centennial to decadal scale monsoon precipitation variability in the semi humid region northern china during the last 1860 years records from stalagmites in huangye cave we developed a composite oxygen isotopic record of cave calcite for the last 1860 years based on three stalagmites from the huangye cave in eastern gansu province northern china the d 18 o values reflect monsoon precipitation changes with lower d 18 o values representing higher precipitation and vice versa three intervals of high precipitation were identified at ad 138 450 ad 730 1200 and ad 1860 1960 two intervals of low precipitation occurred at ad 1320 1410 and ad 1530 1860 the reconstructed monsoon precipitation variations correlate well with other records further east in the eastern yellow river basin suggesting synchronous precipitation changes during the late holocene in the semi humid region of northern china on decadal to centennial scales peak periods of warfare in dynastic transition times such as at ad 391 420 ad 601 630 ad 1111 1140 ad 1351 1380 and ad 1621 1650 correspond to sharp declines in precipitation or temperature in semi humid northern china indicating a strong connection between climatic and societal changes our study suggests that climatic deterioration in semi humid northern china has played an important role in chinese societal evolution

gas hydrate reservoirs and gas migration mechanisms in the terrebonne basin gulf of mexico highlights identify new fine grained hydrate filled fracture units in the terrebonne basin identify new hydrate bearing thin sands mostly within fractured muds present detailed seismic amplitude maps of the new hydrate bearing units discuss methane migration mechanisms and hydrate formation in thin sands identify and discuss source reservoir relationships between thick muds and thin sands abstract the interactions of microbial methane generation in fine grained clay rich sediments methane migration and gas hydrate accumulation in coarse grained sand rich sediments are not yet fully understood the terrebonne basin in the northern gulf of mexico provides an ideal setting to investigate the migration of methane resulting in the formation of hydrate in thin sand units interbedded with fractured muds using 3d seismic and well log data we have identified several previously unidentified hydrate bearing units in the terrebonne basin two units are 100 m thick fine grained clay rich units where gas hydrate occurs in near vertical fractures in some locations these fine grained units lack fracture features and they contain 1 4 m thick hydrate bearing sands in addition several other thin sand units were identified that contain gas hydrate including one sand that was intersected by a well at the location of a discontinuous bottom simulating reflector using correlation of well log data to seismic data we have mapped and described these new units in detail across the extent of the available data allowing us to determine the variation of seismic amplitudes and investigate the distribution of free gas and or hydrate we present several potential source reservoir scenarios between the thick fractured mud units and thin hydrate bearing sands we observe that hydrate preferentially forms within thin sand layers rather than fractures when sands are present in larger marine mud units based on regional mapping showing the patchy lateral extent of the thin sand layers we propose that diffusive methane migration or short migration of microbially generated methane from the marine mud units led to the formation of hydrate in these thin sands as discontinuous sands would not be conducive to long range migration of methane from deeper reservoirs

v p v s in unconsolidated oil sands shear from stoneley the sdt a sonic tool was tested in a borehole in the orinoco heavy oil belt eastern venezuela the sonically slow reservoir consists of unconsolidated quartz sand interbedded with shale full waveform analysis yields both compressional and shear slownesses we calculated the shear wave slowness from the stoneley slowness compressional and stoneley slownesses were determined using modified semblance techniques the compressional velocity is relatively fast in the heavy oil pay zone compared to the remainder of the well heavy oil 8 api possesses a finite rigidity at sonic frequencies and the rigidity of the hydrocarbon adds to the stiffness of the poorly consolidated sand the sand would not otherwise yield such a high velocity compressional and shear velocities of samples from eight whole cores were measured in the laboratory and the core velocities were found to be consistent with the logs especially encouraging is the agreement of the laboratory shear with the shear log derived from stoneley th

on the use of combined geophysical methods to assess water content and water conductivity of near surface formations abstract we propose to deepen the interpretation of combined geophysical methods georadar seismic and electric to assess physical properties characterizing the near surface porous formations especially the influence of water velocity analysis of multioffset georadar data are used together with seismic methods to estimate lateral and vertical ground water fluctuations this enables us to identify transitions from non saturated to fully saturated porous layers with certainty furthermore the accurate knowledge of seismic velocities helps to estimate the porosity of the ground water formations finally we show how the radar technique may be useful in solving the problem of trade off between bulk resistivity which is deduced from electrical measurements and that of water ionic conductivity and water content these theoretical considerations are illustrated using various measurements conducted at the same test site our interpretation is compared with a few laboratory measurements on water and soil samples this study displays the impact of combined geophysical approaches for providing models of water and ionic transfers down to a depth of several metres

direct hydrocarbon detection using comparative p wave and s wave seismic sections two field experiments in the sacramento basin and one in the green river basin demonstrate that comparative p wave and s wave cdp seismic sections can be used to detect gas directly in sandstone reservoirs the lines in the sacramento basin were shot over producing gas fields known to correlate with amplitude anomalies on p wave sections reflections of comparable strength are present on the p wave and s wave sections at lithologic boundaries but gas saturated zones correlating with p wave bright spots show no equivalent s wave amplitude anomalies the responses are consistent with laboratory observations that p wave velocity is more sensitive to the introduction of gas into liquid saturated pore space than s wave velocity the line in the green river basin was shot over a relatively deep gas field producing from an overpressured reservoir not associated with a conventional p wave bright spot the p wave reflection strength of the reservoir is about 50 percent greater than the s wave reflection strength

estimates of velocity dispersion between seismic and ultrasonic frequencies it is generally accepted that acoustic velocities in fluid saturated rocks vary with frequency evidence comes from experimental measurements and from theoretical causality arguments we have developed a simple analysis technique that gives estimates of total velocity dispersion between zero frequency and any measurement frequency the technique requires compressional p and shear s wave velocity measurements on dry and fully saturated rock assuming that the dry velocities are independent of frequency the biot gassmann equations are used to calculate the zero frequency velocities in the fully saturated rock any difference between the measured velocities and the calculated zero frequency velocities is interpreted as evidence of dispersion application of this analysis technique to a variety c ultrasonic data sets gives consistent results in many rocks dispersion between zero frequency and ultrasonic frequencies is on the order of 10 percent at low effective stress and it decreases to only a few pe

a simple method of predicting s wave velocity prediction of shear wave velocity plays an important role in seismic modeling amplitude analysis with offset and other exploration applications this paper presents a method for predicting s wave velocity from the p wave velocity on the basis of the moduli of dry rock elastic velocities of water saturated sediments at low frequencies can be predicted from the moduli of dry rock by using gassmann s equation hence if the moduli of dry rock can be estimated from p wave velocities then s wave velocities easily can be predicted from the moduli dry rock bulk modulus can be related to the shear modulus through a compaction constant the numerical results indicate that the predicted s wave velocities for consolidated and unconsolidated sediments agree well with measured velocities if differential pressure is greater than approximately 5 mpa an advantage of this method is that there are no adjustable parameters to be chosen such as the pore aspect ratios required in some other methods the predicted s wave velocity depends only on the measured p wave velocity and porosity

linearized avo and poroelasticity the technique of amplitude variation with offset avo allows geoscientists to extract fluid and lithology information from the analysis of prestack seismic amplitudes various avo parameterizations exist all of which involve the sum of three weighted elastic constant terms in present day avo approaches the weighting terms involve either knowledge of the incidence angle only or knowledge of both the incidence angle and the in situ vp vs ratio we have used the theory of poroelasticity to derive a generalized avo approximation that provides the estimation of fluid rigidity and density parameters we have combined two previously independent avo formulations thus reducing instead of adding to the total number of formulations this new approach requires knowledge of a third parameter to compute the weights the dry rock vp vs ratio we have derived a new equation and applied it to model and real data sets the new formulation has allowed us to estimate fluid properties of the reservoir in a more dire

deterministic estimation of gas hydrate resource volume in a small area of the ulleung basin east sea japan sea from rock physics modeling and pre stack inversion abstract we made deterministic estimations of the gas hydrate and in place gas resource volumes in a small area in the northwestern ulleung basin east sea japan sea from 3 d pre stack seismic data and well log and core data from the ubgh2 6 well we modeled the p impedance ip logs at the well for 0 100 pore space gas hydrate saturation from the p wave velocity vp and density logs modeled by the simplified three phase biot type equation stpbe then the ip volume for the gas hydrate bearing zone ghbz was constructed by pre stack inversion and divided into 28 layers the porosity and mineralogy along these layers were assumed to be uniform respectively to the porosity log upscaled to the layers and the sediment constituents at the well determined from the core samples next the pore space gas hydrate saturation at every time sample of each layer was found by matching the ip value of the time sample to the modeled ip logs upscaled to the layers the gas hydrate saturation volume with a cell size of 25 m 6 25 m 1 ms was obtained from the product of the pore space gas hydrate saturation volume and the porosity volume the gas hydrate saturation volume was converted into the depth volume based on the vp value at each cell found by matching the pore space gas hydrate saturation of the cell to the modeled vp logs the estimated total gas hydrate and gas resource volumes are about 8 43 108 m3 and about 1 38 1011 m3 respectively

a rock physics strategy for quantifying uncertainty in common hydrocarbon indicators we present a strategy for quantifying uncertainties in rock physics interpretations by combining statistical techniques with deterministic rock physics relations derived from the laboratory and theory a simple example combines gassmann s deterministic equation for fluid substitution with statistics inferred from log core and seismic data to detect hydrocarbons from observed seismic velocities the formulation identifies the most likely pore fluid modulus corresponding to each observed seismic attribute and the uncertainty that arises because of natural variability in formation properties in addition to the measurement uncertainties we quantify the measure of information in terms of entropy and show the impact of additional data about s wave velocity on the uncertainty of the hydrocarbon indicator in some cases noisy s data along with noisy p data can convey more information than perfect p data alone while in other cases s data do not reduce the uncertainty we apply the formulation to a well log example for detecting the most likely pore fluid and quantifying the associated uncertainty from observed sonic and density logs the formulation offers a convenient way to implement deterministic fluid substitution equations in the realistic case when natural geologic variations cause the reference porosity and velocity to span a range of values

a stratigraphic framework for the pliocene pleistocene continental sediments of the guadix basin betic cordillera s spain the central sector of the guadix basin betic cordillera s spain hosts some of the most significant pleistocene large mammal sites in europe the basin infill is divided into six genetic units the three lower with marine sediments and the remainder mainly continental as the environmental characterization of the sediments is crucial to understand the ecologic and geologic processes leading to the formation of these sites a stratigraphic sedimentologic and petrologic study is presented combining a lithologic correlation with previous biostratigraphic data and new magnetostratigraphic data a detailed litho bio and magnetostratigraphic scheme has been developed for the study area it shows the stratigraphic architecture of the central sector of the guadix basin together with the age and the relative spatial position of the most important mammal sites as well it has been also helpful to identify several isochronous surfaces to establish the exact position and the age of the boundary between the two last units of the continental infill in the guadix basin and to calculate sedimentation rates

sensitivity of reflection seismic data to oil column height in high porosity sandstones tuning is the effect of interference between the reflections from the top and bottom of a thin layer on the amplitude of the composite reflection for a homogeneous sandstone reservoir containing an oil column overlying brine interference between the reflection from the top reservoir and the oil water contact is a function of the height of the oil column if the properties of the sandstone do not vary across the oil water contact the ss ps and sp reflection coefficients from the oil water contact are small in comparison to the pp reflection coefficient this allows analytic expressions for the effective pp and ps reflection coefficients from the reservoir to be derived that include all p wave multiples within the oil column for a given source receiver offset the component of the wavevector inside the oil column normal to the interface is larger for the pppp reflection than for the ppps reflection due to the asymmetry in the raypath for the ppps reflection the ppps reflection is therefore useful for determining oil column heights larger than that discriminated by the pppp reflection especially when used at wider offsets a convenient classification of the avo response of hydrocarbon bearing sandstone reservoirs overlain by shale is the scheme of rutherford and williams class 1 sands have higher acoustic impedance for normal incidence than the overlying shale class 2 sands have nearly the same acoustic impedance as the shale and class 3 sands have lower acoustic impedance synthetic shot gathers calculated for these three classes as a function of oil column height show that a combination of the pppp and the ppps amplitudes can be plotted as a tuning trajectory which can be used to determine the oil column height this method is most sensitive for reservoirs that belong to avo classes 1 and 2 and therefore may be useful in avo analysis of class 1 and 2 reservoirs where the traditional avo indicators developed for class 3 reservoirs do not work very well these results demonstrate the usefulness of shear waves recorded in the marine environment at wide offsets

characterization of a steamed oil reservoir using cross well seismology cross well seismic tomography is developing into an important tool for reservoir management and within the last few years there have been some notable advances in out understanding its imaging capability tomography has evolved from its roots in medicine to encompass a broad range of geophysical applications which include earthquake hypocenter location mining and nuclear waste disposal in addition to oil field operations the fundamental technical requirements for the latter have been demonstrated high frequency seismic waves capable of traveling long interwell distances can be generated without damaging the borehole and mathematical inversion techniques can give reliable images as long as problems associated with nonuniform and incomplete sampling are handled correctly

seismic attenuation due to wave induced flow analytical expressions for three p wave attenuation mechanisms in sedimentary rocks are given a unified theoretical framework two of the models concern wave induced flow due to heterogeneity in the elastic moduli at mesoscopic scales scales greater than grain sizes but smaller than wavelengths in the first model the heterogeneity is due to lithological variations e g mixtures of sands and clays with a single fluid saturating all the pores in the second model a single uniform lithology is saturated in mesoscopic patches by two immiscible fluids e g air and water in the third model the heterogeneity is at microscopic grain scales broken grain contacts and or micro cracks in the grains and the associated fluid response corresponds to squirt flow the model of squirt flow derived here reduces to proper limits as any of the fluid bulk modulus crack porosity and or frequency is reduced to zero it is shown that squirt flow is incapable of explaining the measured level of loss 10 sup 2 q sup 1 10 sup 1 within the seismic band of frequencies 1 to 10 sup 4 hz however either of the two mesoscopic scale models easily produce enough attenuation to explain the field data

the effect of clay distribution on the elastic properties of sandstones the shape and location of clay within sandstones have a large impact on the p wave and s wave velocities of the rock they also have a large effect on reservoir properties and the interpretation of those properties from seismic data and well logs numerical models of different distributions of clay structural laminar and dispersed clay can lead to an understanding of these effects clay which is located between quartz grains structural clay will reduce the p wave and s wave velocities of the rock if the clay particles become aligned or form layers the velocities perpendicular to the alignment will be reduced further s wave velocities decrease more rapidly than p wave velocities with increasing clay content and therefore poisson s ratios will increase as the velocities decrease these effects are more pronounced for compacted sandstones small amounts of clay that are located in the pore space will have little effect on the p wave velocity due to the competing influence of the density effect and pore fluid stiffening the s wave velocity will decrease due to the density effect and thus the poisson s ratio will increase when there is sufficient clay to bridge the gaps between the quartz grains p wave and s wave velocities rise rapidly and the poisson s ratios decrease these effects are more pronounced for under compacted sandstones these general results are only slightly modified when the intrinsic anisotropy of the clay material is taken into account numerical models indicate that there is a strong nearly linear relationship between p wave and s wave velocity which is almost independent of clay distribution s wave velocities can be predicted reasonably accurately from p wave velocities based on empirical relationships however this does not provide any connection between the elastic and petrophysical properties of the rocks numerical modelling offers this connection but requires the inclusion of clay distribution and anisotropy to provide a model that is consistent with both the elastic and petrophysical properties if clay distribution is ignored predicting porosities from p wave or s wave data for example can result in large errors estimation of the clay distribution from p wave and s wave velocities requires good estimates of the porosity and clay volume and verification from petrographic analyses of core or cuttings for a real data example numerical models of the elastic properties suggest the predominance of dispersed clay in a fluvial sand from matching p wave and s wave velocity well log data using log based estimates of the clay volume and porosity this is consistent with an interpretation of other log data

paleomagnetic data bearing on the age of high terrace deposits durance sequence in alpine valleys of southeastern france abstract in the durance valley high terrace levels are stepped below the young villafranchian surface and separated from the middle and low terraces by a step 80 m high a paleomagnetic study was made of a key bed of freshwater silt on the high terrace that contains archaic faunal and floral elements the section contains two sedimentary cycles with opposite polarities which are therefore placed on either side of the brunhes matuyama boundary by comparison with the northern european climato stratigraphy this sequence is correlated with the base of the cromerian complex taking into account the morphological relations to the upland morainic system this youngest high terrace is assigned a gunz age

model based shear wave velocity estimation versus empirical regressions modelling of avo signatures for reservoir characterization requires v s estimation from other available logs when shear wave data are not available we tested various models for predicting v s from p wave velocity porosity and shale volume measured in well logs effective medium models which characterize the pore space in terms of ellipsoidal inclusions were compared with statistical v p v s regressions the inclusion models were calibrated by non linear minimization of the difference between model predicted velocities and actual measured velocities the quality of the v s prediction was quantified in terms of the rms error by comparison with shear wave data in wells where both v p and v s were measured the linear regressions were found to be more robust and the rms error in the prediction was comparable to effective medium model based predictions

correlation of selected late cenozoic european mammal faunas with the magnetic polarity time scale abstract three key late cenozoic european mammal faunas e g valensole faunas of southern france triversa or villafranca d asti faunas of northern italy and the hajnacka fauna of southern slovakia are correlated with the magnetic polarity time scale the valensole sequence with mn 13 17 faunas is from sediments correlated with the upper gilbert gauss and lower to middle matuyama magnetic chrons the triversa sequence includes the arondelli fauna mn 16a and the fornace rdb fauna considered herein as mn 15 the fornace rdb fauna is from sediments correlated with the late gilbert magnetic chron the hajnacka sequence with mn 16a fauna is from sediments correlated with the gauss magnetic chron taken separately these mammal sequences would be very difficult to place relative to the magnetic polarity time scale they can be placed now with confidence only because of the abundant paleomagnetic and faunal data from the cabriel jucar and teruel basins of spain in that context there remain some problems of chronologic resolution but collectively the data illustrate a clear pattern that permits correlation of these european late cenozoic mammal faunas with the magnetic polarity time scale

mineral magnetic properties of chinese paddy soils and its pedogenic implications abstract magnetic susceptibility ms of paddy soils hydragric anthrosols and their corresponding dryland soils formed on the same parent materials in zhejiang province of eastern china were measured in order to understand better magnetic profile discrimination and pedogenic magnetic minerals in paddy soils magnetic measurements showed that the ms values of paddy soils were much lower than those of dryland soils formed on the same parent materials the higher the ms value of the original dryland soil is the more remarkable the decrease in the ms value is when the soil is put under rice cultivation gleyed paddy soils exhibited very low ms value

a poroelastic model for the tidal modulation of seafloor hydrothermal systems 1 time series measurements of the temperature and exit velocity of hydrothermal effluent suggest that seafloor hydrothermal systems are modulated by tidal processes here we apply the theory of poroelasticity to predict the magnitude and phase of tidally induced changes in the temperature and flow rate of hydrothermal effluent at the seafloor we construct a model in which the steady state upwelling of buoyant fluid in the crust is modulated by tidal loading of a one dimensional seafloor by the overlying water column the nature of the solution is controlled by the relative magnitudes of three length scales these are 1 the depth h of the heat source below the seafloor 2 the skin depth d over which pore pressure signals can diffuse during one tidal cycle and 3 the advective length scale a over which the upwelling flow advects thermal signals during one tidal cycle we consider the likely magnitude of the parameters in a real system as well as the limitations of a one dimensional representation of that system we then discuss how observational data on the magnitude and phase lag of temperature and flow rate could be used to constrain the subseafloor parameters that govern hydrothermal circulation within the seafloor

zero offset vertical seismic profiling survey and estimation of gas hydrate concentration from borehole data from the ulleung basin korea abstract gas hydrate has been receiving much attention in recent years as the demand for energy has increased before an economic evaluation of the feasibility of producing gas hydrate can be made an assessment of reserves must be completed to evaluate a reserve of gas hydrate we should accurately know the depth interval of the gas hydrate bearing region and the concentration of gas hydrate in it because vertical seismic profiling vsp data can provide higher resolution than surface seismic data we can improve the reliability of the characterization of the gas hydrate bearing region using vsp data along with surface seismic data in this study we obtained interval velocities and corridor stacked sections from zero offset vsp data acquired at the ubgh2 6 and ubgh2 10 wells in gas hydrate bearing zones of the ulleung basin east sea of korea the interval velocities obtained from the zero offset vsp data agreed well overall with sonic logs and showed a significant decrease at the depth beneath the gas hydrate stability zone most reflection events in the corridor stacked section matched well those in the surface migrated seismic section additionally we computed the concentration of the gas hydrate from the log data using a three phase biot type equation and the result agreed well with the concentration calculated from the resistivity log using archie s equation

influence of the intertropical convergence zone on the east asian monsoon a palaeoclimate record spanning the past 16 000 years with nearly annual time resolution has been obtained from lake huguang maar in china the magnetic properties and titanium content of the lake sediments are thought to reflect changes in the east asian winter monsoon strength thus providing a complement to palaeoclimate archives that record the strength of the rain bearing summer monsoon the records point to an anti correlation between winter and summer monsoon strength best explained by migration of the intertropical convergence zone itcz a belt of low pressure air at the equator interestingly the decline of china s tang dynasty and the mayan civilization broadly coincide with drought periods recorded in lake huguang maar and cariaco basin sediments respectively did major shifts in the position of the itcz catalyse simultaneous events in civilizations on opposite sides of the pacific ocean the asian australian monsoon is an important component of the earth s climate system that influences the societal and economic activity of roughly half the world s population the past strength of the rain bearing east asian summer monsoon can be reconstructed with archives such as cave deposits1 2 3 but the winter monsoon has no such signature in the hydrological cycle and has thus proved difficult to reconstruct here we present high resolution records of the magnetic properties and the titanium content of the sediments of lake huguang maar in coastal southeast china over the past 16 000 years which we use as proxies for the strength of the winter monsoon winds we find evidence for stronger winter monsoon winds before the bolling allerod warming during the younger dryas episode and during the middle and late holocene when cave stalagmites suggest weaker summer monsoons1 2 3 we conclude that this anticorrelation is best explained by migrations in the intertropical convergence zone similar migrations of the intertropical convergence zone have been observed in central america for the period ad 700 to 900 refs 4 6 suggesting global climatic changes at that time from the coincidence in timing we suggest that these migrations in the tropical rain belt could have contributed to the declines of both the tang dynasty in china and the classic maya in central america

scale dependent gas hydrate saturation estimates in sand reservoirs in the ulleung basin east sea of korea abstract through the use of 2 d and 3 d seismic data several gas hydrate prospects were identified in the ulleung basin east sea of korea and thirteen drill sites were established and logging while drilling lwd data were acquired from each site in 2010 sites ubgh2 6 and ubgh2 10 were selected to test a series of high amplitude seismic reflections possibly from sand reservoirs lwd logs from the ubgh2 6 well indicate that there are three significant sand reservoirs with varying thickness two upper sand reservoirs are water saturated and the lower thinly bedded sand reservoir contains gas hydrate with an average saturation of 13 as estimated from the p wave velocity the well logs at the ubgh2 6 well clearly demonstrated the effect of scale dependency on gas hydrate saturation estimates gas hydrate saturations estimated from the high resolution lwd acquired ring resistivity vertical resolution of about 5 8 cm reaches about 90 with an average saturation of 28 whereas gas hydrate saturations estimated from the low resolution a40l resistivity vertical resolution of about 120 cm reaches about 25 with an average saturation of 11 however in the ubgh2 10 well gas hydrate occupies a 5 m thick sand reservoir near 135 mbsf with a maximum saturation of about 60 in the ubgh2 10 well the average and a maximum saturation estimated from various well logging tools are comparable because the bed thickness is larger than the vertical resolution of the various logging tools high resolution wireline log data further document the role of scale dependency on gas hydrate calculations

modulus decomposition of compressional and shear velocities in sand bodies the advent of borehole shear slowness measurements in sonically slow formations has lead to breakthroughs in the subsurface profiling of geological bodies in sand bodies compressional and shear velocities depend predictably on porosity mineralogy grain contacts and fluid saturation an interpretation is best performed by decomposing the velocities into moduli that are intrinsic measures of the rock frame and pore fluid compressibilities careful experiments on pure materials i e pure quartz sandstones demonstrate two simplifying constitutive relationships first the bulk and shear frame moduli are simple functions of the porosity a comparison of the measured shear frame modulus to the prediction for the pure material distinguishes sand from shale second the ratio of the bulk and shear frame moduli is a constant 0 9 independent of the porosity the measured velocities are directly inverted to yield the bulk modulus of the pore fluid the fluid saturation effects are so dramatic at high porosit

seismic evidence of free gas migration through the gas hydrate stability zone ghsz and active methane seep in krishna godavari offshore basin abstract seismic amplitude versus offset avo analysis was performed on the multichannel seismic data in the vicinity of site nghp 01 10 where thick fracture filled gas hydrate is reported to understand the fluid migration in fracture dominated gas hydrate system amplitude preserved common image gathers cig are used in extracting the seismic amplitude with offset for seafloor bottom simulating reflectors bsr and different horizons within the gas hydrate stability zone ghsz the extracted amplitudes are fitted with three term shuey s avo approximation to estimate the intercept and gradient values based on the intercept and gradient values two distinct classes of seismic avo responses are observed class iv avo decreasing negative amplitudes with increasing offsets and class iii avo increasing negative amplitudes with increasing offsets for the bsr and other horizons above the bsr the large negative intercept and gradient values which indicate class iii avo ranges from 0 2 to 1 8 for the inline seismic profile and from 0 01 to 0 3 for the crossline profile the presence of class iii avo anomaly within the gas hydrate stability zone ghsz provides evidence of existence of free gas which may be migrating along the fault system some of the active faults are exposed on the surface and show gas venting signatures such as gas flares and pockmarks in the multibeam data

anthropogenic hillslope terraces and swidden agriculture in jiuzhaigou national park northern sichuan china abstract small irregular terraces on hillslopes or terracettes are common landscape features throughout west central china despite their prevalence there is limited understanding of the nature of these topographic features the processes that form them and the role humans played in their formation we used an interdisciplinary approach to investigate the geology ecology and cultural history of terracette development within jiuzhaigou national park sichuan province china terracettes occur on south facing 20 slopes at 2500 m elevation which appears to coincide with places people historically preferred to build villages ethnographic interviews suggest that traditional swidden agricultural cycles removed tree roots causing the loess sediments to lose cohesion slump and the terrace risers to retreat uphill over time this evidence is supported by landslide debris at terracette faces archaeological analysis of terracette sites reveal remains of rammed spread soil structures bones stone tools and ceramics dating from at least 2200 years before present within a distinct paleosol layer radiocarbon and optically stimulated luminescence dating of terracette sediments ranged in age from between 1500 and 2000 14 c yr bp and between 16 and 0 30 ka respectively these multiple lines of evidence indicate a long history of human habitation within jiuzhaigou national park and taken together suggest strong links between terracette formation and human landuse interactions

reservoir management using 3 d seismic data the geologic detail needed to properly develop most hydrocarbon reservoirs substantially exceeds the detail required to find them this obvious but compelling precept has fueled the steadily increasing application of 3 d seismic analyses to reservoir management a measure of the increase is that 3 d surveys now account for half of the seismic activity in the offshore gulf of mexico and north sea and the percentage has been rising steadily year by year since commercial 3 d surveys were first shot in these areas in 1975 figure 1 three dimensional seismic surveying in other offshore areas and on land likewise is growing rapidly the fall 1988 seg distinguished lecture addressed the general subject of managing reservoirs using 3 d seismic data and this article is a condensed version of that lecture there are three parts to the article a definition of reservoir management a discussion of the various kinds of 3 d seismic analyses that can impact the development and production of a field and a synopsis

yet another vs equation the classical raymer hunt gardner functional form vp 1 2 vps vpf where vps and vpf denote the p wave velocity in the solid and pore fluid phases respectively and is the total porosity can also be used to relate the s wave velocity in dry rock to porosity and mineralogy as vsdry 1 2 vss where vss is the s wave velocity in the solid phase assuming that the shear modulus of rock does not depend on the pore fluid vs in wet rock is vswet vsdry bdry bwet where bdry and bwet denote the bulk density of the dry and wet rock respectively this new functional form for vs prediction reiterates nur s critical porosity concept the vp vs ratio in dry rock equals that in the solid phase it accurately predicts vs in consolidated clastic and carbonate rock two motivations for using it are 1 it is simple and 2 it predicts vs not from vp but directly from and mineralogy

a physical model for shear wave velocity prediction the clay sand mixture model of xu and white is shown to simulate observed relationships between s wave velocity or transit time porosity and clay content in general neither s wave velocity nor s wave transit time is a linear function of porosity and clay content for practical purposes clay content is approximated by shale volume in well log applications in principle the model can predict s wave velocity from lithology and any pair of p wave velocity porosity and shale volume although the predictions should be the same if all measurements are error free comparison of predictions with laboratory and logging measurements show that predictions using p wave velocity are the most reliable the robust relationship between s and p wave velocities is due to the fact that both are similarly affected by porosity clay content and lithology moreover errors in the measured p wave velocity are normally smaller than those in porosity and shale volume both of which are subject to errors introduced by imperfect models and imperfect parameters when estimated from logs because the model evaluates the bulk and shear moduli of the dry rock frame by a combination of kuster and toksoz theory and differential effective medium theory using pore aspect ratios to characterize the compliances of the sand and clay components the relationship between p and s wave velocities is explicit and consistent consequently the model sidesteps problems and assumptions that arise from the lack of knowledge of these moduli when applying gassmann s theory to this relationship making it a very flexible tool for investigating how the v p v s relationship is affected by lithology porosity clay content and water saturation numerical results from the model are confirmed by laboratory and logging data and demonstrate for example how the presence of gas has a more pronounced effect on p wave velocity in shaly sands than in less compliant cleaner sandstones

characteristics and interpretation of fracture filled gas hydrate an example from the ulleung basin east sea of korea abstract through the use of 2 d and 3 d seismic data a total of thirteen sites were selected and drilled in the east sea of korea in 2010 a suite of logging while drilling lwd logs was acquired at each site lwd logs from the ubgh2 3a well indicate significant gas hydrate in clay bearing sediments including several zones with massive gas hydrate with a bulk density less than 1 0 g m 3 for depths between 5 and 103 m below the sea floor the ubgh2 3a well was drilled on a seismically identified chimney structure with a mound feature at the sea floor average gas hydrate saturations estimated from the isotropic analysis of ring resistivity and p wave velocity logs are 80 13 and 47 16 respectively whereas they are 46 17 and 45 16 respectively from the anisotropic analysis modeling indicates that the upper part of chimney between 5 and 45 m below sea floor mbsf is characterized by gas hydrate filling near horizontal fractures 7 dip and the lower part of chimney between 45 and 103 mbsf is characterized by gas hydrate filling high angle fractures on the basis of ring resistivity and p wave velocity the anisotropic analysis using p40h resistivity phase shift resistivity at 32 mhz with 40 inch spacing and the p wave velocity yields a gas hydrate saturation of 46 15 and 46 15 respectively similar to those estimated using ring resistivity and p wave velocity but with quite different fracture dip angles differences in vertical resolution depth of investigation and a finite fracture dimension relative to the tool separation appear to contribute to this discrepancy forward modeling of anisotropic resistivity and velocity are essential to identify gas hydrate in fractures and to estimate accurate gas hydrate amounts

effective avo crossplot modeling a tutorial the ability to crossplot attributes from a 3 d seismic volume permits a geophysicist to identify and high grade subsets of the 3 d volume that warrant detailed inspection in the case of amplitude variation with offset avo crossplotting the seismic attributes are derived from cdp data crossplotting has become a fundamental process in avo analysis just as it is in petrophysical analysis comprehending the intricacies and selection of attributes is essential for successful avo analysis and improved seismic interpretation avo crossplotting of modeled seismic data derived from well logs with the biot gassmann equations provides a basis for understanding fluid substitution effects on avo attribute interactions when crossplotting with these model based understandings improved multi attribute interpretation processes can commence with avo crossplotting of seismic volumes

effects of porosity and clay content on wave velocities in sandstones the ultrasonic compressional vp and shear vs velocities and first arrival peak amplitude ap were measured as functions of differential pressure to 50 mpa and to a state of saturation on 75 different sandstone samples with porosities ranging from 2 to 30 percent and volume clay content c ranging from 0 to 50 percent respectively both vp and vs were found to correlate linearly with porosity and clay content in shaly sandstones at confining pressure of 40 mpa and pore pressure of 1 0 mpa the best least squares fits to the velocity data are vp km s 5 59 6 93 2 18c and vs km s 3 52 4 91 1 89c deviations from these equations are less than 3 percent and 5 percent for vp and vs respectively the velocities of clean sandstones are significantly higher than those predicted by the above linear fits about 7 percent for vp and 11 percent for vs which indicates that a very small amount of clay 1 or a few percent of volume fraction significantly reduces the elastic moduli of sandstones for shaly

shear wave velocity estimation in porous rocks theoretical formulation preliminary verification and applications shear wave velocity logs are useful for various seismic interpretation applications including bright spot analyses amplitude versus offset analyses and multicomponent seismic interpretations measured shear wave velocity logs are however often unavailable we developed a general method to predict shear wave velocity in porous rocks if reliable compressional wave velocity lithology porosity and water saturation data are available the precision and accuracy of shear wave velocity prediction are 9 and 3 respectively the success of our method depends on 1 robust relationships between compressional and shear wave velocities for water saturated pure porous lithologies 2 nearly linear mixing laws for solid rock constituents 3 first order applicability of the biot gassmann theory to real rocks we verified these concepts with laboratory measurements and full waveform sonic logs shear wave velocities estimated by our method can improve formation evaluation our method has been successfully tested with data from several locations

downhole log evidence for the coexistence of structure ii gas hydrate and free gas below the bottom simulating reflector in the south china sea abstract stratigraphic layered pore filling gas hydrates are identified above the bottom simulating reflector bsr using the well log and core data acquired at sites w11 and w17 during the third gas hydrate drilling expedition conducted by china s geological survey guangzhou marine geological survey gmgs3 in the south china sea a seismic profile near site w17 reveal the presence of two bsrs i e double bsr which we show to relate to zones of structure i gas hydrate i bsr and structure ii gas hydrate ii bsr well log data from site w17 between the i bsr projected depth of 250 mbsf and ii bsr projected depth of 330 mbsf showed anomalous responses for gas hydrate bearing sediments with high resistivity high s wave velocity and alternating high and low p wave velocities pressure core data support the interpretation that structure ii gas hydrate occurs at a depth of 263 mbsf at site w17 the cross plot between log derived neutron and density porosities reveals a free gas bearing layer at a depth of 258 270 mbsf suggesting gas hydrate coexists with free gas between the i bsr and the ii bsr synthetic seismograms generated from the p wave velocity and density logs further support the presence of free gas in this section based on the coexistence of hydrate free gas and water the simplified three phase equation stpe was modified to simultaneously estimate free gas and hydrate saturations beneath the i bsr from p wave and s wave velocity logs assuming uniform or patchy distributions of free gas the estimated free gas and hydrate saturations together with gas compositions from pressure core samples collectively indicate that structure ii gas hydrate and free gas are interbedded and coexist below the i bsr at site w17 our study of the coexistence of gas hydrate and free gas between the double bsr at site w17 provides new insights into gas hydrate systems in nature that contain more complex gas chemistries

age determination of petra s engineered landscape optically stimulated luminescence osl and radiocarbon ages of runoff terrace systems in the eastern highlands of jordan abstract the unfavourable mountainous environment of the petra region in southern jordan was modified by ancient engineers to supply the nabataean roman city of petra with food and water the area was reclaimed by installing extended runoff terrace systems and hydraulic structures the agricultural terrace systems have so far been dated based on surface pottery and the chronology of the systems is under debate in this study optically stimulated luminescence osl and radiocarbon dating techniques were successfully applied to date these terrace systems samples were taken from the fills of agricultural terraces and underneath their walls to determine the chronology of the construction use and abandonment of the agricultural terraces the results suggest that runoff farming in the petra region started around the beginning of the common era and construction use and maintenance lasted at least until 800 ad

viscosite magnetique potentielle aimantation detritique et viscosite reelle d un sediment lacustre quaternaire the magnetic viscosity of sediments as indicated by thermal demagnetization of natural remanent magnetization nrm depends on two main parameters i the specific magnetic viscosity of the material and ii the efficiency of the primary magnetization process in an attempt to determine the relative importance of these two variables we studied a lower pleistocene lacustrine sequence bearing a primary reversed detrital remanent magnetization drm using natural samples and small cores made of crushed sediment our study is based on the thermodynamic theory of rock magnetism developed by l neel who established an equivalence between time and temperature that is between viscous remanent vrm and thermoremanent trm magnetization processes the determination of the blocking temperature spectra from 20 to 152 c allows us to calculate the maximum theoretical vrm acquired in situ at each horizon while the detailed thermal study of the nrm permits an appraisal of the drm quality this leads us to defi

seismic attenuation effects of pore fluids and frictional sliding seismic wave attenuation in rocks was studied experimentally with particular attention focused on frictional sliding and fluid flow mechanisms sandstone bars were resonated at frequencies from 500 to 9000 hz and the effects of confining pressure pore pressure degree of saturation strain amplitude and frequency were studied observed changes in attenuation and velocity with strain amplitude are interpreted as evidence for frictional sliding at grain contacts since this amplitude dependence disappears at strains and confining pressures typical of seismic wave propagation in the earth we infer that frictional sliding is not a significant source of seismic attenuation in situ partial water saturation significantly increases the attenuation of both compressional p and shear s waves relative to that in dry rock resulting in greater p wave than s wave attenuation complete saturation maximizes s wave attenuation but causes a reduction in p wave attenuation these effects can be interpreted in term

evidence of the theoretically predicted seismo magnetic conversion summary seismo electromagnetic phenomena in porous media arise from seismic wave induced fluid motion in the pore space which perturbs the equilibrium of the electric double layer this paper describes with details the original experimental apparatus built within the ultra shielded chamber of the low noise underground laboratory of rustrel france we measured seismo magnetic conversions in moist sand using two induction magnetometers and a pneumatic seismic source to generate the seismic wave propagation we ensured to avoid the magnetometer vibrations which could induce strong disturbances from induction origin interpretation of the data is improved by an analytical description of phase velocities for fast pf and slow ps longitudinal modes transverse mode s as well as the extensional mode due to the cylindrical geometry of the sample the purpose of this paper is to provide elements to measure correctly coseismic seismomagnetic fields and to specify their amplitude the seismic arrivals recorded in the sample showing a 1200 1300 m s 1 velocity have been associated to p and extensional waves the measured seismo magnetic arrivals show a velocity of about 800 m s 1 close to the calculated phase velocity of s waves therefore we show that the seismo magnetic field is associated to the transverse part of the propagation as theoretically predicted by pride 1994 but never measured up to now moreover the combined experimental and analytical approaches lead us to the conclusion that the measured seismo magnetic field is probably about 0 035 nt for a 1 m s 2 seismic source acceleration 0 1 g

seismic monitoring of fluid fronts an experimental study seismic signatures of time dependent reservoir processes necessary for the interpretation of 4 d seismic data are still insufficiently described this experiment was designed to monitor fluid front movements and saturation changes and to identify the related seismic signatures ultrasonic p and s wave transmission and reflection measurements were used to monitor the waterflooding of a porous sandstone the sandstone was flooded in steps by filling a tank in which the room dry cubic 50 cm side block of rock was placed waterflooding caused the velocity amplitude and frequency of the transmitted waves to diminish significantly however the changes were reversible by drying the maximum reduction of the velocities was 7 and 12 for p and s waves respectively the velocity and amplitude behavior can be explained by the biot gassmann s theory local fluid flow and grain surface effects the correct interpretation of seismic signatures of fluid processes in reservoirs thus involves a knowledge of rock

identification of gas hydrate dissociation from wireline log data in the shenhu area south china sea wireline logs were acquired in eight wells during china s first etas hydrate drilling expedition gmgs 1 in april june of 2007 well logs obtained from site sh3 indicated gas hydrate was present in the depth range of 195 206 in below seafloor with a maximum pore space gas hydrate saturation calculated from pore water freshening of about 26 assuming gas hydrate is uniformly distributed in the sediments resistivity calculations using archie s equation yielded hydrate saturation trends similar to those from chloride concentrations however the measured compressional p wave velocities decreased sharply at the depth between 194 and 199 mbsf dropping as low as 1 3 km s indicating the presence of free gas in the pore space possibly caused by the dissociation of gas hydrate during drilling because surface seismic data acquired prior to drilling were not influenced by the in situ gas hydrate dissociation surface seismic data could be used to identify the cause of the low p wave velocity observed in the well log to determine whether the low well log p wave velocity was caused by in situ free gas or by gas hydrate dissociation synthetic seismograms were generated using the measured well log p wave velocity along with velocities calculated assuming both gas hydrate and free gas in the pore space comparing the surface seismic data with various synthetic seismograms suggested that low p wave velocities were likely caused by the dissociation of in situ gas hydrate during drilling

estimation of rock physics properties from seismic attributes part 1 strategy and sensitivity analysis abstractthe quantitative estimation of rock physics properties is of great importance in any reservoir characterization we have studied the sensitivity of such poroelastic rock physics properties to various seismic viscoelastic attributes velocities quality factors and density because we considered a generalized dynamic poroelastic model our analysis was applicable to most kinds of rocks over a wide range of frequencies the viscoelastic attributes computed by poroelastic forward modeling were used as input to a semiglobal optimization inversion code to estimate poroelastic properties porosity solid frame moduli fluid phase properties and saturation the sensitivity studies that we used showed that it was best to consider an inversion system with enough input data to obtain accurate estimates however simultaneous inversion for the whole set of poroelastic parameters was problematic due to the large number of parameters and their trade off consequently we restricted the sensitivity tests to

a new approach to petroelastic modeling of carbonate rocks using an extended pore space stiffness method with application to a carbonate reservoir in central luconia sarawak malaysia

the genesis and paleoenvironmental records of longji agricultural terraces southern china a pilot study of human environment interaction ancient agricultural terraces distributed over the mountainous areas in southern china can be taken as new objects for studies on human environment interaction in the past however this type of study is rare this study selected longji terraces longsheng guilin one of the famous ancient agricultural terraces to explore its genesis and paleoenvironmental records three soil profiles including an original hillslope woodland soil profile and two terrace profiles were sampled for analyses of physical and geochemical proxies and radiocarbon dating cultivated soil in the terrace profile shows aggradation making the cultivated horizon a bottom up chronological sedimentary sequence two basal samples in the cultivated horizons of two parallel terrace profiles show calibrated ages of 1361 1406 ad and 1335 1384 ad respectively indicating that the genesis of longji terraces occurred during the late yuan dynasty in chinese history strong survival pressure and desire for new living space in the turbulent social situation of the late yuan dynasty might have been the direct factor forcing some people of national minorities to migrate to the longji mountain area where they effectively utilized the hillslope land and water resources for cultivation by building agricultural terraces and constructing a gravity irrigation network multi proxy analyses of the cultivated horizon of one selected terrace profile reveal that human farming activity has experienced a staged strengthening process including four stages since the late 14th century moreover some changes of the intensity of human farming activity found within each stage might reflect the impact of climatic fluctuations on human farming activity during the little ice age and the later warming period

relationships between compressional wave and shear wave velocities in clastic silicate rocks new velocity data in addition to literature data derived from sonic log seismic and laboratory measurements are analyzed for clastic silicate rocks these data demonstrate simple systematic relationships between compressional and shear wave velocities for water saturated clastic silicate rocks shear wave veloci ty is approximately linearly related to compressional wave velocity and the compressional to shear velocity ratio decreases with increasing compressional velocity laboratory data for dry sandstones indicate a nearly constant compressional to shear velocity ratio with rigidity approximately equal to bulk modulus ideal models for regular packings of spheres and cracked solids exhibit behavior similar to the observed water saturated and dry trends for dry rigidity equal to dry bulk modulus gassmann s equations predict velocities in close agreement with data from the water saturated

paleomagnetic correlation of the mediterranean upper neogene biochronology and villafranchian vertebrate sites of the massif central france magnetostratigraphic studies of pliocene pleistocene continental deposits on the plateau de perrier and in the le puy en velay basin massif central as well as in the riez valensole basin alpes de haute provence mediterranean south provide decisive reference dating of several well known fossil bearing villafranchian sites these are well distributed in the tephrostratigraphy on both sides of the gauss matuyama geomagnetic boundary the roca neyra fauna falls within the matuyama chron while older sediments at les etouaires are situated just above the normal reversed transition the vialette section lies below this reversal within the gauss chron in the riez valensole basin the discovery of the gauss matuyama boundary toward the top of the puimoisson section leads to an excellent correlation with the massif central localities rodents of the seynes zone occur below the reversal while equus appears just above this calibration improves the mediterranean upper neogene biochronology and allows a direct correlation with villafranchian vertebrate sites of the massif central

the character and amplitude of discontinuous bottom simulating reflections in marine seismic data abstract bottom simulating reflections bsrs identified in seismic data are well documented and are commonly interpreted to indicate the presence of gas hydrates along continental margins as well as to estimate regional volumes of gas hydrate a bsr is defined as a reflection that sub parallels the seafloor but is opposite in polarity and cross cuts dipping sedimentary strata bsrs form as a result of a strong negative acoustic impedance contrast bsrs however are a diverse seismic phenomena that manifest in strikingly contrasting ways in different geological settings and in different seismic data types we investigate the characteristics of bsrs using conventional and high resolution 2d and 3d seismic data sets in three locations the terrebonne and orca basins in the gulf of mexico and blake ridge on the us atlantic margin the acquisition geometry and frequency content of the seismic data significantly impact the resultant character of bsrs as observed with depth and amplitude maps of the bsrs furthermore our amplitude maps reinforce the concept that the bsr represents a zone over which the transition from hydrate to free gas occurs as opposed to the conventional model of the bsr occurring at a single interface our results show that a bsr can be mapped in three dimensions but it is not spatially continuous at least not at the basin scale rather a bsr manifests itself as a discontinuous or patchy reflection and only at local scales is it continuous we suggest the discontinuous nature of bsrs is the result of variable saturation and distribution of free gas and hydrate acquisition geometry and frequency content of the recorded seismic data the commonly accepted definition of a bsr should be broadened with careful consideration of these factors to represent the uppermost extent of enhanced amplitude at the shallowest occurrence of free gas trapped by overlying hydrate bearing sediments

modern technology in an old area bay marchand revisited bay marchand a giant oil field in the gulf of mexico is undergoing renewed drilling activity as the result of a recently acquired 3 d seismic survey this classic mature field had seemingly entered its last stages of production in the mid 1980s however an integrated team effort by geologists geophysicists and engineers revitalized the area by innovative use of the new 3 d data

the last appearance datum of hipparion in western europe magnetostratigraphy along the pliocene pleistocene boundary in the villarroya basin northern spain the villarroya basin in northern spain contains one of the classic villafranchian localities of europe and allows about 100 m of sediments to be explored by magnetostratigraphic techniques besides the occurrence of some other macro and micro mammifera fossils becomes villarroya in one of the most important villafranchian localities of southern europe therefore we have sampled two sections one contains the classic locality to shed some light on its chronostratigraphy detailed stepwise demagnetization has revealed primary and reliable directions carried by magnetite and hematite in more than 250 samples and allows us building a robust local polarity scale the correlation with the geologic time scale locates the local sequence along the gauss and matuyama chrons a reunion chron c2r 1n equivalent age 2 128 2 148 ma is here proposed for the villarroya fossil locality since it is clearly located above the gauss matuyama boundary very well defined in the magnetostratigraphic section and displays normal polarity this correlation implies a new age assignment for the classic paleontological fossil locality of villarroya and a last appearance datum of hipparion sensu lato in western europe significantly younger than previously established taking into account this new dating the hipparion and equus horses could have coexisted in europe up to the complete extinction of hipparion in early matuyama times

modulus ratio of dry rock based on differential effective medium theory there are two points of view with regard to the relationship between the dry framework modulus ratio of bulk to shear modulus and porosity one view assumed by most empirical models such as the critical porosity model is that the dry framework modulus ratio is independent of porosity however this assumption is sometimes in disagreement with experiments another view supported by the differential effective medium dem theory is that the dry framework modulus ratio depends on porosity ordinary differential equations ode for bulk and shear moduli are coupled and there is no analytical formula for the modulus ratio of dry porous rock by assuming that the difference between the polarization factors of dry inclusions for bulk and shear modulus within the ode has an almost linear relationship with the effective modulus ratio an analytical solution for the dry rock modulus ratio is derived from the ode the above linear relationship characterized by intercept a and gradient b is confirmed by the numerical results calculated from the analytical formulas for the three specific pore shapes spherical pores needle shaped pores and penny shaped cracks then the validity of this assumption is tested by integrating the full dem equation numerically the analytical approximation gives a good estimate of the numerical results for the cases of the three given pore shapes the dry frame modulus ratio is monotonic with respect to porosity and its monotonicity depends on the mineral grain modulus ratio and the pore shape the analytical formula can be simplified further to some popular models such as the critical porosity model by setting constants a and b to specific values predictions of the analytical formula compare favorably with real and synthetic sandstone data and granite data measured in the laboratory

remanent magnetization of ash from the 18 may 1980 eruption of mount st helens ash from the may 1980 eruption of mount st helens deposited from air faithfully records the direction of the local geomagnetic field in eastern washington whereas ash settled from suspension in water in fluvial environments exhibits significant inclination and current rotation errors in magnetic direction similar to those reported in other subaqueously deposited sediments the current rotation errors are associated with partial alignment of the major axes of magnetic susceptibility in the direction of water currents subaerial deposition produces strong stable remanent magnetization in ash with or without postdepositional wetting by rain

explicit use of the biot coefficient in predicting shear wave velocity of water saturated sediments predicting the shear wave s wave velocity is important in seismic modelling amplitude analysis with offset and other exploration and engineering applications under the low frequency approximation the classical biot gassmann theory relates the biot coefficient to the bulk modulus of water saturated sediments if the biot coefficient under in situ conditions can be estimated the shear modulus or the s wave velocity can be calculated the biot coefficient derived from the compressional wave p wave velocity of water saturated sediments often differs from and is less than that estimated from the s wave velocity owing to the interactions between the pore fluid and the grain contacts by correcting the biot coefficients derived from p wave velocities of water saturated sediments measured at various differential pressures an accurate method of predicting s wave velocities is proposed numerical results indicate that the predicted s wave velocities for consolidated and unconsolidated sediments agree well with measured velocities

fundamentals of seismic rock physics during the past 50 years or so tremendous progress has been made in studying physical properties of rocks and minerals in relation to seismic exploration and earthquake seismology during this period many theories have been developed and many experiments have been carried out some of these theories and experimental results have played important roles in advancing earth sciences and exploration technologies this tutorial paper attempts to summarize some of these results

the connection between natural gas hydrate and bottom simulating reflectors bottom simulating reflectors bsrs on marine seismic data are commonly used to identify the presence of natural gas hydrate in marine sediments although the exact relationship between gas hydrate and bsrs is undefined to clarify this relationship we compile a data set of probable gas hydrate occurrence as appraised from well logs of 788 industry wells in the northern gulf of mexico we combine the well log data set with a data set of bsr distribution in the same area identified from 3 d seismic data we find that a bsr increases the chances of finding gas hydrate by 2 6 times as opposed to drilling outside a bsr and that the wells within a bsr also contain thicker and higher resistivity hydrate accumulations even so over half of the wells drilled through bsrs have no detectable gas hydrate accumulations and gas hydrate occurrences and bsrs do not coincide in most cases