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At least 649 records · Page 36Linked to original sources
USGS2005

High-resolution surveys for geohazards and shallow gas: NW Adriatic (Italy) and Iskenderun Bay (Turkey)

The need for quantifying and understanding the distribution of shallow gas is both of academic interest and of relevance to offshore facilities. The combination of seafloor mapping, subbottom profiling, and multi-channel seismic data can provide information on regions of possible shallow gas, where the gas impacts the acoustic properties of the host material and the seafloor. In this paper, we present two case studies - one academic and one industry - that evaluate the distribution of shallow gas in two field areas in the Mediterranean. In the first case study, geophysical data from Iskenderun Bay, southeastern Turkey, indicate the presence and distribution of shallow gas. Pockmarks on the seafloor are associated with acoustic wipeout in the shallow subbottom data. Although deeper seismic data do not show bright spots or other indicators of possible gas, instantaneous frequency analysis clearly shows laterally restricted anomalies indicating gas-rich zones. The interpretation of possible shallow gas resulted in moving a proposed drilling location to a nearby area characterized by fewer (but still present) shallow gas signatures. In the second case study, cores acquired in the Po Delta, Adriatic Sea, provide quantitative ground-truthing of shallow gas - as suggested by geophysical data - and provide minimum estimates of the percentage of gas in the subsurface. Cores targeted on anomalous subbottom data yielded up to 41,000 ppm methane; cores with anomalous gas content are associated with thick recent flood deposits which may effectively isolate reactive terrigenous organic matter from biologic and physical re-working. ?? Springer 2005.

Marine Geophysical Research↗
USGS1984

Observations on Cretaceous abyssal hills in the northeast Pacific

An abyssal hills area of 50 × 60 km in the northeast Pacific was studied using bottom transponder navigation, closely spaced survey lines, and long-traverse oblique photography. The block-faulted north—south hills are bounded by scarps, commonly with 40° slopes. On these steep scarps sedimentation is inhibited and pillow basalts often crop out. An ash layer of high acoustic reflectivity at about 7 m subbottom depth blankets the area. This ash occurs in multiple beds altered to phillipsite and is highly consolidated. A 24 m.y. age for the ash is based on ichthyolith dates from samples in the overlying sediments. Acoustically transparent Neogene sediments above the ash are thickest in trough bottoms and are absent or thin on steep slopes. These Neogene sediments are composed of pale-brown pelagic clays of illite, quartz, smectite, chlorite and kaolinite. Dark-brown pelagic clays, rich in smectite and amorphous iron oxides, underlie the Neogene surficial sediments. Manganese nodules cover the bottom in varying percentages. The nodules are most abundant near basement outcrops and where the subbottom ash layer is absent.

Marine Geology↗
USGS1987

Seafloor gas seeps triggered by a northern California earthquake

A series of marine surveys using high-resolution seismic-reflection and side-scan sonar profiling was conducted on the Klamath River delta from 1977 to 1985. In November 1980, near the middle of this 8-year time period, a major earthquake (magnitude 7) occurred in the area. The earthquake caused liquefaction of shelf deposits off the Klamath River, and while mapping the sediment failures, we observed an inordinate number of gas vents and acoustic anomalies in the sediment (gas charging) and water column (gas clouds). Our analyses indicate that a change in gas seepage occurred after the earthquake. There was a marked increase in gas abundance immediately after the earthquake, a gradual decrease over the next year, and stabilization at pre-earthquake gas levels within five years or less. The acoustic anomalies were observed in pre-Quaternary rocks as well as in surficial Holocene deposits, and gas seeps overlay truncated anticlines, faults, and syncline exposures. Thus, the gas released may have been in part thermogenic in origin. A large area (20 km 2 ) on the delta failed during the earthquake, and the abundance of gas seeps and craters in the area suggests that sediment degassing may have been an important process contributing to failure.

Marine Geology↗
USGS2023

Linking vocal behaviours to habitat structure to create behavioural landscapes

The recent development of animal-borne sensors coupled with location data can provide insights into how individuals modify their behaviour with respect to specific habitat features. Animals can express a diverse array of behaviours as they navigate heterogenous landscapes, yet few studies have specifically evaluated the interaction of behaviours with habitat characteristics. We used a novel broadcast acoustic transmitter to investigate the interaction between vocal behaviours of an endemic Hawaiian thrush, the ʻōmaʻo, Myadestes obscurus , and habitat features across a naturally fragmented forest landscape. Through the development of behavioural landscape models that link specific vocalizations with space use, we found that the use of different vocalization types (calls, songs, whisper songs) were highly variable across the landscape but were associated with distinct habitat features. The likelihood of calls increased in an open lava matrix between forest patches, while whisper songs were more strongly associated with the dense interior areas of forest fragments. In contrast, the rate of ʻōmaʻo vocalizations overall decreased in the open lava matrix, suggesting that ʻōmaʻo may shift behaviours from territory defence to foraging as they transition through different habitats. Our study revealed context-specific changes in behaviour across ʻōmaʻo home ranges, including courtship, aggression and social interactions between individuals. Combining the use of a novel acoustic tool with automated radiotelemetry allowed us to overcome challenges associated with detection and analysis of variation in behaviour and resource selection across a highly heterogeneous landscape that would have been otherwise difficult to impossible.

Hawaii↗
USGS2006

Shear velocity estimates on the inner shelf off Grays Harbor, Washington, USA

Shear velocity was estimated from current measurements near the bottom off Grays Harbor, Washington between May 4 and June 6, 2001 under mostly wave-dominated conditions. A downward-looking pulse-coherent acoustic Doppler profiler (PCADP) and two acoustic-Doppler velocimeters (field version; ADVFs) were deployed on a tripod at 9-m water depth. Measurements from these instruments were used to estimate shear velocity with (1) a modified eddy-correlation (EC) technique, (2) the log-profile (LP) method, and (3) a dissipation-rate method. Although values produced by the three methods agreed reasonably well (within their broad ranges of uncertainty), there were important systematic differences. Estimates from the EC method were generally lowest, followed by those from the inertial-dissipation method. The LP method produced the highest values and the greatest scatter. We show that these results are consistent with boundary-layer theory when sediment-induced stratification is present. The EC method provides the most fundamental estimate of kinematic stress near the bottom, and stratification causes the LP method to overestimate bottom stress. These results remind us that the methods are not equivalent and that comparison among sites and with models should be made carefully. ?? 2006 Elsevier Ltd. All rights reserved.

Oregon, Washington↗
USGS2008

Time-averaged near-bed suspended sediment concentrations under waves and currents: Comparison of measured and model estimates

Profiles of suspended sediment concentration and velocity were measured over a 15-day period at a near-shore site off Santa Cruz, CA in Monterey Bay. The concentration and velocity data were collected with an Acoustic Backscattering System (ABS) and Acoustic Current Profiler (ACP) that were mounted on a bottom tripod. High-resolution bottom scanning sonar was also attached to the tripod to provide images of bed features during the experiment. Hourly time-averaged near-bed concentrations of suspended sediment were calculated from three models and compared with the measurements. Surface waves and currents that were generated by a storm of moderate intensity caused bed stresses that exceeded threshold stress for D 50 =0.02 cm, the median size of the moderately well-sorted bottom sediment, over a period of about 7 days. Estimates of the concentration at 1 cm above the bottom, C a1 , were obtained using the ABS measurements. These observations have been compared with predictions for the concentration at 1 cm above the bottom, C 1 . Nielsen's models for reference concentration C o [Nielsen, P., 1986. Suspended sediment concentrations under waves. Coastal Engineering 10, 32-31; Nielsen, P., 1992. Coastal Bottom Boundary Layers and Sediment Transport, Advanced Series on Ocean Engineering. World Scientific, Hackensack, NJ.] are purely wave-based and do not include effects of bottom currents on bed stress and bedform scales. C 1 calculated from this model compared well with measured C a1 when currents were weak and small oscillatory ripples were observed in the sonar images. However, during the 3-day period of highest bottom stresses modeled C 1 did not compare well to C a1 . The other two models for C 1 , Glenn and Grant [Glenn, S.M., Grant, W.D., 1987. A suspended sediment stratification correction for combined wave and current flows. Journal of Geophysical Research 92(C8), 8244-8264.] and van Rijn and Walstra [Van Rijn, L.C., Walstra, D.J.R., 2004. Description of TRANSPOR2004 and implementation in Delft3D-ONLINE. Interim Report prepared for DG Rijkswaterstaat, Rijksinstituut voor Kust en Zee. Delft Hydraulics Institute, The Netherlands.], accounted for combined wave-current stresses and included different formulations for predicting bedform scales. C 1 calculated from these models compared well with measurements throughout the early and middle period of the storm, but underpredicted measured values during the latter part of the experiment. An alternative method based on dimensional analysis provided a straightforward way to relate C 1 to bed stress and sediment parameters. This new relationship based on dimensional analysis explained about 77% of the variability in C a1 derived from the ABS data.

California↗
USGS2018

Seafloor fluid seeps on Kimki Ridge, offshore southern California: Links to active strike-slip faulting

The Kimki Ridge fluid seeps are located in western Catalina Basin about 60 km southwest of the southern California mainland and at a water depth of approximately 1100 m. Multichannel seismic reflection profiles collected by the U.S. Geological Survey (USGS) in 2014 show acoustic transparency within the Kimki Ridge, suggesting the possibility of fluid seeps and possible sub-seafloor fluid pathways. Subsequent multibeam bathymetric and backscatter intensity data collected during a cooperative University of Washington/USGS cruise in early 2016 show subtle seafloor buildups with high acoustic backscatter (reflectivity) in three places along Kimki Ridge, supporting the existence of fluid seepage. A Remotely Operated Vehicle (ROV) dive, conducted as part of the Nautilus Exploration Program, took place in August 2016 to confirm the presence of these previously unknown seeps and document their characteristics as well as those of any associated biological communities. Two of the three seeps were explored by ROV, and showed abundant evidence of fluid seepage, including characteristic algal mats, chemosynthetic clams, and authigenic carbonate formation. The seeps are comprised of carbonate buildups 1–3 m thick and 300–500 m across. Within these areas, we interpret broad crater-like depressions 30–50 m across and 1–2 m deep to be individual seep vents. The seep areas appear to be broad zones of diffuse seepage that support chemosynthetic biologic communities; however, active venting was not observed. Geochemical analyses of rock samples collected from the seeps indicate microbially driven anaerobic oxidation of methane at or near the sediment water interface. Seismic-reflection profiles show chimney-like fluid pathways along the limbs and in the axis of the fold forming Kimki Ridge, and evidence of methane in shallow sediments can be traced into the adjacent Catalina Basin. A system of closely spaced faults located at the axis of the Kimki Ridge anticline may serve as pathways to allow fluid flow to the seafloor. Our data are consistent with other studies that suggest that transpression is an important component in the formation and localization of fluid seeps in a strike-slip setting, implying that seep formation may be a common occurrence at fault stepovers or transpressional bends in strike-slip systems.

California↗
USGS2025

Two-stage models improve machine learning classifiers in wildlife research: A case study in identifying false positive detections of Ruffed Grouse

Autonomous recording units are increasingly being used to monitor wildlife on large geographic and temporal scales, paired with machine learning (ML) to automate detection of wildlife. However, false positive detections from ML classifiers can result in erroneous ecological models that can lead to misguided management and conservation actions. We used a two-stage general approach to understand and reduce false positive detections, a technique in which outputs of the primary classification model are passed to a secondary classification model to yield the probability that a detection from the primary model is a true positive detection. This approach is demonstrated on two open-source models that detect Ruffed Grouse ( Bonasa umbellus ). We analyzed over 9500 h of acoustic data collected in 2022–2023 from the Green Mountain National Forest in Vermont, USA, and found the two models detected different types of acoustic signals associated with differing life history traits. The first model yielded 4106 detections (71.5 % true positives) while the second model yielded 524 detections (17.0 % true positives). Secondary logistic regression models separated true positives and false positives with high accuracy (84.5 % and 89.8 % respectively). Our findings go beyond improving Ruffed Grouse monitoring and conservation efforts to, more broadly, illustrate how two-stage ML approaches can improve the use of model-derived detections in wildlife research.

Vermont↗
USGS2012

Relationship between mid-water trawling effort and catch composition uncertainty in two large lakes (Huron and Michigan) dominated by alosines, osmerids, and coregonines

Because it is not possible to identify species with echosounders alone, trawling is widely used as a method for collecting species and size composition data for allocating acoustic fish density estimates to species or size groups. In the Laurentian Great Lakes, data from midwater trawls are commonly used for such allocations. However, there are no rules for how much midwater trawling effort is required to adequately describe species and size composition of the pelagic fish communities in these lakes, so the balance between acoustic sampling effort and trawling effort has been unguided. We used midwater trawl data collected between 1986 and 2008 in lakes Michigan and Huron and a variety of analytical techniques to develop guidance for appropriate levels of trawl effort. We used multivariate regression trees and re-sampling techniques to i. identify factors that influence species and size composition of the pelagic fish communities in these lakes, ii. identify stratification schemes for the two lakes, iii. determine if there was a relationship between uncertainty in catch composition and the number of tows made, and iv. predict the number of tows required to reach desired uncertainty targets. We found that depth occupied by fish below the surface was the most influential explanatory variable. Catch composition varied between lakes at depths <38.5 m below the surface, but not at depths ≥38.5 m below the surface. Year, latitude, and bottom depth influenced catch composition in the near-surface waters of Lake Michigan, while only year was important for Lake Huron surface waters. There was an inverse relationship between RSE [relative standard error = 100 × (SE/mean)] and the number of tows made for the proportions of the different size and species groups. We found for the fifth (Lake Huron) and sixth (Lake Michigan) largest lakes in the world, 15–35 tows were adequate to achieve target RSEs (15% and 30%) for ubiquitous species, but rarer species required much higher, and at times, impractical effort levels to reach these targets.

Fisheries Research↗
USGS2016

Five-year evaluation of habitat remediation in Thunder Bay, Lake Huron: Comparison of constructed reef characteristics that attract spawning lake trout

Degradation of aquatic habitats has motivated construction and research on the use of artificial reefs to enhance production of fish populations. However, reefs are often poorly planned, reef design characteristics are not evaluated, and reef assessments are short-term. We constructed 29 reefs in Thunder Bay, Lake Huron, in 2010 and 2011 to mitigate for degradation of a putative lake trout spawning reef. Reefs were designed to evaluate lake trout preferences for height, orientation, and size, and were compared with two degraded natural reefs and a high-quality natural reef (East Reef). Eggs and fry were sampled on each reef for five years post-construction, and movements of 40 tagged lake trout were tracked during three spawning seasons using acoustic telemetry. Numbers of adults and spawning on the constructed reefs were initially low, but increased significantly over the five years, while remaining consistent on East Reef. Adult density, egg deposition, and fry catch were not related to reef height or orientation of the constructed reefs, but were related to reef size and adjacency to East Reef. Adult lake trout visited and spawned on all except the smallest constructed reefs. Of the metrics used to evaluate the reefs, acoustic telemetry produced the most valuable and consistent data, including fine-scale examination of lake trout movements relative to individual reefs. Telemetry data, supplemented with diver observations, identified several previously unknown natural spawning sites, including the high-use portions of East Reef. Reef construction has increased the capacity for fry production in Thunder Bay without apparently decreasing the use of the natural reef. Results of this project emphasize the importance of multi-year reef assessment, use of multiple assessment methods, and comparison of reef characteristics when developing artificial reef projects. Specific guidelines for construction of reefs focused on enhancing lake trout spawning are suggested.

Lake Huron, Thunder Bay↗
USGS2018

Bat community response to silvicultural treatments in bottomland hardwood forests managed for wildlife in the Mississippi Alluvial Valley

Silvicultural treatments (e.g., selective timber harvests) that are prescribed to promote wildlife habitat are intended to alter the physical structure of forests to achieve conditions deemed beneficial for wildlife. Such treatments have been advocated for management of bottomland hardwood forests on public conservation lands in the Mississippi Alluvial Valley. Although some songbirds respond positively to these management actions, and wildlife-forestry indirectly promotes bat prey availability, bat response is largely unknown. Forest structure may affect bat use of bottomland forests due to differences in foraging space or roost sites. We examined the effects of silvicultural treatments that were implemented to promote wildlife habitat on bat species activity. We conducted vegetation surveys and sampled insect biomass within 64 treated and 64 reference stands located on 15 public conservation areas in Arkansas, Louisiana, and Mississippi, USA. We examined the influence of vegetation metrics and insect biomass on acoustic detections of bats during passive nocturnal surveys in these stands. Detections of bat activity were similar between silviculturally treated stands and reference stands, indicating that both managed and reference stands provide habitat for generalist and forest interior bat species. Generalist bat species (e.g., evening bats, eastern red bats, and Seminole bats) were positively associated with increased insect biomass and the amount of dead wood within a stand. Basal area of large trees was positively associated with detection of tri-colored bats and bottomland specialists (Rafinesque’s big-eared bats and myotine bats). Conversely, acoustic detection of bats was negatively associated with increased vegetative density (i.e., clutter). Managers that implement silvicultural treatments to improve desired forest conditions for wildlife can provide habitat for both generalist and forest interior bat species by providing heterogeneous forest structure that includes dead wood, high basal area of large trees, high tree species diversity, and gaps that are sufficiently thinned to allow unimpeded flight by bats.

Arkansas, Louisiana, Mississippi↗
USGS2021

Exploring how vessel activity influences the soundscape at a navigation lock on the Mississippi River

Vessel sound is now globally recognized as a significant and pervasive pollutant to aquatic life. However, compared to marine environments, there is a paucity of data on sound emitted by vessel activity in freshwater habitats. The Upper Mississippi River (UMR) is home to a diverse array of aquatic life as well as being a key route for barge transportation with 29 locks and dams. In this study, passive acoustic monitoring was conducted at Lock and Dam 19 near Keokuk, Iowa, on the UMR between 20 June – August 28, 2019 to coincide with peak navigation use. There was a significant increase in median sound pressure level (SPL; 50–12,000 Hz) recorded during vessel passages (123 dB re. 1μPa for recreational vessels and 137 dB re. 1μPa for commercial vessels) compared to median background levels (111 dB re. 1μPa). Results provide information on the ambient soundscape at a navigation lock, providing a baseline essential for future studies gauging the effect of anthropogenic sound on aquatic life. Lock 19 has also been identified as a potential site for acoustic deterrent deployment to prevent invasive fish movements. The results of this study can help determine the sound level or frequency deterrents would need to emit, to avoid those currently produced during vessel passage.

Illinois, Iowa↗
USGS2012

Small-scale lacustrine drifts in Lake Champlain, Vermont

High resolution CHIRP (Compressed High Intensity Radar Pulse) seismic profiles reveal the presence of two lacustrine sediment drifts located in Lake Champlain's Juniper Deep. Both drifts are positive features composed of highly laminated sediments. Drift B sits on a basement high while Drift A is built on a trough-filling acoustically-transparent sediment unit inferred to be a mass-transport event. These drifts are oriented approximately north–south and are parallel to a steep ridge along the eastern shore of the basin. Drift A, located at the bottom of a structural trough, is classified as a confined, elongate drift that transitions northward to become a system of upslope asymmetric mudwaves. Drift B is perched atop a structural high to the west of Drift A and is classified as a detached elongate drift. Bottom current depositional control was investigated using Acoustic Doppler Current Profilers (ADCPs) located across Drift A. Sediment cores were taken at the crest and at the edges of the Drift A and were dated. Drift source, deposition, and evolution show that these drifts are formed by a water column shear with the highest deposition occurring along its crest and western flank and began developing circa 8700–8800 year BP.

Vermont↗
USGS2023

Large-scale variation in lakebed properties interpreted from single-beam sonar in two Laurentian Great Lakes

Acoustic seabed classification (ASC) is an important method for understanding landscape-level physical and biological patterns in the aquatic environment. Bottom habitats in the Laurentian Great Lakes are poorly mapped to date, and will require a variety of contributors and data sources to complete. We repurposed a long-term split-beam echosounder dataset gathered for purposes of fisheries assessment to estimate lakebed properties utilizing unsupervised classification of echo return data. We interpreted first echo properties representing lakebed hardness and roughness to define and map three statistically supported lakebed classes revealed through cluster analysis. Our results indicate coherent and logical class boundaries and suggest that the dataset has promise for expanded use in ASC. To improve inferences using repeated measures, future work should focus on collecting ground truth information for areas previously surveyed and on collecting concurrent ground truth information when sampling acoustic data moving forward.

Michigan, Ontario, Wisconsin↗
USGS2018

Computing under-ice discharge: A proof-of-concept using hydroacoustics and the Probability Concept

Under-ice discharge is estimated using open-water reference hydrographs; however, the ratings for ice-affected sites are generally qualified as poor. The U.S. Geological Survey (USGS), in collaboration with the Colorado Water Conservation Board, conducted a proof-of-concept to develop an alternative method for computing under-ice discharge using hydroacoustics and the Probability Concept. The study site was located south of Minturn, Colorado (CO), USA, and was selected because of (1) its proximity to the existing USGS streamgage 09064600 Eagle River near Minturn, CO, and (2) its ease-of-access to verify discharge using a variety of conventional methods. From late September 2014 to early March 2015, hydraulic conditions varied from open water to under ice. These temporal changes led to variations in water depth and velocity. Hydroacoustics (tethered and uplooking acoustic Doppler current profilers and acoustic Doppler velocimeters) were deployed to measure the vertical-velocity profile at a singularly important vertical of the channel-cross section. Because the velocity profile was non-standard and cannot be characterized using a Power Law or Log Law, velocity data were analyzed using the Probability Concept, which is a probabilistic formulation of the velocity distribution. The Probability Concept-derived discharge was compared to conventional methods including stage-discharge and index-velocity ratings and concurrent field measurements; each is complicated by the dynamics of ice formation, pressure influences on stage measurements, and variations in cross-sectional area due to ice formation. No particular discharge method was assigned as truth. Rather one statistical metric (Kolmogorov-Smirnov; KS), agreement plots, and concurrent measurements provided a measure of comparability between various methods. Regardless of the method employed, comparisons between each method revealed encouraging results depending on the flow conditions and the absence or presence of ice cover. For example, during lower discharges dominated by under-ice and transition (intermittent open-water and under-ice) conditions, the KS metric suggests there is not sufficient information to reject the null hypothesis and implies that the Probability Concept and index-velocity rating represent similar distributions. During high-flow, open-water conditions, the comparisons are less definitive; therefore, it is important that the appropriate analytical method and instrumentation be selected. Six conventional discharge measurements were collected concurrently with Probability Concept-derived discharges with percent differences (%) of −9.0%, −21%, −8.6%, 17.8%, 3.6%, and −2.3%. This proof-of-concept demonstrates that riverine discharges can be computed using the Probability Concept for a range of hydraulic extremes (variations in discharge, open-water and under-ice conditions) immediately after the siting phase is complete, which typically requires one day. Computing real-time discharges is particularly important at sites, where (1) new streamgages are planned, (2) river hydraulics are complex, and (3) shifts in the stage-discharge rating are needed to correct the streamflow record. Use of the Probability Concept does not preclude the need to maintain a stage-area relation. Both the Probability Concept and index-velocity rating offer water-resource managers and decision makers alternatives for computing real-time discharge for open-water and under-ice conditions.

Colorado↗
USGS2010

Tectonic controls on nearshore sediment accumulation and submarine canyon morphology offshore La Jolla, Southern California

CHIRP seismic and swath bathymetry data acquired offshore La Jolla, California provide an unprecedented three-dimensional view of the La Jolla and Scripps submarine canyons. Shore-parallel patterns of tectonic deformation appear to control nearshore sediment thickness and distribution around the canyons. These shore-parallel patterns allow the impact of local tectonic deformation to be separated from the influence of eustatic sea-level fluctuations. Based on stratal geometry and acoustic character, we identify a prominent angular unconformity inferred to be the transgressive surface and three sedimentary sequences: an acoustically laminated estuarine unit deposited during early transgression, an infilling or “healing-phase” unit formed during the transgression, and an upper transparent unit. Beneath the transgressive surface, steeply dipping reflectors with several dip reversals record faulting and folding along the La Jolla margin. Scripps Canyon is located at the crest of an antiform, where the rocks are fractured and more susceptible to erosion. La Jolla Canyon is located along the northern strand of the Rose Canyon Fault Zone, which separates Cretaceous lithified rocks to the south from poorly cemented Eocene sands and gravels to the north. Isopach and structure contour maps of the three sedimentary units reveal how their thicknesses and spatial distributions relate to regional tectonic deformation. For example, the estuarine unit is predominantly deposited along the edges of the canyons in paleotopographic lows that may have been inlets along barrier beaches during the Holocene sea-level rise. The distribution of the infilling unit is controlled by pre-existing relief that records tectonic deformation and erosional processes. The thickness and distribution of the upper transparent unit are controlled by long-wavelength, tectonically induced relief on the transgressive surface and hydrodynamics.

California↗
USGS2009

Occurrence of gas hydrate in Oligocene Frio sand: Alaminos Canyon Block 818: Northern Gulf of Mexico

A unique set of high-quality downhole shallow subsurface well log data combined with industry standard 3D seismic data from the Alaminos Canyon area has enabled the first detailed description of a concentrated gas hydrate accumulation within sand in the Gulf of Mexico. The gas hydrate occurs within very fine grained, immature volcaniclastic sands of the Oligocene Frio sand. Analysis of well data acquired from the Alaminos Canyon Block 818 #1 ("Tigershark") well shows a total gas hydrate occurrence 13??m thick, with inferred gas hydrate saturation as high as 80% of sediment pore space. Average porosity in the reservoir is estimated from log data at approximately 42%. Permeability in the absence of gas hydrates, as revealed from the analysis of core samples retrieved from the well, ranges from 600 to 1500 millidarcies. The 3-D seismic data reveals a strong reflector consistent with significant increase in acoustic velocities that correlates with the top of the gas-hydrate-bearing sand. This reflector extends across an area of approximately 0.8??km 2 and delineates the minimal probable extent of the gas hydrate accumulation. The base of the inferred gas-hydrate zone also correlates well with a very strong seismic reflector that indicates transition into units of significantly reduced acoustic velocity. Seismic inversion analyses indicate uniformly high gas-hydrate saturations throughout the region where the Frio sand exists within the gas hydrate stability zone. Numerical modeling of the potential production of natural gas from the interpreted accumulation indicates serious challenges for depressurization-based production in settings with strong potential pressure support from extensive underlying aquifers.

Marine and Petroleum Geology↗
USGS2012

Gulf of Mexico Gas Hydrate Joint Industry Project Leg II logging-while-drilling data acquisition and analysis

One of the objectives of the Gulf of Mexico Gas Hydrate Joint Industry Project Leg II (GOM JIP Leg II) was the collection of a comprehensive suite of logging-while-drilling (LWD) data within gas-hydrate-bearing sand reservoirs in order to make accurate estimates of the concentration of gas hydrates under various geologic conditions and to understand the geologic controls on the occurrence of gas hydrate at each of the sites drilled during this expedition. The LWD sensors just above the drill bit provided important information on the nature of the sediments and the occurrence of gas hydrate. There has been significant advancements in the use of downhole well-logging tools to acquire detailed information on the occurrence of gas hydrate in nature: From using electrical resistivity and acoustic logs to identify gas hydrate occurrences in wells to where wireline and advanced logging-while-drilling tools are routinely used to examine the petrophysical nature of gas hydrate reservoirs and the distribution and concentration of gas hydrates within various complex reservoir systems. Recent integrated sediment coring and well-log studies have confirmed that electrical resistivity and acoustic velocity data can yield accurate gas hydrate saturations in sediment grain supported (isotropic) systems such as sand reservoirs, but more advanced log analysis models are required to characterize gas hydrate in fractured (anisotropic) reservoir systems. In support of the GOM JIP Leg II effort, well-log data montages have been compiled and presented in this report which includes downhole logs obtained from all seven wells drilled during this expedition with a focus on identifying and characterizing the potential gas-hydrate-bearing sedimentary section in each of the wells. Also presented and reviewed in this report are the gas-hydrate saturation and sediment porosity logs for each of the wells as calculated from available downhole well logs.

Gulf Of Mexico↗