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High-resolution seismic experiments, employing arrays of closely spaced, four-component ocean-bottom seismic recorders, were conducted at a site off western Svalbard and a site on the northern margin of the Storegga slide, off Norway to investigate how well seismic data can be used to determine the concentration of methane hydrate beneath the seabe...
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... Heightened attenuation in the GHBS has been reported from laboratory experiments (Best et al., 2013;Priest et al., 2006), and field data in the Mallik site in Canada and the Nankai Trough in Japan (Bellefleur et al., 2007;Guerin & Goldberg, 2002;Matsushima, 2005;Pratt et al., 2005). Conversely, attenuation was suppressed due to the gas hydrate presences in the Blake Ridge in America (Wood et al., 2000), the Svalbard site of Norway (Rossi et al., 2007;Westbrook et al., 2008), the Krishna-Godavari Basin in India (Dewangan et al., 2014;Jaiswal et al., 2012;Jyothi et al., 2017), the Vestnesa Ridge in Svalbard (Singhroha et al., 2016), and the Shenhu area in the South China Sea (X. Wang et al., 2018). ...
... The attenuation anomaly observed in our study is in accordance with the previously field observations that the presence of gas hydrate in fine-grained clay-rich sediments would weaken the attenuation of host sediments (e.g., Dewangan et al., 2014;Jaiswal et al., 2012;Rossi et al., 2007;Singhroha et al., 2016). This phenomenon has drawn much attention because it contradicts the observations in gas hydrate-bearing sands (e.g., Rossi et al., 2007;Westbrook et al., 2008;Jaiswal et al., 2012;J. Wang et al., 2017). ...
... The method of Q estimation is another factor that may affect the absolute value of Q. Another field study of the fine-grained silty clays in the Northwestern Svalbard site of Norway (e.g., Rossi et al., 2007;Westbrook et al., 2008) produced Q values varying between 25 and 200, which cover the Q value range in our result. However, these values are obtained in gas-hydrates stability zone and the result for hydrate-free sediments is lacking, which impedes direct comparison with our result. ...
An improved understanding of the effects of gas hydrate presence on seismic attenuation is important for accurate hydrate characterization and quantification. Based on a rock‐physics model recently presented for gas hydrate‐bearing fine‐grained clay‐dominated sediments, here we establish an integrated workflow for surface seismic data from extracting seismic attenuation to estimating gas hydrate concentration (Ch) in the sediment. We apply this workflow to the high‐resolution seismic data acquired at southern Hydrate Ridge, offshore Oregon, to reveal the hydrate distribution and clarify the controlling factor of hydrate formation. We first present an adaptive‐bandwidth spectral ratio method to robustly measure attenuation. The attenuation measurements show that the presence of hydrate suppresses the attenuation of the host sediment. We then calculate Ch by applying the rock‐physics model to the attenuation measurements. The estimated Ch are mostly low (<5%) in Hydrate Ridge and agrees well with the in‐situ Ch measured from core‐ or well log‐based data. Our result also suggests that the lithology and stratigraphic structures together control the distribution of gas hydrate at Hydrate Ridge, where relatively high Ch is found in the region where a gas‐charged conduit exists and in an anticlinal structure overlying a strong bottom simulating reflection. Adjacent to the anticline, however, a low amount of gas hydrate appears present, possibly due to the gas migration blocked by the anticlinal structure or the lack of gas conduits. Our study offers an effective strategy for detecting and quantifying gas hydrate in fine‐grained clayey sediments through surface seismic data.
... Hydrate-related Bottom Simulating Reflectors (BSR) commonly occur in the region with a fluid-flow system composed of free gas and gas hydrates (Mienert et al., 1998;Bünz et al., 2003;Bünz and Mienert, 2004;Westbrook et al., 2008). The Nyegga area on the continental slope off mid-Norway is an area known for its particularly dense distribution of pockmarks (Hjelstuen et al., 2010), ranging from small circular depressions with diameters <5 m (the unit-pockmarks as named by Hovland et al., 2010) to large complex structures with diameters of several hundreds of meters showing up to 10-m-high carbonate ridges inside their depressions (Hovland et al., 2005). ...
... The bottom-simulating seismic reflector that follows the base of the methane-hydrate stability zone and marks the transition from sediment containing hydrate, above, to sediment containing free gas, below, is not observed in most of the study area. The BSR, which is seismically visible further downslope, only occurs in units with sufficiently high permeability to be invaded by gas and so it is discontinuous and stratigraphically controlled (e.g., Bouriak et al., 2003;Bünz et al., 2003;Westbrook et al., 2008). The limits of the BSR observed on seismic sections in this part of the Nyegga slope are shown in Figure 1B. ...
We investigated fluid seepage within the Nyegga pockmark field (600–900 m water depths) off mid-Norway from Remotely Operated Vehicle dives at the so-called CNE sites (CNE01 to CNE17). The seafloor morphology of some of these features corresponds to pockmarks and adjacent ridges, with the latter being the focus of present seepage activity. These structures are underlain by chimneys above a gas-charged zone with, in some cases, a substantial body of hydrate-invaded sediment (down to 1.3 s in two-way travel time at CNE03). Present-day methane-rich fluid seepage through the seabed is indicated by chemosynthetic fauna, in particular Siboglinidae polychaetes (Oligobrachia haakonmobiensis webbi and Sclerolinum contortum), microbial mats and associated Rissoidae gastropod (Alvania sp.) grazers, and confirmed by measured in situ bottom-water methane anomalies, up to 2,130 nL/L. No free-gas bubble emissions were observed or acoustically identified. The presence of authigenic carbonates reveals past seepage with very low δ¹³C values (down to −58‰) indicating that the major source of carbon was methane carried by the venting fluids. The ages of major periods of methane venting are provided by vesicomyid bivalve shells (Isorropodon nyeggaensis) present in two sedimentary layers, 14,930 and 15,500 ¹⁴C yr BP (ca. 17,238 and 17,952 cal yr BP), respectively, corresponding to the time of Melt Water Pulse IA. The seafloor morphology and pattern of seepage -chemosynthetic fauna and microbial mat distribution and dissolved methane concentration-are remarkably heterogeneous. Pore-water chemistry profiles in a gravity core taken only 40 m from major seepage sites indicate no seepage and anaerobic methane oxidation at a sub-bottom depth of about 2 m. Present-day seepage from the studied pockmark-chimney fluid-flow system charged with gas hydrate is dominated by the advection of methane solution in pore water. Some of this methane could result from the dissolution of hydrate in the chimney, most of which would have formed during an earlier period (post-LGM times) of history of the chimney, when it was venting free gas. However, the presence of free gas beneath this chimney is probably why the water entering the chimney is already saturated with methane and the process of hydrate formation in the chimney continues today.
... In the Kumano forearc basin, a low V p zone is associated with a Bottom Seismic Reflector (BSR) at 400 mbsf (Bangs et al., 2010;Miyakawa et al., 2014) and with a turbiditic zone saturated with free gas extending down to 547 mbsf below (Chhun et al., 2018;Saito et al., 2010). Sonic anomalies related to gas hydrates have been modeled to extend over several hundreds of meters (Westbrook et al., 2008). In the upper accretionary prism, V p peaks to 3,700 m/s at 1,600 m ( Figure 2c) in a quartz-rich sand interval (Figure 1d) where a positive peak in resistivity occurs (Strasser et al., 2014). ...
Plain Language Summary
Effective porosity reflects the network of pores that affects the transport properties and the history of sediment compaction. However, evaluating this property is not straightforward given that standard methods provide total porosity, which also includes clay‐bound water. We can estimate the effective porosity using cation exchange capacity of the rocks. This method was used at Site C0002, the deepest hole in history of the International Ocean Discovery Program, drilled down to the heart of the Nankai accretionary prism, located southwest of Japan. We show that the effective porosity follows a simple exponential decay law with depth. We also demonstrate that there is a single relationship between effective porosity and P‐wave velocity. We use this relationship to fill the data gaps in the P‐ and S‐wave velocity profiles at Site C0002. Conversely, this provides a direct method to estimate effective porosity, and hence, the compaction state, from seismic profiles acquired across the Nankai accretionary prism.
... The decomposed P-wave potential can serve as acoustic pressure as per traditional TS exploration, whereas the decomposed S-wave can usually provide higher resolution, bringing about a higher sensitivity to changes in subsurface fractures and pores and improving images in the presence of gas clouds (DeVault et al., 1998;Gaiser et al., 2001;Hanyi et al., 2002;Mavor, 2011). The underlying parameters of subsurface formation, such as Poisson's ratio, density, and porosity, can be acquired by performing a comprehensive analysis of the decomposed P-and S-wave data, thereby offering support for predicting oil and gas reservoirs or gas hydrate stability zones (GHSZ) (Chabert et al., 2011;Chand et al., 2006;Robertson, 1987;Westbrook et al., 2008). ...
Properly decomposed P- and S-waves are indispensable for the imaging and inversion of multicomponent ocean bottom seismic (OBS) data. Hence, multicomponent OBS data decomposition is fundamental to subsequent data processing procedures. We designed separate calibration filters that separate the procedure of pressure-to-particle velocity (or displacement) calibration to time-difference elimination and amplitude compensation. With separate calibration filters, the kinematic characteristics of the particle velocity (or displacement) components are calibrated first, followed by the dynamic characteristics of the particle velocity (or displacement) components, which makes the calibration more precise, and the energy distribution inside each component can be appropriately adjusted. Finally, the proposed decomposition techniques were applied to an active-source deep-water multicomponent OBS dataset to obtain high-quality upgoing and downgoing P- and S-waves. The field data decomposition results show that the expected effects of decomposition, such as water-column-related multiple wave attenuation, have been realized.
... For almost 40 years, scientists have studied the global phenomena of natural methane release from underwater seepage areas at different geological settings (see overview in Suess, 2014). This research has covered different aspects of methane seepage, including their ability to sustain chemoautotrophic ecosystems and microbial communities (Boetius et al., 2000;Sahling et al., 2002;Levin et al., 2016), their geological past and their manifestation in methane derived carbonates (Greinert et al., 2001;Campbell, 2006;Liebetrau et al., 2010), their link to gas hydrates and hydrocarbon reservoirs (Westbrook et al., 2008;Smith et al., 2014;Ruppel and Kessler, 2017), or the potential transport of methane into the atmosphere and its relevance to global atmospheric methane concentrations (Etiope, 2009;Shakhova et al., 2010;Kirschke et al., 2013;Pohlman et al., 2017;Römer et al., 2017;Weber et al., 2019). To answer these questions, scientists have tried to elucidate in detail the mechanism of bubble seepage and its triggers including internal forcing (source depletion and refilling and clogging of pathways) or external forcing (pressure changes, e.g., due to tides, tectonic activity, or sea-level changes) (Westbrook et al., 2009;Berndt et al., 2014;Shakhova et al., 2014;Wallmann et al., 2018). ...
Two lander-based devices, the Bubble-Box and GasQuant-II, were used to investigate the spatial and temporal variability and total gas flow rates of a seep area offshore Oregon, United States. The Bubble-Box is a stereo camera–equipped lander that records bubbles inside a rising corridor with 80 Hz, allowing for automated image analyses of bubble size distributions and rising speeds. GasQuant is a hydroacoustic lander using a horizontally oriented multibeam swath to record the backscatter intensity of bubble streams passing the swath plain. The experimental set up at the Astoria Canyon site at a water depth of about 500 m aimed at calibrating the hydroacoustic GasQuant data with the visual Bubble-Box data for a spatial and temporal flow rate quantification of the site. For about 90 h in total, both systems were deployed simultaneously and pressure and temperature data were recorded using a CTD as well. Detailed image analyses show a Gaussian-like bubble size distribution of bubbles with a radius of 0.6–6 mm (mean 2.5 mm, std. dev. 0.25 mm); this is very similar to other measurements reported in the literature. Rising speeds ranged from 15 to 37 cm/s between 1- and 5-mm bubble sizes and are thus, in parts, slightly faster than reported elsewhere. Bubble sizes and calculated flow rates are rather constant over time at the two monitored bubble streams. Flow rates of these individual bubble streams are in the range of 544–1,278 mm³/s. One Bubble-Box data set was used to calibrate the acoustic backscatter response of the GasQuant data, enabling us to calculate a flow rate of the ensonified seep area (∼1,700 m²) that ranged from 4.98 to 8.33 L/min (5.38 × 10⁶ to 9.01 × 10⁶ CH4 mol/year). Such flow rates are common for seep areas of similar size, and as such, this location is classified as a normally active seep area. For deriving these acoustically based flow rates, the detailed data pre-processing considered echogram gridding methods of the swath data and bubble responses at the respective water depth. The described method uses the inverse gas flow quantification approach and gives an in-depth example of the benefits of using acoustic and optical methods in tandem.
... Interval velocities available from seismic data (P-wave velocities) were used to estimate the amount of gas hydrate in sediments using rock physics models linking seismic velocity to the internal rock structure (e. g., Chand et al., 2004;Westbrook et al., 2008). The Hydrate-Bearing Effective Sediment (HBES) model described by Marín-Moreno et al. (2017) was used to calculate the P-wave velocity based on a sediment composed by mineral grains and variable amounts of brine, gas, and gas hydrate in the pore space. ...
The offshore Bangladesh includes the northern Bengal fan, where sediment supply from the Ganges and Brahmaputra rivers has resulted in the accumulation of up to 20 km of shallow-marine, fluvio-deltaic and slope sediments that have accumulated during rapid tectonic subsidence since the late Miocene. The high sedimentation rates, along with high organic matter content, make this area favorable for the formation of natural gas from both microbial and thermogenic sources. Here we use multichannel seismic reflection profiles and modelling of the gas hydrate stability zone (GHSZ) to present the first evidence for the occurrence of natural gas hydrate in the offshore Bangladesh. First, we analyze the sediments of the shelf and slope areas, which are characterized by downslope sediment transport features and by the presence, in places, of faults/fractures as well as widely distributed amplitude anomalies and seismic facies that we relate to the presence of gas. A high-amplitude reversed polarity reflection of variable continuity that mimics the seafloor and cross-cut stratigraphy is interpreted as a Bottom Simulating Reflector (BSR). The BSR is observed in several areas that are predominantly located in the E-SE of the study area, in water depths of 1300–1900 m and at depths below seafloor of 250–440 m. Sediments above BSR locations generally show higher seismic interval velocities reaching values of ∼1920–1940 m/s, which are consistent with the presence of gas hydrate in shallow marine sediments. Furthermore, the BSR lies at approximately the same depth as the theoretical base of the gas (methane) hydrate stability zone (BGHSZ), calculated assuming a 3.5 % wt pore water salinity and using existing geothermal gradient and seafloor temperature data from the study region. However, in places, the BSR lies deeper or shallower than the base of the modelled BGHSZ. These discrepancies include areas where faults/fractures and seismic evidence linked to fluid flow from deeper reservoirs reach the GHSZ disrupting its stratigraphic continuity. At these locations, we suggest that faults/fractures act as fluid migration pathways causing localized heat-flow perturbations and/or changes in the hydrate-forming gas composition both likely affecting the depth of the GHSZ. Our results provide the first evidence of the gas hydrate potential in the offshore Bangladesh and should drive future research and data acquisition aiming to understand the composition, saturation and thickness of the gas hydrate-bearing sediments in this region.
... Seismic wave attenuation (represented by Q − 1 hereinafter) is a crucial physical factor in seismic interpretation that can be more sensitive to sediment property variations than velocity (Winkler and Nur, 1982). The presence of gas hydrates in host sediment can cause significant attenuation anomalies (e.g., Guerin and Goldberg, 2005;Westbrook et al., 2008;Wang et al., 2018), which suggests attenuation as an effective proxy to characterize the distribution of hydrates. However, it was likely controversial in observations whether hydrate occurrence enhances or suppresses attenuation. ...
... For example, attenuation has been found elevated in data from sonic logging Goldberg, 2002, 2005), cross hole (Pratt et al., 2005) and vertical seismic profiling (VSP) at the Mallik site in Canada (Bellefleur et al., 2007), as well as the sonic logging data in the Nankai Trough in Japan (Matsushima, 2005) and laboratory measurements on synthetic hydrate-bearing sands (Priest et al., 2006;Best et al., 2013). In contrast, suppressed attenuation in hydrate-bearing sediments was observed from high-resolution seismic data in the Blake Ridge region of North America (Wood et al., 2000), the Northwestern Svalbard site of Norway (Rossi et al., 2007;Westbrook et al., 2008), the Krishna-Godavari (KG) Offshore Basin in India (Jaiswal et al., 2012;Dewangan et al., 2014;Jyothi et al., 2017;Nittala et al., 2017), and the Vestnesa Ridge of Svalbard (Singhroha et al., 2016). ...
... Many previous studies attributed the contradictory observations of attenuation anomalies to frequency-dependency of attenuation or source coupling (e.g., Lee and Waite, 2007;Westbrook et al., 2008;Wang et al., 2017). The effects of gas hydrate morphology, however, were typically ignored, though several studies involved the role of different types of host sediments in interpretation (Rossi et al., 2007;Westbrook et al., 2008;Madrussani et al., 2010;Jaiswal et al., 2012). ...
Understanding the attenuation mechanism in gas hydrate-bearing sediments (GHBS) is essential for accurate quantification of natural gas hydrates, yet previous rock physics models fail to explain the observed attenuation suppression at seismic frequencies when hydrates occur as fracture-filling morphology. Here we present a new model to decipher the viscoelastic performance of fracture-filling GHBS, by treating it as a composite of host sediment and pure gas hydrate. We first characterize the attenuation in the host sediment without hydrate by the viscous fluid flow in swelling clay minerals. Then, based on laboratory data, we quantify the mechanical performance of pure gas hydrate, exhibiting that hydrate is a consolidated aggregate with properties different from unconsolidated host sediment. After incorporating fracture-filling gas hydrates into the host sediment, the properties of host sediment are significantly modified and the resulting model reproduces the suppressed attenuation observed in field. We apply this model to the field attenuation extracted from multichannel seismic data in the Krishna-Godavari Offshore Basin in India, where gas hydrate was found filled in fractures. The results show that the attenuation and hydrate concentration predicted by our model agree well with the field measurements, which suggests the promise of our model in detection and quantification of nature gas hydrates in clay-dominated sediments using seismic data.
... At first, the peculiar "vertical sub-bottom zones of acoustic wipe-out", were, according to Ivanov et al. (2010) identified as vertical seismic zones with: "…widths between 150 and 500 m and vertical extensions up to 700 -800 ms TWT ". Later, these gas chimneys were targeted by seafloor tomography technology, imaged in 3D by ocean bottom seismometers (Hustoft et al., 2007;Ivanov et al., 2007;Westbrook et al., 2008;Plaza-Faverola et al., 2010). This resulted in a careful velocity investigation: "In seismic reflection sections, chimneys are represented by zones of low coherence, scattering and low amplitude that is, at least in part, a consequence of the seismic scattering in the shallowest parts of the chimneys. ...
... Several studies have been undertaken to estimate the true content of pore-space gas hydrates and free gas at Nyegga. Velocity analyses of seismic data provided data for such estimates (Bünz and Mienert, 2004;Bünz et al., 2005;Plaza-Faverola et al., 2010;Westbrook et al., 2008). Hydrate saturations have been estimated from OBS (ocean bottom seismic) data and range from 2 to15% of pore-space. ...
... Martin Hovland thanks his previous employer, Statoil (now Equinor) for all support in the research into the nature of seafloor seepage, over a period of 40 years. The investigations reported by Westbrook et al. (2008), andPlaza-Faverola et al. (2010) and others, were supported financially by the same company. This support made it possible to hire the survey vessel Professor Logachev for several weeks in order of performing the 3D-tomography studies at Nyegga (in 2006). ...
Geophysical features such as bottom-simulating reflectors and acoustic wipe-out zones are common at locations where natural gas hydrates form in deep-sea sediments. This is also the case at two locations off mid-Norway in the Norwegian Sea: the Nyegga hydrocarbon seep area and the Husmus shallow gas location. In addition to the aforementioned features, the Nyegga area at 730 m water depth boasts complex pockmarks (up to 300 m wide and 12 m deep), gas hydrate pingoes, giant blocks of carbonate-cemented sediments, and exotic fauna. In contrast, the Husmus shallow gas location on the nearby continental shelf at 330 m water depth also contains a strong and very shallow bottom-simulating reflector (located only 4–5 m below seafloor), some distinct pockmarks, and putative coral reefs. But the deepest location, the Ormen Lange producing hydrocarbon field at 950 m water depth, contains no seep features on the seafloor or geophysical expressions of gas hydrates in the sediments. Here, formation of natural gas hydrates was triggered by a small amount of methane seepage from a drilled well. The methane spontaneously formed hydrate-coated bubbles in addition to some unstable hydrate cement. Thus, these three locations demonstrate the wide range of features and effects caused by gas hydrates in situ. This article describes these settings and discusses concerns related to drilling, production, transport technology and the environment in general. Perhaps one of the least studied aspects of deep-sea natural gas hydrates is their impact on local and regional biodiversity and fauna, which may represent an important topic for future consideration.
... The distribution of gas hydrates and free gas below the BGHSZ has been primarily studied in the area using P-and S-wave seismic velocity (Chabert et al. 2011;Hustoft et al. 2009;Madrussani et al. 2010;Singhroha et al. 2019Singhroha et al. , 2020Westbrook et al. 2008), seismic attenuation (Madrussani et al. 2010;Singhroha et al. 2016) and electrical resistivity (Goswami et al. 2015(Goswami et al. , 2016(Goswami et al. , 2017. Various studies estimate gas hydrate saturation levels in the GHSZ at sites away from focused fluid flow features to vary from $ 0 to 30% of the pore space, with the highest hydrate saturation levels observed near the base of the GHSZ. ...
... Gas hydrate saturation estimates are higher for focused fluid flow features like gas chimneys (up to $ 73% of pore space), probably due to a larger input of gas into the system (Goswami et al. 2015(Goswami et al. , 2017. Gas hydrate saturation estimates from the electrical resistivity analysis (Goswami et al. 2015(Goswami et al. , 2016 are higher than saturation estimates from the seismic velocity analysis (Chabert et al. 2011;Hustoft et al. 2009;Singhroha et al. 2019;Westbrook et al. 2008). Electrical resistivity methods tend to overestimate gas hydrate saturation levels for gas hydrates occurring in faults and fractures (Cook et al. 2010). ...
Gas hydrate systems along the west-Svalbard margin exist in a unique polar margin setting characterized by: (1) changes in sub-seabed pressure due to glacial isostatic rebound and flexure since the onset of glaciations at 2.7 Ma; (2) rapid fluctuations in sediment delivery towards the continental slope associated with glacial dynamics; (3) bottom current variations in both temperature and position along the slope; and (4) geothermal gradient variations (80–160 °C/km) and near-surface deformation associated with ultra-slow mid-ocean ridge spreading. Along this polar margin, the base of the gas hydrate stability zone and thus the depth of the bottom simulating reflection (BSR) adapts to these changes in both pressure and temperature. However, it is the presence of a focused fluid flow system sustained by microbial, thermogenic and possibly even abiotic gas sources reaching the near-surface that determines the distribution of BSRs across the margin. Thus, gas hydrate stability, and therefore BSR dynamics in this polar margin, respond to a mixture of complex mechanisms affecting the system from the top and from the bottom. In this article we describe the structural, sedimentary and oceanographic setting, the type of gas hydrate-related BSRs and the different geological footprints (e.g., gas chimneys, pockmarks, brecciated pathways, mounds) associated with the migration of shallow gas through the gas hydrate stability zone over geological time along the west-Svalbard margin.
... The physical and chemical properties of gas hydrates are already quite well understood from a theoretical and laboratorial perspective (Sloan 1998), and their formation and recognition criteria have seen continuous developments over the past decades (e.g., Shipley and Houston 1979;Ginsburg and Soloviev 1998;Bünz et al. 2005;Westbrook et al. 2008). Nevertheless, certain technical challenges must be addressed before gas hydrthe formates can be considered a viable energy source. ...
For the past 50 years, gas hydrates have been regarded by scientists as part of the hydrocarbon reserves, particularly at governmental institutions. A better understanding of the processes controlling the distribution and dynamics of gas hydrates in nature, especially their sensitivity to changes in gas composition, pressure and temperature, requires both theoretical knowledge of their stability and dynamic behavior and knowledge of how gas hydrates form and where they occur in the sediment. Geophysical data, geochemical data and thermodynamic models indicate that both the rate of response and the total integrated response to climate change in the ocean depend on the location and forms in which hydrates are distributed. Thus, mapping gas hydrates by indirect geophysical methods or through dedicated drilling campaigns is fundamental to all research involving gas hydrates. This includes studies of their role in climate change, their consequences for slope stability, their role at the base of the food web for benthic ecosystems and their potential as a future energy resource. Here we provide a brief introduction to the occurrence of gas hydrates on Earth, and how this information may assist in detecting them on other planetary bodies.
