Gas-saturated Sediments Research Articles

Sound field in a shallow-water arctic-type waveguide with a bottom containing a gas-saturated sediment layer

We have analyzed the possibility of mode description of a sound field in a shallow-water Arctic-type waveguide with a bottom containing a gas-saturated sediment fluid layer lying on an elastic half-space (permafrost). It has been established that the modes, including quasimodes, calculated using Pekeris cuts, yield the best description of the field in the water layer at small distances from the sound source on the order of 1–10 waveguide depths. Calculations of propagation losses in the waveguide for a thickness of the sediment layer comparable to or larger than the length of a sound wave in the sediments have shown that the sea bottom behaves like a homogeneous fluid half-space. Propagation losses sharply increase as the sound speed in the sediments approaches the sound speed in water. We have proposed a technique for estimating the sound speed in the sediment layer based on analysis of the attenuation curves of the sound field components corresponding to different sums of waveguide modes.

Effect of Hydrate Formation Conditions on Thermal Conductivity of Gas-Saturated Sediments

The paper summarizes experimental data on thermal conductivity variations in gas-saturated sediments exposed to hydrate formation at various conditions. All experiments were performed on a specially designed gas hydrate system maintaining a high gas pressure and a steady-state thermal regime, with a built-in unit for thermal conductivity measurements. The measurements were applied to natural samples of fine sand and silty sand collected from gas emanation sites in permafrost and to artificial sand and sand–clay mixtures. The results show that thermal conductivity can either increase or decrease depending upon hydrate formation conditions. Namely, it increases if gas hydrates form at positive temperatures (t > 0 °C) but decreases during hydrate formation in frozen samples. Freezing and thawing of hydrate-bearing sediments above the equilibrium pressure reduce their thermal conductivity as a result of additional hydrate formation. Experimental results are used to model hydrate and ice formation in gas-satur...

STUDY OF PRIORITY GEOLOGICAL HAZARDS IN PREPARATION FOR PROSPECTING AND EXPLORATION ON THE LAPTEV SEA SHELF

To select the most rational types of drilling rigs and safe locations for their installation, regional geophysical site surveys were conducted in particular areas of theLaptev Sea. They included bathymetric survey with a multi-beam echo sounder, side-scan sonar, continuous seismo-acoustic profiling with a parametric narrow-beam profiler SES in a two-frequency version (8 kHz, 100 kHz). Studies have shown that the greatest threats to the stability of offshore constructions are icebergs (numerous ice furrows were found on the shelf), faults, gas-saturated sediments and frozen soils. Priority geological hazards are frozen soils (FS). The problem of risk assessment for FS is exacerbated by the significant difficulties in their identification in a geological section and, in particular, in the detection of the boundary between frozen and thawed sediments (both in plan and in depth) in gas-saturated strata. The acoustic reflections from the frozen soils are very similar to the reflections from gas-saturated soils. To avoid mistakes in determining the sediment type and the depth of the upper boundary of the FS, the method of seismo-acoustic profiling in several frequency ranges has been developed in theMSUSeismicDataAnalysisCenter. In addition, a dynamic analysis of the seismic data has been carried out. Studies have shown that relatively reliable criteria for identifying gas-saturated sediments are the reverse polarity of reflections and the low values of seismic velocities. The amplitude anomalies associated with gas saturation are also identified by low values of acoustic impedance and quality-factor Q (with regard to the enclosing strata). At the same time, the direct polarity and high velocities of seismic waves are characteristic features of the FS. The magnitude of shear wave velocity Vs, the ratio Vp/Vs are the most reliable parameters for the identification of the FS from seismo-acoustic data. It is shown that the value of quality-factor Q for frozen sediments considerably exceeds the one for gas-saturated sediments.

Geochemical and acoustic evidence for the occurrence of methane in sediments of the Polish sector of the southern Baltic Sea

This paper presents the results of geochemical and acoustic investigations of sediments in the Polish sector of the southern Baltic Sea. Its objective was to indicate areas of gas bubble formation and the occurrence of methane. Over 3000 nautical miles of transects were recorded using a variety of hydroacoustic instruments, and five coring points were selected for further analyses of pore waters (CH4, SO42−, H2S, NH4+, total alkalinity) and sediments (grain size distribution, Corg, Ntot, LOI and WC). Gas turned out to be present at shallow depths in different forms such as recent and buried pockmarks, and gas-saturated sediments (including gas pockets and seepages). It was found that methane was widespread in the sediments of the study area, both in the surface sediments, e.g. in the vicinity of the Hel Peninsula or in the central Gulf of Gdańsk, and in deeper sediment layers, e.g. in the Gdańsk Deep and the Słupsk Furrow. Chemical analysis showed that as a result of the rapid decomposition of organic matter, sulphates were depleted in the top 20cm layer of sediments and that methane was produced at relatively shallow depths (in some areas even at depths of 20–30cm bsf) compared to other regions of the Baltic, reaching concentrations of>6mmol l−1 in the 30–40cm layer below the sediment surface. The sulphate-methane transition zone (SMTZ) was 4–37cm thick and was situated in the uppermost 50cm of the sediments.

동해 울릉분지 심해 탄성파 탐사자료 진폭변화분석

탄성파 탐사자료의 진폭변화를 분석하면 지층의 유체를 탐지하고 석유 가스 저류층의 정밀한 물성 도출이 가능하다. 본 연구는 동해 울릉분지의 심해 탄성파 탐사자료에 대하여 진폭변화를 분석하고 정리하였다. 중합단면에서 반사신호가 강하게 기록된 영역의 탄성파 공통깊이점-벌림 모음과 공통깊이점-반사각 모음을 관찰하여 진폭변화가 뚜렷한 영역을 선별하였다. 울릉분지의 중앙부 탄성파 탐사 반사각 모음의 주시 3200과 3300 ms 구간 탄성파 신호에 대한 종축절편과 진폭구배 속성을 계산하여 벌림에 따른 진폭 증가 및 감소를 확인하였다. 종축절편과 진폭구배를 곱한 속성과 합한 속성을 도출하여 울릉분지 퇴적층의 가스부존 가능 영역 상부와 하부 경계를 구분하였다. 가스로 포화된 퇴적층의 탄성파 진폭변화 특성을 보이는 영역은 탄성파주시 3 s 인근에서 간헐적으로 나타났다. 교차도표를 이용하여 울릉분지 탄성파자료의 진폭변화를 유형별로 확인할 수 있었다. 배경매질의 종축절편과 진폭구배는 함수 퇴적층의 일반적인 특징인 반비례관계를 보였고 가스함유 퇴적층의 진폭변화를 보이는 표본은 교차도표 단면상에서 1사분면과 3사분면에 위치하였다. 교차도표에서 선택된 표본들을 중합단면에서 추적한 결과 울릉분지 중앙부의 심해 퇴적지층 중 진폭변화 유형 3에 해당하는 영역이 수평연장 150 m 내로 분포함을 유추할 수 있었다. The amplitude variation with offset of seismic data can detect fluids in the sediment and resolve the petrophysical properties of hydrocarbons in the subsurface. We analyzed and described the amplitude variation in deep sea seismic data obtained from the Ulleung Basin, East Sea. By inspecting seismic CDP-offset and CDP-angle gathers which show a bright reflection event, we decided a target zone for amplitude variation analysis. From the seismic angle gather at the middle of Ulleung Basin, we recognized amplitude increase or decrease versus offset on the intercept-gradient curve. Using the product attribute and Poisson's ratio change attribute computed in terms of intercept with gradient, the top and the base of gas saturated sediments were described. The area of amplitude variation suggestive of the presence of gas saturated sediments is shown at the depth of 3 s traveltime. Anomalous features of seismic amplitude in the Ulleung Basin were classified by the crossplot of intercept and gradient. The background trend of crossplot between intercept and gradient shows an inverse proportional relation that is common for wet sediments. Anomalous amplitudes of Class III fall into the first and the third quadrants on crossplots. We inferred regional gas/water saturated area with the horizontal dimension of 150 m in the Ulleung Basin by cross-section with respect to cross-plot anomaly.

Acoustic properties of natural gas hydrates and the geophysical assessment of the subsurface distribution of hydrates in the Gulf of Mexico and Atlantic.

Natural gas hydrates are a solid form of natural gas found in the deep water marine margins of continents and under permafrost in Arctic regions worldwide. They have been recognized as a very significant potential energy source in the future. They form under high pressure and low temperature. Hydrate saturated sediments are acoustically faster and slightly less dense than water saturated sediments, but much faster and denser than gas saturated sediments. These properties allow for the identification of marine hydrate saturated sediments that are underlain by gas saturated sediments. The resulting geophysical reflector, referred to as a bottom simulating reflector, or BSR, often mimics the seafloor in areas where geothermal gradient is laterally consistent. The Bureau of Ocean Energy Management, Regulation, and Enforcement has used three-dimensional seismic data in the Gulf of Mexico and two-dimensional seismic data in the Atlantic to (1) map the distribution of BSRs, (2) drill six wells in the GOM with moderate to high hydrate saturations in sand reservoirs, and (3) assess the resource potential of hydrates.

Seep mounds on the Southern Vøring Plateau (offshore Norway)

Multidisciplinary study of seep-related structures on Southern Vøring Plateau has been performed during several UNESCO/IOC TTR cruises on R/V Professor Logachev. High-resolution sidescan sonar and subbottom profiler data suggest that most of the studied fluid discharge structures have a positive relief at their central part surrounded by depression. Our data shows that the present day fluid activity is concentrated on the top of these “ seep mounds”. Number of high hydrocarbon (HC) gas saturated sediment cores and 5 cores with gas hydrate presence have been recovered from these structures. δ 13C of methane (between −68 and −94.6‰ VPDB) and dry composition of the gas points to its biogenic origin. The sulfate depletion generally occurs within the upper 30–200 cm bsf and usually coincides with an increase of methane concentration. Pore water δ 18O ranges from 0.29 to 1.14‰ showing an overall gradual increase from bottom water values (δ 18O ∼ 0.35‰). Although no obvious evidence of fluid seepage was observed during the TV surveys, coring data revealed a broad distribution of living Pogonophora and bacterial colonies on sea bottom inside seep structures. These evidences point to ongoing fluid activity (continuous seepage of methane) through these structures. From other side, considerable number and variety of chemosynthetic macro fauna with complete absence of living species suggest that present day level of fluid activity is significantly lower than it was in past. Dead and subfossil fauna recovered from various seep sites consist of solemyid ( Acharax sp.), thyasirid and vesicomyid (cf. Calyptogena sp.) bivalves belonging to chemosymbiotic families. Significant variations in δ 13C (−31.6‰ to −59.2‰) and δ 18O (0.42‰ and 6.4‰) of methane-derived carbonates collected from these structures most probably related to changes in gas composition and bottom water temperature between periods of their precipitation. This led us to ideas that: (1) seep activity on the Southern Vøring Plateau was started with large input of the deep thermogenic gas and gradually decries in time with increasing of biogenic constituent; (2) authigenic carbonate precipitation started at the near normal deep sea environments with bottom water temperature around +5 °C and continues with gradual cooling up to negative temperatures recording at present time.

Microbiological and biogeochemical processes in a pockmark of the Gdansk depression, Baltic Sea

Comprehensive microbiological and biogeochemical investigation of a pockmark within one of the sites of gas-saturated sediments in the Gdansk depression, Baltic Sea was carried out during the 87th voyage of the Professor Shtokman research vessel. Methane content in the near-bottom water and in the underlying sediments indicates stable methane flow from the sediment into the water. In the 10-m water layer above the pockmark, apart from methane anomalies, elevated numbers of microorganisms and enhanced rates of dark CO2 fixation (up to 1.15 micromol C/(1 day)) and methane oxidation (up to 2.14 nmol CH4/(1 day)) were revealed. Lightened isotopic composition of suspended organic matter also indicates high activity of the near-bottom microbial community. Compared to the background stations, methane content in pockmark sediments increased sharply from the surface to 40-60 ml/dm3 in the 20-30cm horizon. High rates of bacterial sulfate reduction (SR) were detected throughout the core (0-40 cm); the maximum of 74 micromol/(dm3 day) was located in subsurface horizons (15-20 cm). The highest rates of anaerobic methane oxidation (AMO), up to 80 micromol/(dm3 day), were detected in the same horizon. Good coincidence of the AMO and SR profiles with stoichiometry close to 1:1 is evidence in favor of a close relation between these processes performed by a consortium of methanotrophic archaea and sulfate-reducing bacteria. Methane isotopic composition in subsurface sediments of the pockmark (from -53.0 to -56.5% per hundred) does not rule out the presence of methane other than the biogenic methane from the deep horizons of the sedimentary cover.

Unique problems associated with seismic analysis of partially gas-saturated unconsolidated sediments

Gas hydrate stability conditions restrict the occurrence of gas hydrate to unconsolidated and high water-content sediments at shallow depths. Because of these host sediments properties, seismic and well log data acquired for the detection of free gas and associated gas hydrate-bearing sediments often require nonconventional analysis. For example, a conventional method of identifying free gas using the compressional/shear-wave velocity ( V p/ V s) ratio at the logging frequency will not work, unless the free-gas saturations are more than about 40%. The P-wave velocity dispersion of partially gas-saturated sediments causes a problem in interpreting well log velocities and seismic data. Using the White, J.E. [1975. Computed seismic speeds and attenuation in rocks with partial gas saturation. Geophysics 40, 224–232] model for partially gas-saturated sediments, the difference between well log and seismic velocities can be reconciled. The inclusion of P-wave velocity dispersion in interpreting well log data is, therefore, essential to identify free gas and to tie surface seismic data to synthetic seismograms.

Integrated analysis of well logs and seismic data to estimate gas hydrate concentrations at Keathley Canyon, Gulf of Mexico

Accurately detecting and quantifying gas hydrate or free gas in sediments from seismic data require downhole well-log data to calibrate the physical properties of the gas hydrate-/free gas-bearing sediments. As part of the Gulf of Mexico Joint Industry Program, a series of wells were either cored or drilled in the Gulf of Mexico to characterize the physical properties of gas hydrate-bearing sediments, to calibrate geophysical estimates, and to evaluate source and transport mechanisms for gas within the gas hydrates. Downhole acoustic logs were used sparingly in this study because of degraded log quality due to adverse wellbore conditions. However, reliable logging while drilling (LWD) electrical resistivity and porosity logs were obtained. To tie the well-log information to the available 3-D seismic data in this area, a velocity log was calculated from the available resistivity log at the Keathley Canyon 151-2 well, because the acoustic log or vertical seismic data acquired at the nearby Keathley Canyon 151-3 well were either of poor quality or had limited depth coverage. Based on the gas hydrate saturations estimated from the LWD resistivity log, the modified Biot–Gassmann theory was used to generate synthetic acoustic log and a synthetic seismogram was generated with a fairly good agreement with a seismic profile crossing the well site. Based on the well-log information, a faintly defined bottom-simulating reflection (BSR) in this area is interpreted as a reflection representing gas hydrate-bearing sediments with about 15% saturation overlying partially gas-saturated sediments with 3% saturation. Gas hydrate saturations over 30–40% are estimated from the resistivity log in two distinct intervals at 220–230 and 264–300 m below the sea floor, but gas hydrate was not physically recovered in cores. It is speculated that the poor recovery of cores and gas hydrate morphology are responsible for the lack of physical gas hydrate recovery.

Seismic velocities and quantification of gas-hydrates from AVA modeling in the western continental margin of India

The most commonly used marker for the investigation of gas-hydrates is the bottom simulating reflector (BSR), which is caused by gas-hydrate laden sediment underlain by either brine or gas-saturated sediment. A BSR has been identified by seismic experiment in the Kerala-Konkan Basin of the western continental margin of India. Here we perform AVA modeling of seismic reflection data from a BSR to investigate the seismic velocities for quantitative assessment of gas-hydrates and to understand the origin of the BSR. The result reveals a P-wave velocity of 2.245 km/s and an S-wave velocity of 0.895 km/s for the sediments above the BSR. This corresponds to a Poisson ratio of 0.406 and hydrates saturation of ∼30% in the study area. The comparison of estimated P-wave velocity (1.77 km/s) above the hydrates-bearing sediment to that (1.78 km/s) below the BSR implies that the origin of the BSR is mainly due to gas-hydrates, as the presence (even in small quantities) of free-gas reduces the P-wave velocity considerably.

Hydrogen from hydrogen sulphide in Black Sea

Hydrogen sulphide, an acid gas, is generally considered an environmental pollutant. As an industrial byproduct, it is produced mostly during fuel processing. Hydrogen sulphide occurs naturally in many gas wells and also in gas hydrates and gas-saturated sediments especially at the bottom of the Black Sea where 90% of the sea water is anaerobic. The anoxic conditions exist in the deepest parts of the basin since nearly 7300 years, caused by the density stratification following the significant influx of the Mediterranean water through the Bosphorous nearly 9000 years ago. Here, H 2 S is believed to be produced by sulphur reducing bacteria at an approximate rate of 10 000 tons per day, and it poses a serious threat since it keeps reducing the life in the Black Sea. An oxygen–hydrogen sulphide interface is established at 150–200 m below the surface after which H 2 S concentration starts increasing regularly until 1000 m, and finally reaches a nearly constant value of 9.5 mg/l around 1500 m depth. Hydrogen sulphide potentially has economic value if both sulphur and hydrogen can be recovered. Several methods are studied for H 2 S decomposition, including thermal, thermochemical, electrochemical, photochemical and plasmochemical methods. In the present work, H 2 S potential in the Black Sea is investigated as a source of hydrogen, an evaluation of the developing prominent techniques for hydrogen production from H 2 S is made, and an engineering assessment is carried out regarding hydrogen production from H 2 S in the Black Sea using a process design based on the catalytic solar thermolysis approach. Possibility of a modular plant is considered for production at larger scale.

Study of gas biogeochemical parameters of gaseous fluids from the bottom of the Black Sea

Gaseous fluid flows from sediments into bottom water of seas and oceans attract attention of oceanographers, particularly specialists in gas geochemistry [1]. Fluid flows from the Black Sea bottom were the main study object in Cruise 61 of the R/V Professor Vodyanitskii , carried out in the Ukrainian segment of the Black Sea in June‐July, 2004. The expedition was organized and conducted by the Division of Marine Geology and Sedimentary Ore Formation, National Academy of Sciences of Ukraine. Gas biogeochemical parameters were investigated at the Institute of Oceanology (Moscow). These works were aimed at studying spatial variations in gas biochemical parameters in seawater and bottom sediments in the zone of local fluid flows. Bottom water was sampled in fluid discharge areas detected by multibeam sonar as a gaseous plume. Samples were taken closest to the fluid discharge site in the depth interval of 200‐2000 m. Specially designed equipment and methods were used for the study of gas biogeochemical parameters [2]. Water samples were taken with a hermetic bottom bathometer from the bottom layer of 1.5‐2.0 m above the seafloor, with the bathometer cartridge at several meters above the bottom, and by integration of the bathometer from the water column located 100‐150 m above the bottom. Bottom sediments were sampled by a gravity corer with a plastic container that fills with sediments in situ. In the case of spontaneous degassing of gas-saturated sediment after dismantling of the corer on deck, the released gas mixture inflated the container and the gas sample was taken by syringe for chromatographic analysis. Then, sediment samples were taken at different levels of the core. The content of gas components (hydrocarbons, helium, hydrogen, carbon dioxide, and permanent gases) and biochemical parameters, such as the ATP content and alkaline phosphomonoesterase activity (APHA), were simultaneously determined in water and sediment samples. The article presents new factual data on underwater fluid discharge areas. Methane and carbon dioxide appeared to be most informative among gas components. Hydrogen, helium, and homologues of methane were not detected, because their concentration in the analyzed gas mixture was below the sensitivity of the chromatograph. The gas mixture was either sampled immediately from the plastic container of the gravity corer (or hermetic core catcher) or was extracted by phase-equilibrium degassing of seawater and sediment samples. In both cases, air was the main gas carrier. Therefore, oxygen and nitrogen were also measured methodologically. In total, approximately 100 samples were analyzed. The gas mixture contained methane (0.0‐89.8 vol %) and carbon dioxide (0.0‐15.5 vol %). The total content of oxygen and nitrogen varied from 8.65 to 100 vol %. Table 1 presents data on various gas mixtures obtained from the gravity corer container and hermetic core catcher or extracted from water and sediment samples. Biochemical parameters were measured in the same water and sediment samples. In addition, certain intervals of the core and bottom and surface water samples were analyzed to estimate biological activity of seawater medium in study areas. Table 2 presents gasometric and biochemical data. The data from Tables 1 and 2 make it possible to disclose some peculiarities in the distribution of gas biogeochemical parameters. In the study areas, bottom water is saturated with methane and carbon dioxide. The biological activity of water is sufficiently high and

A new species of Calyptogena (Bivalvia: Vesicomyidae) from a recently discovered methane seepage area off Concepción Bay, Chile (∼36°S)

Calyptogena gallardoi sp. nov. is described from eight articulated and 15 separated valves collected by dredge at ∼760 m depth off the Bay of Concepción, central Chile (∼36°S). In general outline, C.gallardoi sp. nov. is close to C. pacifica and to a new species of Calyptogena from Peru, from which it differs in details of the shell shape and hinge margin. Bivalves of the genus Calyptogena are typical constituents of marine chemosynthesis-based communities, and are therefore indicators of reducing environments. In the area of occurrence, the presence of C. gallardoi sp. nov. is related to methane seepage, associated in turn with the extensive gas–hydrate fields recently reported for the Chilean margin along 35°S to 45°S. Gas-saturated sediments as well as fragments of other chemosynthetic endosymbiont-containing clams of the families Vesicomyidae, Lucinidae, Thyasiridae and Solemyidae were also retrieved in the area.Calyptogena gallardoi sp. nov. is the first species of Calyptogena s.s. and the second species of the family Vesicomyidae so far described for the south-eastern Pacific area.

Elastic velocities of partially gas-saturated unconsolidated sediments

Fluid in sediments significantly affects elastic properties of sediments and gas in the pore space can be identified by a marked reduction of P-wave velocity or a decrease of Poisson's ratio. The elastic properties of gas-saturated sediments can be predicted by the classical Biot–Gassmann theory (BGT). However, parameters for the BGT such as the Biot coefficient or moduli of dry frame of unconsolidated and high porosity sediments are not readily available. Dependence of velocities on differential pressure or porosity for partially gas-saturated sediments is formulated using properties derived from velocities of water-saturated sediments. Laboratory samples for unconsolidated and consolidated sediments and well log data acquired for unconsolidated marine sediments agree well with the predictions. However, because the P-wave velocity depends highly on how the gas is saturated in the pore space such as uniform or patch, the amounts of gas estimated from the P-wave velocity contains high uncertainty. The modeled V p / V s ratio of partially gas-saturated sediment using the patch distribution is usually greater than 1.6, whereas the ratio modeled assuming a uniform distribution is about 1.6. Thus, Poisson's ratio or V p / V s ratio may be used to differentiate patch from uniform saturation, but differences between various models of patch saturation cannot be easily identified.

Effects of temperature cycling on the phase transition of water in gas-saturated sediments

Experimental results on hydrate- and ice-formation conditions in the pores of sandy sediments that have undergone temperature cycles are presented. Thermodynamic parameters of gas hydrate and ice formation in porous space were determined for CH4 and CO2 saturated sandy sediments. The experiments indicate that temperature and freezing cycles affect the thermodynamics of hydrate and water–ice in gas-saturated sediments. Temperature cycles increased the hydrate accumulation in the pore space of sediments and reduce the freezing temperature of the remaining pore water. PACS Nos.: 91.60Hg, 92.40Sn

Three generations of mud volcanoes on the Louisiana continental slope

Three mud volcanoes were identified in a small area of the upper continental slope of the Gulf of Mexico. One is actively accreting by small, intermittent eruptions of gas and fluid mud. Eruptions occur from a pool of gas- and oil-saturated muds that upwell and spill over the central crater edge and down the flanks of the coneshaped buildup. A second structure, a possible sand volcano, is in the last stages of activity, with only slight evidence of gas-saturated sediments. The third structure is a dormant mud volcano with furrowed sides and scattered authigenic carbonate plates and rubble but no venting of gas or fluids.

Gas hydrates in marine sediments and the problem of their practical application

The exploitation of marine sedimentary gas hydrates is problematic for two reasons. A study of the gas regime of sediments showed that gas-saturated sediments, in which sedimentary gas hydrates may be found, have not developed widely, but occupy local facies zones, favorable for biochemical gas formation. The patchy distribution of sediments with possible gas hydrates is, in turn, responsible for the interlayering of the gas-saturated and nonsaturated varieties in the section of the sedimentary sequence. This lowers the calculated reserves of gas hydrates in the World's oceans, and also makes their exploitation difficult. In addition, the exploitation of gas hydrates from marine sediments will lead to the liquation of the surface layer along with the benthos inhabiting them, and the elimination of bottom organisms, which will injure the biological resources of the oceans. (JMT)