Seismic detection of methane hydrate often relies on indirect or equivocal methods. New multichannel seismic reflection data from the Blake Ridge, located approximately 450 km east of Savannah, Georgia, show three direct seismic indicators of methane hydrate: (1) a paleo bottom‐simulating reflector (BSR) formed when methane gas froze into methane hydrate on the eroding eastern flank of the Blake Ridge, (2) a lens of reduced amplitudes and high P‐wave velocities found between the paleo‐BSR and BSR, and (3) bright spots within the hydrate stability zone that represent discrete layers of concentrated hydrate formed by upward migration of gas. Velocities within the lens (∼1910 m/s) are significantly higher than velocities in immediately adjacent strata (1820 and 1849 m/s). Conservative estimates show that the hydrate lens contains at least 13% bulk methane hydrate within a 2‐km 3 volume, yielding 3.2 × 10 10 kg [1.5 TCF (4.2 × 10 10 m 3 ] of methane. Low seismic amplitudes coupled with high interval velocities within the lens offer evidence for possible methane hydrate “blanking.” Hydrate bright spots yield velocities as high as 2100 m/s, with bulk hydrate concentrations predicted as high as 42% in an approximately 15‐m thick layer. Our results show that, under certain circumstances, hydrate in marine sediments can be directly detected in seismic reflections but that quantification of hydrate concentrations requires accurate velocity information.
New Zealand fjords (Fig. 1e) contain submerged, relict, proglacial lacustrine and marine deltas that formed after glaciers retreated from the west coast by 17 ka (Pickrill et al. 1992). Many of these deltas, for example those in Bradshaw and George sounds, are exceptionally well preserved because postglacial sedimentation rates have been low.
Fig. 1.
Relict proglacial deltas in Bradshaw and George sounds, Fiordland, New Zealand. ( a ) Swath-bathymetric image of the head of George Sound superimposed on a hill-shaded DEM. Acquisition system Kongsberg EM300. Frequency 30 kHz. Grid-cell size 5 m. EB, Elder Basin; WB, Whitewater Basin; SWB, South-West Basin; AD, Anchorage Delta; WD, Whitewater Delta; SWAD, South-West Arm Delta; ALD, Alice Delta; AF, Alice Falls. ( b ) Head of Bradshaw Sound. BB, Bradshaw Basin; GAD, Gaer Arm Delta; PD, Precipice Delta. ( c ) Long-axis profile from EB to Lake Alice (LA), across the relict proglacial ALD and WB. VE×11. ( d ) Long-axis profile across the modern and relict proglacial components of GAD. VE ×10. ( e ) Location of study area (red box; map from GEBCO_08). ( f ) Enlargement showing bathymetric details of the AD foreset slope off Anchorage Cove. ( g ) Head …
Fiordland, on the southwest coast of New Zealand’s South Island, hosts 15 distinct fjords that extend up to 40 km inland from the Tasman Sea into mountainous terrain consisting primarily of hard crystalline igneous and metamorphic rocks. All of these fjords have seaward entrance sills, and most have glacially eroded and overdeepened basins that contain sediments deposited following the retreat of the glaciers that carved the valleys out. These sedimentary basins preserve a record of post-glacial environments that can be used to evaluate changes in regional sea level, climate, vegetation and other conditions. For example, in cases where the entrance sills were higher than the last glacial maximum sea level, the present-day fjords would have previously been isolated glacial lakes prior to marine ingression due to post-glacial sea level rise; this lacustrine-marine transition is recorded in the fjord sediments, e.g., as flooded deltas and beaches.Over the last 10 years, we have collected high-resolution boomer-sourced seismic reflection data in most of the fjords of Fiordland using a 75-m-long 24-channel Geometrics MicroEel array recording signals from an acoustic boomer source (initially a Ferranti system and then, more recently, one from Applied Acoustics). Processing has been undertaken using commercial (GLOBE Claritas) seismic processing software. We present a summary of this work, showing profiles along a number of the fjords including, from north to south, Milford, Nancy, George, Thompson/Bradshaw, Doubtful, Dagg, Dusky and Long sounds. Seismic sections show a wide variety of sediment accumulations in the fjords depending on periglacial conditions, sill depth, catchment size, catchment rock types and vegetation history, etc. Sediment thicknesses are observed to exceed several hundred metres in some of the basins – which supports an interpretation of interbedded strata of muds, silts, and fine sands. The depositional history of the sedimentary units imaged by these data, in conjunction with additional seafloor mapping, direct seafloor sampling and shallow cores, will be confirmed by deep drilling efforts in the fjords.
The distribution of gas hydrate on a continental slope is often characterized as a wedge that pinches out on the seafloor. This part of the hydrate stability zone is particularly relevant for studies of the dynamics of hydrate accumulations, such as processes related to slope stability or hydrate dissociation leading to methane release into the overlying ocean. For regions with very low geothermal gradients, we have produced a series of thermobaric models of the shallow hydrate stability zone that contain an unexpected geometrical distribution of hydrated sediments. In these models, the shallowest part of the stability field thickens and bulges landward. Such a feature is more likely to happen in regions where low geothermal gradients are further lowered by high sedimentation rates. Also, the effect is greater beneath colder oceans. Although a hydrate stability zone bulge would be difficult to image with conventional seismic methods, there are numerous locations around the world where such a system could develop.
<p>Submarine groundwater discharge into coastal areas is a common global phenomenon and is rapidly gaining scientific interest due to its influence on marine biology and the coastal sedimentary environment, and it's potential as a future freshwater resource. We conducted an integrated study of hydroacoustic surveys combined with geochemical porewater and water column investigations at a well-known freshwater seep site in Eckernf&#246;rde Bay (Germany).</p><p>The location and distribution of pockmarks in this area have been the focus of many studies since their discovery in 1966 including numerous investigations of their geochemical, geological and geophysical behavior. Despite several intense and extensive research campaigns (e.g. Sub-GATE/CBBL) their internal morphology and structure presented in this study were poorly constrained to date. With recent advances in shallow high-frequency multibeam echosounder methods combined with highly accurately positioned sediment cores, we can provide new insights on the influence of shallow gas and freshwater on the formation and internal morphology of the pockmarks. We show that high-frequency multibeam data can be used to detect free shallow gas in areas of enhanced freshwater advection in muddy sediments. Intra-pockmarks, forming due to ascending gas and freshwater, pose a new form of &#8216;eyed&#8217; pockmarks revealed by their acoustic backscatter response. Our data suggest that in muddy sediments morphological lows combined with a strong multibeam backscatter signal can be indicative of free shallow gas and the subsequent advection of freshwater.</p>
New Zealand’s (NZ) entire coastline is at risk of tsunami from local, regional, and distant sources. With more than 75% of New Zealanders living or working within 10 km of the coast, the tsunami risk is significant. The Rapid Characterization of Earthquakes and Tsunamis (RCET) research programme is being undertaken to better understand, mitigate and respond to tsunami events in NZ. Within this project, my PhD focuses on improving the communication of tsunami threats to local stakeholders and the emergency response sector by creating a new concept: a time-dependent forecast for tsunami. I have been using the software ComMIT (a tsunami model developed by the NOAA Center for Tsunami Research) to create a catalogue of synthetic tsunamis focusing on the cities of Tauranga and Whangarei, situated on the northeast coast of the North Island. These two cities have been selected due to their exposure to tsunamis: flat topography, densely populated, infrastructure-rich harbour, exposed coastline, proximity to the Kermadec-Tonga trench.  Using Python, I have generated a diverse assembly of forecasts where the tsunami waves amplitude  measured on the coastline are linked to threat levels, resulting in the creation of the final product: a time-dependent forecast. I have also been engaging with stakeholders and various end user communities with the aim of adapting these models to their needs. We anticipate that this new tool will help them to respond to these threats more efficiently.
Abstract The New Zealand Alpine Fault is a major plate boundary that is expected to be close to rupture, allowing a unique study of fault properties prior to a future earthquake. Here we present 3‐D seismic data from the DFDP‐2 drill site in Whataroa to constrain valley structures that were obscured in previous 2‐D seismic data. The new data consist of a 3‐D extended vertical seismic profiling (VSP) survey using three‐component and fiber optic receivers in the DFDP‐2B borehole and a variety of receivers deployed at the surface. The data set enables us to derive a detailed 3‐D P wave velocity model by first‐arrival traveltime tomography. We identify a 100–460 m thick sediment layer (mean velocity 2,200 ± 400 m/s) above the basement (mean velocity 4,200 ± 500 m/s). Particularly on the western valley side, a region of high velocities rises steeply to the surface and mimics the topography. We interpret this to be the infilled flank of the glacial valley that has been eroded into the basement. In general, the 3‐D structures revealed by the velocity model on the hanging wall of the Alpine Fault correlate well with the surface topography and borehole findings. As a reliable velocity model is not only valuable in itself but also crucial for static corrections and migration algorithms, the Whataroa Valley P wave velocity model we have derived will be of great importance for ongoing seismic imaging. Our results highlight the importance of 3‐D seismic data for investigating glacial valley structures in general and the Alpine Fault and adjacent structures in particular.
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