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At least 505 records · Page 28Linked to original sources
USGS2011

Electrical properties of polycrystalline methane hydrate

Electromagnetic (EM) remote-sensing techniques are demonstrated to be sensitive to gas hydrate concentration and distribution and complement other resource assessment techniques, particularly seismic methods. To fully utilize EM results requires knowledge of the electrical properties of individual phases and mixing relations, yet little is known about the electrical properties of gas hydrates. We developed a pressure cell to synthesize gas hydrate while simultaneously measuring in situ frequency-dependent electrical conductivity (σ). Synthesis of methane (CH 4 ) hydrate was verified by thermal monitoring and by post run cryogenic scanning electron microscope imaging. Impedance spectra (20 Hz to 2 MHz) were collected before and after synthesis of polycrystalline CH 4 hydrate from polycrystalline ice and used to calculate σ. We determined the σ of CH 4 hydrate to be 5 × 10 −5 S/m at 0°C with activation energy (E a ) of 30.6 kJ/mol (−15 to 15°C). After dissociation back into ice, σ measurements of samples increased by a factor of ~4 and E a increased by ~50%, similar to the starting ice samples.

Geophysical Research Letters↗
USGS2012

Displacement fields from point cloud data: Application of particle imaging velocimetry to landslide geodesy

Acquiring spatially continuous ground-surface displacement fields from Terrestrial Laser Scanners (TLS) will allow better understanding of the physical processes governing landslide motion at detailed spatial and temporal scales. Problems arise, however, when estimating continuous displacement fields from TLS point-clouds because reflecting points from sequential scans of moving ground are not defined uniquely, thus repeat TLS surveys typically do not track individual reflectors. Here, we implemented the cross-correlation-based Particle Image Velocimetry (PIV) method to derive a surface deformation field using TLS point-cloud data. We estimated associated errors using the shape of the cross-correlation function and tested the method's performance with synthetic displacements applied to a TLS point cloud. We applied the method to the toe of the episodically active Cleveland Corral Landslide in northern California using TLS data acquired in June 2005–January 2007 and January–May 2010. Estimated displacements ranged from decimeters to several meters and they agreed well with independent measurements at better than 9% root mean squared (RMS) error. For each of the time periods, the method provided a smooth, nearly continuous displacement field that coincides with independently mapped boundaries of the slide and permits further kinematic and mechanical inference. For the 2010 data set, for instance, the PIV-derived displacement field identified a diffuse zone of displacement that preceded by over a month the development of a new lateral shear zone. Additionally, the upslope and downslope displacement gradients delineated by the dense PIV field elucidated the non-rigid behavior of the slide.

California↗
USGS2013

Evaluation of permeability and non-Darcy flow in vuggy macroporous limestone aquifer samples with lattice Boltzmann methods

Lattice Boltzmann flow simulations provide a physics-based means of estimating intrinsic permeability from pore structure and accounting for inertial flow that leads to departures from Darcy's law. Simulations were used to compute intrinsic permeability where standard measurement methods may fail and to provide better understanding of departures from Darcy's law under field conditions. Simulations also investigated resolution issues. Computed tomography (CT) images were acquired at 0.8 mm interscan spacing for seven samples characterized by centimeter-scale biogenic vuggy macroporosity from the extremely transmissive sole-source carbonate karst Biscayne aquifer in southeastern Florida. Samples were as large as 0.3 m in length; 7–9 cm-scale-length subsamples were used for lattice Boltzmann computations. Macroporosity of the subsamples was as high as 81%. Matrix porosity was ignored in the simulations. Non-Darcy behavior led to a twofold reduction in apparent hydraulic conductivity as an applied hydraulic gradient increased to levels observed at regional scale within the Biscayne aquifer; larger reductions are expected under higher gradients near wells and canals. Thus, inertial flows and departures from Darcy's law may occur under field conditions. Changes in apparent hydraulic conductivity with changes in head gradient computed with the lattice Boltzmann model closely fit the Darcy-Forchheimer equation allowing estimation of the Forchheimer parameter. CT-scan resolution appeared adequate to capture intrinsic permeability; however, departures from Darcy behavior were less detectable as resolution coarsened.

Water Resources Research↗
USGS2019

Authigenic mineral texture in submarine 1979 basalt drill core, Surtsey volcano, Iceland

Micrometer-scale maps of authigenic microstructures in submarine basaltic tuff specimens from a 1979 Surtsey volcano, Iceland, drill core acquired 15 years after eruptions terminated provide fresh perspectives for deciphering the initial alteration of oceanic basalt in a low temperature hydrothermal system. A novel investigative approach integrates synchrotron source X-ray microdiffraction (µXRD), X-ray microfluoresence (µXRF), micro-computed tomography (µCT), and scanning transmission electron microscopy (S/TEM) coupled with Raman spectroscopy to create finely resolved spatial frameworks that record a continuum of alteration in glass and olivine. Micro-analytical maps of vesicular and fractured lapilli in specimens from 157.1, 137.9, and 102.6 m depth, and borehole temperatures of 83, 93.9 and 141.3 °C measured in 1980, respectively, record the production of nanocrystalline clay mineral, zeolites, and Al-tobermorite in diverse microenvironments. Nanocrystalline clay mineral (nontronite) and zeolite (amicite) texture in linear microstructures have concentrically-oriented crystallographic preferred orientation. Raman spectra indicating degraded organic carbonaceous matter are associated with nanocrystalline clay mineral in 10–25 nm, sub-circular nanoscale cavities in altered glass at 137.9 m depth and in a concentrically-layered, crystallographically-oriented linear microstructure in altered olivine at 102.6 m. These features have little resemblance to previously described alteration features in basalt. Irregular alteration fronts between fresh and altered glass at 157.1 depth, however, show a resemblance to microchannels in older basalts. The integrated analyses describe the complex organization of previously unrecognized mineral textures in very young basalt and provide a foundational mineralogical reference for longitudinal, time-lapse characterizations of palagonitized basalt in oceanic environments.

Geochemistry, Geophysics, Geosystems↗
USGS2019

The effect of brine on the electrical properties of methane hydrate

Gas hydrates possess lower electrical conductivity (inverse of resistivity) than either seawater or ice, but higher than clastic silts and sands, such that electromagnetic methods can be employed to help identify their natural formation in marine and permafrost environments. Controlled laboratory studies offer a means to isolate and quantify the effects of changing individual components within gas‐hydrate‐bearing systems, in turn yielding insight into the behavior of natural systems. Here we investigate the electrical properties of polycrystalline methane hydrate with ≥25% gas‐filled porosity and in mixture with brine. Initially, pure methane hydrate was synthesized from H 2 O ice and CH 4 gas while undergoing electrical impedance measurement, then partially dissociated to assess the effects of pure pore water accumulation on electrical conductivity. Methane hydrate + brine mixtures were then formed by either adding NaCl (0.25–2.5 wt %) to high‐purity ice or by using frozen seawater as a reactant. Conductivity was obtained from impedance measurements made in situ throughout synthesis while temperature cycled between +15 °C and −25 °C. Several possible conduction mechanisms were subsequently determined using equivalent circuit modeling. Samples with low NaCl concentration show a doping/impurity effect and a log linear conductivity response as a function of temperature. For higher salt content samples, conductivity increases exponentially with temperature and the log linear relationship no longer holds; instead, we observe phase changes within the samples that follow NaCl–H 2 O–CH 4 phase equilibrium predictions. Final samples were quenched in liquid nitrogen and imaged by cryogenic scanning electron microscopy (cryo‐SEM) to assess grain‐scale characteristics.

Journal of Geophysical Research↗
USGS2020

Reconstructing the velocity and deformation of a rapid landslide using multiview video

Noncontact measurements of spatially varied ground surface deformation during landslide motion can provide important constraints on landslide mechanics. Here, we present and test a new method for extracting measurements of rapid landslide surface displacement and velocity (accelerations of approximately 1 m/s 2 ) using sequences of stereo images obtained from a pair of inexpensive, stationary 4K video cameras with nominal frame rates of 29.97 Hz. The method combines elements of Structure from Motion with those of optical flow to extract data on 3‐D evolution of the ground surface during slope failure. We apply the method to an experiment at the U.S. Geological Survey debris‐flow flume in which a high‐speed, liquefying landslide was triggered by gradually adding water to a 6‐m 3 prism of loosely packed sediment on a 31° slope. Strip‐scanning lidar measurements made during the experiment corroborate our video‐based measurements, but the latter encompassed the entire landslide surface and were much lower in cost. Our video‐based measurements enabled computation of depth‐integrated landslide dilation/contraction rates. The range of computed rates was within the ranges inferred from independent measurements of evolving pore water pressures and reasonable estimates of the hydraulic permeability of the sediment. Dilation and contraction rates play a crucial role in landslide mechanics. The dilation and contraction we observe contradict the incompressible flow assumption used in many studies that have employed noncontact methods to infer landslide properties.

Oregon↗
USGS2020

Laboratory electrical conductivity of marine gas hydrate

Methane hydrate was synthesized from pure water ice and flash frozen seawater, with varying amounts of sand or silt added. Electrical conductivity was determined by impedance spectroscopy, using equivalent circuit modeling to separate the effects of electrodes and to gain insight into conduction mechanisms. Silt and sand increase the conductivity of pure hydrate, we infer by contaminant NaCl contributing to conduction in hydrate, to values in agreement with resistivities observed in well logs through hydrate The addition of silt and sand lowers the conductivity of hydrate synthesized from seawater by an amount consistent with Archie's Law. All samples were characterized using cryogenic scanning electron microscopy and energy dispersive spectroscopy, which show good connectivity of salt and brine phases. Electrical conductivity measurements of pure hydrate and hydrate mixed with silt during pressure‐induced dissociation supports previous conclusions that sediment increases dissociation rate.

Geophysical Research Letters↗
USGS2022

Microbially induced anaerobic oxidation of magnetite to maghemite in a hydrocarbon-contaminated aquifer

Iron mineral transformations occurring in hydrocarbon-contaminated sites are linked to the biodegradation of the hydrocarbons. At a hydrocarbon-contaminated site near Bemidji, Minnesota, USA, measurements of magnetic susceptibility (MS) are useful for monitoring the natural attenuation of hydrocarbons related to iron cycling. However, a transient MS, previously observed at the site, remains poorly understood and the iron mineral phases acting as reactants and products associated with this MS perturbation remain largely unknown. To address these unknowns, we acquired mineral magnetism measurements, including hysteresis loops, backfield curves, and isothermal remanent magnetizations on sediment core samples retrieved from the site and magnetite-filled mineral packets installed within the aquifer. Our data show that the core samples and magnetite packs display decreasing magnetization with time and that this loss in magnetization is accompanied by increasing bulk coercivity consistent with decreased average grain size and/or partial oxidation. Low-temperature magnetometry on all samples displayed behavior consistent with magnetite, but samples within the plume also show evidence of maghemitization. This interpretation is supported by the occurrence of shrinkage cracks on the surface of the grains imaged via scanning electron microscopy. Magnetite transformation to maghemite typically occurs under oxic conditions, here, we propose that maghemitization occurs within the anoxic portions of the plume via microbially mediated anaerobic oxidation. Mineral dissolution also occurs within the plume. Microorganisms capable of such anaerobic oxidation have been identified within other areas at the Bemidji site, but additional microbiological studies are needed to link specific anaerobic iron oxidizers with this loss of magnetization.

Minnesota↗
USGS2025

A method to obtain remotely sensed grain size distributions from nonplanar granular deposits

Constraining the grain size distribution of granular deposits with complex surfaces is difficult with existing approaches. Field and laboratory techniques are time consuming and limited by the maximum grain size that laboratories can accommodate. In this study, we present a new method to identify the coarse fraction of the grain size distribution at a debris-flow fan deposit surveyed with terrestrial laser scanning (TLS) in Glenwood Canyon, Colorado, USA. This method is a novel grain segmentation algorithm developed for application to point cloud data of deposits with complex surfaces and angular grains ranging in size from centimeters to a meter. This approach combines an existing random forest machine learning method with a novel iterative clustering algorithm. We compared the grain size distribution from our algorithm with a Wolman pebble count conducted in the field, and found a root mean squared error of less than 2 cm from the 5th to 95th percentile of the grain size distribution of grains ranging from cobble to boulder sized (6.3–78 cm in our application). Finally, we compared our new algorithm with an existing open-source grain segregation algorithm, and our method outperformed the selected alternative when applied to the debris-flow deposit point cloud.

Colorado↗
USGS2012

Upper crustal densities derived from sea floor gravity measurements: Northern Juan De Fuca Ridge

A transect of sea floor gravity stations has been analyzed to determine upper crustal densities on the Endeavour segment of the northern Juan de Fuca Ridge. Data were obtained using ALVIN along a corridor perpendicular to the axis of spreading, over crustal ages from 0 to 800,000 years. Calculated elevation factors from the gravity data show an abrupt increase in density with age (distance) for the upper 200 m of crust. This density change is interpreted as a systematic reduction in bulk porosity of the upper crustal section, from 23% for the axial ridge to 10% for the off-axis flanking ridges. The porosity decrease is attributed to the collapse and filling of large-scale voids as the abyssal hills move out of the crustal formation zone. Forward modeling of a plausible density structure for the near-axis region agrees with the observed anomaly data only if the model includes narrow, along-strike, low-density regions adjacent to both inner and outer flanks of the abyssal hills. The required low density zones could be regions of systematic upper crustal fracturing and faulting that were mapped by submersible observers and side-scan sonar images, and whose presence was suggested by the distribution of heat flow data in the same area.

Geophysical Research Letters↗
USGS2010

Turbidite systems: State of the art and future directions

The study of turbidite systems covering a wide range of physical scales has led to confus ion regarding the use of certain key terms and hence a breakdown in communication between workers involved in turbidite research. There are three fundamentally different scales and types of observations derived from the study of outcrop data (ancient systems), high-resolution seismic reflection and side scan sonar data (modern systems), and multichannel seismic reflection data (modern and older buried systems). Despite the variability of scale the same terms are used to describe features that may have little in common. Consequently, turbidite system terminology has become imprecise and even misleading in some cases, thus providing impediments to developing useful predictive models for processes, depositional environments, and lateral and vertical distribution of sand bodies within turbidite systems. To address this concern, we review the principal elements critical to deepwater systems: slump scars, submarine canyons, channels, channel fill deposits, overbank deposits, and lobes and discuss some of their recognition criteria with each different type of data base. Local and regional tectonic setting, relative sea level variations, and bottom current activity are probably the main factors that control size, external geometry, internal stratal configuration, and facies characteristics of both modern and ancient turbidite systems. These factors ultimately control the timing and bounding characteristics between stages of growth of deepwater systems. If comparison of elements from different turbidite deposits using various data types is carried out at similar physical and temporal scales, predictive models eventually may be improved.

Reviews of Geophysics↗
USGS1996

Submarine landslides

Landslides are common on inclined areas of the seafloor, particularly in environments where weak geologic materials such as rapidly deposited, finegrained sediment or fractured rock are subjected to strong environmental stresses such as earthquakes, large storm waves, and high internal pore pressures. Submarine landslides can involve huge amounts of material and can move great distances: slide volumes as large as 20,000 km3 and runout distances in excess of 140 km have been reported. They occur at locations where the downslope component of stress exceeds the resisting stress, causing movement along one or several concave to planar rupture surfaces. Some recent slides that originated nearshore and retrogressed back across the shoreline were conspicuous by their direct impact on human life and activities. Most known slides, however, occurred far from land in prehistoric time and were discovered by noting distinct to subtle characteristics, such as headwall scarps and displaced sediment or rock masses, on acoustic-reflection profiles and side-scan sonar images. Submarine landslides can be analyzed using the same mechanics principles as are used for occurrences on land. However, some loading mechanisms are unique, for example, storm waves, and some, such as earthquakes, can have greater impact. The potential for limited-deformation landslides to transform into sediment flows that can travel exceedingly long distances is related to the density of the slope-forming material and the amount of shear strength that is lost when the slope fails.

Reviews of Geophysics↗
USGS2011

Airborne laser study quantifies El Niño-induced coastal change

Winter storms during the 1997–1998 El Niño caused extensive changes to the beaches and cliffs of the west coast of the United States, a NASA-NOAA-USGS investigation using a scanning airborne laser has found. For example, near Pacifica in central California, the cliff eroded locally as much as 10–13 m landward during the El Niño winter, at least 40 times the long term average erosion rate. However, only several hundred meters away the cliff was stable. This variability in cliff response may be related to differences in local beach changes where an accreting beach protected part of the cliff and an eroding beach exposed another part to attack by waves.

Pacific Coast↗
USGS2012

The giant submarine alika debris slide, Mauna Loa, Hawaii

A 4000‐km 2 area of submarine slump and slide deposits along the west flank of Mauna Loa volcano has been mapped with GLORIA side‐scan sonar images, seismic reflection profiles, and new bathymetry. The youngest deposits are two debris avalanche lobes that travelled from their breakaway area near the present shoreline as much as 100 km into the Hawaiian Deep at water depths of 4800 m. The two lobes partly overlap and together are designated the Alika slide. They were derived from the same source area and probably formed in rapid succession. Distinction hummocky topography, marginal levees, and other features on lower slopes (0.3°–0.6°) of these deposits resemble subaerial volcanic debris avalanche deposits such as 1980 Mount St. Helens and suggest high emplacement velocities. The breakaway area for the Alika slide (10°–15° slopes) is characterized by large block slumps, bounded by normal faults, that probably represent multiple subsidence events before, during, and after the debris avalanches. Lower slopes of the slide contain distinctive lobate‐terraced deposits that are interpreted as having been emplaced more slowly, prior to the debris avalanches. Estimated thicknesses of 50–200 m suggest volumes of 200–600 km 3 for the two lobes. The combined volume of the entire slide and slump terrane is probably 1500–2000 km 3 . The slide deposits predate a 13‐ka coral reef and probably postdate the block‐faulted Ninole Basalt, roughly dated as a few hundred thousand years old. The Alika slide, or a similar deposit recognized on GLORIA images further north along the Hawaiian Ridge, probably triggered a giant wave that washed 325 m high on Lanai at about 100 ka. Slumping on Mauna Loa has been most intense adjacent to the large arcuate bend in its southwest rift zone, as the rift zone migrated westward away from the growing Kilauea volcano. Slumping events were probably triggered by seismic activity accompanying dike injection along the rift zone. Such massive slumps, landslides, and distal submarine turbidity flows appear to be widespread on the flanks of Hawaiian volcanoes.

Journal of Geophysical Research↗
USGS2012

Morphology and growth history of Delgada Fan: Implications for the Neogene evolution of Point Arena Basin and the Mendocino Triple Junction

Long-range side scan (GLORIA) sonographs and seismic reflection data acquired during a survey of the western U.S. Exclusive Economic Zone in 1984, coupled with information from Deep Sea Drilling Project sites, provide new insights into the growth and evolution of the Delgada Fan. Construction of the fan commenced in the latest Miocene (∼6 Ma) following the filling of the Neogene Point Arena Basin. The fan presently covers more than 50×10 3 km 2 of the Pacific plate and contains approximately 15×10 3 km 3 of predominantly terrigenous detritus. The large size of the fan is incompatible with the small present-day supply of sediment to the canyon system. The GLORIA data show the Delgada Fan to be a hybrid-type fan, exhibiting characteristics of both elongate and radial fans. The morphology and volume of the fan, along with evidence for a decline in accumulation rates on the lower fan during the Quaternary period, suggest that the fan experienced an early growth phase (latest Miocene and Pliocene) characterized by relatively rapid progradation of elongate fan lobes followed by a period (Quaternary) of slower growth that has featured a shift of depocenters to sites closer to the canyons and a transition to distributary channels bordered by less prominent levees and overbank deposits. We examine the growth of Delgada Fan in relation to the Neogene evolution of the North American-Pacific plate boundary using a series of paleogeographic reconstructions based on recently published time displacement histories of the Mendocino triple junction (MTJ), the San Andreas fault (SAF), and the Pacific plate, upon which the fan rests. The time displacement curves for the SAF and the MTJ suggest that the MTJ and Mendocino Fracture Zone overtook and passed Point Arena Basin at about 10 Ma when the basin lay immediately southwest of the present San Francisco Bay area. We suggest that the MTJ joined the SAF at approximately that time and location, thus making the SAF the master fault in the transform system. This interpretation is compatible with evidence from seismic reflection profiles over the fan, which demonstrate that the fan and the canyon system and therefore Point Arena Basin have moved as a unit since the inception of fan growth (∼6–7 Ma). Point Arena Basin lay southwest of the San Francisco area at 10–12 Ma, and the passage of the MTJ caused the disruption of the forearc shelf and slope and the development of local uplifted and subsiding blocks. In particular, uplift of the “bay block” immediately east of the SAF may have provided the source area for the late Miocene sediments that filled Point Arena Basin and set the stage for the growth of Delgada Canyon and Fan system. The growth rate of the fan has decreased, and the style of deposition has changed as the system was tectonically transported to its present location adjacent to the small youthful drainages of the King Range.

Journal of Geophysical Research Solid Earth↗
USGS1970

Geological significance of coccoliths in fine-grained carbonate bands of postglacial Black Sea sediments

The origin of fine carbonate muds in deep parts of the Black Sea has been explained in various ways, but details of how the carbonate was formed are poorly understood. We have studied samples containing fine carbonate from cores obtained during the cruise of Atlantis II (Woods Hole Oceanographic Institute) to the Black Sea in April and May 1969. Examination of the light-coloured bands and darker bands by scanning electron micrographs and light microscopy shows that Holocene coccoliths are the overwhelmingly dominant constituents in the light bands, whereas Cretaceous and Eocene coccoliths are important components of the carbonate in the darker matrix.

Nature↗
USGS2020

Mechanics of near-field deformation during co- and post-seismic shallow fault slip

Poor knowledge of how faults slip and distribute deformation in the shallow crust hinders efforts to mitigate hazards where faults increasingly intersect with the expanding global population at Earth’s surface. Here we analyze two study sites along the 2014 M 6.0 South Napa, California, earthquake rupture, each dominated by either co- or post-seismic shallow fault slip. We combine mobile laser scanning (MLS), active-source seismic tomography, and finite element modeling to investigate how deformation rate and mechanical properties of the shallow crust affect fault behavior. Despite four orders-of-magnitude difference in the rupture velocities, MLS-derived shear strain fields are remarkably similar at the two sites and suggest deceleration of the co-seismic rupture near Earth’s surface. Constrained by the MLS and seismic data, finite element models indicate shallow faulting is more sensitive to lithologic layering and plastic yielding than to the presence of fault compliant zones (i.e., regions surrounding faults with reduced stiffness). Although both elastic and elastoplastic models can reproduce the observed surface displacement fields within the uncertainty of MLS data, elastoplastic models likely provide the most reliable representations of subsurface fault behavior, as they produce geologically reasonable stress states and are consistent with field, geodetic, and seismological observations.

California↗
USGS2020

Morphological, elemental, and boron isotopic insights into pathophysiology of diseased coral growth anomalies

Growth anomalies (GAs) impact both coral skeleton and soft tissues and are detrimental to reef health. This tumor-like disease is increasingly found throughout the tropics and is commonly associated with high human population density, yet little is known about the etiology, pathology, or calcification behavior of the disease. Here, we investigate potential mechanisms involved in the development of GAs through chemical and morphological characterization of GA skeletons in Porites compressa from a site of high disease prevalence (Coconut Island, Hawaii). A comprehensive suite of trace elements and boron isotopes (δ11B) were measured in skeletal GAs to assess calcification behavior and uptake of essential and toxic metals. Scanning electron microscopy of GA skeleton revealed it to be highly porous consisting of a matrix with a disorganized crystal structure, in contrast to the dense well-organized normal skeleton of P. compressa. Elemental analyses revealed decreased Mg/Ca and increased U/Ca in GA skeletons relative to paired unaffected samples, suggesting a decreased abundance of rapidly accreting microstructures “centers of calcification” in the GAs. Estimates of carbonate system parameters based on δ11B and B/Ca measurements indicate reduced pH (–0.05 units) and [CO32–] within the calcifying fluid of GAs, which may have implications for GA calcification. Higher levels of essential (V/Ca and Mo/Ca) elements in GAs potentially indicate increased abundance of holobiont-associated, nitrogen-fixing bacteria and higher Sb/Ca and Nd/Ca indicate alteration in the accumulation/depuration of these toxic metals. In aggregate, our findings show that dystrophic calcification processes could explain structural differences seen in GA vs unaffected skeletons and highlight the use of approaches herein to shed light on disease pathophysiology in corals.

Scientific Reports↗