disparity between measured and bsr heat flow in the xisha trough of the south china sea and its implications for the methane hydrate abstract we calculate the heat ow from the depth of bottom simulating seismic re ectors bsrs on a seismicpro le in the xisha trough of the south china sea and compare them with the probe heat ow measure ments the bsr heat ow turn out to be 32 80 mw m 2 signi cantly lower than the measurements of 83 112 mw m 2 such big disparity cannot be ascribed only to the errors from parameters parameter errors that traditionally believed to in uence the bsr heat ow besides the parameter errors we discussemphatically the errors coming from the theoretical assumption for the bsr heat ow determination theoretical errors which occur when the bsr depth does not coincide with the base of the methanehydrate stability zone mhsz if bsr stays bellow the base of mhsz lying at the top of free gas zone the derived heat ow would be underestimated compared with the parameter errors the theoreticalerrors would be relatively larger in some geological settings the disparity between measured and bsrheat ow in the xisha trough might be mainly due to the theoretical error based on the theoreticalmodel assuming that the bsr lying at the top of the free gas zone the methane ux along the xisha seis mic pro le is estimated and the thickness of the methane hydrate occurrence zone is predicted 2008 elsevier ltd all rights reserved

sediment dewatering in the makran accretionary prism abstract sediments within the makran accretionary prism are considerably more compacted than the material entering the subduction zone due to the additional tectonic stresses resulting from plate convergence and accretionary processes tectonic compaction of the offscraped sediments has been documented by deriving in situ porosities in the upper 4 km from their seismic velocities wide angle seismic reflection refraction profiles were analysed from a transect crossing both the makran accretionary prism and the undeformed abyssal plain sediments to the south the 6 7 km thick abyssal plain sediments exhibit normal compaction with depth the sediments incorporated into the accretionary prism have higher seismic velocities at all depths corresponding to reductions in porosity of 30 40 mapping the porosity depth gradient across the prism shows that there is a marked change in porosity at the seaward toe of the prism the frontal fold there is then little further change until further folding begins to occur 70 km landward of the toe the dewatering and consolidation of the sediments is an important control on deformation occurring within the sedimentary prism

estimates of heat flow derived from gas hydrates a gas hydrate phase boundary in marine sediments is often observed as an anomalous reflector in seismic profiling data from the depth of the reflectors heat flow values can be estimated using the phase relation of the gas hydrate system and inferred density and thermal conductivity of sediments this method is applied to the data in the nankai trough around central america and along the blake outer ridge the estimated values consistent with those measured by conventional means suggest that derivation of heat flow by this method is feasible on the landward slope of the nankai trough we find that heat flow increases downslope toward the floor of the trough

heat flow and age of the gulf of oman we present the results of twenty heat flow stations in the gulf of oman which are used to infer the first reliable age estimates for the basin a mean surface heat flux of 42 6 3 6 mw m 2 exhibits no significant regional variation after correction for thick and rapidly deposited sediments this yields an age of 70 to 100 ma according to oceanic thermal models this age is also consistent with the sediment corrected basement depths of 5 5 6 0 km and with formation during the cretaceous quiet zone the latter can explain the absence of magnetic sea floor spreading lineations heat flow measurements are also used to confirm the presence of gas hyrdate layers the measured thermal gradient yields a depth for the solid to free gas phase transition which is the same as that deduced from bright spots seen on seismic reflection profiles

effect of thermal non equilibrium seafloor topography and fluid advection on bsr derived geothermal gradient abstract the seafloor and bottom simulating reflectors bsrs are interpreted from the 3d seismic data acquired in krishna godavari kg offshore basin in the vicinity of sites drilled cored during national gas hydrate program nghp expedition 01 the shallow structures such as inner toe thrust fault system regional and local linear fault systems and mass transport deposits are inferred from attributes of seafloor time structure as well as from the seismic profiles the geothermal gradient is estimated from the depths and temperatures of the seafloor and the bsr the temperature at the bsr depth is estimated from the methane hydrate and seawater salinity phase boundary assuming that the bsr represents the base of the gas hydrate stability zone the spatial variations in geothermal gradient gtg show a strong correlation with seafloor topography in the kg basin the gtg decreases by 13 30 over the topographic mounds formed due to inner toe thrust faults and recent mass transport deposits the gtg decreases by only 5 10 over the mounds likely due to defocusing of heat flux based on one dimensional topographic modeling hence the gtg perturbation due to topography alone cannot explain the observed gtg anomaly the temperature profile beneath these mounds may not be in equilibrium with the surroundings either due to the recent upliftment of sediments along the inner toe thrust faults or rapid deposition of sediments due to slumping sliding in contrast an increase in gtg by 10 15 is observed in the vicinity of major fault systems we presume that the likely mechanism for the increase in gtg is fluid advection from a deeper part of the basin a detailed thermal modeling involving the effect of surface topography high sedimentation rates fluid advection and sediment thickening due to tectonics is required to understand the thermal profile in kg offshore basin

deep ocean field test of methane hydrate formation from a remotely operated vehicle we have observed the process of formation of clathrate hydrates of methane in experiments conducted on the remotely operated vehicle rov ventana in the deep waters of monterey bay a tank of methane gas acrylic tubes containing seawater and seawater plus various types of sediment were carried down on ventana to a depth of 910 m where methane gas was injected at the base of the acrylic tubes by bubble stream prior calculations had shown that the local hydrographic conditions gave an upper limit of 525 m for the p t boundary defining methane hydrate formation or dissociation at this site and thus our experiment took place well within the stability range for this reaction to occur hydrate formation in free seawater occurred within minutes as a buoyant mass of translucent hydrate formed at the gas water interface in a coarse sand matrix the filling of the pore spaces with hydrate turned the sand column into a solidified block which gas pressure soon lifted and ruptured in a fine grained black mud the gas flow carved out flow channels the walls of which became coated and then filled with hydrate in larger discrete masses our experiment shows that hydrate formation is rapid in natural seawater that sediment type strongly influences the patterns of hydrate formation and that the use of rov technologies permits the synthesis of large amounts of hydrate material in natural systems under a variety of conditions so that fundamental research on the stability and growth of these substances is possible

mud volcanoes at the front of the makran accretionary complex pakistan abstract conical mounds 1 1 5 km in diameter and up to 65 m high were mapped at the foot of the active makran continental margin the mounds developed seaward of the accretionary front in a relatively planar zone where the beginning of build up of tectonic pressure initiates deformation based on shallow high resolution 4 khz sediment echosounding the sedimentary sequence in this area is generally well stratified as indicated by closely spaced horizontal reflections however in the vicinity of the mounds the sediment is characterised by many acoustically transparent zones which are 100 300 m in diameter and cut near vertically through the horizontal reflectors two sediment cores from the top of the largest cone and a neighbouring acoustically transparent zone reveal small scale post depositional deformation in a stratified sequence and methane concentrations up to 40 000 ng g this deformation and disruption of potential reflectors provides a clue to explain the acoustic transparency we interpret it as caused by the rise of charged fluids and mud leading initially to the slight disturbance of the generally good acoustic reflectors and eventually to the formation of conical mud mounds mud volcanoes mcs data showing a buried mound in an analogous structural position support the idea of tectonically induced mud fluid expulsion seaward of the accretionary front

gas hydrate stability and the assessment of heat flow through continental margins summary a prominent feature across some continental margins is a bottom simulating reflector bsr this seismic reflection generally coincides with the depth predicted for the base of the gas hydrate stability field because the occurrence of gas hydrates is controlled by temperature and pressure conditions it has been suggested that bsrs mark an isotherm and they have therefore been used to estimate the heat flow through continental margins crucial parameters are the temperature at bsr depth and at the seafloor and the thermal conductivity structure between the bsr and the seabed however very often the required parameters are not available and therefore they have been derived from models for gas hydrate stability and empirical relationships to obtain thermal conductivities from seismic velocities here we use downhole temperature thermal conductivity porosity and logging data from 10 ocean drilling program odp sites drilled into and through the gas hydrate field to investigate the quality of estimates our analyses and application of constraints to the makran margin off pakistan indicate the following i the temperature at bsr depth could be approximated by a seawater methane system although capillary forces chemical impurities or non equilibrium conditions can lower or increase the temperature if calibration by heat probe measurements is possible errors of geothermal gradients are less than 10 per cent otherwise uncertainties of 20 per cent or even higher may arise in addition seasonal variations of bottom water temperature have to be considered because they may affect thermal gradients by up to y10 per cent ii the impact of typical quantities of low thermal conductivity gas hydrate on the bulk thermal conductivity is insignificant iii the thermal conductivity profile between the bsr and the seabed can generally be approximated by a mean value thus iv seabed measurements should be used instead of empirical relationships which may produce errors of 5 30 per cent consequently in addition to high quality seismic data a prerequisite should be a large data set of thermal conductivities and oceanographic data heat probe measurements are recommended to constrain geothermal gradients in this case the uncertainty of heat flow is 5 10 per cent of the estimated heat flow if these data are not available errors uncertainties can reach 50 60 per cent of the calculated value

internal deformation and compaction of the makran accretionary wedge the makran accretionary wedge is one of the largest on earth a 7 km thick column of sands and quartzolithic turbidites are incorporated into this wedge in a series of deformed thrust sheets we present the results of prestack depth migration and focusing error analysis migration velocity analysis performed on a profile across the makran wedge the depth section shows the deformation style of the accreted sediments and the migration velocities allow us to estimate porosity variations in the sediments the thrust sheets show evidence of fault propagation folding with a long wavelength of deformation 12 km and secondary thrusting in the kink bands of the folds such that the central part of each thrust sheet is elevated to form an additional ridge this deformation style and the 15 steep surface slope of the first ridge suggest a high degree of consolidation porosities were calculated from the seismic migration velocities and the ratio of fluid pressure to lithostatic pressure was estimated for 5 locations along the profile rather than being undercompacted and overpressured as in most accretionary wedges the sedimentary input is normally compacted exponential porosity decay throughout almost the whole wedge however a slight increase in porosity and at depth with respect to the normal compaction curve indicates that the turbiditic sequence might be overpressured landward of the deformation front

thermal effects of sediment thickening and fluid expulsion in accretionary prisms model and parameter analysis we investigate the thermal consequences of sediment thickening and fluid expulsion in subduction zone accretionary prisms using a model where the rate of fluid expulsion in a uniformly thickening wedge is obtained analytically and the fluid flow and temperature fields are computed numerically variations landward across the wedge in the porosity depth function are included in the model the most important contribution to the thermal regime arises from the thickening of the wedge which can reduce the heat flow at the seafloor relative to the deep lithospheric heat flow by up to a few tens of percent this effect is countered by an opposite but lesser contribution of the heat advected upward by the fluid expelled from the reconsolidating sediments the total thermal perturbation is sensitive to the prism taper angle the incoming sediment thickness and the convergence velocity anomalously low seismic velocities observed in a zone several tens of kilometers wide near the toe of some accretionary prisms suggest that there is a delay between the initial thickening of the sediment section and reconsolidation toward equilibrium porosity at depth the thermal consequences of retarded fluid expulsion are significant and result in locally depressed seafloor heat flow in this region for the full observed range of accretion parameters the rates of fluid expulsion are not sufficient to cause a detectable depth variation in heat flow on a depth scale of less than 1 km

natural gas hydrates on the continental slope off pakistan constraints from seismic techniques summary in october 1997 new vertical incidence wide angle reflection and refraction data were obtained on the convergent continental margin ov pakistan the most prominent feature across the entire margin into the gulf of oman is a bottom simulating reflector bsr this reflection generally coincides with the depth predicted for the base of the gas hydrate stability field on the accretionary prism the bsr is a clear reflection with inverse polarity and strata beneath the bsr show enhanced reflectivities suggesting that gas is trapped between stratigraphic layers joint inversion of wide angle and refraction data yielded a 200 m thick low velocitiy zone at bsr depth using the born expansion band limited impedance logs were calculated from deconvolved and scaled single channel data suggesting that velocity at the bsr drops by 200 m s 1 some 40 km towards the coast cdp4400 of minshull white 1989 yielded a velocity inversion of about 600 m s 1 indicating that bsr characteristics vary significantly across the continental margin ov pakistan beneath the gulf the bsr was sampled at a similar subbottom depth where seismic bright spots occur however its reflection amplitude is unusually low and seismic stratigraphy beneath the bsr did not show enhanced reflectivities joint traveltime inversion of wide angle and refraction data revealed continuously increasing velocities suggesting that an adequate supply of methane for forming free gas and gas hydrate is mainly confined to sediments incorporated into the accretionary prism we believe that these variations are related to an evolutionary process due to tectonic uplift and ongoing sedimentation as sediments are incorporated into the accretionary prism sedimentation and uplift produce an upward migration of isotherms and therefore lead to a dissociation of gas hydrates which releases methane gas and thus causes an enhancement of the bsr our survey imaged bsr properties of the abyssal plain and within the first slope basin at water depths of 3300 m and 2490 m respectively cdp4400 was located farther landwards at a depth of 1730 m within a buried and uplifted and hence more evolved slope basin we therefore suggest that in the accretionary wedge tectonic uplift and ongoing sedimentation cause hydrate recycling by upward migration of the stability field in addition tectonic dewatering may accumulate hydrate at the base of the stability field by advection of methane from depth

comparison of measured and bsr derived heat flow values makran accretionary prism pakistan abstract during the german research cruise so 124 on rv sonne fall 1997 on the makran accretionary wedge off pakistan geophysical investigations were carried out to study the thermal regime at a gas hydrate bearing sediment in a tectonically deformed accretionary wedge on a transect perpendicular to the strike of the deformation front 42 heat flow measurements were carried out accompanied by seismic reflection experiments the investigations start in the south in the abyssal plain and cover the continental slope up to 2300 m water depth the aim of this study is to compare the bsr derived heat flow denoted as estimated heat flow with the values from measurements at the seafloor this requires the calculation of sediment physical properties at depth using empirical relationships between velocity and porosity the value measured and corrected for sedimentation of 47 mw m 2 south of the deformation front is slightly higher than values reported by hutchison et al earth planetary sci lett 56 1981 252 262 in all basins the estimated heatflow is significantly higher than the measured values as a result temperatures at the bsr extrapolated from seafloor measurements are 5 6 k lower than those taken from gas hydrate stability considerations as an overall trend the estimated as well as the measured heat flow show a small decrease from the deformation front to the northward thickening prism a similar observation was made at other accretionary wedges and described by wang et al j geophys res 98 b3 1993 4121 4142 and ferguson et al j geophys res 98 b6 1993 9975 9984 within the slope basins heat flow values show little variation indicating predominantly conductive heat transport fluid flow might occur at the bounding faults where we have little control the effect of rapid sedimentation on the dynamic behavior of the bsr might also have a significant influence on the estimated heat flow values our data set shows clearly that detailed seismic surveys and good control of the subsurface velocity are absolute necessities for the comparison of measured and bsr derived heat flow values however the uncertainty of the velocity porosity relationship together with a high and only approximately established sedimentation rate represent crucial but missing constraints which can be gained only by drilling

thermal regime of the costa rican convergent margin 1 along strike variations in heat flow from probe measurements and estimated from bottom simulating reflectors the thermal structure of convergent margins provides information related to the tectonics geodynamics metamorphism and fluid flow of active plate boundaries we report 176 heat flow measurements made with a violin bow style probe across the costa rican margin at the middle america trench the probe measurements are collocated with seismic reflection lines these seismic reflection lines show widespread distribution of bottom simulating reflectors bsrs to extend the spatial coverage of heat flow measurements we estimate heat flow from the depth of bsrs comparisons between probe measurements and bsr derived estimates of heat flow are generally within 10 and improve with distance landward of the deformation front together these determinations provide new information on the thermal regime of this margin consistent with previous studies the margin associated with the northern nicoya peninsula is remarkably cool we define better the southern boundary of the cool region the northern extent of the cool region remains poorly determined a regional trend of decreasing heat flow landward of the deformation front is apparent consistent with the downward advection of heat by the subducting cocos plate high wave number variability at a scale of 5 10 km is significantly greater than the measurement uncertainty and is greater south of the northern nicoya peninsula these heat flow anomalies vary between approximately 20 and 60 mw m 2 and are most likely due to localized fluid flow through mounds and faults on the margin simple one dimensional models show that these anomalies are consistent with flow rates of 7 15 mm yr 1 across the margin toe variability is significant and likely due to fluid flow through deformation structures associated with the frontal sedimentary prism

deformation at plate boundaries around the gulf of oman abstract seismic studies during charles darwin cruise 18 86 focused on three zones of deformation the makran accretionary prism the murray ridge owen fracture zone system and the continental margin of east oman a multichannel seismic reflection profile across the makran margin clearly imaged the imbricate thrust slices and their bounding faults a bottom simulating reflector bsr 500 800 m beneath the seabed marking the base of a gas hydrate stability zone may be traced across most of the profile the heat flow variation estimated from changes in the depth of this reflector shows evidence for localised fluid expulsion reflection profiles across the murray ridge show evidence for active extension along a series of major normal faults the rift structure is segmented with the fault polarity changing between segments a combined reflection and wide angle study of the east oman margin shows a deep offshore basin underlain by crust only 5 km thick and bounded by a basement ridge interpreted as a tilted fault block the wide angle data suggest a sleeply dipping moho beneath the margin the large lateral changes in crustal thickness may be explained by episodes of margin parallel shear

characteristics of gas hydrate occurrences associated with mud diapirism and gas escape structures in the northwestern sea of okhotsk abstract an interpretation of 8450 line km of seismic reflection data acquired during four cruises in the northwestern sea of okhotsk shows widespread occurrence of free gas or gas hydrates in the sediment this occurrence is documented seismically by gas escape structures acoustic blanking and by a bottom simulating reflector bsr gas escape structures and vents are concentrated on the northern sakhalin continental margin here they are associated with local tectonic movements along a n s trending dextral shear system the inessa shear zone mud diapirism is common in the derugin basin where compression dominates this diapirism is probably closely related to the gas accumulations in the sediment below the base of the gas hydrate stability zone bghsz conductive heat flow derived from the bsr depth distribution averages about 30 8 mw m 2 only adjacent to basement highs and around the inessa shear zone do values higher than 80 22 mw m 2 occur these high values are attributed to an elevated geothermal gradient in the tectonically active zone where fluid venting takes place areas of drift sedimentation and mass wasting where sedimentation rates are high are characterised by computed heat flow values below 30 8 mw m 2 however these values must be corrected upwards by 3 20 depending on the sedimentation rate applicable 3 8 100 cm kyr the total amount of methane preserved in the hydrate stability zone hsz and trapped as free gas beneath the bsr is estimated at 17 14 10 12 m 3 for the northwestern sea of okhotsk and 15 12 10 13 m 3 for the entire sea of okhotsk the latter figure represents about 0 8 of the global reservoir of methane gas from hydrates our study documents that the semi enclosed sea of okhotsk offers favourable conditions for the accumulation of gas in its sediments on account of its subarctic climate and the prevailing hydrologic regime these conditions include high primary productivity low bottom water temperatures and high sedimentation rates

interaction between accretionary thrust faulting and slope sedimentation at the frontal makran accretionary prism and its implications for hydrocarbon fluid seepage 1 using high resolution seismic profiles and other geophysical data collected during r v meteor cruise m74 2 we investigate the distribution patterns of shallow sediments their structure and deformation processes and their role in the migration accumulation and seepage of hydrocarbon rich fluids here we show that rapid syn kinematic sedimentation at the frontal makran accretionary prism provides a mechanism by which emerging imbricated thrust packets override the footwall at the seafloor without significant mass wasting and destruction of fault related anticline in the hanging wall these anticlines may rise high above the seafloor to form plate boundary parallel ridges and distinguish from simple thrust blocks seen at convergent margins elsewhere with the fast burial of many thrust faults by the syn kinematic sediments anticlinal growth structures form in these syn kinematic sediments by continuous thrust activity the anticlinal structures preserved within the cores of the ridges or formed from these syn kinematic sediments act as structural traps for migrating hydrocarbon rich fluids above which fluid escape structures are generated leading to seafloor seeps most of the discovered hydrocarbon seeps around sixth ridge are sourced from these traps despite the compressional environment and the rapid syn kinematic sedimentation rates shallow subsurface of the frontal makran is a normally pressured regime in which the buoyancy of hydrocarbons may account for the fluid migration in this important respect the makran accretionary prism differs from many other convergent margins and accretionary prisms where fluid flow is largely driven by tectonically induced overpressure

atypical heat flow near gas hydrate irregularities and cold seeps in the baikal rift zone abstract in this paper we address the irregular behaviour and geometry of the gas hydrate stability zone hsz inferred from reflection seismic data in relation to heat flow measurements the study area lies in the hanging wall of the posolsky fault in the southern baikal basin sbb side scan sonar imagery already revealed an undulating antithetic active fault structure and several isolated active vent structures remarkably these fluid discharge structures occur only where the base of the hydrate stability zone bhsz as inferred from seismic reflection profiles is fluctuating and discontinuous independent of lake floor morphology the correlation between the interpreted bhsz and heat flow data across the malenki seep is reasonable on a seismic profile south of the fluid escape features the bhsz is expressed as an oscillatory but continuous reflection and shows poor correlation with heat flow measurements in nearly all cases measured heat flow exceeds inferred heat flow additionally the local inferred minima are anomalously low compared to the expected background values in the sbb these observations suggest that the present day hydrate accumulation and its meta stability are more complicated than originally suspected the limited area of these anomalies their amplitudes and their occurrence in the immediate vicinity of faults and fluid escape features suggest that fluid convection cells disturb local gas hydrate stability conditions

newly identified strike slip plate boundary in the northeastern arabian sea the first regional swath bathymetry survey of the makran accretionary wedge revealed a sinistral strike slip fault named herein the sonne fault obliquely crossing the wedge and continuing into the abyssal plane this fault separates the western part of the makran subduction zone where plate boundary events are absent from the eastern part that does show plate boundary seismicity most events are concentrated along the sonne fault little murray ridge a basement high and related magnetic anomalies are offset along the sonne fault together these observations identify the newly discovered sonne strike slip fault as a plate boundary that has been active 2 m y this finding suggests that what has been considered the northeasternmost part of the arabian plate is actually a separate microplate named herein the ormara plate the formation of which resulted from tearing of the arabian plate along the sonne fault with this concept the different dips of the downgoing plate below the western and eastern parts of the makran margin and the related different distances between the trench and quaternary arc volcanic centers can be unequivocally explained

sediment compaction and fluid migration in the makran accretionary prism the makran continental margin in the gulf of oman forms the seaward extremity of an accretionary sediment prism which extends several hundred kilometers inland a recently acquired multichannel seismic reflection profile shot across the margin imaged the structure of the prism in greater detail than was previously possible and allowed us to investigate the relationship between deformation and pore fluid motion in the region velocity analyses of the common midpoint gathers reveal a marked change in velocity structure at the toe of the accretionary wedge as seen in previous sonobuoy wide angle data accreted sediments show significantly higher vertical velocity gradients than those of sediments entering the prism this change is interpreted as due to porosity reduction as pore fluids are squeezed out of the compacting sediment a prominent bottom simulating reflector appears 500 800 m beneath the sea bed several lines of evidence suggest that this reflector represents the base of a gas hydrate zone underlain by widespread free gas which may be exsolved from pore water migrating from deep within the sediment pile up permeable fault planes imaged in the profile the hydrate reflector appears to shallow in the region of some faults suggesting a temperature anomaly due to the presence of warm pore fluids a heat flow profile derived from the depth of the hydrate reflector does not show the expected landward decrease as the sediment pile thickens simple thermal modeling suggests that advective heat flow within the prism may explain this anomaly the inferred presence of overpressured pore fluids in the makran suggests that accreted sediments have a low permeability the seismic evidence suggests a two stage compaction process with rapid initial dewatering through intergranular permeability as sediment enters the prism followed by a buildup of pore pressure as the permeability decreases and fluid migration is restricted to fault zones

velocity porosity relationships for water saturated siliciclastic sediments seeking a global empirical relationship between compressional wave velocity and porosity for siliciclastic sediments we have brought together an extensive suite of both new and published log and core based data we undertook a detailed statistical analysis of ocean drilling program data from amazon fan to examine variables affecting compressional velocity in shallow unconsolidated sediments we identify three dominant variables porosity shale fraction and consolidation history and present two empirically determined boundary curves one for normally consolidated sediments and a second for highly consolidated environments e g accretionary prisms these two empirical relationships predict the compressional velocity of siliciclastic sedimentary rocks with water filled pores as a function of porosity and clay content for the full range of observed porosities velocities of siliciclastic sedimentary rocks decrease rapidly with both increasing porosity and increasing clay content at fractional porosities higher than about 0 4 fluid dominates the elastic properties and velocity exhibits a subtle dependence on porosity remarkably the amazon fan data show that clay content has no direct influence on velocity at high porosities both clay content and sorting do indirectly affect velocity through their control of porosity burial affects velocity not only by compaction related porosity decrease but also by pressure induced increase of intergrain coupling because of the sensitivity of velocity to consolidation history particularly at intermediate fractional porosities of about 0 30 0 40 no single velocity porosity relationship can apply to all high porosity sediments the two proposed relationships fit the majority of published and new data they are applicable however only for normally pressured in situ conditions and water filled pores

mineralogy porosity and fluid control on thermal conductivity of sedimentary rocks summary estimates of thermal conductivity of the main non clay and clay sedimentary minerals are presented especially for the case of argillaceous ones these estimates are averaged estimates based on the interpretation of laboratory conductivity porosity and mineralogy measurements performed on small volumes of the order of 100cm3 of water saturated moist and air saturated samples the sampling is representative of the main sedimentary rocks sandstones carbonates evaporites and shales and it is composed of samples considered as structurally isotropic material first we experimentally verify the first order control of mineralogy porosity and fluid content on bulk conductivity and we demonstrate that such influences may be predicted accurately using a geometric mean model as long as isotropic samples are used then we interpret the data using an inverse method in order to estimate the average mineral conductivities of the main non clay and clay minerals which give the best fit to the individual laboratory measurements through the geometric mean model the analysis is based on measurements on 82 non clay samples and 29 clay samples taken on oil well cores or cuttings on outcrops and on artificially recompacted samples estimates of average mineral conductivities returned by the inversion process for the main non clay minerals are similar to generally admitted values 7 7 w m k for quartz 3 3 w m i k for calcite 5 3 w m l k for dolomite and 6 3 w m k for anhydrite values obtained for the clay minerals are systematically lower than those obtained for non clay ones they are of the order of 1 9 w m k for illite and smectite 2 6 w m k for kaolinite and 3 3 w m l k for chlorite these estimates combined with porosity and mineralogy data are used with the geometric mean model in order to verify its first order validity for predicting thermal conductivity of small volumes of porous isotropic sedimentary rocks the bulk conductivity is predicted with an accuracy of the order of f10 per cent for moist or water saturated samples and of the order of f20 per cent for air saturated samples

seismic velocities and inferred porosities in the accretionary wedge sediments at the cascadia margin large offset multichannel seismic reflection data from the subduction zone margin off vancouver island define the geometry internal structure and deformation style of the cascadia accretionary prism a very detailed seismic velocity analysis has been performed to quantitatively assess the velocity structure and thus to infer porosity variations and pore fluid expulsion across the deformation front within the sediments of cascadia basin approaching the deformation front and within the frontal thrust zone of the accretionary prism velocities increase landward probably as a result of sediment consolidation associated with horizontal compression a remarkable conclusion is that more than one third of the initial pore fluid content of the incoming sediments is lost by the time they are incorporated into the wedge in contrast a pronounced velocity decrease is evident in the lower slope region 15 20 km landward of the deformation front where the wedge sediment thickness is more than doubled compared with the section in cascadia basin the low velocity is explained by underconsolidation resulting from rapid horizontal shortening and vertical thickening of the sediment column accommodated by displacements along thrust faults or by distributed deformation as interred from the velocity data sediment porosities are high in this lower to midslope region of the wedge the thickening of the wedge results in sediment elements being moved to greater depths landward with little porosity loss and velocity change in the rapidly thickening section the overburden load stress increases faster than the sediments can respond by fluid expulsion and overpressures likely result

the effects of sedimentation and compaction on oceanic heat flow summary the estimation of environmental effects forms an important part of the interpretation of oceanic heat flow measurements in particular the perturbations associated with sedimentation and surface temperature changes must be taken into account analytical solutions can be obtained only for individual simplified versions of these problems whereas any real example is complicated by the process of sediment compaction which changes the bulk thermal properties with depth a physical model is developed which uses sediment porosity trends to predict the thermal parameters and material advection rates for an evolving sediment basement system these values are then used in a numerical solution to the heat flow equation to give estimates of the perturbed surface heat flux through time in addition to variations in sedimentation rate sediment type radioactive heat production and surface temperature changes are considered heat flow corrections may vary by up to a factor of 2 according to sediment type while radioactive heat production can offset the effects of sedimentation by as much as 40 per cent the results also indicate that alterations determined from simple analytical models tend to over estimate the true perturbation to the flux

methane hydrate and free gas on the blake ridge from vertical seismic profiling seismic velocities measured in three drill holes through a gas hydrate deposit on the blake ridge offshore south carolina indicate that substantial free gas exists to at least 250 meters beneath the bottom simulating reflection bsr both methane hydrate and free gas exist even where a clear bsr is absent the low reflectance or blanking above the bsr is caused by lithologic homogeneity of the sediments rather than by hydrate cementation the average methane hydrate saturation above the bsr is relatively low 5 to 7 percent of porosity which suggests that earlier global estimates of methane in hydrates may be too high by as much as a factor of 3

high resolution heat flow density in the lower congo basin 1 heat flow density hfd has been derived in the lower congo basin from 293 conventional probe measurements 424 oil exploration data and about 1 000 000 bottom simulating reflector bsr values each set of data provides independent estimates of temperature gradients associated with specific uncertainties and geographic distributions but a statistical comparison in overlapping domains shows a relatively good consistency in situ conductivity has been measured in conventional surveys and estimated from rock composition and porosity in oil exploration boreholes in the deep offshore measurements and estimates are consistently low and increase significantly toward the shelf domain thermal conductivity does not change significantly in the interval between the water bottom and the bsr and can be estimated from the surface measurements to provide an inference of hfd the combined sets of hfd in the lower congo basin cover the overall domain of the continental margin and a part of the oceanic domain high resolution imaging of the bsr by three dimensional 3 d seismic studies provides in addition good quality nominal resolution of 500 m in the deep offshore at short length scales on the order of kilometers large variations of hfd from 30 to 140 mwm 2 exist above salt diapirs and canyons related to heat refraction on the other hand no small scale effect related to fluid flow has been detected although active venting is known at larger length scale hfd in the deep offshore is different on both sides of the congo submarine channel 54 8 mwm 2 to the north and 48 12 mwm 2 to the south which are also characterized by different basin tectonic styles at the margin scale hfd is significantly different in the oceanic domain 42 3 mwm 2 the deep continental offshore 52 10 mwm 2 and the continental shelf 65 15 mwm 2 a progressive reduction of the radiogenic heat contribution in the extended continental crust is the more likely the cause of these differences mantle heat flow below the continental margin is consequently high compared to that expected 15 mwm 2 for old shields such as congo

anomalously cold temperatures observed at the base of the gas hydrate stability zone on the u s atlantic passive margin in situ measurements to depths of 415 m below sea floor in methane hydrate bearing sediments on the u s atlantic passive margin indicate that temperatures at the bottom simulating reflector bsr are anomalously low by 0 5 2 9 c if the bsr marks the base of gas hydrate stability bghs several hypotheses may explain the occurrence of the bsr at inappropriate pressure temperature p t conditions 1 if the bsr does not mark the bghs then p t conditions need not be sufficient to dissociate gas hydrate at this depth 2 the bsr may lie at nonequilibrium p t conditions due to incomplete readjustment in response to upper pleistocene holocene climate change however the occurrence of the blake ridge bsr at an overly shallow depth cannot be easily explained by realistic combinations of pressure driven deepening sea level rise and temperature driven shoaling bottom water temperature changes 3 the properties of sediments or pore fluids may inhibit the stability of gas hydrate in particular capillary forces arising in the fine grained montmorillonite rich sediments of the blake ridge may lead to shoaling of the bsr in this setting

the arctic frontal zone as seen in the ncep ncar reanalysis abstract calculations of a thermal front parameter using ncep ncar reanalysis data over the period 1979 98 reveal a relative maximum in frontal frequencies during summer along northern eurasia from about 60 to 70 n best expressed over the eastern half of the continent a similar relative maximum is found over alaska which is present year round although best expressed in summer these high latitude features can be clearly distinguished from the polar frontal zone in the midlatitudes of the pacific basin and collectively resemble the summertime arctic frontal zone discussed in several early studies while some separation between high and midlatitude frontal activity is observed in all seasons the summer season is distinguished by the development of an attendant mean baroclinic zone aligned roughly along the arctic ocean coastline and associated wind maxima in the upper troposphere the regions of maximum summer frontal frequency correspond to preferred areas of cyclogenesis and to where the summertime

regional spectral analysis of three moderate earthquakes in northeastern north america abstract we analyze fourier spectra obtained from the horizontal components of broadband and accelerogram data from the 1997 cap rouge the 2002 ausable forks and the 2005 riviere du loup earthquakes recorded by canadian and american stations sited on rock at hypocentral distances from 23 to 602 km we check the recorded spectra closely for anomalies that might result from site resonance or source effects we use beresnev and atkinson s 1997 near surface velocity structures and boore and joyner s 1997 quarter wave method to estimate site response at hard and soft rock sites we revise the street et al 1975 model for geometrical spreading adopting a crossover distance of r o 50 km instead of 100 km we obtain an average attenuation of q 410 25 f 0 50 0 03 for s l g surface waves with ray paths in the appalachian and southeastern grenville provinces we correct the recorded spectra for attenuation and site response to estimate source spectral shape and radiated energy for these three earthquakes and the 1988 m 5 8 saguenay earthquake the brune stress drops range from 130 to 419 bars and the apparent stresses range from 39 to 63 bars the corrected source spectral shapes of these earthquakes are somewhat variable for frequencies from 0 2 to 2 hz falling slightly below the fitted brune spectra

seismic and heat flow constraints from the gas hydrate system in the krishna godavari basin india abstract high resolution multi channel seismic reflection data acquired in the offshore domain of the krishna godavari kg basin provide seismic indicators of gas hydrate including bottom simulating reflectors bsr and high reflectivity zones below the bsr as an indication of free gas the recent recovery of gas hydrate by drilling across various sites in the kg basin together with the seismic indicators suggests overall widespread favourable gas hydrate stability conditions the bsrs provide indirect estimates of geothermal gradients and heat flow a simple conductive model is used to calculate heat flow from the depth of the bsr bsr derived heat flow values range from about 30 to 102 mw m2 across the kg basin we also modeled the base of gas hydrate stability zone bghsz to analyse the linkage between gas hydrate occurrences in the basin and its relation to the fault systems in the basin the influence of bottom water temperature pore water salinity and geothermal gradient variation on gas hydrate stability zone ghsz thickness is analysed in relation to both geological formations and tectonics our analysis suggests a highly variable ghsz in the kg basin controlled by local variations in the parameters of stability condition and faulting the bsr derived heat flow increases towards deeper water depths across the basin consistent local variations were also observed with low heat flow values over prominent topographic highs and high heat flow values over the flanks of the topographic highs this variation may be due to focusing and defocusing effects of the topography however local bsr depths patterns may also be due to dynamic effects including the displacement of isotherms by thrust faulting additionally recent mass wasting events and normal faulting resulted in the bghsz being too shallow relative to the predicted model which assumed equilibrium conditions

heat flow and thermal models of the barbados ridge accretionary complex the pattern of heat flow from the barbados ridge accretionary complex has been derived from marine surveys with heat flow probes from measurements in drill holes and from the depths of bottom simulating seismic reflectors caused by gas hydrate the heat flow from the accretionary complex has been simulated using a finite difference model to investigate how heat flow responds to changes in the cross sectional shape of the complex and the rate of convergence and to variations in pore fluid pressure within the complex and along the decollement at its base in the south of the complex heat flow decreases westward from the toe of the wedge towards the island arc because the downward movement of subducting lithosphere beneath the wedge and the active thickening of the accreted sedimentary sequence reduce the geothermal gradient more rapidly than thermal diffusion can maintain it localised high anomalies attributed to the flow of warm fluids along fault zones are superimposed on the conductive heat flow pattern farther west heat flow increases arcward produced partly by thermal diffusion re establishing a steeper thermal gradient where the rate of thickening is decreased but mainly by the arcward increase of frictional heating along the base of the wedge caused by the increase of slip rate and increased normal stress arising from thickening of the wedge in the north of the complex around the odp leg 110 drill sites the conductive model cannot reproduce the anomalously high heat flow measurements which can only be explained by the advection of warm pore fluids