and Delineation of a Major Avon Park Permeable Zone in Central and Southern Florida

The carbonate Floridan aquifer system of central and southern Florida (south of a latitude of about 29 degrees north) is an invaluable resource with a complex framework that has previously been mapped and managed primarily in a subregional context according to geopolitical boundaries. As interest and use of the Floridan aquifer system in this area increase, a consistent regional hydrogeologic framework is needed for effective management across these boundaries. This study synthesizes previous studies on the Floridan aquifer system and introduces a new regional hydrogeologic conceptual framework, linking physical relations between central and southern Florida and between the west and east coastal areas. The differences in hydrogeologic nomenclature and interpretation across the study area from previous studies were identified and resolved. The Floridan aquifer system consists of the Upper Floridan aquifer, middle confining unit, and Lower Floridan aquifer. This study introduces and delineates a new major, regional productive zone or subaquifer, referred to as the Avon Park permeable zone. This zone is contained within the middle confining unit and synthesizes an extensive zone that has been referred to differently in different parts of the study area in previous studies. The name of this zone derives from the description of this zone as the ?Avon Park highly permeable zone? in west-central Florida in a previous study. Additionally, this zone has been identified previously in southeastern Florida as the ?middle Floridan aquifer.? An approximately correlative or approximate time-stratigraphic framework was developed and was used to provide guidance in the identification and determination of aquifers, subaquifers, and confining units within the Floridan aquifer system and to determine their structural relations. Two stratigraphic marker horizons within the Floridan aquifer system and a marker unit near the top of the aquifer system were delineated or mapped. The marker horizons are correlative points in the stratigraphic section rather than a unit with upper and lower boundaries. The two marker horizons and the marker unit originated from previous studies, wherein they were based on lithology and correlation of geophysical log signatures observed in boreholes. The depths of these marker horizons and the marker unit were extended throughout the study area by correlation of natural gamma-ray logs between wells. The Floridan aquifer system includes, in ascending order, the upper part of the Cedar Keys Formation, Oldsmar Formation, Avon Park Formation, Ocala Limestone, Suwannee Limestone, and in some areas the lower part of the Hawthorn Group. The first marker horizon is in the lower part of the aquifer system near the top of the Oldsmar Formation and is associated with the top of distinctive glauconitic limestone beds that are present in some regions; the second marker horizon is near the middle of the aquifer system in the middle part of the Avon Park Formation. The marker unit lies at the top of a basal unit in the Hawthorn Group and provides a stratigraphic constraint for the top of the Floridan aquifer system. The marker horizons do not have distinguishing lithologic characteristics or a characteristic gamma-ray log pattern in all areas but are still thought to be valid because of correlation of the entire section and correlation of all sufficiently deep wells with gamma-ray logs. The Avon Park permeable zone is contained entirely within the Avon Park Formation; its position within the section is either near the middle Avon Park marker horizon or within a thick part of the section that extends several hundred feet above the marker horizon. This subaquifer is present over most of the study area and characteristically consists of thick units of dolostone and interbedded limestone, and limestone in its upper part. Permeability is primarily associated with fracturing. This subaquifer is well developed in west-cen

Scientific Investigations Report↗
USGS1998

Paleoseismic investigations in the Santa Cruz mountains, California: Implications for recurrence of large-magnitude earthquakes on the San Andreas fault

Trenching, microgeomorphic mapping, and tree ring analysis provide information on timing of paleoearthquakes and behavior of the San Andreas fault in the Santa Cruz mountains. At the Grizzly Flat site alluvial units dated at 1640–1659 A.D., 1679–1894 A.D., 1668–1893 A.D., and the present ground surface are displaced by a single event. This was the 1906 surface rupture. Combined trench dates and tree ring analysis suggest that the penultimate event occurred in the mid-1600 s, possibly in an interval as narrow as 1632–1659 A.D. There is no direct evidence in the trenches for the 1838 or 1865 earthquakes, which have been proposed as occurring on this part of the fault zone. In a minimum time of about 340 years only one large surface faulting event (1906) occurred at Grizzly Flat, in contrast to previous recurrence estimates of 95–110 years for the Santa Cruz mountains segment. Comparison with dates of the penultimate San Andreas earthquake at sites north of San Francisco suggests that the San Andreas fault between Point Arena and the Santa Cruz mountains may have failed either as a sequence of closely timed earthquakes on adjacent segments or as a single long rupture similar in length to the 1906 rupture around the mid-1600 s. The 1906 coseismic geodetic slip and the late Holocene geologic slip rate on the San Francisco peninsula and southward are about 50–70% and 70% of their values north of San Francisco, respectively. The slip gradient along the 1906 rupture section of the San Andreas reflects partitioning of plate boundary slip onto the San Gregorio, Sargent, and other faults south of the Golden Gate. If a mid-1600 s event ruptured the same section of the fault that failed in 1906, it supports the concept that long strike-slip faults can contain master rupture segments that repeat in both length and slip distribution. Recognition of a persistent slip rate gradient along the northern San Andreas fault and the concept of a master segment remove the requirement that lower slip sections of large events such as 1906 must fill in on a periodic basis with smaller and more frequent earthquakes.

Journal of Geophysical Research B: Solid Earth↗
USGS1981

Significant results from using earth observation satellites for mineral and energy resource exploration

A large number of Earth-observation satellites orbit our world several times each day, providing new information about the land and sea surfaces and the overlying thin layer of atmosphere that makes our planet unique. Meteorological satellites have had the longest history of experimental use and most are now considered operational. The geologic information collected by the Landsat, Polar Orbiting Geophysical Observatory (POGO), Magsat, Heat Capacity Mapping Mission (HCMM) and Seasat land and ocean observation systems is being thoroughly tested, and some of these systems are now approaching operational use. Landsat multispectral images provide views of large areas of the Earth under uniform lighting conditions and can be obtained at a variety of scales and formats. Not only do the Landsat data provide highly useful images showing surficial materials and structures such as folds and faults, but also measurements and computer-derived ratios of the brightness of different rock types, alteration zones, and mineral associations. These data have led to the finding of a variety of new ore deposits. In addition, the combination of Landsat digital data and aeromagnetic data has extended the use of Landsat as an exploration tool which can be used to readily relate surface features to subsurface anomalies. Magsat data, now being collected, are helping refine information on major crustal anomalies that were first recognized during the analysis of POGO data. The more nearly circular orbit, lower altitude, and increased sophistication of its vector magnetometer enable Magsat to provide more precise information than POGO. Information of this type is required to develop crustal models. Although Magsat is designed to operate for only 4–8 months, the number of orbits that it should be able to make will be sufficient to accomplish its mission and to record a major magnetic storm expected in 1980. HCMM is a two-band visible to near-IR (0.55–1.1 μm) and thermal infrared (10.2–12.5 μm) system designed to measure reflected solar energy, determine the heat capacity of rocks and to monitor soil moisture, thermal effluents, plant canopy temperatures and snow cover. Launched in April 1978, it is in sun-synchronous, circular orbit at an altitude of 620 km. It is a relatively low-resolution system with an instantaneous field of view (IFOV) of 500–600 m and a swath width of 716 km. However, the system is designed to detect objects in the range of 260°–340° K with a sensitivity (NEδT) of 0.4°K at 280°. Recording the thermal radiation of urban heat islands and high thermal inertia of quartzite strata in the Appalachian region are two examples of its land applications. Launched in June 1978, Seasat operated for only 100 days, but successfully acquired much information over both sea and land. The collection of synthetic aperture radar (SAR) imagery and radar altimetry was particularly important to geologists. Although there are difficulties in processing and distributing these data in a timely manner, initial evaluations indicate that the radar imagery supplements Landsat data by increasing the spectral range and offering a different look angle. The radar altimeter provides accurate profiles over narrow strips of land (1 km wide) and has demonstrated usefulness in measuring icecap surfaces (Greenland, Iceland, and Antarctica). The Salar of Uyuni in southern Bolivia served as a calibration site for the altimeter and has enabled investigators to develop a land-based smoothing algorithm that is believed to increase the accuracy of the system to 10 cm. Data from the altimeter are currently being used to measure subsidence resulting from ground water withdrawal in the Phoenix-Tucson area.

Advances in Space Research↗
USGS2001

Surface-geophysical characterization of ground-water systems of the Caloosahatchee River basin, southern Florida

The Caloosahatchee River Basin, located in southwestern Florida, includes about 1,200 square miles of land. The Caloosahatchee River receives water from Lake Okeechobee, runoff from the watershed, and seepage from the underlying ground-water systems; the river loses water through drainage to the Gulf of Mexico and withdrawals for public-water supply and agricultural and natural needs. Water-use demands in the Caloosahatchee River Basin have increased dramatically, and the Caloosahatchee could be further stressed if river water is used to accommodate restoration of the Everglades. Water managers and planners need to know how much water will be used within the river basin and how much water is contributed by Lake Okeechobee, runoff, and ground water. In this study, marine seismic-reflection and ground-penetrating radar techniques were used as a means to evaluate the potential for flow between the river and ground-water systems. Seven test coreholes were drilled to calibrate lithostratigraphic units, their stratal geometries, and estimated hydraulic conductivities to surface-geophysical profiles. A continuous marine seismic-reflection survey was conducted over the entire length of the Caloosahatchee River and extending into San Carlos Bay. Lithostratigraphic units that intersect the river bottom and their characteristic stratal geometries were identified. Results show that subhorizontal reflections assigned to the Tamiami Formation intersect the river bottom between Moore Haven and about 9 miles westward. Oblique and sigmoidal progradational reflections assigned to the upper Peace River Formation probably crop out at the floor of the river in the Ortona area between the western side of Lake Hicpochee and La Belle. These reflections image a regional-scale progradational deltaic depositional system containing quartz sands with low to moderate estimated hydraulic conductivities. In an approximate 6-mile length of the river between La Belle and Franklin Lock, deeper karstic collapse structures are postulated. These structures influence the geometries of parallel reflections that intersect the river channel. Here, reflections assigned to the Buckingham Limestone Member of the Tamiami Formation (a confining unit) and reflections assigned to the clastic zone of the sandstone aquifer likely crop out at the river bottom. Beneath these shallow reflections, relatively higher amplitude parallel reflections of the carbonate zone of the sandstone aquifer are well displayed in the seismic-reflection profiles. In San Carlos Bay, oblique progradational reflections assigned to the upper Peace River Formation are shown beneath the bay. Almost everywhere beneath the river, a diffuse ground-water flow system is in contact with the channel bottom. Ground-penetrating radar profiles of an area about 2 miles north of the depositional axis of the deltaic depositional system in the Ortona area show that progradational clinoforms imaged on seismic reflection profiles in the Caloosahatchee River are present within about 17 feet of the ground surface. Ground-penetrating radar profiles show southward dipping, oblique progradational reflections assigned to the upper Peace River Formation that are terminated at their tops by a toplapping or erosional discontinuity. These clinoformal reflections image clean quartz sand that is probably characterized by moderate hydraulic conductivity. This sand could be mapped using ground-penetrating radar methods.

Water-Resources Investigations Report↗