permeability prediction from geologic models permeability is a critical parameter for the petroleum geologist by simulating the processes of compaction and cementation in a model porous medium we have gained a new understanding of how permeability is controlled in reservoir sandstones this understanding can be used predictively for simple sandstones the geometry of the model pore system is completely defined so permeability can be calculated directly using a network model for flow the calculation is based on first principles and is physically rigorous in contrast to many previous efforts to predict permeability there are no adjustable parameters in the calculations and no additional measurements or correlations e g capillary pressure data or pore system data from thin sections are required the model derived porosity permeability trend for a compacted or quartz cemented sandstone or a sandstone having a combination of these processes closely matches measurements on fontainebleau sandstone samples whose permeabilities span nearly five orders of magnitude the model also correctly predicts mercury injection measurements of pore throat size distribution for the fontainebleau sandstone pore scale geometric features of the model are found to be spatially correlated and this departure from randomness significantly affects macroscopic properties such as permeability the agreement between predictions and measurements suggests that spatial correlation is inherent in granular porous media and consequently uncorrelated or arbitrarily correlated models of transport in such media are unlikely to be physically representative we also discuss extending the model to predict properties of more complicated rocks

empirical relationships among seismic velocity effective pressure porosity and clay content in sandstone we use a multivariate analysis to investigate the influence of effective pressure pe porosity and clay content c on the compressional velocity vp and shear velocity vs of sandstones laboratory measurements on water saturated samples of 64 different sandstones provide a large data set that was analyzed statistically for each sample relationships between effective pressure and vp and vs have been determined all samples were well fit by relationships that have an exponential increase in velocity at low pe tapering to a linear increase with pe for pe greater than 0 2 kbar there are differences in the pressure dependences of velocity for different rocks particularly at very low pressures however the differences cannot be attributed to or c for the combined set of measurements from all samples the best fitting formulations are vp 5 77 6 94 1 73c 0 446 pe e 16 7pe and vs 3 70 4 94 1 57c 0 361 pe e 16 7pe while this is admittedly a very simplified parameterization it is remarkable how well

numerical study of airflow in the unsaturated zone induced by sea tides 1 the air pressures in coastal unsaturated zones fluctuate in response to tidal fluctuations two phase air water flow induced by sea tides in a coastal two layered subsurface system is investigated through numerical simulations the system consists of upper and lower layers with permeabilities of ku and kl respectively a water table exists in the lower layer the study demonstrates that airflow in the unsaturated zone is controlled by permeabilities of both layers and the fluctuation of water level the tide induced water level fluctuation may result in significant air pressure fluctuation in the unsaturated zone in an air confined system that is the permeability of the upper layer is about 2 orders of magnitude lower than that of the lower layer in such an air confined system horizontal airflow is dominant in the lower layer and vertical airflow is dominant in the upper layer the simulation results show that the hydraulic head rather than the sea tide is the direct driving force of the air pressure fluctuation above the water table the study on the relation between the air pressure amplitude and the landward distance demonstrates that the air pressure amplitudes increase to a maximum and then decrease with distance the amplitude of air pressure fluctuation decreases progressively with ku in the lower layer in the upper layer however the amplitude tends to increase with ku when ku is very low and decrease with ku when ku is relatively high overall the air flux fluctuations across the atmosphere soil interface attenuate landward gradually and this attenuation becomes faster as the permeability of the upper layer increases

atmospheric pumping a mechanism causing vertical transport of contaminated gases through fractured permeable media as the barometer falls gases are drawn upward out of the permeable earth into the atmosphere conversely a rising barometer pushes air downward in a homogeneous permeable medium these cyclical gas motions are piston like and nearly reversible so they contribute only modestly to the net transport of contaminant gases in a fractured permeable medium however the fractures will generally serve as breathing passages for all of the gas filled porosity greatly increasing the amplitude and nonuniformity of vertical motions the resulting transport process may be orders of magnitude more significant than molecular diffusion according to the theoretical analysis presented here analytical solutions are first derived for the sinusoidal pressure response of a medium containing identical vertical fractures equally spaced by slabs of permeable matrix material these solutions are then used to constrain the relationship between fracture aperture and fracture spacing based on field comparisons between surface and subsurface pressure variations the final phase of the analysis addresses the diffusive and advective transport of an inert trace gas which is carried by an oscillatory flow along a fracture having permeable walls a maximum rate of transport is predicted to occur for an intermediate fracture spacing which is typically a few meters

airflow induced by pumping tests in unconfined aquifer with a low permeability cap 1 most analytical and numerical models developed to analyze pumping test data focus on saturated flow below the water table traditionally the soil above the initial water table prior to pumping has been thought to have little influence on the test results and has usually been ignored it is hypothesized that if the unsaturated zone is capped by low permeability soil airflow in the unsaturated zone may be developed during pumping and may have impact on the drawdown in the aquifer a transient three dimensional and variably saturated flow model is employed to simulate the pumping induced air and groundwater flows in both the saturated zone and unsaturated zone with a low permeability layer the results demonstrate that negative pressure in the unsaturated zone can be generated by pumping the negative pressure begins to appear as the drawdown rate increases to a maximum approaches a peak before the drawdown rate becomes zero and then gradually disappears drawdown obtained from the capped aquifer is much greater because the water in the pores in the unsaturated zone is sucked by the negative pressure and the gravity drainage from the pores is hampered consequently the drawdown versus time curve does not conform to the traditional s shaped curve for an unconfined aquifer but is similar to that of a confined aquifer if the airflow caused by the low permeability cap is ignored the error in estimated drawdown could be over 80 for the specific parameters and aquifer configuration used in the study the possible errors in parameter estimation when airflow is ignored are explored overall the hydraulic conductivity of the aquifer can be overestimated and the specific yield of the aquifer underestimated if airflow is ignored the estimation error for specific yield tends to be greater than that in hydraulic conductivity

gas transport in a confined unsaturated zone during atmospheric pressure cycles gas transport induced by temporal pressure fluctuations in the atmosphere can be an important mechanism for transport of atmospheric oxygen within the unsaturated zone moreover the presence of oxygen in the unsaturated zone may be a factor controlling oxidation of sulphide minerals and other redox processes a field study was carried out in a glacial aquifer with a 10 12 m thick sandy unsaturated zone to explore gas exchange between the atmosphere and the unsaturated zone the exchange occurs through a geological window in a till layer which covers the sandy unsaturated zone observed pressure distribution and oxygen concentrations within the unsaturated zone were compared to numerical simulations with sutra a finite element and fluid density dependent groundwater flow model the simulations were carried out by modeling the gas pressure distribution within the unsaturated zone based on atmospheric pressure time series the spatial variation in permeability observed from borehole logging was implemented in the model the analysis demonstrated a good match between the field observations and the numerical simulations during an atmospheric pressure cycle atmospheric oxygen migrated more than 10 md 1 horizontally in the capped unsaturated zone the analysis shows that both the amplitude and the length of the period of pressure variations are important for the transport of oxygen and it shows that the combined effects determine the extent of a subsurface zone where atmospheric oxygen can reach

erosional response of south china to arc rifting and monsoonal strengthening a record from the south china sea abstract ocean drilling program sampling of the distal passive margin of south china at sites 1147 and 1148 has yielded clay rich hemipelagic sediments dating to 32 ma oligocene just prior to the onset of seafloor spreading in the south china sea the location of the drill sites offshore the pearl river suggests that this river or its predecessor may have been the source of the sediment in the basin which accounts for only 1 8 of the total neogene sediment in the asian marginal seas a mean erosion depth of 1 km over the current pearl river drainage basin is sufficient to account for the sediment volume on the margin two dimensional backstripping of across margin seismic profiles shows that sedimentation rates peaked during the middle miocene 11 16 ma and the pleistocene since 1 8 ma nd isotopic analysis of clays yielded nd values of 7 7 to 11 0 consistent with the south china block being the major source of sediment more positive nd values during and shortly after rifting compared to later sedimentation reflect preferential erosion at that time of more juvenile continental arc rocks exposed along the margin as the drainage basin developed and erosion shifted from within the rift to the continental interior nd values became more negative a rapid change in the clay mineralogy from smectite dominated to illite dominated at 15 5 ma synchronous with middle miocene rapid sedimentation mostly reflects a change to a wetter more erosive climate evidence that the elevation of the tibetan plateau and erosion in the western himalaya both peaked close to this time supports the suggestion that the asian monsoon became much more intense at that time much earlier than the 8 5 ma age commonly accepted

air and water flows induced by pumping tests in unconfined aquifers with low permeability zones air flows from the atmosphere into an unconfined aquifer when the water table falls during pumping tests pumping test results in unconfined aquifers may be significantly affected by low permeability zones lpzs near the initial water table position because they restrict the downward movement of air a transient three dimensional air water two phase flow model is employed to investigate numerically the effects of local heterogeneity on pumping test results in unconfined aquifers two cases of local heterogeneities are considered herein a lpz around the pumping well and on one side of the pumping well results show that the drawdown with the lpz is significantly greater than that of the homogeneous aquifer the differences in drawdown are the most significant at intermediate times and gradually diminish at later times the lpz significantly reduces air flow from the atmosphere to the aquifer the pore air velocity in the lpz is very low the air pressure at the observation point under the lpz when air begins to enter is significantly lower than the air pressure of the homogeneous aquifer at the same point after that the air pressure increases quickly and then increases slowly the time for the air pressure to reach the atmospheric pressure is significantly longer copyright 2013 john wiley sons ltd

air and water flows in a vertical sand column 1 the unsteady state drainage of water from a vertical sand column with and without a finer layer on the top was studied theoretically and experimentally to investigate the airflow generated by the finer layer the sand column saturated at its lower portion and initially in the condition of hydrostatic equilibrium is drained at its bottom at constant head the results show that significant vacuum can be generated in the vadose zone of the column with a finer layer on the top the vacuum increases quickly in the earlier stage of the drainage reaches a maximum and gradually becomes zero because of the effect of the vacuum in the vadose zone water is held in and the cumulative outflow from the column with the finer layer is much smaller than without the layer during most of the drainage process ordinary differential equations ode which require only saturated hydraulic properties of the porous media are derived to predict the location of the surface of saturation and vacuum in the vadose zone in air water two phase flow the solutions of ode match very satisfactorily with the experimental data and give better results than tough2

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

pore geometric modeling for petrophysical interpretation of downhole formation evaluation data 1 an accurate description of water or oil bearing reservoirs strongly depends on a robust determination of their petrophysical parameters e g porosity permeability and fluid distribution downhole logging measurements are the primary means to formation evaluation however they do not directly provide the petrophysical properties of interest to interpret well logging data a range of empirical models are usually employed these empirical relationships however lack scientific basis and usually represent generalizations of the observed trends since macroscopic rock properties vary depending on their microstructure we suggest using a pore scale approach to establish links between various petrophysical properties of sedimentary rocks we outline a method for computing formation permeability using the proposed rock models the method utilizes nmr nuclear magnetic resonance logging data for the information about porosity and grain size we also present an approach for prediction of acoustic velocities of model rocks the proposed methodology is applied to the field data and the corresponding interpretation results are included in this paper

tidal dynamics of the water table in beaches tidal motions of the water table height inside a sloping beach are investigated via field measurements and theoretical considerations only the movements forced by the tide are considered so a beach with negligible wave activity was chosen for the field measurements the data show that even in the absence of precipitation the time averaged inland water table stands considerably above the mean sea level also the water table at a fixed point inside the beach is far from sinusoidal even though its variation is forced by an essentially sinusoidal tide this latter effect is due to the boundary condition along the sloping beach face which acts as a highly nonlinear filter the observed behavior of the water table is explained in terms of perturbation extensions to the classical deep aquifer solution one extension deals with the nonlinearity in the interior the other with the boundary condition at the sloping beach face

astronomical forcing of the east asian monsoon mirrored by the composition of pliocene south china sea sediments abstract high precision major and minor element records of pliocene core intervals from ocean drilling program odp site 1145 located in the northern part of the south china sea were generated at a 5 cm approximately 2 kyr resolution from the geochemical data distinct proxies those indicating changes in detrital matter provenance as well as those indicating productivity variations could be derived both the terrigenous input to the northern south china sea k si and variations in productivity ba al show a response to insolation forced monsoon variability while the fluvial input k si responds to changes in the summer monsoon productivity increases as documented by ba enrichments seem to reflect variations in winter monsoon intensity a stronger winter monsoon may have increased nutrient availability via dust input and or upwelling phenomena according to our geochemical proxy records summer and winter monsoons are approximately 180 out of phase this may imply forcing of the winter monsoon through southern hemisphere summer insolation maxima the k si ratio shows a linear correlation with the la90 1 0 5 northern hemisphere summer insolation record and has therefore been used to create an astronomical timescale for the investigated sediments of odp site 1145 carbonate contents and corresponding color reflectances display 41 and 100 kyr cycles we propose that these do not reflect glacial cycles but are rather the result of an interference signal of dilution by fluvial and eolian terrigenous material and carbonate production burial flux a sudden onset of gradual changes in sediment composition occurs after 3 0 ma this is also documented by an increase in sedimentation rates and may be caused by enhanced fluvial input from the pearl river due to intensified weathering rates on the asian continent this may be linked to tectonic processes around the himalayan tibetan complex and its consequences regarding land sea heating and atmospheric circulation

pliocene pleistocene stable isotope and paleoceanographic changes in the northern south china sea abstract based on the stable isotopic analysis of planktonic and benthic foraminifers from ocean drilling program core 1148 of the northern south china sea scs pliocene pleistocene isotope stratigraphy and events have been reconstructed the benthic foraminiferal 18o record shows that the pacific intermediate water had a greater influence upon the scs or the pacific deep water above 2600 m was warmer before 3 2 ma than at present after that the benthic 18o conspicuously increased during the 3 2 2 5 ma period in correspondence to the formation of the northern hemisphere ice sheet whereas the planktonic 18o signal suggests a stepwise overall decrease of sea surface temperature during the 2 2 0 9 ma period compared to the equatorial pacific records the decrease in planktonic globigerinoides ruber 13c during the 3 2 2 2 ma period is particularly striking suggesting that fertility of surface water increased noticeably according to the modern 13c distribution of g ruber in the northern scs it is inferred that the east asian winter monsoon strengthened during this interval afterwards there were several conspicuous decreases of g ruber 13c at 1 7 1 3 0 9 0 45 and 0 15 ma bp that is about every 0 4 ma suggesting that the east asian winter monsoon became episodically stronger this is confirmed by changes in relative abundance of planktonic foraminifer species neogloboquadrina dutertrei a typical east asian winter monsoon proxy the deep water 13c of the scs is close to that of the pacific but lighter than that of the atlantic implying that the pattern of deep water originating mainly from the atlantic and through the pacific entering the scs existed at least since the early pliocene after 1 4 ma the benthic 13c signal decreased conspicuously but with a periodicity of 100 ka suggesting that the deep water ventilation of the scs was reduced probably corresponding to a decrease of the north atlantic deep water and or further isolation of the scs deep basin from the pacific during glaciations

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

magnetic signature of environmental changes in the last 1 2 myr at odp site 1146 south china sea abstract a detailed magnetic analysis has been done on the upper 145 m of the splice of odp site 1146 drilled during leg 184 in the south china sea first a magnetostratigraphic record was obtained the bruhnes matuyama reversal preceded by a precursor and the two jaramillo reversals were identified at 115 133 and 137 5 mcd meter composite depth respectively several known excursions during the brunhes period as well as the kamikatsura event during the matuyama chron were also identified coupled with the oxygen isotope record obtained from benthic foraminifera this allows us to establish a precise age model showing that the studied interval extends to 1 18 myr long term and short term variations have been documented in the magnetic parameters which appear at least in part related to changes in monsoon activity on the long term trend three main intervals with different magnetic trends were identified 1 2 0 7 0 7 0 2 and the most recent 0 2 myr during the oldest period the magnetic content decreases while the grain size of the magnetic mineral increases with large fluctuations the coercivity of the magnetic minerals also increases consistently with data obtained by other authors from pacific cores during the following 500 kyr all the magnetic properties are rather uniform with rather low abundance and relatively large grain sizes and low s ratio this probably illustrates a period of enhanced winter monsoon then in the most recent period the magnetic grain size decreases again with pronounced fluctuations on the short term scale a 23 kyr component is identified by power spectrum analysis in all magnetic parameters in the bottom part of the studied interval this component is correlated with the oxygen isotope record as shown by coherence analysis except for the most recent 200 kyr where the pattern changes cold warm periods coincide with low high magnetic content and magnetic grains are coarser during cold stages and finer during warm periods this is consistent with the clay mineral analysis and illustrates changes in the balance between arid humid periods corresponding to dominant winter summer monsoon and giving rise to physical erosion and wind blowing chemical weathering in the main fluvial plains and transport by oceanic current

development of east asian summer monsoon environments in the late miocene radiolarian evidence from site 1143 of odp leg 184 abundant radiolarians are preserved in the relatively complete upper miocene section of odp site 1143 three radiolarian zones rn6 rn7 and rn8 are recognized on the basis of diartus petterssoni total range in rn6 6 7 ma variations in the abundance of radiolarians especially pyloniid forms radiolarian flux and species diversity are good proxies of upwelling which similar to today s was likely driven by summer monsoons these radiolarian paleomonsoon proxies indicate that the east asian summer monsoon first initiated close to the middle late miocene boundary at similar to 12 11 ma and reached a maximum strength at similar to 8 24 ma therefore the initiation of the east asian summer monsoon was likely earlier than the first indian monsoon which appeared similar to 8 myr ago c 2003 elsevier b v all rights reserved

seismic properties of rocks with distributions of small cracks summary hudson s theory of multiple scattering for the dynamic properties of cracked rock is applied to media containing cracks of varying orientation radius and aspect ratio angular variation of velocity and attenuation due to parallel and nearly parallel cracks and cracks with normals at a fixed angle to a given axis are presented anisotropy gradually decreases as the distribution of orientations is widened from exactly parallel to random increasing the aspect ratio of thin saturated cracks generally causes the elastic properties to approach those due to dry gas filled cracks for fluid filled cracks the line singularity where the birefringent shear waves have equal velocity moves closer to the symmetry axis as either the distribution of orientations is widened or the aspect ratio is increased calculated attenuations due to rayleigh scattering and viscous dissipation in crack filling fluids are many orders of magnitude less than reported attenuations in the earth viscous dissipation varies linearly with frequency and aspect ratio of thin fluid filled cracks a power law distribution of crack radii intended to represent that found in the earth s crust has no effect on the angular variations of seismic velocities or viscous attenuation but scattering attenuation is sensitive to the power law exponent lognormal distributions of aspect ratio also corresponding to observations in real rock affect velocity as well as attenuation velocities derived from the theory are compared with those measured in laboratory specimens with known geometries of cracks

an approximate analytical solution for measuring air permeability of asphalt samples partially saturated with water in coastal areas abnormally high pressure may be caused by the tide induced water table variation under extensive pavements particularly during rainfall to simulate the rainfall infiltration effects on the air permeability of asphalt pavements in coastal area column shaped asphalt sample was fixed in the upper part of a steel cylinder with its upper surface saturated with ponding water depth 5 mm and open to the atmosphere the cylinder s lower part formed an air chamber the chamber was pressurized and then the air therein was released naturally through the sample the pressure variation with time in the chamber was recorded for analysis based on the green ampt piston model for the surface water infiltration an approximate analytical solution was derived to describe the pressure time relationship in the chamber a new parameter called the escape pressure was introduced to describe the air pressure needed for the chamber air to break through the capillary pressure induced by the ponding water the analytical solution gave good estimations of both the escape pressures and the harmonic averages of the permeabilities of the wet and dry parts of 14 samples in the sense that excellent fittings were obtained between the observed and predicted air pressures in the air chamber the estimated escape pressure ranges from 0 0 to 1 74 kpa the harmonic average of the permeabilities of the wet and dry parts is 5 94 of the dry sample s permeability

prediction of elastic wave velocities in sandstones using structural models elastic wave propagation in fluid saturated sandstones depends upon two sets of rock features 1 the volume fractions and elastic constants of the rock constituents quartz clay water etc and 2 microstructural geometry grain contacts pore aspect ratios while the former data are usually obtainable the latter are relatively inaccessible we present a new method for determining microstructural data using idealized but physically representative models of sandstone the key to the method is the simulation of certain depositional and diagenetic processes in a manner that completely specifies the geometry of the resulting models hence the geometric features of the grain space and void space required for various theories of elastic propagation can be calculated directly from the models we find good agreement between predictions and measurements of compressional and shear wave velocities in both clean and clay bearing saturated sandstones in contrast with previous efforts at predicting velocitie

tidal effects on sea water intrusion in unconfined aquifers abstract a variable density groundwater model is used to analyse the effects of tidal fluctuations on sea water intrusion in an unconfined aquifer it is shown that the tidal activity forces the sea water to intrude further inland and it also creates a thicker interface than would occur without tidal effects moreover the configuration of the interface is radically changed when the tidal fluctuations are included this is because of the dramatic changes in the flow pattern and velocity of the groundwater near the shoreline for aquifer depths much larger than tidal amplitudes the tidal fluctuation does not have much effect on how far the sea water intrudes into the aquifer nevertheless a significant change in the configuration of concentration contours because of the effect of tidal fluctuations is observed this change is more noticeable at the top of the aquifer near the water table than at the bottom of the aquifer and is caused by the infiltration of salt water into the top of the aquifer at higher tidal levels a flatter beach slope therefore intensifies this phenomenon the interface configurations do not change noticeably over the course of a tidal cycle neglecting tidal fluctuation effects results in an inaccurate evaluation of the water table elevation at the land end of the aquifer although no distinguishable difference is seen between the water tables near the shoreline where the landward boundary condition is a constant head the effects of tidal fluctuations on sea water intrusion are more pronounced than for cases where the landward boundary condition is a specified flux also it is shown that the effects of tidal fluctuations are more significant for a sloping beach than for a vertical shoreline and the salt water intrudes further inland for the sloping case

groundwater response to tidal fluctuation in a two zone aquifer summary a coastal aquifer may consist of two zones of distinctly different hydraulic properties with the coastal zone of limited width and the inland zone of infinite width the analytical solutions for groundwater level response to tidal fluctuation in such a two zone aquifer are derived and the underlying physical meanings are discussed a dimensionless zone contrast parameter r is introduced which is controlled by ratio between the product of the transmissivity and storativity of the inland zone and that of the coastal zone in the coastal zone the amplitude of groundwater head fluctuation attenuates more quickly when r becomes greater the amplitude declines exponentially with distance from the coastline and the phase lag increases linearly only when r 1 in the inland zone however the head fluctuation always exhibits an exponential attenuation of the amplitude and a linear increase of the phase lag for any value of r thus the diffusivity of the inland zone can be readily assessed with measurements of fluctuation amplitudes or lags as are frequently used when estimating aquifer parameters in homogeneous aquifers compared with jacob 1950 solution for a semi infinite homogeneous aquifer the existence of the coastal zone reduces the amplitude in the inland zone by a spatially constant coefficient and increases the phase lag by a spatially constant shift inverse estimation of aquifer parameters are discussed based on the analytical solutions the analytical solutions are used to estimate the aquifer parameters in a coastal two zone aquifer located in the west part of dongzhai harbor hainan province china

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

nitrogen isotope and chlorin paleoproductivity records from the northern south china sea remote vs local forcing of millennial and orbital scale variability abstract variations in nitrogen isotopic composition 15n and total chlorin accumulation rate ar are employed as proxies to reconstruct oceanic nitrate inventory the balance between denitrification and n fixation and paleoproductivity in a rapidly accumulating sediment drift deposit beneath the western pacific warm pool for the last 145 kyr subsurface and deep waters of the northern south china sea scs are sourced from the shallow kuroshio current kc and pacific intermediate water respectively their relative importance in determining sedimentary 15n and paleoproductivity have been altered by changes in equatorial circulation summer and winter monsoon intensity and relative sea level the location and basin configuration of the marginal scs renders it especially sensitive to such changes assuming complete annual nitrate utilization low 15n values during glacial stages are interpreted as a reflection of reduced remote denitrification in the eastern tropical north pacific etnp source waters while much of marine isotope stage mis 3 and the last interglacial were characterized by high denitrification however intervals of anomalously low 15n values are interpreted as reflecting the contribution from regional n fixation in west pacific surface waters transmitted to the site by the shallow kc unusually the holocene is characterized by declining 15n values and an inverse correlation with organic matter content since ca 8 2 ka millennial scale variations during mis 3 indicate higher frequency variations in both etnp denitrification and local n fixation which may be coherent with a hemispheric response to dansgaard oeschger events recorded at high latitudes for much of the last 145 kyr paleoproductivity was decoupled from 15n and instead seems to reflect the extent of the global nitrate inventory stimulated by elevated dust fertilization and regional mixed layer deepening associated with the relative intensity of the se asian winter monsoon despite evaluation of possible conflicting influences on the record of each proxy we interpret our data as clear evidence of glacial interglacial changes in marine nutrient inventory across the whole of the north pacific with a corresponding biogeochemical response and important implications for global co2 drawdown via an invigorated biological pump the relative importance of local regional and global contributions to our records appears to be strongly modulated by relative sea level controlling trans and extra basinal circulation in the scs

deep sea pollen from the south china sea pleistocene indicators of east asian monsoon abstract a high resolution pollen record sampling interval averages 820 years has been obtained from odp site 1144 water depth 2037 m northern south china sea the 504 m sequence in composition length covers the last 1 03 million years according to micropaleontological and isotopic stratigraphy the pollen assemblages are characterized by high proportions of pinus and herb pollen and by their frequent alternations based on these alternations 29 pollen zones have been recognized that are closely correlated to the marine oxygen isotope stages mis 1 29 pinus dominant pollen zones correspond to interglacial periods with lighter 18o values while herb marked ones relate to the heavier 18o stages assigned to glacials judging from the pollen data the exposed northern continental shelf of the south china sea during the glacials was covered by grassland and the extensive northern shelf has formed only since mis 6 ca 150 ka probably as a result of tectonic subsidence tree pollen influx values are indicative of winter monsoon which began to intensify 600 ka ago the summer monsoon variations can be approximated by the fern percentage within the total pollen and spore abundance and the result shows high values in general occurring at interglacials with the maxima at mis 15 5e and 1 the relatively high fern percentage with smaller amplitude in variations before 600 ka might suggest more stable humid conditions before the intensification of winter monsoon

changes in terrestrial ecosystem since 30 ma in east asia stable isotope evidence from black carbon in the south china sea a 30 m y stable isotopic record of marine deposited black carbon from regional ter restrial biomass burning from the northern south china sea reveals photosynthetic path way evolution for terrestrial ecosystems in the late cenozoic this record indicates that c3 plants negatively adjusted their isotopic discrimination and c 4 plants appeared grad ually as a component of land vegetation in east asia since the early miocene a long time before sudden c4 expansion occurred during the late miocene to the pliocene the changes in terrestrial ecosystems with time can be reasonably related to the evolution of east asian monsoons which are thought to have been induced by several intricate mechanisms during the late cenozoic and could contribute significantly to the post miocene marine carbonate isotope decline

an analytical solution of groundwater response to tidal fluctuation in a leaky confined aquifer an analytical solution is derived to investigate the influence of leakage on tidal response in a coastal leaky confined aquifer system the analytical solution developed here is more general than the traditional solution obtained by ferris 1951 which can be regarded as a special case of the solution presented in this paper this solution is based on a conceptual model under the assumption that the groundwater level in the confined aquifer fluctuates in response to sea tide while that of the overlying unconfined aquifer remains constant this conceptual model is supported by numerous field studies by previous researchers which have demonstrated that the tidal response in an unconfined aquifer may be negligible compared to that in a confined aquifer the leakage has a significant impact on the tidal behavior of the confined aquifer hypothetical studies indicate that both tidal amplitude of groundwater head in the aquifer and the distance over which the aquifer can be disturbed by the sea tide will be considerably reduced because of the existence of leakage this analytical solution is used to investigate the tidal and piezometer data at the chek lap kok airport hong kong special administrative region people s republic of china

critical grain size parameters for predicting framework and floating grains in sediments abstract in unconsolidated sediments some grains form a load bearing framework while others may float that is occupy volume without contributing to mechanical strength the latter grains affect the relationship between acoustic velocity and porosity as well as the permeability of the sediment we propose that characteristic features of the grain size distribution are indicators of the fraction of floating grains the criterion for floating involves grain scale geometry not accessible from macroscopic models of grain packing so we support this proposition with an examination of computer generated model sediments dense disordered packings of spheres having two sizes of spheres the analysis reveals two thresholds first the fraction of floating grains is larger when the volume fraction of small grains is below 40 in packings satisfying the first threshold a second threshold exists if the large grain radius is sufficiently greater than the small grain radius by a factor of 3 or half a decade on a logarithmic scale the fraction of floating grains is significantly larger these thresholds are qualitatively consistent with field observations of anomalously slow p wave velocities in several poorly consolidated to semiconsolidated reservoirs the grain size distributions in these reservoirs are sufficiently broad to satisfy the two thresholds and the emergence of a second mode in the distribution correlates with greater anomaly in the velocity porosity trend the maximum fraction of floating grains in the model sediments is comparable to the value needed to explain the anomaly quantitatively significantly the thresholds for floating grains are closely related to those known to affect trends of macroscopic properties of sediments porosity permeability the grain scale model can thus serve more generally as a microstructural complement to established macroscopic models

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

investigating the history of east asian monsoon and climate during the last glacial interglacial period 0 140 000 years mineralogy and geochemistry of odp sites 1143 and 1144 south china sea abstract monsoon climate is an important component of the global climatic system a comprehensive understanding of its variability over glacial interglacial time scales as well as of its effects on the continent and in the ocean is required to decipher links between climate continental weathering and productivity a detailed multiproxy study including bulk and clay mineralogy grain size analysis phosphorus geochemistry sedex extraction organic matter characterization and nitrogen stable isotopes was carried out on samples from odp sites 1143 and 1144 leg 184 south china sea covering the past 140 000 years we tentatively reconstruct the complex sedimentation and climatic history of the region during the last glacial interglacial cycle when sea level variations linked to the growth and melting of ice caps interact with monsoon variability during interglacial periods of high sea level summer monsoon was strong and humid and warm climate characterized the adjacent continent and islands clay minerals bear signals of chemical weathering during these intervals high calcite and reactive phosphorus mass accumulation rates mars indicate high productivity especially in the southern region of the basin during glacial intervals strong winter monsoon provided enhanced detrital input from the continent as indicated by high detrital mar glacial low sea level resulted in erosion of sediments from the exposed sunda shelf to the south and clay mineral variations indicate that warm and humid conditions still prevailed in the southern tropical areas enhanced supply of nutrients from the continent both by river and eolian input maintained high primary productivity reduced circulation during these periods possibly induced active remobilization of nutrients such as phosphorus from the sediments intense and short cold periods recorded during glacial and interglacial stages correlate with loess records in china and marine climatic records in the north atlantic confirming a teleconnection between low and high latitude climate variability

one dimensional airflow in unsaturated zone induced by periodic water table fluctuation the amplitude of p0 t increases with the frequency of the head fluctuation the upper layer s thickness and the unconfined aquifer s air filled porosity and decreases with the upper layer s permeability the phase shift of p0 t ranges from 0 to p 2 indicating a time advance particularly p0 t is approximately proportional to the temporal derivative of the head for sufficiently thin or permeable upper layer and to the head itself for sufficiently thick or less permeable one the fluctuation amplitude of the water table is always less than that of the head and can be only one tenth of the latter if the upper layer is sufficiently thick or less permeable which may slow significantly the landward attenuation speed of the tide induced head fluctuation in a coastal air confined aquifer the analytical solution was used to estimate the value range of the air permeability of the marine sand fill at a coastal reclamation area of hong kong

the stress sensitivity of shaley sandstones the link between the stress sensitivity of shaley sandstones and their porosity and clay content is investigated this is achieved by firstly fitting a compliance based stress sensitivity law to laboratory measurements of ultrasonic velocity taken from four sets of reservoir sandstones extracted from a variety of depositional settings correlations are then sought between the independent parameters of this law and the porosity or clay fraction of the rocks which are then subsequently interpreted in terms of framework or pore space related microstructural clay models the general conclusion drawn from the results is that both of the parameters defining the stress sensitivity law the asymptotic modulus and the stress dependent excess compliance clearly vary with porosity however only the asymptotic modulus shows a convincing trend with clay and there is little observed variation of the stress dependent compliance with clay there is therefore a resultant variation of stress sensitivity with clay but it is controlled only by the asymptotic modulus the analysis also concludes that all four data sets fall into a framework related category of clay model

geochemical and nd isotopic variations in sediments of the south china sea a response to cenozoic tectonism in se asia secular variations in geochemistry and nd isotopic data have been documented in sediment samples at odp site 1148 in the south china sea major and trace elements show significant changes at ca 29 5 ma and 26 23 ma whereas ond values show a single change at ca 26 23 ma increases in al ti al k rb sr and la lu ratios and a decrease in the th la ratio of the sediments beginning at 29 5 ma are consistent with more intense chemical weathering in the source region the abrupt change in nd isotopes and geochemistry at ca 26 23 ma coincides with a major discontinuity in the sedimentology and physical properties of the sediments implying a drastic change in sedimentary provenance and environment at the drill site comparison of the nd isotopes of sediments from major rivers flowing into the south china sea suggests that pre 27 ma sediments were dominantly derived from a southwestern provenance indochina sunda shelf and possibly northwestern borneo whereas post 23 ma sediments were derived from a northern provenance south china this change in provenance from southwest to north was largely caused by ridge jumping during seafloor spreading of the south china sea associated with a southwestward expansion of the ocean basin crust and a global rise in sea level thus the geochemical and nd isotopic changes in the sediments at odp site 1148 are interpreted as a response to a major plate reorganization in se asia at ca 25 ma b 2003 elsevier science b v all rights reserved