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At least 919 records · Page 51Linked to original sources

USGS2016

Blood selenium concentrations in female Pacific black brant molting in Arctic Alaska: Relationships with age and habitat salinity

Blood samples collected from 81 female Pacific black brant ( Branta bernicla nigricans ) molting near Teshekpuk Lake, Alaska, were analyzed for selenium concentration. The concentration of selenium in blood of after second year (hatched two or more years ago) females (0.84 μg/g wet weight) was significantly greater than the concentration in second year (hatched the previous year) females (0.61 μg/g wet weight). The concentrations of selenium we found in blood of black brant were 1.5 to 2 times greater than baseline values typical of freshwater birds, but considerably lower than reported in other marine waterfowl sampled in Alaska. This finding may be attributable in part to the nearly exclusive herbivorous diet of black brant. No relationship was noted between blood selenium concentration and molting habitat salinity. We are unaware of any previous reports of blood selenium concentrations in black brant.

Alaska↗

USGS2011

Interspecies transmission and limited persistence of low pathogenic avian influenza genomes among Alaska dabbling ducks

The reassortment and geographic distribution of low pathogenic avian influenza (LPAI) virus genes are well documented, but little is known about the persistence of intact LPAI genomes among species and locations. To examine persistence of entire LPAI genome constellations in Alaska, we calculated the genetic identities among 161 full-genome LPAI viruses isolated across 4 years from five species of duck: northern pintail ( Anas acuta ), mallard ( Anas platyrhynchos ), American green-winged teal ( Anas crecca ), northern shoveler ( Anas clypeata ) and American wigeon ( Anas americana ). Based on pairwise genetic distance, highly similar LPAI genomes (>99% identity) were observed within and between species and across a range of geographic distances (up to and >1000 km), but most often between isolates collected 0–10 km apart. Highly similar viruses were detected between years, suggesting inter-annual persistence, but these were rare in our data set with the majority occurring within 0–9 days of sampling. These results identify LPAI transmission pathways in the context of species, space and time, an initial perspective into the extent of regional virus distribution and persistence, and insight into why no completely Eurasian genomes have ever been detected in Alaska. Such information will be useful in forecasting the movement of foreign-origin avian influenza strains should they be introduced to North America.

Alaska↗

USGS2017

Observations and first reports of saprolegniosis in Aanaakłiq, broad whitefish (Coregonus nasus), from the Colville River near Nuiqsut, Alaska

We report the first confirmed cases (2013–2016) of saprolegniosis caused by water mold from the genus Saprolegnia in Aanaakłiq, broad whitefish ( Coregonus nasus ), from the Colville River near Nuiqsut, Alaska. While this mold is known to be worldwide, these instances represent the first cases in Nuiqsut and only the second instance on a single fish on the North Slope, occurring in 1980. We describe the collaborative work on monitoring this emerging disease. Because fish constitute a critical component of the diet in Nuiqsut and fishing is an integral part of Inupiaq nutritional and cultural subsistence activities overall, individual subsistence fishers, local governmental entities, and Alaska Native organizations representing Nuiqsut requested an examination of affected fish and information on possible drivers of this emerging disease. The collaborative work described here ranges from recording fishermen observations, acquiring fish and mold specimens, histopathology, and molecular identification of the mold. This work, not currently grant-funded, begins with Native observation that incorporates western scientific methods and involves local, state, and federal departments as well as for-profit and non-profit organizations. Additionally, we report the more recent (2016) observation of this disease in a second species of whitefish, Pikuktuuq, humpback whitefish ( Coregonus pidschain ).

Alaska↗

USGS2012

Early retreat of the Alaska Peninsula Glacier Complex and the implications for coastal migrations of First Americans

The debate over a coastal migration route for the First Americans revolves around two major points: seafaring technology, and a viable landscape and resource base. Three lake cores from Sanak Island in the western Gulf of Alaska yield the first radiocarbon ages from the continental shelf of the Northeast Pacific and record deglaciation nearly 17 ka BP (thousands of calendar years ago), much earlier than previous estimates based on extrapolated data from other sites outside the coastal corridor in the Gulf of Alaska. Pollen data suggest an arid, terrestrial ecosystem by 16.3 ka BP. Therefore glaciers would not have hindered the movement of humans along the southern edge of the Bering Land Bridge for two millennia before the first well-recognized “New World” archaeological sites were inhabited.

Alaska↗

USGS2013

Ice-age megafauna in Arctic Alaska: extinction, invasion, survival

Radical restructuring of the terrestrial, large mammal fauna living in arctic Alaska occurred between 14,000 and 10,000 years ago at the end of the last ice age. Steppe bison, horse, and woolly mammoth became extinct, moose and humans invaded, while muskox and caribou persisted. The ice age megafauna was more diverse in species and possibly contained 6× more individual animals than live in the region today. Megafaunal biomass during the last ice age may have been 30× greater than present. Horse was the dominant species in terms of number of individuals. Lions, short-faced bears, wolves, and possibly grizzly bears comprised the predator/scavenger guild. The youngest mammoth so far discovered lived ca 13,800 years ago, while horses and bison persisted on the North Slope until at least 12,500 years ago during the Younger Dryas cold interval. The first people arrived on the North Slope ca 13,500 years ago. Bone-isotope measurements and foot-loading characteristics suggest megafaunal niches were segregated along a moisture gradient, with the surviving species (muskox and caribou) utilizing the warmer and moister portions of the vegetation mosaic. As the ice age ended, the moisture gradient shifted and eliminated habitats utilized by the dryland, grazing species (bison, horse, mammoth). The proximate cause for this change was regional paludification, the spread of organic soil horizons and peat. End-Pleistocene extinctions in arctic Alaska represent local, not global extinctions since the megafaunal species lost there persisted to later times elsewhere. Hunting seems unlikely as the cause of these extinctions, but it cannot be ruled out as the final blow to megafaunal populations that were already functionally extinct by the time humans arrived in the region.

Alaska↗

USGS2014

A deglacial and Holocene record of climate variability in south-central Alaska from stable oxygen isotopes and plant macrofossils in peat

We used stable oxygen isotopes derived from bulk peat (δ 18 O TOM ), in conjunction with plant macrofossils and previously published carbon accumulation records, in a &sim;14,500 cal yr BP peat core (HT Fen) from the Kenai lowlands in south-central Alaska to reconstruct the climate history of the area. We find that patterns are broadly consistent with those from lacustrine records across the region, and agree with the interpretation that major shifts in δ 18 O TOM values indicate changes in strength and position of the Aleutian Low (AL), a semi-permanent low-pressure cell that delivers winter moisture to the region. We find decreased strength or a more westerly position of the AL (relatively higher δ 18 O TOM values) during the Bølling-Allerød, Holocene Thermal Maximum (HTM), and late Holocene, which also correspond to warmer climate regimes. These intervals coincide with greater peat preservation and enhanced carbon (C) accumulation rates at the HT Fen and with peatland expansion across Alaska. The HTM in particular may have experienced greater summer precipitation as a result of an enhanced Pacific subtropical high, a pattern consistent with modern δ 18 O values for summer precipitation. The combined warm summer temperatures and greater summer precipitation helped promote the observed rapid peat accumulation. A strengthened AL (relatively lower δ 18 O TOM values) is most evident during the Younger Dryas, Neoglaciation, and the Little Ice Age, consistent with lower peat preservation and C accumulation at the HT Fen, suggesting less precipitation reaches the leeward side of the Kenai Mountains during periods of enhanced AL strength. The peatlands on the Kenai Peninsula thrive when the AL is weak and the contribution of summer precipitation is higher, highlighting the importance of precipitation seasonality in promoting peat accumulation. This study demonstrates that δ 18 O TOM values in peat can be applied toward understand large-scale shifts in atmospheric circulation over millennial timescales.

Alaska↗

USGS2018

Aeolian stratigraphy describes ice-age paleoenvironments in unglaciated Arctic Alaska

Terrestrial paleoenvironmental records with high dating resolution extending into the last ice age are rare from the western Arctic. Such records can test the synchronicity and extent of ice-age climatic events and define how Arctic landscapes respond to rapid climate changes. Here we describe the stratigraphy and sedimentology of a yedoma deposit in Arctic Alaska (the Carter Section) dating to between 37,000 and 9000 calibrated radiocarbon years BP (37–9 ka) and containing detailed records of loess and sand-sheet sedimentation, soil development, carbon storage, and permafrost dynamics. Alternation between sand-sheet and loess deposition provides a proxy for the extent and activity of the Ikpikpuk Sand Sea (ISS), a large dune field located immediately upwind. Warm, moist interstadial times (ca. 37, 36.3–32.5, and 15–13 ka) triggered floodplain aggradation, permafrost thaw, reduced loess deposition, increased vegetation cover, and rapid soil development accompanied by enhanced carbon storage. During the Last Glacial Maximum (LGM, ca. 28–18 ka), rapid loess deposition took place on a landscape where vegetation was sparse and non-woody. The most intense aeolian activity occurred after the LGM between ca. 18 and 15 ka when sand sheets fringing the ISS expanded over the site, possibly in response to increasingly droughty conditions as summers warmed and active layers deepened. With the exception of this lagged LGM response, the record of aeolian activity at the Carter Section correlates with other paleoenvironmental records from unglaciated Siberia and Alaska. Overall, rapid shifts in geomorphology, soils, vegetation, and permafrost portray an ice-age landscape where, in contrast to the Holocene, environmental change was chronic and dominated by aeolian processes.

Alaska↗

USGS2009

Integration of MODIS-derived metrics to assess interannual variability in snowpack, lake ice, and NDVI in southwest Alaska

Impacts of global climate change are expected to result in greater variation in the seasonality of snowpack, lake ice, and vegetation dynamics in southwest Alaska. All have wide-reaching physical and biological ecosystem effects in the region. We used Moderate Resolution Imaging Spectroradiometer (MODIS) calibrated radiance, snow cover extent, and vegetation index products for interpreting interannual variation in the duration and extent of snowpack, lake ice, and vegetation dynamics for southwest Alaska. The approach integrates multiple seasonal metrics across large ecological regions. Throughout the observation period (2001-2007), snow cover duration was stable within ecoregions, with variable start and end dates. The start of the lake ice season lagged the snow season by 2 to 3??months. Within a given lake, freeze-up dates varied in timing and duration, while break-up dates were more consistent. Vegetation phenology varied less than snow and ice metrics, with start-of-season dates comparatively consistent across years. The start of growing season and snow melt were related to one another as they are both temperature dependent. Higher than average temperatures during the El Ni??o winter of 2002-2003 were expressed in anomalous ice and snow season patterns. We are developing a consistent, MODIS-based dataset that will be used to monitor temporal trends of each of these seasonal metrics and to map areas of change for the study area.

Remote Sensing of Environment↗

USGS2024

Molecular detection and characterization of highly pathogenic H5N1 clade 2.3.4.4b avian influenza viruses among hunter-harvested wild birds provides evidence for three independent introductions into Alaska

We detected and characterized highly pathogenic avian influenza viruses among hunter-harvested wild waterfowl inhabiting western Alaska during September–October 2022 using a molecular sequencing pipeline applied to RNA extracts derived directly from original swab samples. Genomic characterization of 10 H5 clade 2.3.4.4b avian influenza viruses detected with high confidence provided evidence for three independent viral introductions into Alaska. Our results highlight the utility and some potential limits of applying molecular processing approaches directly to RNA extracts from original swab samples for viral research and monitoring.

Alasak↗

USGS1998

Carbon isotopic comparisons of oil products used in the developmental history of Alaska

Studies of the fate of oil released into Prince William Sound, AK, as a result of the 1989 Exxon Valdez oil spill, have led to an unexpected discovery. In addition to oil-like residues attributed to the spill, the ubiquitous presence of flattened tar balls, the carbon isotopic compositions of which fall within a surprisingly narrow range [??13C(PDB) = -23.7 ?? 0.3??? (n = 65)], were observed on the shorelines of the northern and western parts of the sound. These compositions are similar to those of some oil products [-23.7 ?? 0.7??? (n = 35)] that were shipped from California and used in Alaska for fuel, lubrication, construction, and paving before ~ 1970. These products include fuel oil, asphalt, and lubricants [-23.8 ?? 0.5??? (n = 11)], caulking, sealants, and roofing tar [-23.7 ?? 0.7??? (n = 16)], and road pavements and airport runways [-23.5 ?? 0.9??? (n = 8)]. Fuel oil and asphalt [-23.5 ?? 0.1??? (n = 3)], stored at the old Valdez town site and spilled during the 1964 Alaskan earthquake, appear to be the source of most of the beached tar balls. Oil products with lighter carbon isotopic compositions, between -25 and -30??? (n = 18), appear to have been used more recently in Alaska, that is, after ~ 1970. The source of some of the products used for modern pavement and runways [-29.3 ?? 0.2??? (n = 6)] is likely Alaskan North Slope crude oil, an example of which was spilled in the 1989 oil spill [-29.2??? (n = 1)].

Chemical Geology↗

USGS1997

Mass-mortality of guillemots ( Uria aalge ) in the Gulf of Alaska in 1993

During the first six months of 1993, about 3500 dead and moribund guillemots ( Uria aalge ) were observed throughout the northern Gulf of Alaska coast ( ca 1800 km range). Mortality peaked during March. Highest numbers were observed in western Prince William Sound and along the south coast of the Kenai Peninsula. Large flocks of live guillemots gathered in nearshore waters, in contrast to most winters when guillemots reside offshore. Most guillemots recovered were extremely emaciated ( ca 60% of normal weight) and sub-adult (80%). Based on carcass deposition and persistence experiments, we calculate that about 10 900 birds eventually came ashore on beaches that were surveyed. Even if most birds killed made it to shore, only a fraction of beaches in the Gulf of Alaska were surveyed and we estimate that a minimum total of 120 000 guillemots died. Results of other investigations on potential causes of mortality (biotoxins, pathogens, parasites, metals, etc.) were either negative or inconclusive, and necropsies lead us to believe that starvation was the proximate cause of death. Reduced food availability could have been related to anomalous sea conditions found during the prolonged 1990–1995 El Niño-Southern Oscillation event.

Alaska↗

USGS1997

An 18 million year record of vegetation and climate change in northwestern Canada and Alaska: Tectonic and global climatic correlates

We reconstruct long-term vegetation/paleoclimatic trends, spanning the last 18 million years, in Alaska. Yukon and far western Northwest Territories. Twenty-one average percentage spectra for pollen and spores are assembled from eight surface/subsurface sections. The sections are dated independently or by correlation. Pollen and spore ratios indicate the direction of change in vegetation and climatic parameters growing season temperature (T(est)), tree canopy density (C(est)) and paludification at study sites (P(est)). A global warm peak ca. 15 Ma is shown by the abundance of thermophilous taxa, including Fagus and Quercus. A temperature decline immediately following 15 Ma parallels climatic reconstructions based on marine oxygen isotopes. Subsequent declines correlate to the Messinian event and the onset of late Pliocene Pleistocene glaciation. After 7 Ma herbs and shrubs become more important elements of the palynological assemblages, suggesting a more continental, colder/drier climate. However, a late Pliocene warm interval is evident. Vegetation/climatic changes during the early to late Miocene show synchrony with, and are most economically attributable to, global events. After 7 Ma, vegetation/climate change is attributed primarily to latest Miocene-to-Pleistocene uplift of the Alaska Range and St. Elias Mrs. The continuing influence of global climatic patterns is shown in the late Pliocene warm interval, despite uplift to the south. The opening of the Bering Strait ca. 3 Ma may have moderated the climate in the study area.

Palaeogeography, Palaeoclimatology, Palaeoecology↗

USGS2000

Distribution, speciation, and transport of mercury in stream-sediment, stream-water, and fish collected near abandoned mercury mines in southwestern Alaska, USA

Concentrations of total Hg, Hg (II), and methylmercury were measured in stream-sediment, stream-water, and fish collected downstream from abandoned mercury mines in south-western Alaska to evaluate environmental effects to surrounding ecosystems. These mines are found in a broad belt covering several tens of thousands of square kilometers, primarily in the Kuskokwim River basin. Mercury ore is dominantly cinnabar (HgS), but elemental mercury (Hg(o)) is present in ore at one mine and near retorts and in streams at several mine sites. Approximately 1400 t of mercury have been produced from the region, which is approximately 99% of all mercury produced from Alaska. These mines are not presently operating because of low prices and low demand for mercury. Stream-sediment samples collected downstream from the mines contain as much as 5500 ??g/g Hg. Such high Hg concentrations are related to the abundance of cinnabar, which is highly resistant to physical and chemical weathering, and is visible in streams below mine sites. Although total Hg concentrations in the stream-sediment samples collected near mines are high, Hg speciation data indicate that concentrations of Hg (II) are generally less than 5%, and methylmercury concentrations are less than 1% of the total Hg. Stream waters below the mines are neutral to slightly alkaline (pH 6.8-8.4), which is a result of the insolubility of cinnabar and the lack of acid- generating minerals such as pyrite in the deposits. Unfiltered stream-water samples collected below the mines generally contain 500-2500 ng/l Hg; whereas, corresponding stream-water samples filtered through a 0.45-??m membrane contain less than 50 ng/l Hg. These stream-water results indicate that most of the Hg transported downstream from the mines is as finely- suspended material rather than dissolved Hg. Mercury speciation data show that concentrations of Hg (II) and methylmercury in stream-water samples are typically less than 22 ng/l, and generally less than 5% of the total Hg. Muscle samples of fish collected downstream from mines contain as much as 620 ng/g Hg (wet wt.), of which 90-100% is methylmercury. Although these Hg concentrations are several times higher than that in fish collected from regional baseline sites, the concentration of Hg in fish is below the 1000 ng/g action level for edible fish established by the US Food and Drug Administration (FDA). Salmon contain less than 100 ng/g Hg, which are among the lowest Hg contents observed for fish in the study, and well below the FDA action level. (C) 2000 Elsevier Science B.V.

Science of the Total Environment↗

USGS2002

Holocene tephrochronology of the Cold Bay area, southwest Alaska Peninsula

The major-element glass geochemistry of 92 tephra samples from the southwest Alaska Peninsula provides the basis for establishing a Holocene tephrochronology for the region. Electron microprobe analysis has been combined with field descriptions of samples, stratigraphic relationships between tephra samples and sample localities, and glass shard micro-morphology to correlate these sampled distal tephra units throughout the area of Cold Bay and adjacent Morzhovoi Bay. Radiocarbon dating provides age constraints on correlated horizons. Previous research had clearly delineated only one horizon in the region, the so-called 'Funk/Fisher' ash, dating to between 8425 ± 350 and 9130 ± 140 14 C yr BP. In addition to constraining the bimodal andesitic and dacitic glass chemistry of that horizon, this study has recognized six additional tephra layers in the area. Two horizons pre-date the Funk/Fisher ash and four are younger than it. A tephra containing dacitic and andesitic components was identified in the vicinity of Morzhovoi Bay, with a minimum age of 9300 ± 80 14 C yr BP and a maximum age of 10,200 ± 75 14 C yr BP. A rhyolitic horizon composed of cm-sized, rounded pumice clasts was identified in the vicinity of Cold Bay; it has been correlated to the ca 9500 BP eruption of Roundtop volcano on Unimak Island. The four younger tephra beds date to between 6070 ± 340 and 3600 ± 140 14 C yr BP. The oldest of the four is rhyodacitic, followed by a mixed rhyodacitic-andesitic horizon, another rhyodacitic horizon, and finally an andesitic layer. Comparison of all the correlated horizons to proximal samples collected on Unimak Island provides conclusive geochemical evidence that the ca 9100 BP Caldera-forming eruption of Fisher volcano is the source of the Funk/Fisher ash. Correlation between the rhyodacitic tephra horizons and proximal samples from Fisher volcano suggests that Fisher Caldera is the source of one of the rhyodacitic tephra horizons that post-dates the Funk/Fisher ash. Additional tephra samples from the southwest Alaska Peninsula and Unimak Island that were collected prior to this study correlate to the tephra horizons identified in the Cold Bay area and identify one additional horizon.

Quaternary Science Reviews↗

USGS2017

Geometry, mass balance and thinning at Eklutna Glacier, Alaska: an altitude-mass-balance feedback with implications for water resources

We analyzed glacier surface elevations (1957, 2010 and 2015) and surface mass-balance measurements (2008–2015) on the 30 km 2 Eklutna Glacier, in the Chugach Mountains of southcentral Alaska. The geodetic mass balances from 1957 to 2010 and 2010 to 2015 are −0.52 ± 0.46 and −0.74 ± 0.10 m w.e. a −1 , respectively. The glaciological mass balance of −0.73 m w.e. a −1 from 2010 to 2015 is indistinguishable from the geodetic value. Even after accounting for loss of firn in the accumulation zone, we found most of the mass loss over both time periods was from a broad, low-slope basin that includes much of the accumulation zone of the main branch. Ice-equivalent surface elevation changes in the basin were −1.0 ± 0.8 m a −1 from 1957 to 2010, and −0.6 ± 0.1 m a −1 from 2010 to 2015, shifting the glacier hypsometry downward and resulting in more negative mass balances: an altitude-mass-balance feedback. Net mass loss from Eklutna Glacier accounts for 7 ± 1% of the average inflow to Eklutna Reservoir, which is entirely used for water and power by Anchorage, Alaska's largest city. If the altitude-mass-balance feedback continues, this ‘deglaciation discharge dividend’ is likely to increase over the short-term before it eventually decreases due to diminishing glacier area.

Alaska↗

USGS2021

Late Quaternary deglaciation of Prince William Sound, Alaska

To understand the timing of deglaciation of the northernmost marine-terminating glaciers of the Cordilleran Ice Sheet (CIS), we obtained 26 10 Be surface-exposure ages from glacially scoured bedrock surfaces in Prince William Sound (PWS), Alaska. We sampled six elevation transects between sea level and 620 m and spanning a distance of 14 to 70 km along ice flow paths. Most transect age–elevation patterns could not be explained by a simple model of thinning ice; the patterns provide evidence for lingering ice cover and possible inheritance. A reliable set of 20 ages ranges between 17.4 ± 2.0 and 11.6 ± 2.8 ka and indicates ice receded from northwestern PWS around 14.3 ± 1.6 ka, thinned at a rate of ~120–160 m/ka, and retreated from sea-level sites at 12.9 ± 1.1 ka at a rate of 20 m/yr. The retreat rate likely slowed as glaciers retreated into northern PWS. These results are consistent with the growing body of reported deglacial constraints on collapse of ice sheets along the Alaska margin indicating collapse of the CIS soon after 17 ka. These data are consistent with paleotemperature data indicating that a warming North Pacific Ocean caused catastrophic collapse of this part of the CIS.

Alaska↗

USGS2023

Late glacial–Younger Dryas climate in interior Alaska as inferred from the isotope values of land snail shells

The isotope values of fossil snail shells can be important archives of climate. Here, we present the first carbon (δ 13 C) and oxygen (δ 18 O) isotope values of snail shells in interior Alaska to explore changes in vegetation and humidity through the late-glacial period. Snail shell δ 13 C values were relatively consistent through the late glacial. However, late-glacial shell δ 13 C values are 2.8‰ higher than those of modern shells. This offset is best explained by the Suess effect and changes in the δ 13 C values of snail diet. Snail shell δ 18 O values varied through the late glacial, which can be partially explained by changes in relative humidity (RH). RH during the snail growing period was modeled based on a published flux balance model. Results suggest a dry period toward the beginning of the Bølling–Allerød (~14 ka) followed by two distinct stages of the Younger Dryas, a wetter stage in the early Younger Dryas from 12.9 to 12.3 ka, and subsequent drier stage in the late Younger Dryas between 12.3 and 11.7 ka. The results show that land snail isotopes in high-latitude regions may be used as a supplementary paleoclimate proxy to help clarify complex climate histories, such as those of interior Alaska during the Younger Dryas.

Alaska↗

USGS2022

Advanced distributed acoustic sensing vertical seismic profile imaging of an Alaska North Slope gas hydrate field

Gas hydrates are found in significant quantities on the North Slope of Alaska in subpermafrost sand units and intermixed in lower portions of permafrost within the hydrate stability window. While conventional surface seismic data and established imaging methods can indicate the presence of gas hydrate reservoirs, producing high-resolution images of (seismically) thin layers remains challenging due to the preferential attenuation of the higher-frequency data components. An alternative strategy is to use distributed acoustic sensing (DAS) involving cementing optical fibers into boreholes to measure seismic wavefield energy closer to the strata of interest using vertical seismic profiling (VSP). DAS VSP imaging takes advantage of the shorter travel paths and reduced attenuation to generate higher-resolution near-well images. We illustrate these benefits on a DAS VSP data set acquired at the Hydrate-01 stratigraphic test well located in the Prudhoe Bay Unit of Alaska where significant gas hydrate deposits have been detected in two subpermafrost sand layers that are intended for long-duration production testing. Our DAS data preprocessing workflow effectively isolates the upgoing compressional-wave (P-wave) reflections required for subsurface acoustic imaging. After applying three-dimensional (3-D) tomography to improve the quality of the 3-D migration velocity model, we use 3-D reverse-time migration (RTM) to develop high-quality images of the two target sands and minor near-well faulting. We validate our RTM images through highly accurate well-ties with previously acquired petrophysical log data. This study demonstrates that combining 3-D RTM imaging with DAS VSP data provides significant value to gas hydrate and similar projects, and it suggests that more advanced inversion approaches such as (elastic) least-squares RTM could recover higher-resolution and more quantitative estimates of subsurface reflectivity, which would be valuable for refining the understanding of gas hydrate systems.

Alaska↗