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Results for “Exploration Geophysics”

Search indexed USGS publications on groundwater, aquifers, geologic maps, mineral resources and earthquakes. Explore source records by subject and place.

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At least 451 records · Page 25Linked to original sources
USGS2015

Evolution of Mars’ Northern Polar Seasonal CO 2 deposits: variations in surface brightness and bulk density

Small scale variations of seasonal ice are explored at different geomorphic units on the Northern Polar Seasonal Cap (NPSC). We use seasonal rock shadow measurements, combined with visible and thermal observations, to calculate density over time. The coupling of volume density and albedo allows us to determine the microphysical state of the seasonal CO 2 ice. We find two distinct endmembers across the NPSC: 1) Snow deposits may anneal to form an overlying slab layer that fractures. These low density deposits maintain relatively constant densities over springtime. 2) Porous slab deposits likely anneal rapidly in early spring and fracture in late spring. These high density deposits dramatically increase in density over time. The endmembers appear to be correlated with latitude.

Journal of Geophysical Research E: Planets↗
USGS2020

Simultaneous observations of geoelectric and geomagnetic fields produced by magnetospheric ULF waves

Geomagnetic perturbations ( B G E O ) related to magnetospheric ultralow frequency (ULF) waves induce electric fields within the conductive Earth—geoelectric fields ( E G E O )—that in turn drive geomagnetically induced currents. Though numerous past studies have examined ULF wave B G E O from a space weather perspective, few studies have linked ULF waves with E G E O . Using recently available magnetotelluric impedance and E G E O measurements in the contiguous United States, we explore the relationship between ULF waves and E G E O . We use satellite, ground‐based radar, B G E O , and E G E O measurements in a case study of a plasmaspheric virtual resonance (PVR), demonstrating that the PVR E G E O has significant spatial variation in contrast to a relatively uniform B G E O , consistent with spatially varying Earth conductivity. We further show ULF wave E G E O measurements during two moderate storms of ∼ 1 V/km. We use both results to highlight the need for more research characterizing ULF wave E G E O .

Geophysical Research Letters↗
USGS2023

Geophysical data provide three dimensional insights into porphyry copper systems in the Silverton caldera, Colorado, USA

The Silverton caldera in southwest Colorado, USA hosts polymetallic veins and pervasively altered rocks indicative of porphyry copper systems. Nearly a kilometer of erosion has exposed multiple levels of the hydrothermal systems from shallow lithocaps down to quartz-sericite-pyrite veins. New airborne electromagnetic and magnetic survey data are integrated with previous alteration mapping and porphyry models to show the subsurface geophysical response of shallow to deep levels of the porphyry system. Qualitative map views show lateral changes in the magnetization and resistivity of the hydrothermally altered rocks. The volcanic terrain exhibits high magnetization and high amplitude anomalies map near-surface plutonic rocks associated with porphyry systems. Magnetic susceptibility measurements on outcrops of hydrothermally altered rocks indicate magnetite content decreases upward and outward from the source intrusions where magnetic anomaly lows are observed over the lithocaps. The resistivity maps highlight hydrothermal alteration as resistivity lows with exception being rocks having propylitic alteration. Quantitative resistivity models show low resistivity zones with an apparent thickness around 50–150 m beneath quartz-sericite-pyrite veins interpreted to be the result of supergene processes that may continue today, and the calculated magnetic source depths occur near the top of this zone. The resistivity models also show rocks having propylitic, silicic, and quartz-alunite-pyrophyllite assemblages exhibit high resistivity with depth, and argillic alteration assemblages had high resistivity due to high quartz content. This integrated approach presented in a three-dimensional environment provides guidance when exploring for porphyry copper systems in less exposed terrains.

Colorado↗
USGS2014

Ambient tremors in a collisional orogenic belt

Deep-seated tectonic tremors have been regarded as an observation tied to interconnected fluids at depth, which have been well documented in worldwide subduction zones and transform faults but not in a collisional mountain belt. In this study we explore the general features of collisional tremors in Taiwan and discuss the possible generation mechanism. In the 4 year data, we find 231 ambient tremor episodes with durations ranging from 5 to 30 min. In addition to a coseismic slip-induced stress change from nearby major earthquake, increased tremor rate is also highly correlated with the active, normal faulting earthquake swarms at the shallower depth. Both the tremor and earthquake swarm activities are confined in a small, area where the high attenuation, high thermal anomaly, the boundary between high and low resistivity, and localized veins on the surfaces distributed, suggesting the involvement of fluids from metamorphic dehydration within the orogen.

Geophysical Research Letters↗
USGS2013

Seasonal circulation over the Catalan inner-shelf (northwest Mediterranean Sea)

This study characterizes the seasonal cycle of the Catalan inner-shelf circulation using observations and complementary numerical results. The relation between seasonal circulation and forcing mechanisms is explored through the depth-averaged momentum balance, for the period between May 2010 and April 2011, when velocity observations were partially available. The monthly-mean along-shelf flow is mainly controlled by the along-shelf pressure gradient and by surface and bottom stresses. During summer, fall, and winter, the along-shelf momentum balance is dominated by the barotropic pressure gradient and local winds. During spring, both wind stress and pressure gradient act in the same direction and are compensated by bottom stress. In the cross-shelf direction the dominant forces are in geostrophic balance, consistent with dynamic altimetry data.

Journal of Geophysical Research C: Oceans↗
USGS2011

Exploring Martian polar atmospheric circulation and surface interactions

The northern and southern seasonal polar caps of Mars are formed in the polar night, during their respective autumn and winter seasons, by the condensation of atmospheric CO 2 directly to the solid phase as ice, snow, and possibly frost. During spring and summer, the seasonal ice sublimes, returning CO 2 to the atmosphere. The caps advance and recede in response to seasonal variations in solar insolation, extending as far as 40° from the poles, and have been noted by telescopic observers since the 17th century Roughly 25% of the atmosphere, which is 95% CO 2 by volume, is cycled into and out of the seasonal caps. The CO 2 cycle dominates atmospheric circulation on Mars, and must be thoroughly understood in order to answer fundamental questions about climate history and the global distribution of near‐surface water.

Eos, Transactions, American Geophysical Union↗
USGS2007

Mars reconnaissance orbiter's high resolution imaging science experiment (HiRISE)

The HiRISE camera features a 0.5 m diameter primary mirror, 12 m effective focal length, and a focal plane system that can acquire images containing up to 28 Gb (gigabits) of data in as little as 6 seconds. HiRISE will provide detailed images (0.25 to 1.3 m/pixel) covering ∼1% of the Martian surface during the 2‐year Primary Science Phase (PSP) beginning November 2006. Most images will include color data covering 20% of the potential field of view. A top priority is to acquire ∼1000 stereo pairs and apply precision geometric corrections to enable topographic measurements to better than 25 cm vertical precision. We expect to return more than 12 Tb of HiRISE data during the 2‐year PSP, and use pixel binning, conversion from 14 to 8 bit values, and a lossless compression system to increase coverage. HiRISE images are acquired via 14 CCD detectors, each with 2 output channels, and with multiple choices for pixel binning and number of Time Delay and Integration lines. HiRISE will support Mars exploration by locating and characterizing past, present, and future landing sites, unsuccessful landing sites, and past and potentially future rover traverses. We will investigate cratering, volcanism, tectonism, hydrology, sedimentary processes, stratigraphy, aeolian processes, mass wasting, landscape evolution, seasonal processes, climate change, spectrophotometry, glacial and periglacial processes, polar geology, and regolith properties. An Internet Web site (HiWeb) will enable anyone in the world to suggest HiRISE targets on Mars and to easily locate, view, and download HiRISE data products.

Journal of Geophysical Research E: Planets↗
USGS2007

Alvin explores the deep northern Gulf of Mexico Slope

Many of the world's productive deepwater hydrocarbon basins experience significant and ongoing vertical migration of fluids and gases to the modern seafloor. These products, which are composed of hydrocarbon gases, crude oil, formation fluids, and fluidized sediment, dramatically change the geologic character of the ocean floor, and they create sites where chemosynthetic communities supported by sulfide and hydrocarbons flourish. Unique fauna inhabit these sites, and the chemosynthetic primary production results in communities with biomass much greater than that of the surrounding seafloor.

Gulf of Mexico↗
USGS2006

Erosion rates at the Mars Exploration Rover landing sites and long-term climate change on Mars

Erosion rates derived from the Gusev cratered plains and the erosion of weak sulfates by saltating sand at Meridiani Planum are so slow that they argue that the present dry and desiccating environment has persisted since the Early Hesperian. In contrast, sedimentary rocks at Meridiani formed in the presence of groundwater and occasional surface water, and many Columbia Hills rocks at Gusev underwent aqueous alteration during the Late Noachian, approximately coeval with a wide variety of geomorphic indicators that indicate a wetter and likely warmer environment. Two-toned rocks, elevated ventifacts, and perched and undercut rocks indicate localized deflation of the Gusev plains and deposition of an equivalent amount of sediment into craters to form hollows, suggesting average erosion rates of ???0.03 nm/yr. Erosion of Hesperian craters, modification of Late Amazonian craters, and the concentration of hematite concretions in the soils of Meridiani yield slightly higher average erosion rates of 1-10 nm/yr in the Amazonian. These erosion rates are 2-5 orders of magnitude lower than the slowest continental denudation rates on Earth, indicating that liquid water was not an active erosional agent. Erosion rates for Meridiani just before deposition of the sulfate-rich sediments and other eroded Noachian areas are comparable with slow denudation rates on Earth that are dominated by liquid water. Available data suggest the climate change at the landing sites from wet and likely warm to dry and desiccating occurred sometime between the Late Noachian and the beginning of the Late Hesperian (3.7-3.5 Ga). Copyright 2006 by the American Geophysical Union.

Journal of Geophysical Research E: Planets↗
USGS2014

Deep‐well salinity‐exploration

One of the outstanding accomplishments of recent years in connection with the study of the geologic source of groundwater and the quality of water yielded by different formations has been the development of methods and equipment for the exploration of wells. The description and use of the deep ‐ well current‐meter on well ‐ exploration work in Hawaii and New Mexico has already appeared in published literature (John McCombs and A. G. Fiedler, Methods of exploring and repairing leaky artesian wells, U.S. Geol. Sur. Water‐Supply Paper 596, pp. 1–32, 1928) on hydrology but recent developments in the application of electrical conductivity‐measurements for determining the salinity of water in wells have not heretofore been presented. In addition to being of considerable scientific importance in connection with problems of ground‐water hydrology, the practical value of locating the place at which salt water is entering a well and contaminating a potable water‐supply is great. Such information may be the means of preventing the loss of large investments in industries, irrigation‐projects, and cities that are dependent upon well ‐water supplies that may at some time become contaminated by salty or other highly mineralized water.

Nevada, Hawaii↗
USGS2011

Effort explores 130 Million years of Antarctic paleoenvironment

Antarctic climate history has been dominated by events and turning points with causes that are poorly understood. To fill the gaps in our knowledges new effort is underway in the international geologic community to acquire and coordinate the circum‐Antarctic geologic data needed to derive and model paleoenvironments of the past 130 m.y. The effort, which focuses principally on using shallow (<100 m) stratigraphic drilling and coring to acquire the geologic data, is being led by the Antarctic Offshore Stratigraphy Project (ANTOSTRAT), a group that works under the aegis of the Scientific Committee on Antarctic Research (SCAR). About 40 scientists from 12 countries met this past summer in Wellington, New Zealand, at an ANTOSTRAT meeting to discuss strategies for implementing the desired paleoenvironmental field and modeling studies. The meeting was held in conjunction with the 8th International Symposium on Antarctic Earth Sciences.

Eos, Transactions, American Geophysical Union↗
USGS2018

Identifying physics‐based thresholds for rainfall‐induced landsliding

Most regional landslide warning systems utilize empirically derived rainfall thresholds that are difficult to improve without recalibration to additional landslide events. To address this limitation, we explored the use of synthetic rainfall to generate thousands of possible storm patterns and coupled them with a physics‐based hydrology and slope stability model for various antecedent soil saturation scenarios to analyze pore‐water pressure and factor of safety metrics. We used these metrics to generate two‐tiered alert thresholds that can be employed to assess shallow landslide potential for any given combination of storm and antecedent wetness. When applied to the San Francisco Bay region (California, USA), the results are consistent with events that caused widespread landsliding. Our deterministic modeling approach, which accounts for plausible ranges in soil hydraulic and mechanical properties, can inform the development of the next generation of warning systems for rainfall‐induced landsliding.

Geophysical Research Letters↗
USGS2012

Using cluster analysis to organize and explore regional GPS velocities

Cluster analysis offers a simple visual exploratory tool for the initial investigation of regional Global Positioning System (GPS) velocity observations, which are providing increasingly precise mappings of actively deforming continental lithosphere. The deformation fields from dense regional GPS networks can often be concisely described in terms of relatively coherent blocks bounded by active faults, although the choice of blocks, their number and size, can be subjective and is often guided by the distribution of known faults. To illustrate our method, we apply cluster analysis to GPS velocities from the San Francisco Bay Region, California, to search for spatially coherent patterns of deformation, including evidence of block-like behavior. The clustering process identifies four robust groupings of velocities that we identify with four crustal blocks. Although the analysis uses no prior geologic information other than the GPS velocities, the cluster/block boundaries track three major faults, both locked and creeping.

Geophysical Research Letters↗
USGS2006

Geomorphic control of landscape carbon accumulation

We use the CREEP process-response model to simulate soil organic carbon accumulation in an undisturbed prairie site in Iowa. Our primary objectives are to identify spatial patterns of carbon accumulation, and explore the effect of erosion on basin-scale C accumulation. Our results point to two general findings. First, redistribution of soil carbon by erosion results in a net increase in basin-wide carbon storage relative to a noneroding environment. Landscape-average mean residence times are increased in an eroding landscape owing to the burial/preservation of otherwise labile C. Second, field observations taken along a slope transect may overlook significant intraslope variations in carbon accumulation. Spatial patterns of modeled deep C accumulation are complex. While surface carbon with its relatively short equilibration time is predictable from surface properties, deep carbon is strongly influenced by the landscape's geomorphic and climatic history, resulting in wide spatial variability. Convergence and divergence associated with upland swales and interfluves result in bimodal carbon distributions in upper and mid slopes; variability in carbon storage within modeled mid slopes was as high as simulated differences between erosional shoulders and depositional valley bottoms. The bimodality of mid-slope C variability in the model suggests that a three-dimensional sampling strategy is preferable over the traditional two-dimensional analog or "catena" approach. Copyright 2006 by the American Geophysical Union.

Journal of Geophysical Research G: Biogeosciences↗
USGS2021

Exploring GPS observations of postseismic deformation following the 2012 MW7.8 Haida Gwaii and 2013 MW7.5 Craig, Alaska Earthquakes: Implications for viscoelastic Earth structure

The Queen Charlotte-Fairweather Fault (QC-FF) system off the coast of British Columbia and southeast Alaska is a highly active dextral strike-slip plate boundary that accommodates ∼50 mm/yr of relative motion between the Pacific and North America plates. Nine M W ≥ 6.7 earthquakes have occurred along the QC-FF system since 1910, including a M S (G-R) 8.1 event in 1949. Two recent earthquakes, the October 28, 2012 Haida Gwaii ( M W 7.8) and January 5, 2013 Craig, Alaska ( M W 7.5) events, produced postseismic transient deformation that was recorded in the motions of 25 nearby continuous Global Positioning System (cGPS) stations. Here, we use 5+ yr of cGPS measurements to characterize the underlying mechanisms of postseismic deformation and to constrain the viscosity structure of the upper mantle surrounding the QC-FF. We construct forward models of viscoelastic deformation driven by coseismic stress changes from these two earthquakes and explore a large set of laterally heterogeneous viscosity structures that incorporate a relatively weak back-arc domain; we then evaluate each model based on its fit to the postseismic signals in our cGPS data. In determining best-fit model structures, we additionally incorporate the effects of afterslip following the 2012 event. Our results indicate the occurrence of a combination of temporally decaying afterslip and vigorous viscoelastic relaxation of the mantle asthenosphere. In addition, our best-fit viscosity structure (transient viscosity of 1.4–2.0 × 10 18 Pa s; steady-state viscosity of 10 19 Pa s) is consistent with the range of upper mantle viscosities determined in previous studies of glacial isostatic rebound and postseismic deformation.

Alaska, British Columbia↗
USGS2022

What to expect when you are expecting earthquake early warning

We present a strategy for earthquake early warning (EEW) alerting that focuses on providing users with a target level of performance for their shaking level of interest (for example, ensuring that users receive warnings for at least 95 per cent of the occurrences of that shaking level). We explore the factors that can affect the accuracy of EEW shaking forecasts including site conditions (which can make shaking more or less varied), the share of earthquake hazard that comes from outlier events, and even the metric of shaking being forecast (with modified Mercalli intensity being the most uncertain quantity to forecast). To provide different locations with the same level of EEW performance, the alert issuance criteria will need to change based on these factors. Finally, we present a hierarchy of information approach with which to communicate EEW system design and performance to the public and other stakeholders, including how frequently alerts will be issued and how accurate those alerts will be.

Geophysical Journal International↗
USGS2025

The role of bedrock circulation depth and porosity in mountain streamflow response to prolonged drought

Quantitative understanding is lacking on how the depth of active groundwater circulation in bedrock affects mountain streamflow response to a multi‐year drought. We use an integrated hydrological model to explore the sensitivity of a variety of streamflow metrics to bedrock circulation depth and porosity under a plausible extreme drought scenario lasting up to 5 years. Endmember depth versus hydraulic conductivity relationships and porosity values for fractured crystalline rock are simulated. With drought, a deeper circulation system with higher drainable porosity more effectively buffers minimum flow and significantly limits perennial stream loss in comparison to a shallow circulation system. Streamflow buffering is accomplished through extensive groundwater storage loss. However, deeper circulation systems experience prolonged recovery from drought in comparison to storage‐limited shallow systems. Research highlights the importance of characterizing the deeper bedrock hydrogeology in mountainous watersheds to better understand and predict drought impacts on stream ecosystem health and water resource sustainability.

Colorado↗
USGS1996

Postaudit of head and transmissivity estimates and ground-water flow models of Avra Valley, Arizona

Ground water from regional alluvial-aquifer systems is the main source of water in the alluvial basins of Arizona, such as Avra Valley. Ground-water flow models are used to assess ground-water availability and the effects of development on the regional ground-water resources. A postaudit of regional-head and transmissivity estimates and the ground-water flow models of Avra Valley was used to evaluate potential errors in the distribution of aquifer properties and recharge that can cause predictive errors in ground-water models. Simulations of predevelopment conditions in 1940 and historical development conditions for 1960-79 provided the basis of comparison for assessing predictive errors of historical conditions for two regional ground-water flow models. Potential errors in the estimation of the regional-head and transmissivity and alternate conceptual models were compared with an existing calibrated two-layer flow model for predevelopment (1940) and developed conditions (1940-85). Measured heads can be subdivided into a north-central region and a region south of the basin constriction. A more variable regional-head surface typical of developed aquifer systems was indicated by kriged developed heads (1985) with about 50 percent more uncertainty than predevelopment heads (1940). Incorporating heads from adjacent basins at the ground-water inflow and outflow regions reduced uncertainty in kriged heads for these boundary areas. Universal cokriging of heads with the strongly correlated land-surface altitudes may improve regional-head estimates and model comparisons where head data are sparse. Local transmissivity estimates can be subdivided into northern and south-central regions that are distributed along the valley axis and the Santa Cruz River. Regional geostatistical estimates of transmissivity, which are based solely on local estimates, are low in the northern part of Avra Valley and are high in the south-central part when compared with the head-conditioned model-derived estimates. These differences may be related to a systematic bias between aquifer-test conditions and methods of aquifer-test analysis. Cokriging transmissivities with specific capacity and silt-and-clay content provided the least uncertainty of all kriged estimates. Predictive errors for the Avra Valley model are the result of a different combination of factors that become significant in the simulation of ground-water flow for the periods representing predevelopment, historical development, and future development conditions. Predictive errors for simulation of predevelopment conditions are caused by potential systematic errors in estimates of local transmissivity, uncertainty in long-term mountain-front recharge, and uncertainty in predevelopment heads along the margins of the basin where recharge and transmissivity estimates are constrained by heads during model calibration. Analog-model historical predictions of future development indicate changes to 1985 were as much as 50 to 100 feet different from actual declines that were caused by errors in the spatial distribution and not the total amount of estimated future pumpage. Predictive errors for simulation of historical development (1960-79) appear to be caused to a greater extent by combined errors in estimates of transmissivity and storage properties and to a lesser extent by estimates of net withdrawal and subsidence. Comparison of the two digital models resulted in differences in transmissivity of as much as 30,000 feet squared per day and differences in specific yield of as much as 0.1. In combination with some differences in net withdrawal, these model-parameter differences resulted in local differences in change in storage of as much as 4,000 acre-feet per square mile and are equivalent to historical predictive errors in water levels of as much as 40 feet. Areas with no differences in model parameters yield comparable simulated water-level declines that are similar to measured declines. The pattern of differences in transmissivity and storage parameters are similar to differences between model-derived estimates conditioned on heads and related geostatistical estimates derived from aquifer-test estimates. A postaudit analysis of alternate conceptual models was explored on the basis of well-by-well comparisons of reductions in mean error and variance, and through the use of standardized calibration-error maps for predevelopment heads (1940) and developed heads (1985). Calibration-error maps provide a useful tool for exploring the spatial structure of model errors and the relative adequacy of model fit that is not available from traditional methods of model comparison. Calibration-error maps indicate estimated heads were too high in the southern part of Avra Valley, and estimated heads were too low in the northern part. Increased transmissivity in the southern part of the lower model layer; decreased hydraulic conductivity in the southwestern part of the upper layer; reduced ground-water inflow from Altar Valley; and increased recharge along the Tortolita Mountains, Tucson Mountains, and Brawley Wash yielded a significantly better model for predevelopment but not for developed conditions (1940-85). This may indicate that alternate conceptual models are different for different time periods or require analysis of time-varying model parameters for developed conditions, such as climatically variable recharge. Predictive errors for future simulations (1986-2025) also could potentially include errors of more than 40 ft from omission of subsidence from the simulation of regional ground-water flow in Avra Valley. Further refinement of the changing conceptual model of an aquifer system under continuing development and variable climate, such as Avra Valley, will require a variety of additional geophysical, geochemical, and hydraulic field data.

Arizona↗