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At least 307 records · Page 17Linked to original sources
USGS2022

Using carbon, nitrogen, and mercury isotope values to distinguish mercury sources to Alaskan lake trout

Lake trout ( Salvelinus namaycush ), collected from 13 remote lakes located in southwestern Alaska, were analyzed for carbon, nitrogen, and mercury (Hg) stable isotope values to assess the importance of migrating oceanic salmon, volcanic activity, and atmospheric deposition to fish Hg burden. Methylmercury (MeHg) bioaccumulation in phytoplankton (5.0–6.9 kg L –1 ) was also measured to quantify the basal uptake of MeHg to these aquatic food webs. Hg isotope values in lake trout revealed that while the extent of precipitation-delivered Hg was similar across the entire study area, volcanic Hg is likely an important additional source to lake trout in proximate lakes. In contrast, migratory salmon ( Oncorhynchus nerka ) deliver little MeHg to lake trout directly, although indirect delivery processes via decay could exist. A high level of variability in carbon, nitrogen, and Hg isotope values indicates niche partitioning in lake trout populations within each lake and that a complex suite of ecological interactions is occurring, complicating the conceptually linear assessment of the contaminant source to the receiving organism. Without connecting energy and contaminant isotope axes, we would not have understood why lake trout from these pristine lakes have highly variable Hg burdens despite consistently low water Hg and comparable age-length dynamics.

Alaska↗
USGS2005

Triggering of tsunamigenic aftershocks from large strike‐slip earthquakes: Analysis of the November 2000 New Ireland earthquake sequence

[1] The November 2000 New Ireland earthquake sequence started with a M w = 8.0 left‐lateral main shock on 16 November and was followed by a series of aftershocks with primarily thrust mechanisms. The earthquake sequence was associated with a locally damaging tsunami on the islands of New Ireland and nearby New Britain, Bougainville, and Buka. Results from numerical tsunami‐propagation models of the main shock and two of the largest thrust aftershocks ( M w > 7.0) indicate that the largest tsunami was caused by an aftershock located near the southeastern termination of the main shock, off the southern tip of New Ireland (Aftershock 1). Numerical modeling and tide gauge records at regional and far‐field distances indicate that the main shock also generated tsunami waves. Large horizontal displacements associated with the main shock in regions of steep bathymetry accentuated tsunami generation for this event. Most of the damage on Bougainville and Buka Islands was caused by focusing and amplification of tsunami energy from a ridge wave between the source region and these islands. Modeling of changes in the Coulomb failure stress field caused by the main shock indicate that Aftershock 1 was likely triggered by static stress changes, provided the fault was on or synthetic to the New Britain interplate thrust as specified by the Harvard CMT mechanism. For other possible focal mechanisms of Aftershock 1 and the regional occurrence of thrust aftershocks in general, evidence for static stress change triggering is not as clear. Other triggering mechanisms such as changes in dynamic stress may also have been important. The 2000 New Ireland earthquake sequence provides evidence that tsunamis caused by thrust aftershocks can be triggered by large strike‐slip earthquakes. Similar tectonic regimes that include offshore accommodation structures near large strike‐slip faults are found in southern California, the Sea of Marmara, Turkey, along the Queen Charlotte fault in British Columbia, and near the Alpine fault of New Zealand. Results from this study and previous stress modeling studies suggest that the likelihood of local tsunamis in these regions may significantly increase after a great strike‐slip earthquake.

New Britain, New Ireland↗
USGS1995

Fe(III) and S0 reduction by Pelobacter carbinolicus

There is a close phylogenetic relationship between Pelobacter species and members of the genera Desulfuromonas and Geobacter , and yet there has been a perplexing lack of physiological similarities. Pelobacter species have been considered to have a fermentative metabolism. In contrast, Desulfuromonas and Geobacter species have a respiratory metabolism with Fe(III) serving as the common terminal electron acceptor in all species. However, the ability of Pelobacter species to reduce Fe(III) had not been previously evaluated. When a culture of Pelobacter carbinolicus that had grown by fermentation of 2,3- butanediol was inoculated into the same medium supplemented with Fe(III), the Fe(III) was reduced. There was less accumulation of ethanol and more production of acetate in the presence of Fe(III). P. carbinolicus grew with ethanol as the sole electron donor and Fe(III) as the sole electron acceptor. Ethanol was metabolized to acetate. Growth was also possible on Fe(III) with the oxidation of propanol to propionate or butanol to butyrate if acetate was provided as a carbon source. P. carbinolicus appears capable of conserving energy to support growth from Fe(III) respiration as it also grew with H 2 or formate as the electron donor and Fe(III) as the electron acceptor. Once adapted to Fe(III) reduction, P. carbinolicus could also grow on ethanol or H 2 with S 0 as the electron acceptor. P. carbinolicus did not contain detectable concentrations of the c -type cytochromes that previous studies have suggested are involved in electron transport to Fe(III) in other organisms that conserve energy to support growth from Fe(III) reduction. These results demonstrate that P. carbinolicus may survive in some sediments as an Fe(III) or S 0 reducer rather than growing fermentatively on rare substrates or syntrophically as an ethanol-oxidizing acetogen. These studies also suggest that the ability to use Fe(III) as a terminal electron acceptor may be an important unifying feature of the Geobacter-Desulfuromonas- Pelobacter branch of the delta Proteobacteria .

Applied and Environmental Microbiology↗
USGS2017

Modeling of high‐frequency seismic‐wave scattering and propagation using radiative transfer theory

This is a study of the nonisotropic scattering process based on radiative transfer theory and its application to the observation of the M 4.3 aftershock recording of the 2008 Wells earthquake sequence in Nevada. Given a wide range of recording distances from 29 to 320 km, the data provide a unique opportunity to discriminate scattering models based on their distance‐dependent behaviors. First, we develop a stable numerical procedure to simulate nonisotropic scattering waves based on the 3D nonisotropic scattering theory proposed by Sato (1995). By applying the simulation method to the inversion of M 4.3 Wells aftershock recordings, we find that a nonisotropic scattering model, dominated by forward scattering, provides the best fit to the observed high‐frequency direct S waves and S ‐wave coda velocity envelopes. The scattering process is governed by a Gaussian autocorrelation function, suggesting a Gaussian random heterogeneous structure for the Nevada crust. The model successfully explains the common decay of seismic coda independent of source–station locations as a result of energy leaking from multiple strong forward scattering, instead of backscattering governed by the diffusion solution at large lapse times. The model also explains the pulse‐broadening effect in the high‐frequency direct and early arriving S waves, as other studies have found, and could be very important to applications of high‐frequency wave simulation in which scattering has a strong effect. We also find that regardless of its physical implications, the isotropic scattering model provides the same effective scattering coefficient and intrinsic attenuation estimates as the forward scattering model, suggesting that the isotropic scattering model is still a viable tool for the study of seismic scattering and intrinsic attenuation coefficients in the Earth.

Bulletin of the Seismological Society of America↗
USGS1987

Petrology and sedimentology of the Horlick Formation (Lower Devonian), Ohio Range, Transantarctic Mountains

The Horlick Formation of Early Devonian age is as thick as 50 m and consists of subhorizontal, interbedded subarkosic sandstone and chloritic shale and mudstone. The Horlick overlies an erosion surface cut into Ordovician granitic rocks and is, in turn, overlain by Carboniferous and Permian glacial and periglacial deposits. Textures, sedimentary structures, and ubiquitous marine body fossils and animal traces suggest that the Horlick was deposited on a shallow shelf having moderate wave energy and a moderate tidal range. The source terrane probably lay to the north, and longshore transport was toward the west.

Bulletin↗
USGS2015

Hydraulic, geomorphic, and trout habitat conditions of the Lake Fork of the Gunnison River in Hinsdale County, Lake City, Colorado, Water Years 2010-2011

Channel rehabilitation, or reconfiguration, to mitigate a variety of riverine problems has become a common practice in the western United States. However, additional work to monitor and assess the channel response to, and the effectiveness of, these modifications over longer periods of time (decadal or longer) is still needed. The Lake Fork of the Gunnison River has been an area of active channel modification to accommodate the needs of the Lake City community since the 1950s. The Lake Fork Valley Conservancy District began a planning process to assess restoration options for a reach of the Lake Fork in Lake City to enhance hydraulic and ecologic characteristics of the reach. Geomorphic channel form is affected by land-use changes within the basin and geologic controls within the reach. The historic channel was defined as a dynamic, braided channel with an active flood plain. This can result in a natural tendency for the channel to braid. A braided channel can affect channel stability of reconfigured reaches when a single-thread meandering channel is imposed on the stream. The U.S. Geological Survey, in cooperation with the Colorado Water Conservation Board and Colorado River Water Conservation District, began a study in 2010 to quantify existing hydraulic and habitat conditions for a reach of the Lake Fork of the Gunnison River in Lake City, Colorado. The purpose of this report is to quantify existing Lake Fork hydraulic and habitat conditions and establish a baseline against which post-reconfiguration conditions can be compared. This report (1) quantifies the existing hydraulic and geomorphic conditions in a 1.1-kilometer section of the Lake Fork at Lake City that has been proposed as a location for future channel-rehabilitation efforts, (2) characterizes the habitat suitability of the reach for two trout species based on physical conditions within the stream, and (3) characterizes the current riparian canopy density. The FaSTMECH computational flow-model within MD_SWMS was selected to characterize the effects of streamflow on hydraulic and habitat-suitability conditions for a study reach of the Lake Fork. Habitat suitability was evaluated for cutthroat ( Oncorhynchus clarkii ) and brown trout ( Salmo trutta morpha fario ) fry, juveniles, and adults. Microscale (point locations) and mesoscale (reach features) habitats were assessed using the combination of field observations, measurements, and hydraulic simulations within the study reach of the Lake Fork. Microscale trout habitat, presented as weighted usable area, generally increased as streamflow increased for both trout species and all life stages. Areas of suitable microscale habitat occur along the banks for flows of 900 cubic feet per second (ft 3 /s) and less. Out-of-bank areas became more substantial contributors to overall habitat availability for flows of 1,300 ft 3 /s or more when compared to other features. Adult habitat, for both trout species, was the most abundant habitat type for nearly all streamflows. In general, the upper reach provided 2–3 times more available habitat than the lower reach for both trout species. Mesoscale trout habitat of the Lake Fork was assessed based on the conditions present in the 150 ft 3 /s flow simulation as well as field observation. Both the upper and lower reach is primarily characterized as riffle/run habitat. The presence of pool habitat was limited throughout both reaches and occurred along the channel margins. For both reaches, the pool habitat was less than 5 percent of the total wetted area, a percentage that is substantially lower than the recommendations for sustainable populations of 40–70 percent. Areas of cover were adjacent to potential drift feeding areas in the lower reach, and often occurred within the same pool habitat. This may favor energy expenditure ratios of both fish species, wherein little energy is needed to acquire adequate food sources. Sediment mobility is an important process for flushing fine sediments from within the gravel frameworks. Evaluations of channel and flow characteristics at cross-section locations 2–8 show a range of streambed mobility. In general, boundary shear stress and streambed mobility increase with increases in streamflow. Within the cross sections, the greatest boundary shear stress occurs towards the center of the channel. Reach-scale assessment of sediment mobility in the lower reach shows increased streambed mobility. This is due in part to smaller grain sizes in the lower reach, but may also reflect the greater extent of channel alterations, specifically the temporary berms constructed by CDOT in the late 1980s and 1990s, present in this reach.

Colorado↗
USGS2024

Underwater flashlights: What light can tell us about water quality

Water is essential for life. The particles and dissolved chemicals found in rivers, lakes, and oceans are constantly changing with weather, seasons, and human activities. The substances found in water can be helpful or harmful to humans and other organisms. New technologies allow scientists to use waterproof computers (called sensors) to record the quality of water as it changes throughout the day or night. Many sensors use the interaction of water with light, or other energy forms, to learn about what is in it. Through observing what happens to light energy in water, scientists can reveal the sources and movements of harmful pollutants or essential nutrients for plants, animals, and bacteria. Networks containing many sensors work together to provide continuous information about the ever-changing conditions that affect all living organisms that depend on water.

Frontiers for Young Minds↗
USGS2022

Spatial models of jaguar energy expenditure in response to border wall construction and remediation

The construction of a wall at the United States-Mexico border is known to impede and deter movement of terrestrial wildlife between the two countries. One such species is the jaguar, in its northernmost range in the borderlands of Arizona and Sonora. We developed an anisotropic cost distance model for jaguar in a binational crossing area of the Madrean Sky Islands at the United States-Mexico border in Southern Arizona as a case study by using previously collected GPS tracking data for jaguars, bioenergetic calculations for pumas, and a digital elevation model. This model describes projected energy expenditure for jaguar to reach key water sources north of the international border. These desert springs and the broader study region provide vital habitat for jaguar conservation and reintroduction efforts in the United States. An emerging impediment to jaguar conservation and reintroduction is border infrastructure including border wall. By comparing walled and un-walled border sections, and three remediation scenarios, we demonstrate that existing border infrastructure significantly increases energy expenditure by jaguars and that some partial remediation scenarios are more beneficial than others. Our results demonstrate opportunities for remediation. Improved understanding of how border infrastructure impacts physiological requirements and resulting impacts to jaguar and other terrestrial wildlife in the United States-Mexico borderlands may inform conservation management.

Arizona↗
USGS2006

Evaluation of kinetic uncertainty in numerical models of petroleum generation

Oil-prone marine petroleum source rocks contain type I or type II kerogen having Rock-Eval pyrolysis hydrogen indices greater than 600 or 300-600 mg hydrocarbon/g total organic carbon (HI, mg HC/g TOC), respectively. Samples from 29 marine source rocks worldwide that contain mainly type II kerogen (HI = 230-786 mg HC/g TOC) were subjected to open-system programmed pyrolysis to determine the activation energy distributions for petroleum generation. Assuming a burial heating rate of 1??C/m.y. for each measured activation energy distribution, the calculated average temperature for 50% fractional conversion of the kerogen in the samples to petroleum is approximately 136 ?? 7??C, but the range spans about 30??C (???121-151??C). Fifty-two outcrop samples of thermally immature Jurassic Oxford Clay Formation were collected from five locations in the United Kingdom to determine the variations of kinetic response for one source rock unit. The samples contain mainly type I or type II kerogens (HI = 230-774 mg HC/g TOC). At a heating rate of 1??C/m.y., the calculated temperatures for 50% fractional conversion of the Oxford Clay kerogens to petroleum differ by as much as 23??C (127-150??C). The data indicate that kerogen type, as defined by hydrogen index, is not systematically linked to kinetic response, and that default kinetics for the thermal decomposition of type I or type II kerogen can introduce unacceptable errors into numerical simulations. Furthermore, custom kinetics based on one or a few samples may be inadequate to account for variations in organofacies within a source rock. We propose three methods to evaluate the uncertainty contributed by kerogen kinetics to numerical simulations: (1) use the average kinetic distribution for multiple samples of source rock and the standard deviation for each activation energy in that distribution; (2) use source rock kinetics determined at several locations to describe different parts of the study area; and (3) use a weighted-average method that combines kinetics for samples from different locations in the source rock unit by giving the activation energy distribution for each sample a weight proportional to its Rock-Eval pyrolysis S2 yield (hydrocarbons generated by pyrolytic degradation of organic matter). Copyright ?? 2006. The American Association of Petroleum Geologists. All rights reserved.

American Association of Petroleum Geologists Bulle↗
USGS1996

Origin of Bermuda's clay-rich Quaternary paleosols and their paleoclimatic significance

Red clayey paleosols that are chiefly the product of aerosolic dust deposition are interbedded in the Quaternary carbonate formations of the Bermuda oceanic island system. These paleosols provide a basis for reconstructing Quaternary atmospheric circulation patterns in the northwestern Atlantic. Geochemical analyses were performed on representative paleosol samples to identify their parent dust source. Fine-grained fractions were analyzed by energy-dispersive X ray fluorescence to determine trace element (Zr, Y, La, Ti, and Nb) concentrations and to derive geochemical signatures based on immobile element ratios. These ratios were compared with geochemical signatures determined for three possible sources of airborne dust: (1) Great Plains loess, (2) Mississippi River Valley loess, and (3) Saharan dust. The Zr/Y and Zr/La ratios provided the clearest distinction between the hypothesized dust sources. The low ratios in the paleosol B horizons most closely resemble Saharan dust in the <2-μm size class fraction. Contributions from the two North American loessial source areas could not be clearly detected. Thus Bermuda paleosols have a predominantly Saharan aerosolic dust signature. Saharan dust deposition on Bermuda during successive Quaternary glacial periods is consistent with patterns of general circulation models, which indicate that during glacial maxima the northeast summer trade winds were stronger than at present and reached latitudes higher than 30°N despite lower-than-present sea surface temperatures in the North Atlantic.

Journal of Geophysical Research D: Atmospheres↗
USGS2022

Categorizing active marine acoustic sources based on their potential to affect marine animals

Marine acoustic sources are widely used for geophysical imaging, oceanographic sensing, and communicating with and tracking objects or robotic vehicles in the water column. Under the U.S. Marine Mammal Protection Act and similar regulations in several other countries, the impact of controlled acoustic sources is assessed based on whether the sound levels received by marine mammals meet the criteria for harassment that causes certain behavioral responses. This study describes quantitative factors beyond received sound levels that could be used to assess how marine species are affected by many commonly deployed marine acoustic sources, including airguns, high-resolution geophysical sources (e.g., multibeam echosounders, sidescan sonars, subbottom profilers, boomers, and sparkers), oceanographic instrumentation (e.g., acoustic doppler current profilers, split-beam fisheries sonars), and communication/tracking sources (e.g., acoustic releases and locators, navigational transponders). Using physical criteria about the sources, such as source level, transmission frequency, directionality, beamwidth, and pulse repetition rate, we divide marine acoustic sources into four tiers that could inform regulatory evaluation. Tier 1 refers to high-energy airgun surveys with a total volume larger than 1500 in 3 (24.5 L) or arrays with more than 12 airguns, while Tier 2 covers the remaining low/intermediate energy airgun surveys. Tier 4 includes most high-resolution geophysical, oceanographic, and communication/tracking sources, which are considered unlikely to result in incidental take of marine mammals and therefore termed de minimis . Tier 3 covers most non-airgun seismic sources, which either have characteristics that do not meet the de minimis category (e.g., some sparkers) or could not be fully evaluated here (e.g., bubble guns, some boomers). We also consider the simultaneous use of multiple acoustic sources, discuss marine mammal field observations that are consistent with the de minimis designation for some acoustic sources, and suggest how to evaluate acoustic sources that are not explicitly considered here.

Journal of Marine Science and Engineering↗
USGS2004

Chapter 4 The meade peak member of the phosphoria formation: Temporal and spatial variations in sediment geochemistry

Variations in the geochemistry of rocks from the Meade Peak Member of the Phosphoria Formation were examined using ratios of elements associated with either the +terrigenous or marine sediment fractions. Inter-element relationships in the terrigenous fraction appear useful for chemo-stratigraphic correlation. A sharp decrease upsection in K 2 O/AI 2 O 3 ratios occurs in the lower half of all but the most northeasterly section, wherein an offset is still evident in average and minimum values. These offsets correspond closely to the lower Guadalupian Series boundary as defined by conodont zonations, coincident with a change from major low-stand to transgressive conditions. The offsets are possibly the result of increased transport distances or flooding of source areas related to transgres- sion of the Phosphoria sea on the Wyoming shelf. A series of intervals displaying high Fe 2 O 3 /Al 2 O 3 , Ba/Al 2 O 3 and Sc/Al 2 O 3 ratios occur in the upper beds of the easternmost sections. The intervals do not appear to reflect amplified marine signals, but rather the introduction of terrigenous sediment from a secondary source, or, simply, reworking of sediments under higher energy conditions. The westernmost section, presumably repre- senting the deepest parts of the Phosphoria basin, contains intervals with high Ba/Al 2 O 3 . We suggest these horizons represent periods of low sediment accumulation during maxi- mum flooding and high-stand conditions. Inter-element relationships in the marine-derived sediment fraction indicate that bottom waters of the Phosphoria basin were dominantly denitrifying (suboxic). Ratios of Cd and Mo to Zn and Cu closely approach those in modern plankton in most of the sections, implying a major biogenic source for these elements. Exceptions occur through- out the westernmost (distal) section, possibly due to changes in the dominant plankton populations and relative nutrient uptakes, and in the upper part of the most northeasterly (shoreward, ramp) section, which we suggest is due to increased oxygen levels. Relatively thick phosphatic layers occur in basinal areas due largely to lack of terrig- enous dilution during deposition. These basinal deposits appear to have lower concentra- tions of many trace elements than more shoreward deposits. This may reflect deposition away from areas of peak primary production. Alternatively, biogenic detritus in these areas may have been derived from differing populations of primary producers with differing nutrient requirements. Both mid-shelf (middle ramp) and marginal environments were sites of accumulation of rich phosphatic units with high concentrations of trace elements. Deposits from marginal areas have the most varied geochemistry, largely because they experienced greater variability in terrigenous sediment influx. Even moderate changes in sea level may have dramatically altered energy levels, sediment mixing, and the amount of organic detritus reaching the sediment surface in these shallower marginal areas.

Colorado, Idaho, Montana, Nebraska, North Dakota, ↗
USGS2020

Spectral wave-driven bedload transport across a coral reef flat/lagoon complex

Coral reefs are an important source of sediment for reef-lined coasts by helping to maintain beaches while also providing protection in the form of wave energy dissipation. Understanding the mechanisms by which sediment is delivered to the coast as well as better constraining the total volumes generated are critical for projecting future coastal change. A month-long hydrodynamics and sediment transport study on a fringing reef/lagoon complex in Western Australia indicates that lower frequency constituents of wave energy are important to the total bedload transport of sediment across the reef flat and lagoon to the shoreline. The reef flat and the lagoon are characterized by distinctly different transport regimes, resulting in an offset in the timing of bedform migration between the two. Short-term storage of sediment is noted on the reef flat, which is subsequently washed out into the lagoon when offshore wave heights increase and strong currents due to wave breaking at the reef crest develop. This sudden influx of sediment is a significant control on bedform migration rates in the lagoon. Infragravity wave energy on the reef flat and lagoon make an important contribution to the migration of bedforms and resultant bedload transport. Given the complexity of the hydrodynamics of fringing reefs, the transfer of energy to lower frequency bands, as well as accurate estimates of sources and sinks of sediment, must but considered in order to correctly model the transport of sediment from the reef to the coast.

Ningaloo Reef↗
USGS2024

New methodology for assessing underground natural gas storage resources – Example from Michigan Basin, United States

Energy consumption in the United States (U.S.) and across the world is shifting away from traditional fossil fuels like coal and oil, and towards natural gas and renewable sources, including hydrogen. Because gas demand is typically greatest during cold seasons and renewable sources sometimes produce variable supplies, it is important to store energy for use when demand exceeds supply. Whereas batteries and tanks typically store energy above ground, geologic (underground) storage may be able to retain much greater quantities of energy over much longer durations (e.g., Matos et al. 2019; Buursink et al. 2023). Consequently, the U.S. Geological Survey (USGS) is developing a methodology to locate new geologic energy storage sites with an initial focus on depleted hydrocarbon reservoirs followed by estimates of associated pore space or gas storage capacity. Depleted hydrocarbon fields in Michigan Basin in the U.S. already host large quantities of injected natural gas (about 681 billion cubic feet of seasonal gas capacity in 42 gas storage facilities; U.S. Energy Information Administration 2024). Therefore, we used this basin as an example to assess potential new storage resources alongside existing storage capacity and carbon dioxide sequestration resources (e.g., Katz and Coats 1968; Haagsma et al. 2020). In Michigan Basin gas storage is taking place in reservoirs within several key hydrocarbon producing formations with different lithologies, including the informal Stray sandstone of the Michigan Formation and the pinnacle reefs of the Niagara Group (Gautier et al. 1995; Swezey et al. 2015). The assessment methodology introduced here consists of two steps relying on both well information and reservoir data.

Michigan↗
USGS1983

A practical, low-noise coil system for magnetotellurics

Magnetotellurics is a geophysical technique which was developed by Cagnaird (1953) and Tikhonov (1950) and later refined by other scientists worldwide. The technique is a method of electromagnetic sounding of the Earth and is based upon the skin depth effect in conductive media. The electric and magnetic fields arising from natural sources are measured at the surface of the earth over broad frequency bands. An excellent review of the technique is provided in the paper by Vozoff (1972). The sources of the natural fields are found in two basic mechanisms. At frequencies above a few hertz, most of the energy arises from lightning in thunderstorm belts around the equatorial regions. This energy is propagated in a wave-guide formed by the earthionospheric cavity. Energy levels are higher at fundamental modes for this cavity, but sufficient energy exists over most of the audio range to be useful for sounding at these frequencies, in which case the technique is generally referred to as audio-magnetotellurics or AMT. At frequencies lower than audio, and in general below 1 Hz, the source of naturally occuring electromagnetic energy is found in ionospheric currents. Current systems flowing in the ionosphere generate EM waves which can be used in sounding of the earth. These fields generate a relatively complete spectrum of electromagnetic energy that extends from around 1 Hz to periods of one day. Figure 1 shows an amplitude spectrum characteristic of both the ionospheric and lightning sources, covering a frequency range from 0.0001 Hz to 1000 Hz. It can be seen that there is a minimum in signal levels that occurs at about 1 Hz, in the gap between the two sources, and that signal level increases with a decrease in frequency.

Open-File Report↗
USGS2002

Geothermal systems of the Great Basin and U.S. Geological Survey plans for a regional resource assessment

Based on current projections, the United States faces the need to increase its electrical power generating capacity by 40% (approximately 300,000 Megawatts-electrical or MWe) over the next 20 years (Energy Information Administration, EIA - Department of Energy). A critical question for the near future is the extent to which geothermal resources can contribute to this increasing demand for electricity. Geothermal energy constitutes one of the nation's largest sources of renewable and environmentally benign electrical power, yet the installed capacity of 2860 MWe falls far short of estimated geothermal resources. This is particularly true for the Great Basin region of the western United States, which has an installed capacity of about 500 MWe, much lower than the 7500 MWe resource estimated by the U.S. Geological Survey (USGS) in the late 1970s. The reasons for the limited development of geothermal power are varied, but political, economic and technological developments suggest the time is ripe for a new assessment effort. Technologies for power production from geothermal systems and scientific understanding of geothermal resource occurrence have improved dramatically in recent years. The primary challenges facing geothermal resource studies are (1) understanding the thermal, chemical and mechanical processes that lead to the colocation of high temperatures and high permeabilities necessary for the formation of geothermal systems and (2) developing improved techniques for locating, characterizing and exploiting these systems. Starting in the fall of 2002, the USGS will begin work with institutions funded by the Department of Energy's (DOE) Geothermal Research Program to investigate the nature and extent of geothermal systems in the Great Basin and to produce an updated assessment of available geothermal resources.

Conference Paper↗
USGS2015

Unconventional energy resources: 2015 review. Shale gas and liquids

Introduction As the source rocks from which petroleum is generated, organic-rich shales have always been considered an important component of petroleum systems. Over the last few years, it has been realized that in some mudrocks, sufficient hydrocarbons remain in place to allow for commercial development, although advanced drilling and completion technology is typically required to access hydrocarbons from these reservoirs. Tight oil reservoirs (also referred to as continuous oil accumulations) contain hydrocarbons migrated from source rocks that are geologically/stratigraphically interbedded with or occur immediately overlying/underlying them. Migration is minimal in charging these tight oil accumulations (Gaswirth and Marra 2014 ). Companies around the world are now successfully exploiting organic-rich shales and tight rocks for contained hydrocarbons, and the search for these types of unconventional petroleum reservoirs is growing. Unconventional reservoirs range in geologic age from Ordovician to Tertiary (Silverman et al. 2005 ; EIA 2013a ).

Natural Resources Research↗