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

equatorial spread f instability conditions as determined from ionograms conditions of plasma irregularity or spread f generation under the rayleigh taylor mechanism in the nighttime equatorial ionosphere are examined from ionogram data registered over fortaleza 4 s 38 w mag dip lat 1 7 brazil for a one year period the results show that local ionospheric conditions such as the bottomside f region electron density scale length and the ion neutral collision frequency at the base of the layer at sunset hours determine the generation of the spread f irregularities over the equator in the post sunset hours

distribution characteristics of topside ionospheric density irregularities equatorial versus midlatitude regions 1 the global distribution of the occurrence rate for density irregularities at 600 km topside ionosphere between 35 geographic latitudes has been studied with the rocsat data during moderate to high solar activity years of 1999 to 2004 the result indicates that the global occurrence distribution of the intermediate scale 0 1 to 50 km density irregularities can be grouped into two different populations one in the equatorial region and the other in the middle to subauroral latitude region the global seasonal longitudinal s l distribution of equatorial irregularities in the current report reproduces the result of mcclure et al 1998 obtained with the ae e observations of the mesoscale 50 to 1000 km plasma bubble structures during high solar activity years of 1978 to 1980 two solar cycles ago this implies that the density irregularities of different scales from multistage cascading process of the large scale 1000 km gravitational rayleigh taylor instability have manifested in same global s l distribution pattern furthermore global variation in seeding mechanism and growth condition of the instability process that results in major features in global irregularity pattern seems to persist for past 25 years in addition the current result further indicates that an upper latitudinal limit of the equatorial irregularity distribution is located at about 30 a different kind of midlatitude irregularity distribution starts to fill in from this dip latitude in other words the equatorial density irregularity inside a depleted flux tube can only rise on statistical average to an apex height of 2000 km different magnetic and solar variability effects as well as the local time dependence are noted for the occurrences of density irregularities in the equatorial region versus that at midlatitudes the occurrence frequency of equatorial density irregularities increases with solar flux intensity whereas the midlatitude density irregularity is more likely to occur during low solar activity period the equatorial density irregularities are more likely to occur during periods of low magnetic activity than during magnetic disturbed times on the other hand the occurrence of midlatitude density irregularities indicates little dependence on geomagnetic activity the local time distribution of equatorial irregularity peaks before midnight while the midlatitude irregularity indicates a plateau of high occurrence rate after midnight such opposite characteristics in the occurrence pattern between these two spatially separated distributions suggest that different instability mechanisms are operated in two different latitude regions for the occurrence of intermediate scale density irregularities

longitudinal characteristics of spread f backscatter plumes observed with the ear and sanya vhf radar in southeast asia 1 the development of equatorial plasma irregularity plumes can be well recorded by steerable backscatter radars operated at and off the magnetic equator due to the fact that the vertically extended plume structures are tracers of magnetically north south aligned larger scale structures from observations during march 2012 using two low latitude steerable backscatter radars in southeast asia the equatorial atmosphere radar ear 0 2 s 100 3 e dip lat 10 4 s and the sanya vhf radar 18 4 n 109 6 e dip lat 12 8 n the characteristics of backscatter plumes over the two sites separated in longitude by 1000 km were simultaneously investigated the beam steering measurements reveal frequent occurrences of multiple plumes over both radar sites of which two cases are analyzed here the observations on 30 march 2012 show plume structures initiated within the radar scanned area followed by others drifting from the west of the radar beam over both stations a tracing analysis on the onset locations of plasma plumes reveals spatially well separated backscatter plumes with a maximum east west wavelength of about 1000 km periodically generated in longitudes between 85 e and 110 e the postsunset backscatter plumes seen by the sanya vhf radar are found to be due to the passage of sunset plumes initiated around the longitude of ear most interestingly the ear measurements on the night of 21 march 2012 show multiple plume structures that developed successively in the radar scanned area with east west separation of 50 km with however no sunset plasma plume over sanya colocated ionogram measurements show that spread f irregularities occurred mainly in the bottomside f region at sanya whereas satellite traces in ionograms that are indications of large scale wave structures were observed on that night at both stations possible causes for the longitudinal difference in the characteristics of radar backscatter plumes are discussed

effects of tidal forcing conductivity gradient and active seeding on the climatology of equatorial spread f over kwajalein plasma structure in nighttime equatorial f layer referred to as equatorial spread f esf displays climatology whose seasonal variation depends on longitude at longitudes where esf favors equinoxes times when maxima occur can be predicted in terms of the day of year when e region sunset is simultaneous in conjugate hemispheres i e sunset nodes aside from occurrences around equinoxes there are only three longitudes where esf also occurs during a solstice one is the central pacific region here esf activity is strong during the june solstice when solar activity is high to understand this puzzling behavior esf climatology over the kwajalein atoll was compared with properties of the postsunset rise pssr of the f layer and seeding activity in the troposphere the key findings are as follows 1 maxima in pssr velocity vpssr are better aligned with equinoxes than with sunset nodes hence seasonal pattern of vpssr not only sunset nodes should be included in interpretation of esf climatology 2 the source of vpssr during solstice appears to differ from that during equinoxes 3 equinoctial maxima in vpssr could be related to a semiannual variation in equatorial electrojet strength and its contribution to polarization of the f region dynamo current 4 enhanced vpssr during the june solstice is interpreted in terms of tidal forcing with a wave number of 2 5 displacements of maxima in esf climatology from maxima in vpssr are shown to be consistent with deep convective activity

spread f occurrence over a southern anomaly crest location in brazil during june solstice of solar minimum activity 1 we present a study of spread f occurrence over a location under the southern crest of the equatorial ionization anomaly cachoeira paulista 22 7 s 45 0 w mag lat 16 s dip angle 32 3 brazil during the last solar cycle which presented an extended solar minimum activity after analyzing hundreds of ionograms obtained with a digital ionosonde dgs 256 between 2001 and 2010 we verified high spread f occurrence around midnight postmidnight during june solstice southern hemisphere winter with a peak occurrence between 2006 and 2009 when the solar flux has reached very low values 70 sfu at the brazilian sector the occurrence of spread f is known to be associated with equatorial plasma bubbles epbs and is observed mainly between september and march on the other hand epbs are rarely observed over cachoeira paulista hereafter referred as cp during june solstice because of the weak magnitudes of the equatorial electric field prereversal enhancement an essential condition to the development of the large scale irregularities at the equatorial region despite the rarity of epbs we observed several strong spread f events occurring under low plasma densities conditions which can extend for several hours we have found evidences that the spread f events over cp during june solstices of low solar activity can be caused by ionospheric disturbances that are unrelated to equatorial processes finally we present a statistical analysis of the events and we suggest that traveling ionospheric disturbances could be one of the main sources of these nonequatorial spread f events

simulation study of the longitudinal variation of evening vertical ionospheric drifts at the magnetic equator during equinox 1 quiet time low latitude ionospheric electric fields and vertical drifts in the evening sector at equinox are examined as a function of longitude and solar activity using simulations from the magnetosphere thermosphere ionosphere electrodynamics general circulation model mtiegcm the model inputs are held fixed with respect to universal time in order to evaluate the contributions to longitudinal variations of the equatorial electric fields and drifts associated with zonal variations in the geomagnetic field the simulated upward evening drift is somewhat larger than observations it increases with solar activity at a similar rate in all longitude sectors this rate about 0 32 0 37 m s per unit of 10 7 cm solar flux is quantitatively consistent with observations in the peruvian sector but is considerably greater than observations in the indian sector the simulations agree with observations in finding larger vertical drifts in the american atlantic sector than in the east asian pacific sector generally associated with longitudinal variations in the strength of the geomagnetic field relations among the longitude variations of the vertical drift the conductivity the eastward wind velocity the geomagnetic declination and gradients of the wind and declination are examined revealing few clear correlations however extrema of many of these quantities are noted in the american atlantic sector

julia radar studies of equatorial spread f this paper presents coherent scatter radar observations of plasma irregularities in the equatorial f region ionosphere made with the julia jicamarca unattended long term studies of the ionosphere and atmosphere radar and discusses model equations and simulations pertinent to their interpretation the data set shows that bottom type and bottomside scattering layers are prevalent throughout solar minimum equinox bottom type layers are undifferentiated and relatively weak layers confined to a narrow range of altitudes falling within regions of the ionosphere driven by the e region dynamo these layers are often precursors to large scale radar plumes as well as to bottomside layers which are broader more structured and more intense layers than the bottom type variety bottomside layers are the source of intermediate scale plasma depletions that can ascend and penetrate through to the topside fluid simulations of the collisional interchange instability suggest that dynamo theory explains why one kind of layer emerges at a given time and altitude

dependence of the evening prereversal enhancement of the vertical plasma drift on geophysical parameters 1 the variability of the evening prereversal enhancement pre of the vertical plasma drift has been investigated by using the average velocity at a fixed local time range but this method does not provide a clear distinction between the pre occurrence frequency and magnitude in this study we examine the dependence of the occurrence frequency occurrence local time and magnitude of the pre on geophysical parameters by using the retrievals of the individual pre from the first republic of china satellite data our result shows that the seasonal longitudinal variation of the average vertical velocity in the evening that has been identified in previous studies is associated with both the pre occurrence frequency and pre magnitude the magnitude of the pre shows an increasing trend with an increase of the solar flux and with a decrease of the conjugate e region sunset time lag the seasonal longitudinal variation of the occurrence local time of the peak pre shows a dependence on the e region sunset time at the magnetic equator rather than on the magnetic declination or the sunset time lag in the conjugate e regions this observation indicates that the optimum time for the creation of the peak pre is when the sunset terminator at the e region height and the magnetic meridian intersect near the magnetic equator

assessment of fluid contacts and compartmentalization in sandstone reservoirs using fluid inclusions an example from the magnus oil field north sea location of an oil water contact is crucial for estimating reserves and for designing production strategies identifying hydrodynamic barriers or baffles is also important for optimizing both recovery and flow rates in some oil fields identifying fluid contacts and barriers is not always possible using conventional wireline and formation testing methods

deep burial dissolution of k feldspars in a fluvial sandstone pentland formation uk central north sea the pentland formation of the uk central north sea is a quartzite in a sequence of more arkosic sandstones provenance sensitive heavy mineral indices are interpreted to indicate that it was probably derived by erosion of arkosic sediments which also supplied the arkosic marine sandstones of the contemporaneous fulmar formation a lack of apatite within the pentland formation suggests that the feldspars could have dissolved during exposure to groundwaters during very shallow burial however petrographic and geochemical evidence supports survival of at least some of the feldspar to burial depths in excess of at least 2 km when the sandstones had been stabilized against compaction by quartz cements or overpressure abundant kaolin which has not reacted to form illite is preserved at burial depths in excess of 4000 m suggesting an unusual lack of available potassium there is a strong contrast in diagenetic pathways with the overlying marine fulmar formation which most probably had the same initial composition but in which kaolin is absent and fibrous illite is present it is suggested that a lower concentration of potassium and aluminium within the pentland formation compared with the probable source and the comparably sourced fulmar formation represents export of k from the sands on a metre scale

equatorial f region vertical plasma drifts seasonal and longitudinal asymmetries in the american sector longitudinal and seasonal asymmetries in the evening ionospheric f region plasma vertical drift v sub z enhancements between two longitudinally separated stations situated along the magnetic equator in the american sector are investigated under solar maximum conditions based on results obtained from the analysis of ionosonde data for these stations the two stations are huancayo peru and fortaleza brazil which have markedly different magnetic declination angles the observed asymmetries are interpreted using a detailed numerical simulation of the e and f region electrodynamic coupling process that takes into account also its asymmetry about the magnetic equator arising from the finite magnetic declination angle the results of the simulation show in agreement with observations that the occurrence time of the evening f region vertical drift prereversal peak and its seasonal variation at a station are controlled by the magnetic declination angle at that station which determines the seasonal variation of the sunset times and hence the integrated pedersen conductivity longitudinal gradient at its magnetic conjugate e layers the amplitude of the prereversal peak on the other hand undergoes the influence of the magnetic declination angle as well as of the thermospheric zonal wind component

analysis of spatial order in sandstones ii grain clusters packing flaws and the small scale structure of sandstones abstract sandstone microstructure at scales of 5 5000 m is a domainal structure consisting of juxtaposed clusters of efficiently packed grains bounded by packing flaws the flaws represent zones of loose packing arising from grain to grain mismatches between well packed clusters flaws tend to propagate throughout the sediment and are marked in thin section by zones of oversized pores known to be associated with enhanced fluid flow using a two dimensional fourier transform it is possible to identify these flawed zones and examine not only their spatial distribution but also the relative contribution of close packed and flawed porosity to the pore network as a whole flaws represent zones of disorder formed through a variety of depositional and postdepositional processes two flaw producing mechanisms have been recognized in random aggregates of spheres created in a laboratory the first arises from grain to grain mismatches at the boundaries of concurrently growing grain clusters the second type is associated with the growth of well packed clusters over a preexisting cohesive substrate in the laboratory this is represented by the walls of the container holding the aggregate the distribution of loose packed and well packed grain domains in natural sands is governed by depositional environment the size and shape of the constituent grains define the fundamental spatial frequency of porosity but factors such as grain orientation and size sorti g affect the orientation of clusters and flaws as well as the scale at which they exist for example flaws in poorly sorted sands arise from the misfit of the finer size classes in a grain matrix controlled by the larger grains the flaws generated by this misfit propagate throughout the fine grained sections of the sediment with a spatial period characteristic of the coarser size classes grain clusters and flaws can be recognized as major textural elements in all sands and sandstones even those with macroporosity as low as 5 not only is the depositional signal preserved as porosity decreases its existence affords the opportunity to closely examine the effects of diagenesis upon the pore network as porosity decreases the relative proportion of flawed porosity increases by the time total porosity has been reduced to 5 flawed porosity has increased to the point that it constitutes 80 of the remainder the increase in relative proportion of loose packed porosity is due to preferential cementation within well packed clusters as well us an increase in loose packed porosity due to dissolution of unstable mineral grains in all the diagenetically altered sandston s examined the only grain molds or partially dissolved grains present were found to be located either within or immediately adjacent to packing flaws

effect of pore geometry on gassmann fluid substitution although there is no assumption of pore geometry in derivation of gassmann s equation the pore geometry is in close relation with hygroscopic water content and pore fluid communication between the micropores and the macropores the hygroscopic water content in common reservoir rocks is small and its effect on elastic properties is ignored in the gassmann theory however the volume of hygroscopic water can be significant in shaly rocks or rocks made of fine particles therefore its effect on the elastic properties may be important if the pore fluids in microspores cannot reach pressure equilibrium with the macropore system assumption of the gassmann theory is violated therefore due to pore structure complexity there may be a significant part of the pore fluids that do not satisfy the assumption of the gassmann theory we recommend that this part of pore fluids be accounted for within the solid rock frame and effective porosity be used in gassmann s equation for fluid substitution integrated study of ultrasonic laboratory measurement data petrographic data mercury injection capillary pressure data and nuclear magnetic resonance t2 data confirms rationality of using effective porosity for gassmann fluid substitution the effective porosity for gassmann s equation should be frequency dependent knowing the pore geometry if an empirical correlation between frequency and the threshold pore throat radius or nuclear magnetic resonance t2 could be set up gassmann s equation can be applicable to data measured at different frequencies without information of the pore geometry the irreducible water saturation can be used to estimate the effective porosity

mineralogical mapping of scanning electron micrographs abstract a technique is described to map regions of back scattered scanning electron images according to the mineralogy of the specimen such a technique may be used to study in detail the micro mineralogy of geological materials in particular the size shape and distribution of the individual components may be studied such information could provide additional evidence on the origin and nature of such materials composite multi layered images are constructed from both back scattered images and x ray maps of the same area of relevant elements training areas representative of the different mineral classes present are defined and the statistics within these areas are used to classify the whole image changes in classification parameters appear to have little effect on the determined composition segmentation of an image by this means is superior to techniques using a single image following segmentation particle size parameters for the constituent mineral grains may be determined examples showing different size frequency distributions for the different minerals are included several practical points on the use of the method are also covered

a satellite derived climatology of the itcz abstract this paper presents fundamental climatological characteristics of the intertropical convergence zone itcz in a simple concise manner using the highly reflective cloud hrc dataset this satellite derived dataset uses both visible and infrared observations to measure the frequency of occurrence of large scale convective systems over the global tropics at a 1 spatial resolution these dataset characteristics make the hrc particularly well suited for climatological analysis of the itcz because the dataset is based on estimates of organized deep convective cloud systems rather than observations of clouds as a whole and it provides the spatial resolution needed to identify thew large wale convective structures furthermore the dataset covers a time period extending nearly two decades which provides for a fairly robust climatology and the opportunity to examine seasonal and interannual variability of both the convection and the latitude of the itcz

coherent and incoherent scatter radar study of the climatology and day to day variability of mean f region vertical drifts and equatorial spread f

the effect of ductile lithic sand grains and quartz cement on porosity and permeability in oligocene and lower miocene clastics south china sea prediction of reservoir quality ductile clay rich sand grains control porosity and permeability in the fluvio deltaic oligocene and lower miocene sandstones of the south china sea ductile grains account for between approximately 5 and 50 of the original sand grain population there is a pronounced loss of porosity with increasing burial depth in the basin at depths of less than 3000 m this is due solely to ductile grain compaction where the rate of porosity loss with depth increases with increasing ductile grain content at depths greater than 3000 m the steep porosity loss with depth is due to combined ductile grain compaction and quartz cementation the amount of quartz cement increases with increasing burial depth however cleaner sandstones tend to have greater amounts of quartz cement at any given depth below 3000 m this leads to convergence of porosity evolution for the clean and ductile rich sandstones below 3000 m there is a rapid loss of permeability with decreasing porosity because compaction of ductile grains smears them between rigid quartzose grains leading to blocked pore throats a consequence of this process is that the lowest permeabilities are found in sandstones with the highest ductile grain contents quartz cement does not have a clear and discernible control on permeability the pronounced loss of porosity with increasing depth and permeability with decreasing porosity leads to low permeability at relatively shallow burial depths reservoir quality is thus controlled by the nature of the primary sand and depth of burial the sediment supply system led to systematic changes in ductile grain content across the basin with ductile content increasing into the more distal part of the sediment system the consequence is that depth of economic basement in terms of porosity or permeability can be predicted as a function of ductile grain content and burial depth for prospects across the basin

simulations of the effects of diagenesis on the evolution of sandstone porosity abstract automated image analysis and image processing techniques make possible the simulation of sandstone diagenesis and prediction of pore system modification diagenetic processes in real rocks are simulated using a combination of element x ray maps and backscattered electron images processes can also be simulated for computer generated sand packs by image analysis and image processing compaction is simulated by moving grain centers closer together overgrowth type cementation is simulated by dilation of free grain surfaces mineral dissolution is simulated by selective removal of specific minerals identified by the combination of element x ray maps and backscattered electron images the original and modified images can be processed and analyzed to reveal all the parameters needed to predict permeability using a kozeny carman type equation the computer modeling and image analysis approach can also be used to determine the specific effects of various diagenetic processes on porosity the results are encouragingly realistic by reference to the few available well characterized empirical datasets

magnetic signatures of medium scale traveling ionospheric disturbances as observed by champ 1 in this work we analyze the global distribution and physical characteristics of nighttime midlatitude magnetic field fluctuations mmfs as observed by the champ satellite from 2001 to 2002 solar maximum and from 2006 to 2007 solar minimum mmfs are defined as medium scale magnetic fluctuations perpendicular to the mean field which are not accompanied by plasma density irregularities at the champ altitude 400 km mmfs occur at 15 40 invariant latitude in the ionospheric f region the occurrence is rare above the southern atlantic ocean and bears little connection to geomagnetic activity the global mmf occurrence rate depends on season the occurrence is generally low in equinox maximizes around east asia oceania and europe northern atlantic ocean in june solstice and peaks above the american continents in december solstice as the solar cycle declines the detected mmf occurrence rate also decreases the mmf occurrence peaks around 2100 lt and slowly decreases toward midnight in the postmidnight sector events are practically absent the mmf occurrence is generally consistent with known features of nighttime medium scale traveling ionospheric disturbances mstids such as the conjugate climatology and premidnight occurrence peak in the east asia oceania region but differences in their distributions also exist implying that factors other than mstids e g ionospheric conductivity sporadic e layer or plasma instabilities may play a nonnegligible role in generating mmfs mmfs have a preferred direction of polarization which is consistent with that of mstids and again corroborates the close connection between these two phenomena we interpret the observed magnetic deflections in terms of field aligned currents facs the estimated wavelength range 200 500 km of associated fac pairs also agrees well with the size of mstid density structures

postsunset vortex in equatorial f region plasma drifts and implications for bottomside spread f recent high resolution incoherent scatter drift measurements conducted at jicamarca during solar minimum conditions indicate that a vortical plasma flow pattern accompanies the well known prereversal enhancement events observed in the equatorial f region the vortex is centered at 250 km altitude and about 2000 3000 km east of the evening terminator and is characterized by upward and downward flows to the west and to the east and eastward and westward flows on the top and the bottom respectively postsunset bottomside spread f events are observed to commence in the interior of the vortex in regions of westward plasma drifts the formation of the vortex implies localized charging of the postsunset bottomside f region in response to eastward neutral wind driven f region dynamo action

influences of geomagnetic fields on longitudinal variations of vertical plasma drifts in the presunset equatorial topside ionosphere on the basis of the measurements from the ion drift meter on the defense meteorological satellite program f13 satellite from 2000 to 2002 the relative longitudinal variations of e x b vertical plasma drifts in the presunset equatorial topside ionosphere at 1745 lt are examined obvious influences of geomagnetic fields on longitudinal variations of e x b vertical plasma drifts which also show a seasonal dependence have been found during the june solstice the relative vertical plasma drifts is most significantly influenced by the geomagnetic field declination however the relative vertical plasma drifts during the december solstice more strongly correlates with the geomagnetic field strength although the relative vertical plasma drifts during equinox are also influenced by the geomagnetic field declination an underlying dependence on magnetic field magnitude as seen in december is also present

seasonal and longitudinal variation of large scale topside equatorial plasma depletions 1 we examine the large scale properties of three classes of equatorial topside plasma depletions referred to as channels in order of increasing zonal width and internal large scale substructuring we study simple channels multichannels and superchannels we find that peaks in channel activity are centered on 0 african sector and 180 western pacific sector superchannels are found to strongly dominate in the african sector whereas simple and multichannels dominate the pacific sector within the african sector the relative proportions of the three channel classes remain stable in all seasons in the pacific sector simple and multichannels are dominant in all seasons of activity though small but significant seasonal variations in channel widths and spacings are seen structure at a zonal scale size of about 100 km is found to be present in all channels examined observational and model evidence suggests that 100 km density structure represents channels such that all multichannels and superchannels consist of multiple contiguous simple channels since seeds that initiate channel formation necessarily influence large scale channel features we utilize our data to draw conclusions regarding bottomside seeding conditions in the various seasonal and longitudinal sectors specifically we find that strong long wavelength 1000 km seeds appear much more frequently in the african sector resulting in the prevalence of superchannels there when long wavelength seeds appear in the pacific sector they result in periodic trains of simple and multichannels a bottomside preseeded at a scale size near 100 km accounts for the ubiquity of 100 km scale size in the ae e data

study of the evening plasma drift vortex in the low latitude ionosphere using san marco electric field measurements san marco d electric field measurements have been averaged in terms of the equivalent plasma drifts for days near the 1988 autumn equinox the observed plasma drifts provide a satellite based composite in altitude and local time of the evening enhancements in the zonal and vertical drifts and the shear in the zonal drift the satellite data reveal details of the postsunset plasma drift vortex which have been studied in radar and chemical release experiments the averaged drift data show a proportional relationship between the prereversal enhancement of the vertical drift and the zonal derivative of the zonal drift this relationship suggests that the vertical drift enhancement is caused by a curl free response to the rapid increase of the zonal drift which in turn is tied to the f region zonal winds and the e region sunset the data also show a curl free drop off in altitude of the vertical plasma drift above and below the peak drift at 450 km altitude during the prereversal enhancement comparisons of the averaged data with results of coupled ionosphere electrodynamics models are used to examine the underlying causes of the evening plasma drifts the evening plasma drift vortex is shown to be a single drift structure with a single source the source of the evening drift vortex resides on the bottomside of the integrated f region just as originally supposed by rishbeth 1971 this dominant single source which is produced by the divergence of the wind driven current of the f region causes the evening enhancement of the eastward plasma drift the shear in the eastward drift and the prereversal enhancement of the vertical drift

cosmic formosat 3 observations of equatorial f region irregularities in the saa longitude sector 1 the data of ionospheric electron density and s4 scintillation index from cosmic formosat 3 gps occultation soundings are used to study the morphology of the equatorial f region irregularity efi in the longitude sector 0 75 w where the south atlantic magnetic anomaly saa is located launched into low earth orbits on 14 april 2006 the six satellite constellation cosmic provides daily global measurements of the ionosphere at altitudes below 800 km the large data set enables us a statistically correlative study of electron density depletion and gps l1 scintillation which allows the global and altitudinal distributions of irregularity to be examined under solar minimum and geomagnetic quiet conditions postsunset efi scintillation events are found to concentrate in the saa longitude sector during northern winter months d months but nearly disappeared in the same longitude sector during the opposite season j months the d months average pattern of efi reveals that most of irregularities occurred in the bottom side f region 200 350 km altitude and at low magnetic latitudes adjacent to the locations of the maximum postsunset nmax and hmax this is consistent with the early postsunset irregularities that are generated in the region where plasma upward drifts have been greatly enhanced by examining the ut snapshots of the global distributions of electron density and scintillation events strong spatial correlations are found to exist among the sunset terminator the westward declined geo magnetic field strong scintillation event and density depletion in the saa longitude sector only during d months where the e region sunset terminator is most possibly parallel to the magnetic field direction at equatorial latitudes density depletion structures within this longitude sector were bounded by two postsunset ionization enhancement bands eia that extended over central saa and its conjugated region respectively such features provide further evidence that the enhanced conductivity gradients due to particle induced ionization in the vicinity of saa contribute additional electric field enhancements in the equatorial f region at the walls of the depletion region near the sunset lines prominent horizontal density gradients exist in both westward and southward directions due to sun setting and saa ionization enhancement respectively the combination of horizontal density gradients with antiparallel north eastward neutral winds during d months can contribute additional growth rate to the efi generation these cosmic observations not only confirm that the sunset equatorial electrodynamics plays a key role in controlling the seasonal and longitudinal occurrences of the quiet time efi but also reveal that seasonally dependent ionospheric responses to the energetic particle precipitation in saa can affect considerably the morphology of efi in the saa longitude sector

the characteristics and generation mechanism of small amplitude and large amplitude esf irregularities observed by the c nofs satellite it is well understood that equatorial plasma bubbles are initiated in the bottomside f region and grow into the topside f region producing equatorial spread f esf irregularities in both bottomside and topside f regions however it is not known whether small amplitude and large amplitude irregularities have the same occurrence pattern and how the generation of small amplitude and large amplitude irregularities is related to the postsunset vertical plasma drift and potential seeding processes in this study we use ion density and velocity data measured by the communications navigation outage forecasting system c nofs satellite during may 2008 july 2014 to derive the longitude month distributions of the occurrence probability of esf irregularities at different amplitudes it is found that the occurrence probability of large amplitude irregularities is well correlated with the postsunset vertical plasma drift and that small amplitude irregularities does not show a clear pattern at low solar activity but is anti correlated with that of large amplitude irregularities at moderate solar activity that is the months and longitudes with high occurrence probability of large amplitude irregularities are exactly those with low occurrence probability of small amplitude irregularities and vice versa esf irregularities are mostly limited below 500 km during low solar activity in 2008 2010 but large amplitude irregularities can reach 700 800 km in altitude during moderate solar activity in 2011 2014 the generation of large amplitude esf irregularities is controlled by the postsunset vertical plasma drift at both low and moderate solar activities in contrast small amplitude esf irregularities at moderate solar activity occur more frequently in the areas where the postsunset vertical plasma drift is small

magnetic declination control of the equatorial f region dynamo electric field development and spread f we have carried out a comparative study of the evening prereversal enhancements in the equatorial f region vertical ionization drift velocities vz over fortaleza 4 s 38 w brazil and jicamarca 12 s 77 w peru two magnetic equatorial stations in the american zone the results show profound dissimilarities in the seasonal trends in the times and widths of the vz prereversal peak which reflect in the spread f characteristics as well at the two stations the dissimilarities are shown to be arising mainly from the difference in the magnetic field declination angles that causes differences in the conjugate e region sunset durations and hence in the f region polarization electric field development rates at the two stations

global distribution of density irregularities in the equatorial ionosphere we analyzed measurements of ion number density made by the retarding potential analyzer aboard the atmosphere explorer e ae e satellite which was in an approximately circular orbit at an altitude near 300 km in 1977 and later at an altitude near 400 km large scale 60 km density measurements in the high altitude regions show large depletions of bubble like structures which are confined to narrow local time longitude and magnetic latitude ranges while those in the low altitude regions show relatively small depletions which are broadly distributed in space for this reason we considered the altitude regions below 300 km and above 350 km and investigated the global distribution of irregularities using the rms deviation n n over a path length of 18 km as an indicator of overall irregularity intensity seasonal variations of irregularity occurrence probability are significant in the pacific regions while the occurrence probability is always high in the atlantic african regions and is always low in the indian regions we find that the high occurrence probability in the pacific regions is associated with isolated bubble structures while that near 0 longitude is produced by large depletions with bubble structures which are superimposed on a large scale wave like background considerations of longitude variations due to seeding mechanisms and due to f region winds and drifts are necessary to adequately explain the observations at low and high altitudes seeding effects are most obvious near 0 longitude while the most easily observed effect of the f region is the suppression of irregularity growth by interhemispheric neutral winds

an analysis of the roles of stress temperature and ph in chemical compaction of sandstones abstract chemical compaction also known as pressure solution involves dissolution along grain contacts and transport of solute to the adjacent pore space by diffusion the driving force for diffusion is a gradient in chemical potential therefore chemical compaction requires persistent gradients in chemical potential along grain contacts in order to drive diffusion continuously such gradients exist in porous rocks because of the heterogeneous distribution of stress over the grain surfaces the rate of chemical compaction depends on stress fluid pressure temperature and the composition of the pore fluid which itself depends on various processes that add or remove solute an alternative model of chemical compaction invokes the effect of sheet silicates on ph to account for localized dissolution of grains adjacent to stylolites or where sheet silicates are present along grain contacts in this model the rate of chemical compaction is independent of stress and fluid pressure depending only on temperature this model is untenable because the proposed mechanism cannot give rise to the persistent gradient in chemical potential required to drive diffusion however sheet silicates may increase the rate of chemical compaction because of their influence on dissolution and diffusion rates the development of anisotropic fabrics as a result of chemical compaction and the widespread occurrence of stylolites in carbonates as well as sandstones are inconsistent with the ph model and provide strong support for the role of stress in chemical compaction

longitudinal variability of annual changes in activity of equatorial spread f and plasma bubbles global distribution maps of equatorial spread f esf activity for various annual periods were derived from the topside soundings by the ionosphere sounding satellite b iss b in 1978 1980 the esf activity during the northern winter period reveals maximum enhancement at the atlantic longitudes of large westward geomagnetic declination and during the northern summer at the pacific longitudes of large eastward declination on several orbits passing over the region of the esf activity enhancement abrupt depletions of the electron density or equatorial plasma bubbles were observed at the satellite altitude of about 1100 km it has also been found that the background electron density distributions tend to be symmetric with respect to the magnetic equator in the region of the enhanced esf activity and asymmetric in the region of the suppressed esf activity these observations are taken account of by the influence of a transequatorial thermospheric wind upon the suppression of the rayleigh taylor type plasma instability

global equatorial plasma bubble occurrence during the 2015 st patrick s day storm an analysis of the occurrence of equatorial plasma bubbles epbs around the world during the 2015 st patrick s day geomagnetic storm is presented a network of 12 global positioning system receivers spanning from south america to southeast asia was used in addition to colocated vhf receivers at three stations and four nearby ionosondes the suppression of postsunset epbs was observed across most longitudes over 2 days the epb observations were compared to calculations of the linear rayleigh taylor growth rate using coupled thermosphere ionosphere modeling which successfully modeled the transition of favorable epb growth from postsunset to postmidnight hours during the storm the mechanisms behind the growth of postmidnight epbs during this storm were investigated while the latter stages of postmidnight epb growth were found to be dominated by disturbance dynamo effects the initial stages of postmidnight epb growth close to local midnight were found to be controlled by the higher altitudes of the plasma i e the gravity term modeling and observations revealed that during the storm the ionospheric plasma was redistributed to higher altitudes in the low latitude region which made the plasma more susceptible to rayleigh taylor growth prior to the dominance of the disturbance dynamo in the eventual generation of postmidnight epbs

effects of the vertical plasma drift velocity on the generation and evolution of equatorial spread f we use radar observations from the jicamarca observatory from 1968 to 1992 to study the effects of the f region vertical plasma drift velocity on the generation and evolution of equatorial spread f the dependence of these irregularities on season solar cycle and magnetic activity can be explained as resulting from the corresponding effects on the evening and nighttime vertical drifts in the early night sector the bottomside of the f layer is almost always unstable the evolution of the unstable layer is controlled by the history of the vertical drift velocity when the drift velocities are large enough the necessary seeding mechanisms for the generation of strong spread f always appear to be present the threshold drift velocity for the generation of strong early night irregularities increases linearly with solar flux the geomagnetic control on the generation of spread f is season solar cycle and longitude dependent these effects can be explained by the response of the equatorial vertical drift velocities to magnetospheric and ionospheric disturbance dynamo electric fields the occurrence of early night spread f decreases significantly during equinox solar maximum magnetically disturbed conditions due to disturbance dynamo electric fields which decrease the upward drift velocities near sunset the generation of late night spread f requires the reversal of the vertical velocity from downward to upward for periods longer than about half an hour these irregularities occur most often at 0400 local time when the prompt penetration and disturbance dynamo vertical drifts have largest amplitudes the occurrence of late night spread f is highest near solar minimum and decreases with increasing solar activity probably due to the large increase of the nighttime downward drifts with increasing solar flux

global s4 index variations observed using formosat 3 cosmic gps ro technique during a solar minimum year 1 in this paper presented for the first time the three dimensional global morphology and seasonal variations of scintillation index s4 index measured from the signal to noise ratio snr intensity fluctuations of l1 channel of gps radio occultation ro signals using formosat 3 cosmic in short f3 c satellites for a low solar activity year 2008 the s4 index which confined around 30 magnetic latitudes is found to start around post sunset hours 1900 mlt magnetic local time and often persists till post midnight hours 0300 mlt between 150 and 350 km altitudes during equinox and northern winter seasons while no activity is observed during southern winter season however high latitudes are characterized with no scintillation activity beyond 150 km during any season which implying that in the solar minimum period the drives of instabilities in the auroral cusp and polar cap regions namely the gradient drift and velocity shear are absent the s4 index at f region altitudes during magnetically quiet times is more intense and extends to higher latitudes than that observed during disturbed time consistent with earlier studies the equatorial s4 index appears below the peak of f2 layer hmf2 during most of the seasons although the associated intensities and the time of maximum occurrences are relatively higher and earlier during vernal equinox followed by autumn equinox this equinoctial asymmetry could be primarily attributed to the asymmetries in eastward drift velocities thermospheric meridional winds and plasma densities further the global maps of s4 index at e region altitudes between 75 and 125 km show strong seasonal variations with highest activity during northern and southern summer solstice in the middle latitudes while it appears on both sides of magnetic equator with less or no activity at and around the equator during equinox seasons the absence of s4 index along the equator can be understood in terms of the vanishing vertical component of the magnetic field lines that can inhibit the vertical movement and layered deposition of ionized particles of thin irregular electron density layers such as es layers keeping in view the importance of these valuable database we would like to emphasize that the f3 c gps ro technique can be used to study the ionospheric irregularities at ghz frequency globally directly from the high rate l1 data which reiterating its importance as a powerful tool to explore the terrestrial ionosphere on a global scale

on monthly seasonal longitudinal variations of equatorial irregularity occurrences and their relationship with the postsunset vertical drift velocities 1 monthly variation of global equatorial density irregularity distribution has been obtained with the data taken by rocsat 1 at the 600 km topside ionosphere from march 1999 to june 2004 during high to moderate solar activity years this global distribution of monthly irregularity occurrence rate not only provides the best spatial temporal distribution in existence so far but also serves to fill the gap of irregularity distribution missing over some eastern pacific region where no ground observation is available the 5 1 2 year result of the monthly occurrence pattern indicates a smooth variation across the longitudes contrary to some beliefs that a drastic change in irregularity occurrence pattern has occurred across the eastern pacific longitudes excellent agreement is noted for the current results with aarons 1993 conjectured sketch of the global scintillation distribution furthermore the seasonal longitudinal s l variations of quiet time postsunset vertical drift velocities are found to track closely with the s l variations of irregularity occurrences linear regression analysis between the vertical drift velocity and the irregularity occurrence rate indicates that the vertical drift velocities at three different longitude zones of magnetic declination have good correlations with the irregularity occurrences for all seasons this implies that the averaged postsunset vertical drift velocity is a good indicator for the occurrences of equatorial density irregularities in a longitude region of similar magnetic declination the smooth variation of monthly seasonal longitudinal distributions of the vertical drift velocities that results in the smooth variation of topside density irregularity occurrences implies that the global occurrence pattern of either distribution is controlled by the smooth global variation of the postsunset ionospheric condition related to the magnetic declination effect and the seasonal variation of the ionospheric density level at the dip equator located with respect to the geographic equator there seems little need to include the global distribution of instability perturbation seeds from atmospheric disturbances to complete the global distribution of topside irregularity occurrences

petrography and reservoir physics i objective classification of reservoir porosity porosity observed in thin section can be objectively classified using a combination of digital acquisition procedures and pattern recognition algorithms pore types are derived from the frequency distributions of sizes and shapes of patches of porosity exposed in thin section each pore type is represented by a characteristic distribution of sizes and shapes found in thin section most sandstone reservoirs contain fewer than six pore types much of the intersample variability in reservoir physics is associated with changes in pore type abundance the advantages of this approach to porosity classification are 1 the criteria for classification are objectively defined 2 classification procedure is rapid accurate and precise 3 pore types are understood easily in ter s of conventional genetic classification schemes and 4 pore type data are related strongly to petrophysical properties

systematic diagenetic changes in the grain scale morphology and permeability of a quartz cemented quartz arenite the material properties of sedimentary rocks are controlled by a range of parameters including grain size sorting and modification of the original sediment through the diagenetic processes of compaction and cementation to isolate the effects of diagenesis and explore how they modify permeability we quantified changes in grain and pore morphology accompanying progressive diagenesis of a simple system a well sorted variably quartz cemented quartz arenite of relatively uniform grain size the most common type of authigenic cement in sandstones quartz overgrowths is responsible for significant porosity and permeability reduction the distribution of overgrowths is controlled by available pore space and the crystallographic orientations of individual quartz grains we show that progressive quartz cementation modifies the grain framework in consistent predictable ways detailed microstructural characterization and multiple regression analyses demonstrate that both the number and length of grain contacts increase as the number of pores increases and the number of large well connected pores decreases with progressive diagenesis the aforementioned changes progressively alter pore shape and reduce pore size variability and bulk permeability these systematic variations in the pore network correlate with changes in permeability such that we can use our data to calibrate the kozeny carmen relation demonstrating that it is possible to refine predictions of permeability based on knowledge of the sedimentary system

quiet time equatorial f region vertical plasma drift model derived from rocsat 1 observations 1 we have used five years of measurements on board the rocsat 1 satellite to develop a detailed quiet time global empirical model for equatorial f region vertical plasma drifts this model describes the local time seasonal and longitudinal dependence of the vertical drifts for an altitude of 600 km under moderate and high solar flux conditions the model results are in excellent agreement with measurements from the jicamarca radar and also from other ground based and in situ probes we show that the longitudinal dependence of the daytime and nighttime vertical drifts is much stronger than reported earlier especially during december and june solstice the late night downward drift velocities are larger in the eastern than in the western hemisphere at all seasons the morning and afternoon december solstice drifts have significantly different longitudinal dependence and the daytime upward drifts have strong wave number four signatures during equinox and june solstice the largest evening upward drifts occur during equinox and december solstice near the american sector the longitudinal variations of the evening prereversal velocity peaks during december and june solstice are anti correlated which further indicates the importance of conductivity effects on the electrodynamics of the equatorial ionosphere

on the enigma of day to day variability in equatorial spread f 1 we show that large scale wave structure lsws in plasma density in the bottomside f layer is a hitherto unheralded contributor to the long standing enigma of day to day variability in equatorial spread f esf little is known about lsws it seems to appear in altitude near a vertical shear in zonal plasma drift during the post sunset rise of the f layer and its growth via an interchange instability appears to predispose quasi periodically spaced regions to development of plasma bubbles first indications are that lsws development is necessary and sufficient for esf occurrence we suggest that variability in lsws development perhaps together with the shear in zonal drift may contribute to day to day esf variability a need revealed by this study is that a cluster of distributed sensors not isolated ones is necessary to pursue the problem of day to day variability