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1.
A wide-spread bottom simulating reflector (BSR), interpreted to mark the thermally controlled base of the gas hydrate stability zone, is observed over a close grid of multichannel seismic profiles in the Krishna Godavari Basin of the eastern continental margin of India. The seismic data reveal that gas hydrate occurs in the Krishna Godavari Basin at places where water depths exceed 850 m. The thickness of the gas hydrate stability zone inferred from the BSR ranges up to 250 m. A conductive model was used to determine geothermal gradients and heat flow. Ground truth for the assessment and constraints on the model were provided by downhole measurements obtained during the National Gas Hydrate Program Expedition 01 of India at various sites in the Krishna Godavari Basin. Measured downhole temperature gradients and seafloor-temperatures, sediment thermal conductivities, and seismic velocity are utilized to generate regression functions for these parameters as function of overall water depth. In the first approach the base of gas hydrate stability is predicted from seafloor bathymetry using these regression functions and heat flow and geothermal gradient are calculated. In a second approach the observed BSR depth from the seismic profiles (measured in two-way travel time) is converted into heat flow and geothermal gradient using the same ground-truth data. The geothermal gradient estimated from the BSR varies from 27 to 67°C/km. Corresponding heat flow values range from 24 to 60 mW/m2. The geothermal modeling shows a close match of the predicted base of the gas hydrate stability zone with the observed BSR depths.  相似文献   

2.
This article provides new constraints on gas hydrate and free gas concentrations in the sediments at the margin off Nova Scotia. Two-dimensional (2-D) velocity models were constructed through simultaneous travel-time inversion of ocean-bottom seismometer (OBS) data and 2-D single-channel seismic (SCS) data acquired in two surveys, in 2004 and 2006. The surveys, separated by ∼5 km, were carried out in regions where the bottom-simulating reflection (BSR) was identified in seismic reflection datasets from earlier studies and address the question of whether the BSR is a good indicator of significant gas hydrate on the Scotian margin. For both datasets, velocity increases by 200–300 m/s at a depth of approximately 220 m below seafloor (mbsf), but the results of the 2006 survey show a smaller velocity decrease (50–80 m/s) at the base of this high-velocity layer (310–330 mbsf) than the results of the 2004 survey (130 m/s). When converted to gas hydrate concentrations using effective medium theory, the 2-D velocity models for both datasets show a gas hydrate layer of ∼100 m thickness above the identified BSR. Gas hydrate concentrations are estimated at approximately 2–10% for the 2006 data and 8–18% for the 2004 survey. The reduction in gas hydrate concentration relative to the distance from the Mohican Channel structure is most likely related to the low porosity within the mud-dominant sediment at the depth of the BSR. Free gas concentrations were calculated to be 1–2% of the sediment pore space for both datasets.  相似文献   

3.
Seismic imaging of gas hydrates in the northernmost South China sea   总被引：1，自引：1，他引：0  
Horizon velocity analysis and pre-stack depth migration of seismic profiles collected by R/V Maurice Ewing in 1995 across the accretionary prism off SW Taiwan and along the continental slope of the northernmost South China Sea were implemented for identifying gas hydrates. Similarly, a survey of 32 ocean-bottom seismometers (OBS), with a spacing of about 500 m, was conducted for exploring gas hydrates on the accretionary prism off SW Taiwan in April 2006. Travel times of head wave, refraction, reflection and converted shear wave identified from the hydrophone, vertical and horizontal components of these OBS data were applied for imaging P-wave velocity and Poisson’s ratio of hydrate-bearing sediments. In the accretionary prism off SW Taiwan, we found hydrate-bearing sediment, with a thickness of about 100–200 m, a relatively high P-wave velocity of 1.87–2.04 km/s and a relatively low Poisson’s ratio of 0.445–0.455, below anticlinal ridges near imbricate emergent thrusts in the drainage system of the Penghu and Kaoping Canyons. Free-gas layer, with a thickness of about 30–120 m, a relatively low P-wave velocity of 1.4–1.8 km/s and a relatively high Poisson’s ratio (0.47–0.48), was also observed below most of the bottom-simulating reflectors (BSR). Subsequently, based on rock physics of the three-phase effective medium, we evaluated the hydrate saturation of about 12–30% and the free-gas saturation of about 1–4%. The highest saturation (30% and 4%) of gas hydrates is found below anticlines due to N–S trending thrust-bounded folds and NE-SW thrusting and strike-slip ramps in the lower slope of the accretionary prism. We suggest that fluid may have migrated through the relay-fault array due to decollement folding and gas hydrates have been trapped in anticlines formed by the basement rises along the thrust faults. In contrast, in the rifted continental margin of the northernmost South China Sea, P-wave velocities of 1.9–2.2 km/s and 1.3–1.6 km/s, and thicknesses of about 50–200 m and 100–200 m, respectively, for a hydrate layer and a free-gas layer were imaged below the remnant and erosional ridges in the upper continental slope. High P-wave velocity of hydrate-bearing sediment below erosional ridges may also indicate high saturation of hydrates there. Normal faults due to rifting in the South China continental crust may have provided conduits for gas migration below the erosional ridges where P-wave velocity of hydrate-bearing sediment in the passive continental margin of the northernmost South China Sea is greater than that in the active accretionary prism off SW Taiwan.  相似文献   

4.
Several cold vents are observed at the northern Cascadia margin offshore Vancouver Island in a 10 km2 region around Integrated Ocean Drilling Program Expedition 311 Site U1328. All vents are linked to fault systems that provide pathways for upward migrating fluids and at three vents methane plumes were detected acoustically in the water column. Downhole temperature measurements at Site U1328 revealed a geothermal gradient of 0.056 ± 0.004°C/m. With the measured in situ pore-water salinities the base of methane hydrate stability is predicted at 218–245 meters below seafloor. Heat-probe measurements conducted across Site U1328 and other nearby vents showed an average thermal gradient of 0.054 ± 0.004°C/m. Assuming that the bottom-simulating reflector (BSR) marks the base of the gas hydrate stability zone variations in BSR depths were used to investigate the linkages between the base of the gas hydrate stability zone and fluid migration. Variations in BSR depth can be attributed to lithology-related velocity changes or variations of in situ pore-fluid compositions. Prominent BSR depressions and reduced heat flow are seen below topographic highs, but only a portion of the heat flow reduction can be due to topography-linked cooling. More than half of the reduction may be due to thrust faulting or to pore-water freshening. Distinct changes in BSR depth below seafloor are observed at all cold vents studied and some portion of the observed decrease in the BSR depth was attributed to fault-related upwelling of warmer fluids. The observed decrease in BSR depth below seafloor underneath the vents ranges between 7 and 24 m (equivalent to temperature shifts of 0.07–0.15°C).  相似文献   

5.
The multichannel seismic data along one long-offset survey line from Krishna-Godavari (K-G) basin in the eastern margin of India were analyzed to define the seismic character of the gas hydrate/free gas bearing sediments. The discontinuous nature of bottom simulating reflection (BSR) was carefully examined. The presence of active faults and possible upward fluid circulation explain the discontinuous nature and low amplitude of the BSR. The study reveals free gas below gas hydrates, which is also indicated by enhancement of seismic amplitudes with offsets from BSR. These findings were characterized by computing seismic attributes such as the reflection strength and instantaneous frequency along the line. Geothermal gradients were computed for 18°C and 20°C temperature at the depth of BSR to understand the geothermal anomaly that can explain the dispersed nature of BSR. The estimated geothermal gradient shows an increase from 32°C/km in the slope region to 41°C/km in the deeper part, where free gas is present. The ray-based travel time inversion of identifiable reflected phases was also carried out along the line. The result of velocity tomography delineates the high-velocity (1.85–2.0 km/s) gas hydrate bearing sediments and low-velocity (1.45–1.5 km/s) free gas bearing sediments across the BSR.  相似文献   

6.
In this study, we present the results of the combined analyses of ocean bottom seismometer and multi-channel seismic reflection data collection offshore southwestern Taiwan, with respect to the presence of gas hydrates and free gas within the accretionary wedge sediments. Estimates of the compressional velocities along EW9509-33 seismic reflection profile are obtained by a series of pre-stack depth migrations in a layer stripping streamlined Deregowski loop. Strong BSR is imaged over most of the reflection profile while low velocity zones are imaged below BSR at several locations. Amplitude versus angle analysis that are performed within the pre-stack depth migration processes reveal strong negative P-impedance near the bottom of the hydrate stability zone, commonly underlain by sharp positive P impedance layers associated with negative pseudo-Poisson attribute areas, indicating the presence of free gas below the BSR. Ray tracing of the acoustic arrivals with a model derived from the migration velocities generally fits the vertical and hydrophone records of the four ocean-bottom seismographs (OBS). In order to estimate the Poisson’s ratios in the shallow sediments at the vicinity of the OBSs, we analyze the mode-converted arrivals in the wide-angle horizontal component. P-S mode converted reflections are dominant, while upward P-S transmissions are observed at large offsets. We observe significant compressional velocity and Poisson’s ratio pull-down in the sediment below the BSR likely to bear free gas. When compared to Poisson’s ratio predicted by mechanical models, the values proposed for the OBSs yield rough estimates of gas hydrate saturation in the range of 0–10% in the layers above the BSR and of free gas saturation in the range of 0–2% just below the BSR.  相似文献   

7.
Bottom simulating reflectors (BSRs), known as the base of gas hydrate stability zone, have been recognized and mapped using good quality three-dimensional (3D) pre-stack migration seismic data in Shenhu Area of northern South China Sea. Additionally, seismic attribute technique has been applied to better constrain on the distribution of gas hydrate. The results demonstrate that gas hydrate is characterized by “blank” zone (low amplitude) in instantaneous amplitude attribute. The thickness of gas hydrate stability zone inferred from BSR ranges from 125 to 355 m with an average of 240 m at sea water depth from 950 to 1,600 m in this new gas hydrate province. The volume of gas in-place bound in hydrate is estimated from 1.7 × 109 to 4.8 × 109 m3, with the most likely value of around 3.3 × 109 m3, using Monte Carlo simulation. Furthermore, geothermal gradient and heat flow are derived from the depths of BSRs using a conductive heat transfer model. The geothermal gradient varies from 35 to 95°C km−1 with an average of 54°C km−1. Corresponding heat flow values range from 43 to 105 mW m−2 with an average of 64 mW m−2. By comparison with geological characteristics, we suggest that the distribution of gas hydrate and heat flow are largely associated with gas chimneys and faults, which are extensively distributed in Shenhu Area, providing easy pathways for fluids migrating into the gas hydrate stability zone for the formation of gas hydrate. This study can place useful constraints for modeling gas hydrate stability zone from measured heat flow data and understanding the mechanism of gas hydrate formation in Shenhu Area.  相似文献   

8.
We investigate the estimation of gas hydrate and free gas concentration using various rock physics models in the Cascadia accretionary prism, which is one of the most intensively studied regions of natural gas hydrate occurrences. Surface seismic reflection data is the most useful and cost-effective in deriving seismic velocity, and hence estimating gas hydrate and free gas across a BSR with depth, if a proper background (without gas hydrate and free gas) velocity is chosen. We have used effective medium theory of Helgerud et al. (EMTH) and, a combination of self-consistent approximation and differential effective medium (SCA-DEM) theory coupled with smoothing approximation for crystalline aggregate. Using the SCA-DEM (non-load-bearing) and EMTH (load-bearing) modeling, we calculate the average saturations of gas hydrate as 17 and 19%, respectively within ~100 m thick sedimentary column using velocity, derived from the surface seismic data. The saturations of gas hydrate are estimated as 15 and 18% using the SCA-DEM, and 20 and 25% using EMTH from the logging-while-drilling and wire-line sonic velocities, respectively. Estimations of gas hydrate from Poisson’s ratio are in average 50% for EMTH and 10% for SCA-DEM theory. We obtain the maximum saturation of free gas as 1–2% by employing the SCA-DEM theory either to seismic or sonic velocities, whereas the free-gas saturation varies between 0.1 and 0.4% for EMTH model. The gas hydrate saturation estimated from the sonic velocity and the free gas saturation derived from both the seismic and sonic velocities using the SCA-DEM modeling match quite well with those determined from the pressure core data in the study region.  相似文献   

9.
Two seismic sections offshore Arauco and Coyhaique, Chile, have been analysed to better define the seismic character of hydrate-bearing sediments. The velocity analysis was used to estimate the gas-phase concentration, which can serve to correlate hydrate presence to the geological features. The velocity model allowed us to recognise the hydrate layer above the bottom simulating reflector (BSR), and the free gas layer below it. The velocity field is affected by strong lateral variation, showing maximum (above the BSR) and minimum (below the BSR) values in the southern sector. Here, highest gas hydrate and free gas concentrations were calculated (15% and 2.7% of total volume respectively). The estimated geothermal gradient ranges from 35 to 95°C/km. In the northern sector, the highest gas hydrate and free gas concentrations are 15% and 0.2% of total volume respectively, and the geothermal gradient is uniform and equal to about 30°C/km.  相似文献   

10.
This paper presents results of a seismic tomography experiment carried out on the accretionary margin off southwest Taiwan. In the experiment, a seismic air gun survey was recorded on an array of 30 ocean bottom seismometers (OBS) deployed in the study area. The locations of the OBSs were determined to high accuracy by an inversion based on the shot traveltimes. A three-dimensional tomographic inversion was then carried out to determine the velocity structure for the survey area. The inversion indicates a relatively high P wave velocity (Vp) beneath topographic ridges which represent a series of thrust-cored anticlines develop in the accretionary wedge. The bottom-simulating reflectors (BSR) closely follow the seafloor and lies at 325 ± 25 m within the well-constrained region. Mean velocities range from ~1.55 km/s at the seabed to ~1.95 km/s at the BSR. We model Vp using an equation based on a modification of Wood’s equation to estimate the gas hydrate saturation. The hydrate saturation varies from 5% at the top ~200 m below the seafloor to 25% of pore space close to the BSR in the survey area.  相似文献   

11.
Satyavani  N.  Shankar  Uma  Thakur  N.K.  Reddi  S.I. 《Marine Geophysical Researches》2002,23(5-6):423-430
Multi-channel seismic reflection data from the western continental margin of India (WCMI) have been analyzed to construct a plausible model for gas hydrate formation. A reflector at 2950 ms two way travel time (TWT) on one of the sections is interpreted to represent the base of the layer of the methane hydrate, identified by a bottom simulating reflector (BSR) that lies almost 500 ms beneath the sea floor. BSRs of similar origin are common world wide, where they are usually interpreted to mark the base of gas hydrate bearing clastic sediment, with or without underlying free gas. In this study we present a model with the contrasting physical properties that produce synthetic wavelets that match with the observed BSR amplitude and waveforms for varying source-receiver offsets of multi-channel seismic reflection data. The preliminary results presented here put important constraints on models that predict the distribution and formation of hydrate. Offset-dependent amplitude recovery also gives an appropriate response for hydrate characterization.  相似文献   

12.
An analysis of 3D seismic data from the Zhongjiannan Basin in the western margin of the South China Sea (SCS) reveals seismic evidence of gas hydrates and associated gases, including pockmarks, a bottom simulating reflector (BSR), enhanced reflection (ER), reverse polarity reflection (RPR), and a dim amplitude zone (DAZ). The BSR mainly surrounds Zhongjian Island, covering an area of 350 km2 in this 3D survey area. The BSR area and pockmark area do not match each other; where there is a pockmark developed, there is no BSR. The gas hydrate layer builds upward from the base of the stability zone with a thickness of less than 100 m. A mature pockmark usually consists of an outside trough, a middle ridge, and one or more central pits, with a diameter of several kilometers and a depth of several hundreds of meters. The process of pockmark creation entails methane consumption. Dense faults in the study area efficiently transport fluid from large depths to the shallow layer, supporting the formation of gas hydrate and ultimately the pockmark.  相似文献   

13.
The presence of gas hydrates, one of the new alternative energy resources for the future, along the Indian continental margins has been inferred mainly from bottom simulating reflectors (BSR) and the gas stability zone thickness mapping. Gas hydrate reserves in Krishna Godawari Basin have been established with the help of gas-hydrate related proxies inferred from multidisciplinary investigations. In the present study, an analysis of 3D seismic data of nearly 3,420 km2 area of Mahanadi deep water basin was performed in search of seismic proxies related with the existence of natural gas hydrate in the region. Analysis depicts the presence of BSR-like features over a large areal extent of nearly 250 km2 in the central western part of the basin, which exhibit all characteristics of a classical BSR associated with gas hydrate accumulation in a region. The observed BSR is present in a specific area restricted to a structural low at the Neogene level. The coherency inversion of pre-stack time migration (PSTM) gathers shows definite inversion of interval velocity across the BSR interface which indicates hydrate bearing sediments overlying the free gas bearing sediments. The amplitude versus offset analysis of PSTM gathers shows increase of amplitude with offset, a common trend as observed in BSR associated with gas hydrate accumulation. Results suggest the possibility of gas hydrate accumulation in the central part of the basin specifically in the area of structural low at the Neogene level. These results would serve as preliminary information for selecting prospective gas hydrate accumulation areas for further integrated or individual study from geophysical, geological, geochemical and microbiological perspectives for confirmation of gas hydrate reserves in the area. Further, on the basis of these results it is envisaged that biogenic gas might have been generated in the region which under suitable temperature and pressure conditions might have been transformed into the gas hydrates, and therefore, an integrated study comprising geophysical, geological, geochemical and microbiological data is suggested to establish the gas hydrate reserves in Mahanadi deep water basin.  相似文献   

14.
东海天然气水合物的地震特征   总被引：1，自引：0，他引：1  
使用中国科学院海洋研究所“科学一号”调查船于2001年以及20世纪80年代在东海地区采集的多道地震资料,以海域天然气水合物研究为目的,对这些资料进行了数据处理并获得了偏移地震剖面。通过对地震剖面的解释,在6条剖面上确定了6段异常反射为BSR,均有振幅强、与海底相位相反的特点。6段BSR基本上都没有出现和沉积地层相交的现象。分析认为,这与东海地区第四纪以来的沉积特征有关,并不能由此否认这些异常反射是BSR。6段BSR出现的水深为750~2 000 m,埋深在0.1~0.5 s(双程时间)之间。随着海底深度的增大,BSR埋深有增大的趋势。计算结果显示,6段BSR所处的温度和压力条件都满足水合物稳定赋存所需要的温度和压力条件。本文的BSR主要与北卡斯凯迪亚盆地以及智利海域水合物的温度、压力条件相似,而与日本南海海槽、美国布莱克海台等海域水合物的温度、压力条件相差比较大。在地震剖面上,6段BSR所处的局部构造位置都和挤压、断层有关,有利于水合物的发育;在空间上,它们主要分布在东海陆坡近槽底的位置以及与陆坡相近的槽底。在南北方向上,除分布在吐噶喇断裂和宫古断裂附近外,还与南奄西、伊平屋和八重山热液活动区相邻。热液活动和水合物虽然没有直接的成因关系,但岩浆活动为水合物气源的形成提供了热源条件,为流体和气体的运移、聚集提供了通道条件,从而有利于水合物的发育与赋存。根据地震剖面反射特征推断,剖面A1A2和A14A23发育BSR的位置应该有气体或者流体从海底流出,可能是海底冷泉发育的位置。剖面A14A23上BSR发育处,振幅比的异常增大和BSR埋深的降低是相关联的。这种关联支持该处发育海底冷泉的推测。  相似文献   

15.
大量研究表明南海北部珠江口盆地是天然气水合物发育区,但是该盆地东部揭阳凹陷水合物研究较少。本文利用揭阳凹陷新采集三维地震资料,对该三维地震资料进行成像道集优化和叠前时间偏移处理,得到针对水合物的新处理地震数据体,并通过高精度网格层析反演得到层速度数据体。利用该数据开展叠后约束稀疏脉冲反演,获得含天然气水合物地层波阻抗异常,综合分析反演与地震属性识别水合物。从新处理地震资料看,该区域似海底反射(bottom simulation reflection,BSR)反射呈连续、不连续与地层斜交等特征,BSR发育在一个继承性小型水道上,且下部断裂和气烟囱发育。通过分析BSR特征及BSR上下地层的速度、波阻抗、振幅、频率、相干等属性异常,结合水合物成藏条件,发现了南海北部新的天然气水合物有利富集区,为该区域水合物勘探提供基础。  相似文献   

16.
Presence of gas hydrate and free gas in Iranian part of Makran accretionary prism changes the elastic properties of unconsolidated sediments and produces sharp bottom simulating reflectors (BSRs) which are observed on the 2-D seismic data. Different methods have been applied to estimate the gas hydrate and free gas saturations in marine sediments based on seismic measurements. Most of these methods are based on relating the elastic properties to the hydrate and free gas saturations and remotely estimating their concentration. In this regard, using the effective medium theory (EMT) which was developed for different modes of hydrate distribution is more considered among other rock physics theories. The main concern about saturation estimations based on EMT is that the velocities of the hydrate-bearing sediments primarily depend on how they are distributed within the pore space. Therefore, understanding the modes of hydrate distribution (at least cementing or non-cementing modes) is necessary to decrease the estimation uncertainties.The first intention of paper is to investigate amplitude variation versus offset (AVO) analysis of BSR to determine the hydrate distribution modes. The results from the probable saturation revealed that if the hydrate cements the sediment grains, BSR would show the AVO class IV and if hydrate does not cement the sediment grains, then BSR would show either the AVO class II or class III depending on the free gas saturation just beneath the BSR. The second intention of paper is to introduce some templates called reflectivity templates (RTs) for quantitative study of hydrate resources. These templates are provided based on the EMT to quantify the hydrate and free gas near the BSR. Validation of this approach by synthetic data showed that a reliable quantification could be achieved by intercept-gradient RTs, only if these attributes are determined with a high accuracy and good assumptions are made about the mineralogical composition and porosity of the unconsolidated host sediments. The results of this approach applied to a 2-D marine pre-stack time migrated seismic line showed that less than 10% of the gas hydrate accumulated near to the BSR in anticlinal-ridge type structure of Iranian deep sea sediments. The free gas saturation near to the BSR by assuming a homogeneous distribution was less than 3% and by assuming patchy distribution was about 3–10%.  相似文献   

17.
During the China’s first gas hydrate drilling expedition -1 (GMGS-1), gas hydrate was discovered in layers ranging from 10 to 25 m above the base of gas hydrate stability zone in the Shenhu area, South China Sea. Water chemistry, electrical resistivity logs, and acoustic impedance were used to estimate gas hydrate saturations. Gas hydrate saturations estimated from the chloride concentrations range from 0 to 43% of the pore space. The higher gas hydrate saturations were present in the depth from 152 to 177 m at site SH7 and from 190 to 225 m at site SH2, respectively. Gas hydrate saturations estimated from the resistivity using Archie equation have similar trends to those from chloride concentrations. To examine the variability of gas hydrate saturations away from the wells, acoustic impedances calculated from the 3 D seismic data using constrained sparse inversion method were used. Well logs acquired at site SH7 were incorporated into the inversion by establishing a relation between the water-filled porosity, calculated using gas hydrate saturations estimated from the resistivity logs, and the acoustic impedance, calculated from density and velocity logs. Gas hydrate saturations estimated from acoustic impedance of seismic data are ∼10-23% of the pore space and are comparable to those estimated from the well logs. The uncertainties in estimated gas hydrate saturations from seismic acoustic impedances were mainly from uncertainties associated with inverted acoustic impedance, the empirical relation between the water-filled porosities and acoustic impedances, and assumed background resistivity.  相似文献   

18.
Seismic tomography is an effective means of estimating velocity and structure from multichannel seismic (MCS) reflection data. In this study we have followed a 2D approach to arrive at the probable velocity field configuration from multichannel seismic data and infer the presence of gas hydrates/free-gas in the offshore Kerala-Konkan region, along the eastern part of a seismic line on which a bottom simulating reflector (BSR) has previously been identified. Tomographic modeling consists of the identification of reflection phases and picking of respective travel times for various source-receiver positions. These picks were then utilized to arrive at a 2D velocity field following a forward and inversion approach using a ray tracing technique. The modeling for the first time brought out the finer scale velocity structure under the region of investigation. Modeling through the 2D approach shows lateral variation in velocity field along the studied segment of the seismic line. The results indicate a thin (∼50–60 m) sedimentary cover with velocity ranging from 1,770 to 1,850 m/s. A sedimentary layer with high P-wave velocity 1,980–2,100 m/s below the sea floor was interpreted as the hydrate layer. The thickness of this layer varies between 110 and 140 m. The hydrate layer is underlain by a low-velocity layer having velocities in the range 1,660–1,720 m/s. This low velocity may represent a free gas layer, whose thickness varies between 50 and 100 m located below the hydrated layer. The investigation suggests the occurrence of gas hydrate underlain by free gas in some parts of the Kerala-Konkan offshore region.  相似文献   

19.
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20.
Travel-time inversion is applied to seismic data to produce acoustic velocity images of the upper 800 m of the South Shetland margin (Antarctic Peninsula) in three different geological domains: (i) the continental shelf; (ii) the accretionary prism; (iii) the trench. The velocity in the continental shelf sediments is remarkably higher, up to 1000 m/s at 600–700 m below seafloor, than that of the other two geological domains, due to the sediment overcompaction and erosion induced by the wax and waning of a grounded ice sheet. Pre-stack depth migration was applied to the data in order to improve the seismic image and to test the quality of the velocity fields. Where the Bottom Simulating Reflector (BSR) is present, positive and negative velocity anomalies were found with respect to a reference empirical velocity profile. The 2D-velocity section was translated in gas hydrate and free gas distribution by using a theoretical approach. The analysis revealed that the BSR is mainly related to the presence of free gas below it. The free gas is distributed in the area with variable concentration and thickness, while the gas hydrate is quite uniformly distributed across the margin.  相似文献   

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