the influence of fluid sensitive dispersion and attenuation on avo analysis summary analysis of seismic data suggests that hydrocarbon deposits are often associated with higher than usual values of attenuation but this is generally ignored during amplitude versus offset avo analysis the effect can be modelled with equivalent medium theory based on the squirt flow concept but the excess attenuation is associated with strong velocity dispersion consequently when we study reflections from the interface between such an equivalent medium and an elastic overburden we find that the reflection coefficient varies with frequency the impact of this variation depends on the avo behaviour at the interface class i reflections tend to be shifted to higher frequency while class iii reflections have their lower frequencies amplified we calculate synthetic seismograms for typical models using the reflectivity method for materials with frequency dependent velocities and attenuations and find that these effects are predicted to be detectable on stacked data two field data sets show frequency anomalies similar to those predicted by the analysis and we suggest that our modelling provides a plausible explanation of the observations

evaluation of carbonate pore system under texture control for prediction of microporosity aspect ratio and shear wave velocity abstract this work evaluates a suite of carbonate rocks from albian age in the campos basin brazil complemented by data from the literature totaling 472 samples with detailed description of diagenetic features quantitative mineralogy analyses and p and s wave velocities vp and vs measured at three ranges of effective pressure loading low 5 7 5 mpa moderate 20 mpa and high 40 50 mpa values digital image analysis dia was applied on microtomography ct images to quantitatively describe the macro mesopore system of the albian carbonates and was extended to characterize different textures from literature data to estimate reference values for carbonates the methodology utilized to predict the aspect ratio of microporosity assumes three pore space scales in two representative scenarios 1 measured macro mesopore aspect ratio from dia and 2 predicted microporosity aspect ratio using vp measurement as the main input parameter the differential effective medium model demm is combined with analytical theories of data analysis to characterize microporosity shear modulus and microporosity aspect ratio calibrated by this methodology were used to predict vs which was compared to experimental data resulting in a good match for all samples polynomial curves are fitted with a variety of carbonate textures by velocities at effective pressure and bulk porosity crossplots establishing important relationships for velocity prediction the effects of effective pressure on the pore system within dry plugs of albian samples were evaluated by combining triaxial measurements at 0 10 mpa relative pore volume reduction rpvr and microporosity aspect ratio prediction according to the results micropores that exhibit low aspect ratio tend to close with stress and cause an increase on vp and vs a wide textural heterogeneity of data base and different digital image analysis and resolutions were employed successfully combining rock physics methodologies and concepts to characterize carbonate pore system as microporosity and pressure effects

carbon concentrations and isotopic ratios in fluid inclusion bearing upper mantle xenoliths along the northwestern margin of north america abstract at least three and perhaps four carbon components have been identified by step heating extraction of upper mantle xenoliths from nunivak island and prindle volcano alaska fort selkirk and alligator lake yukon jacques lake and castle rock british columbia one component released mostly between 200 and 600 c comprises 30 95 of the total carbon extracted and has 13c values between 28 and 23 this carbon corresponds mostly to complex carbon compounds condensed on surface minerals and cracks and will be referred to as isotopically light carbon ilc co2 released between 600 and 800 c is derived from interstitial carbonates it has variable 13c and concentrations that generally do not exeed 24 ppm c the carbon extracted from 1000 to 1450 c is a mixture of co2 from fluid inclusions and the ilc fraction the amount of ilc released in this temperature interval never exceeds 10 ppm because part of this ilc may have been introduced from the host lava the 13c variations of this carbon extracted above 800 c is not only characteristic for the original carbon present in fluid inclusion poor xenoliths however xenoliths enriched in co2 fluid inclusions show c isotopic variations at the highest temperature steps that fall in a relatively narrow range between 10 and 4 depending upon the fluid inclusion abundance the concentration of high temperature c derived from fluid inclusions varies from 0 1 to 66 ppm anhydrous spinel lherzolites and harzburgites generally contain

origin of late cements in oolitic reservoir facies a fluid inclusion and isotopic study mid jurassic southern england abstract during bathonian times a carbonate ramp was constructed over southern england by the progradation of oolitic skeletal and oncoidal grainstones and packstones the grainstones provide a significant hydrocarbon reservoir facies although these carbonates suffered early diagenesis under marine and more significantly meteoric conditions later diagenetic cements such as saddle dolomite and more particularly mildly ferroan calcite are volumetrically more important in causing the occlusion of porosity on a regional scale we have analysed systematically these late cements employing a number of techniques standard petrography including cl and uvl c and o isotopes sr isotopes and fluid inclusion studies integration of the results of these approaches constrains both the pore fluid composition and the temperature regime at the time of cement precipitation saddle dolomite was emplaced before the mildly ferroan calcite at temperatures between 75 and 85 c and from highly saline brines 18 19 5 wt nacl equivalents mildly ferroan calcite precipitated over a generally lower temperature range and from less saline brines 6 5 15 wt nacl equivalents five types of fluid inclusion have been recognised 1 monophase aqueous liquid 2 monophase oil liquid 3 two phase aqueous liquid vapour 4 two phase oil liquid hydrocarbon vapour 5 three phase aqueous oil liquid hydrocarbon aqueous liquid vapour during burial warm 75 c brines passed through the limestones and precipitated saddle dolomite later slightly cooler oil rich brines precipitated mildly ferroan calcite wherever hydrocarbon accumulation had not already occurred our sr isotopic studies suggest that both these brines were derived from underlying and more deeply buried lias shales and that these brines were generated from late jurassic times onwards brine movements are likely to have been largely complete by the late cretaceous

the transformation of amphibolite facies gneiss to charnockite in southern karnataka and northern tamil nadu india amphibolite facies metamorphic grade gives way southward to the granulite grade in southern karnataka as acid gneisses develop charnockite patches and streaks and basic enclaves develop pyroxenes petrologic investigations in the transitional zone south of mysore have established the following points 1 the transition is prograde amphibole bearing gneisses intimately associated with charnockite at kabbal and several similar localities are not retrogressive after charnockite as proved by patchy obliteration of their foliation by transgressive very coarse grained charnockite high fluorine content of biotite and amphibole in gneisses and high large ion lithophile element contents in gneisses and charnockites these features are in contrast to very low fluorine in retrogressive amphiboles and biotites very low large ion lithophile element contents and zonal bleaching of charnockite in clearly retrogressive areas as at bhavani sagar tamil nadu 2 metamorphic temperatures in the transitional areas were 700 800 c pressures were 5 7 kbar and h2o pressures were 0 1 0 3 times total pressures based on thermodynamic calculations using mineral analyses dense co2 rich fluid inclusions in the kabbal rocks confirm the low h2o pressures at the first appearance of orthopyroxene farther to the south in the nilgiri hills and adjacent granulite massif areas peak metamorphic temperatures were 800 900 c pressures were 7 9 kbar and water pressures were very low so that primary biotites and amphiboles those with high f contents are rare 3 the incipient granulite grade metamorphism of the transitional areas was introduced by a wave of anatexis and k metasomatism this process was arrested by drying out under heavy co2 influx charnockites so formed are hybrids of anatectic granite and metabasite of metabasite and immediately adjacent gneiss or are virtually isochemical with pre existing gneiss despite gross recrystallization to granulite mineralogy these features show that partial melting and metasomatism are attendant rather than causative in charnockite development copious co2 from a deep crustal or mantle source pushed ahead of it a wave of more aqueous solutions which promoted anatexis granulite metamorphism of both neosome and paleosome followed the process is very similar to that deduced for the madras granulites by weaver 1980 the massif charnockites for the most part extremely depleted in lithophile minor elements show many evidences of having gone through the same process

seismic wave attenuation in carbonates 1 the effect of pore fluids on seismic wave attenuation in carbonate rocks is important for interpreting remote sensing observations of carbonate reservoirs undergoing enhanced oil recovery here we measure the elastic moduli and attenuation in the laboratory for five carbonate samples with 20 to 30 porosity and permeability between 0 03 and 58 1 mdarcy contrary to most observations in sandstones bulk compressibility losses dominate over shear wave losses for dry samples and samples fully saturated with either liquid butane or brine this observation holds for four out of five samples at seismic 10 1000 hz and ultrasonic frequencies 0 8 mhz and reservoir pressures attenuation modeled from the modulus data using cole cole relations agrees in that the bulk losses are greater than the shear losses on average attenuation increases by 250 when brine substitutes a light hydrocarbon in these carbonate rocks for some of our samples attenuation is frequency dependent but in the typical exploration frequency range 10 100 hz attenuation is practically constant for the measured samples

kiglapait geochemistry vi oxygen isotopes abstract the kiglapait layered intrusion is the first major intrusion found to have all whole rock and calculated liquid 18o values close to a normal uncontaminated gabbroic value of 6 0 the intrusion experienced no detectable oxygen isotope exchange with its surrounding rocks and cooling of the magma was conductive the 18o values of average whole rocks vary smoothly from 6 0 at the base of the lower zone to 6 3 at the top of the upper zone the calculated liquid 18o values lie practically superimposed on the whole rock trend the whole rock data and the modelled 18o of the magma and cumulates rigorously demonstrate that the effect of incoming cumulus phases such as magnetite and augite on the 18o of the liquid and rocks during fractional crystallization is negligible the cancelling effects of complementary modal variations among the mafic mineral phases and feldspar keep the 18o of the whole rocks constant to within 0 1 the minor change in 18o that does occur with fractionation is consistent with the enrichment of residual liquids in feldspar component and the increasing fractionation factor liquid fsp with falling temperature the 18o values of the country rocks bracket the estimated 18o of the kiglapait magma modelling with oxygen isotopes indicates that contamination of the intrusion indicated by published radiogenic sr and nd isotopic data was minor the most probable contaminant had 18o 7 7 and the contamination most likely occurred at 99 solidified subsolidus oxygen isotope exchange with an external source appears to have been very minor

co 2 melts and granulite metamorphism the discovery that granulites contain co2 rich fluid inclusions while amphibolites in the same terrane contain h2o rich or mixed co2 h2o inclusions1 has led to a debate over the processes that produce granulite metamorphism one theory2 4 maintains that granulites form as the result of a massive influx of co2 from lower crustal or mantle sources such an influx could be responsible for the marked depletion many granulite terranes show in large ion lithophile elements2 5 others however maintain that granulite facies metamorphism is a necessary complement of crustal melt ing6 9 recently lamb and valley10 11 have argued that granulite metamorphism may be a product of any one of three processes co2 streaming partial melting or recrystallization of originally dry rocks here we suggest that these processes need not be independent and that they all may be related to the passage of melts through the crust

seismic attenuation and pore fluid viscosity in clay rich reservoir sandstones the frequency dependence of seismic attenuation in a suite of clay rich reservoir sandstones was investigated in the laboratory compressional and shear wave velocities vp and vs and quality factors qp and qs were measured as functions of pore fluid viscosity at an effective pressure of 50 mpa and at an experimental frequency of about 0 8 mhz using the pulse echo technique the experimental viscosity ranged from 0 3 to 1000 centipoise which gives equivalent frequencies for a water saturated sandstone of 2 6 mhz to 780 hz assuming a global flow loss mechanism two types of behavior were observed high permeability greater than 100 millidarcies sandstones tend to show variable qp and qs which are similar in magnitude to those predicted by the biot theory over the viscosity range 0 3 to about 20 centipoise equivalent frequency range 2 6 mhz to about 39 khz low permeability less than 50 millidarcies sandstones tend to show almost constant qp and qs over the experimental viscosity range that are

scientific drilling in the great rift valley the 2005 lake malawi scientific drilling project an overview of the past 145 000years of climate variability in southern hemisphere east africa abstract the recovery of detailed and continuous paleoclimate records from the interior of the african continent has long been of interest for understanding climate dynamics of the tropics and also for constraining the environmental backdrop to the evolution and spread of early homo sapiens in 2005 an international team of scientists collected a series of scientific drill cores from lake malawi the first long and continuous high fidelity records of tropical climate change from interior east africa the paleoclimate records which include lithostratigraphic geochemical geophysical and paleobiological observations documented in this special issue of palaeo3 indicate an interval of high amplitude climate variability between 145 000 and 60 000 years ago when several severe arid intervals reduced lake malawi s volume by more than 95 these intervals of pronounced tropical african aridity in the early late pleistocene around lake malawi were much more severe than the last glacial maximum lgm a well documented period of drought in equatorial and northern hemisphere tropical east africa after 70 000 years ago climate shifted to more humid conditions and lake levels rose during this latter interval however wind patterns shifted rapidly and perhaps synchronously with high latitude shifts and changes in thermohaline circulation this transition to wetter more stable conditions coincided with diminished orbital eccentricity and a reduction in precession dominated climatic extremes the observed climate mode switch to decreased environmental variability is consistent with terrestrial and marine records from in and around tropical africa

shear wave velocity and attenuation structure of ischia island using broad band seismic noise records abstract in volcanic areas accurate localization of earthquakes requires detailed velocity and possibly attenuation models taking into account wide lithological variability and high geothermal gradients ischia island campania region italy is a seismically active volcano recently affected by a mw 3 9 event casamicciola august 21 2017 1 km depth due to the lack of a specific velocity model the earthquakes occurred on the island were localized using the one developed for the nearby campi flegrei caldera the aim of this work is the definition of a mean representative 1d shear wave velocity vs and attenuation q model of the shallower crust up to 2 km depth of ischia seismic noise array and spectral ratios techniques were applied to broad band seismic signals recorded by temporary and permanent networks updated after the august 2017 earthquake the values of both shear wave velocity vs and quality factor q are realistic with q values comparable with those obtained for campi flegrei and stromboli volcanic areas by taking into account stratigraphic information from deep wells and ultrasonic measurements of velocity on granite and trachytic lava samples a geological interpretation of the resulting velocity model is provided such a model can have significant implications for understanding the dynamics of a volcano mainly those leading to seismic activity

the measurement of velocity dispersion and frequency dependent intrinsic attenuation in sedimentary rocks a series of experiments to determine the elastic properties of a sequence of saturated sedimentary rocks over as wide a frequency range as possible was carried out at the imperial college borehole test site these experiments fall into four categories vertical seismic profiles vsps within the frequency range 30 280 hz crosshole surveys 0 2 2 3 khz sonic logging 8 24 khz and laboratory measurements 300 900 khz the intrinsic attenuation and velocity of compressional and shear waves were measured whenever possible velocity dispersion is observed for both compressional and shear waves the intrinsic attenuation of compressional waves is frequency dependent with a peak in the attenuation in the sonic frequency band the data were modeled assuming the attenuation is caused by local fluid flow in pores of small aspect ratio the modeling indicates that the intrinsic attenuation may be dominated by cracks with aspect ratios of around 10 super 3 to 10 super 4

modelling the growth of natural diamonds abstract in the following article we have reviewed the mineralogical chemical physical and isotopic evidence concerning the growth of coated diamonds and have combined it to produce a comprehensive model for their formation we suggest that the coatings formed during a period of metasomatic activity which preceded the eruption of their host kimberlites ultrapotassic co21bh2o rich fluids resulted from the crystallisation of diamond free kimberlitic magmas intruded into the deep lithosphere the kimberlite magmas were derived from an asthenospheric source which was globally quite uniform in terms of the isotopic composition of carbon and nitrogen 13c and 15n both being close to 5 the fluids produced during crystallisation invaded overlying diamondiferous lithosphere in which different regions could contain diamond populations with different mineralogical isotopic and physical characteristics depending on the history of their source region either intrinsic to their formation or due to reaction with reduced lithosphere the fluids were super saturated with carbon although not to such an extent as to allow for the nucleation of new diamonds when these super saturated fluids encountered older diamonds new and fibrous diamond growth commenced immediately this period of metasomatic activity was followed rapidly by one or more successful kimberlite eruptions which transported the now coated diamonds to the surface in the crust carbonates within hypabyssal facies kimberlites will have 13c values which reflect the deep seated source 5 whereas those within diatreme facies may be more variable due to outgassing of co2 during eruption finally we consider isotope fractionation related to growth we suggest that approaches based on equilibrium are not appropriate to diamonds but rather that it is better to consider the nature and kinetics of the surface reactions leading to growth where diamond growth is very rapid as may be the case for fibrous diamond we propose that there will be no difference in isotopic composition between the fluid and diamond and not the 3 4 13c depletion in the latter predicted by theoretical treatments octahedral diamonds are more problematic since far less is known about their growth conditions however we show in principle how the content and perhaps isotopic composition of nitrogen within a diamond may be controlled by its growth rate we are uncertain whether 13 c would be affected

effects of heterogeneities and anisotropy on sonic and ultrasonic attenuation in rocks we compare the attenuation obtained by the resonant bar technique in the sonic frequency range 3 10 khz with that obtained by the pulse transmission technique in the ultrasonic frequency range 500 khz on a set of 30 rocks including sandstones and limestones the sonic and ultrasonic measurements were performed under high effective pressure on adjacent water saturated samples from each rock block compressional ultrasonic attenuation was typically larger than extensional sonic attenuation particularly in limestones this is attributed to the presence of density heterogeneities that can be seen on x ray scanner imagery heterogeneities at the scale of the ultrasonic wavelength produce diffraction of the waves and as a consequence high attenuation values at ultrasonic frequencies large apparent shear attenuation values compared to compressional attenuation values are explained by shear wave splitting

experimental measurements of seismic attenuation in microfractured sedimentary rock ultrasonic compressional and shear wave attenuation in water saturated carrara marble increase with increasing crack density and decreasing effective pressure between 0 4 and 1 0 mhz empirical linear relationships between 1 q and crack density cd were found to be cd 1 96 or 0 63 x 1 q for compressional waves andcd 6 7 or 1 5 x 1 q for shear waves

carbon isotopes in xenoliths from the hualalai volcano hawaii and the generation of isotopic variability abstract the isotopic composition of carbon has been determined in a suite of xenoliths from lava of the 1800 1801 kaupulehu eruption of hualalai volcano hawaii several lithologies are represented in the suite including websterite dunite wehrlite pyroxenite and gabbro in addition there are composite xenoliths in which contacts between lithologies are preserved most of the xenoliths represent deformed cumulates the contact relations in the composite samples indicate that the lithologies originated from the same source region which based on pressures determined from fluid inclusions is estimated to be at a depth of 20 km or near the crust mantle boundary samples were heated in steps from 200 to 1475 c to obtain separation of the different carbonaceous phases and the isotopic composition of carbon released at each step was determined grossular glass was found to be a suitable flux to fuse refractory samples upon heating carbon exhibits the typical bimodal evolution behavior observed in other studies of xenoliths and basalts carbon extracted from all samples at temperatures below 900 c is characterized by a 13 c of about 25 vs pdb and is thought to be composed dominantly of graphitic and organic material which is known to be present on virtually all cracks the 13 c of the carbon fraction extracted at 1200 c and above from wehrlite and dunite is in the range 1 5 to 5 2 whereas that extracted from websterite ranges from 22 to 26 similarly in one composite sample the compositions of dunite and websterite were found to be 2 4 and 7 0 respectively the large difference can be associated with specific petrographic features unique to each lithology in wehrlite and dunite carbon exists mostly as co 2 rich inclusions in arrays representing partially annealed microcracks the websterite xenoliths contain megascopic zones of large irregularly shaped inclusions the zones traverse entire thinsections and are interpreted to represent fractures annealed at depth most of the carbon is believed to exist in the inclusion rich zones and to consist of carbonaceous material precipitated from fluid the observations and isotopic results demonstrate that isotopic variability can be generated by multistage fractionation processes such as degassing of co 2 from magma and precipitation of co 2 rich fluids to form graphitic compounds such processes operated over regions the scales of which were determined by style and intensity of deformation and by lithology

high resolution seismic attenuation imaging from wide aperture onshore data by visco acoustic frequency domain full waveform inversion summary here we assess the potential of the visco acoustic frequency domain full waveform inversion fwi to reconstruct p wave velocity vp and p wave attenuation factor q from surface onshore seismic data first we perform a sensitivity analysis of the fwi based upon a grid search analysis of the misfit function and several synthetic fwi examples using velocity and q models of increasing complexity subsequently we applied both the acoustic and viscoacousticfwitorealsurfacewide apertureonshoreseismicdatafromthepolishbasin wherea strongattenuationoftheseismicdataisobserved thesensitivityanalysisofthevisco acoustic fwi suggests that the fwi can jointly reconstruct the velocity and the attenuation factor if the signatureoftheattenuationissufficientlystronginthedata asyntheticexamplecorresponding to a homogeneous background model with an inclusion shows a reliable reconstruction of vp and q in the inclusion when q is as small as 90 and 50 in the background model and in the inclusion respectively a first application of acoustic fwi to real data shows that a heuristic normalization of the data with offset allows us to compensate for the effect of the attenuation in the data and reconstruct a reliable velocity model alternatively we show that visco acoustic fwi allows us to reconstruct jointly both a reliable velocity model and a q model from the true amplitude data we propose a pragmatical approach based upon seismic modelling and source wavelet estimation to infer the best starting homogeneous q model for visco acoustic fwi we find the source wavelet estimation quite sensitive to the quality of the velocity and attenuation models used for the estimation for example source to source wavelets are significantly more consistent when computed in the final fwi model than in the initial one a good kinematic and amplitude match between the early arriving phases of the real and time domain synthetic seismograms computed in the final fwi model provides an additional evidence of the reliability of the final fwi model we find the recovered velocity and attenuation models consistent with the expected lithology and stratigraphy in the study area we link high attenuation zones with the increased clay content and the presence of the mineralized fluids

the relationships between the velocities attenuations and petrophysical properties of reservoir sedimentary rocks1 the authors have measured the velocities and attenuations of compressional and shear waves in 29 water saturated samples of sandstones and shales at a confining pressure of 60 mpa and at frequencies of about 0 85 mhz the measurements were made using a pulse echo method in which the samples diameter 5 cm length 1 5 cm to 2 5 cm were placed between perspex buffer rods inside a high pressure cell the velocity of each seismic wave was determined from the travel time difference of equivalent phase points corrected for diffraction effects of the signals reflected from the top and from the base of each samples attenuation was determined in a similar way by comparison of the diffraction corrected amplitudes of the signals the attenuation data are presented as quality factors q sub p and q sub s for compressional and shear waves respectively and shear waves respectively the results show that qs is strongly correlated with v sub s that q sub p is weakly correlated with v sub p and that q sub p is strongly correlated with q sub s center dot q sub p is strongly dependent on the volume percentage of the assemblage of intra pore minerals whether they are clays or carbonates it is concluded that themore attenuation mechanism is due to the local fluid flow arising from the differential dilation of the solid rock frame and the intra pore mineral assemblage which is a results of their very different elastic moduli less

laboratory measurements of the effect of fluid saturation on elastic properties of carbonates at seismic frequencies abstract a significant portion of the world s hydrocarbon reserves are found in carbonate reservoirs yet analysis of the petrophysical properties of these reservoirs is associated with a number of challenges some of these challenges stem from physical and chemical interactions between the carbonate rock matrix and pore fluids which can affect elastic properties of the rock hence the study of the pore fluid effects on the elastic properties of carbonates is important for understanding a change of the field performance properties of carbonate reservoir caused by fluid movements during hydrocarbon extraction in producing fields in this laboratory study we investigate the applicability of gassmann s model for predictions of the elastic moduli of water and hydrocarbon saturated savonnieres limestone and the influence of partial water saturation on elastic and anelastic properties of the rock we present the results of two sets of laboratory experiments on the savonnieres oolitic limestone where we i evaluate the effect of full water and n decane saturation on elastic moduli and attenuation at seismic 0 1 hz 120 hz and ultrasonic 0 5 mhz frequencies and ii quantify the dependence of elastic moduli and extensional attenuation on water saturation at two seismic frequencies of 1 hz and 10 hz we demonstrate that the change in the bulk modulus of limestone fully saturated either with n decane or water is in agreement with gassmann s fluid substitution theory whereas the shear modulus is noticeably reduced the measurements with partial saturation show that the bulk modulus decreases with increasing water saturation to a lesser extent than the young s and shear moduli our results show that extensional attenuation in the samples with closed boundaries is insignificant under dry and fully saturated conditions but is influenced greatly by the liquid content when saturation is between 0 and 20 or 95 and 100

acoustic properties of ancient shallow marine carbonates effects of depositional environments and diagenetic processes middle jurassic paris basin france abstract examination of petrophysical properties acoustic velocity porosity permeability and density and petrographical characteristics texture facies composition and diagenesis of more than 250 core plugs from the middle jurassic carbonates of the eastern paris basin provides insights into the parameters controlling acoustic velocities in relatively low porosity carbonate rocks 20 the pore type observations reveal distinct acoustic velocities in samples with intergranular macropores and samples with micropores in subhedral micrite such that velocities in microporous mudstone wackestone lagoonal deposits are greater than in macroporous grainstone shoal samples at a given porosity range 15 20 the standard wyllie and raymer transforms fit very well with the linear regression between acoustic velocity and porosity from mudstone or lagoonal facies marls and fine grained deposits interpreted as lagoonal facies include statistically significant correlation r 0 9 between velocity and porosity however the data suggest that the wide scatter in velocity porosity relationship from grainstones are not the result of different sorting grain size pore type dolomite content or clay content instead early cementation greatly influences acoustic properties during diagenesis and are interpreted to account for the high variability of velocities over a given porosity range specifically at a given porosity acoustic velocities in compacted grainstone that did not undergo early cementation are higher than in early cemented grainstone petrographic observations suggest that early cementation limits mechanical compaction creating a heterogeneous medium from the earliest stages of diagenesis non touching grains preservation of intergranular macropores that are partially to totally filled by later blocky calcite cement the abundant interfaces between micritized ooids early cement fringes and blocky calcites in grainstones may induce significant wave attenuation as a result the standard time average equations fail to predict the effect of diagenetic features such as early cementation on sonic velocity conversely an absence of early cementation favors mechanical compaction grain to grain contact and suturing the result is a homogeneous micritized grain supported network that may facilitate wave propagation through demonstration of the key role of early cementation in the explanation of variability in acoustic properties the results of this study illustrate the complicated factors influencing velocity transforms in carbonates wyllie and raymer i e classical tools for predicting reservoir properties these insights on the interpretation of vp and the refinement of velocity porosity transforms in grainstone units may be broadly applicable to enhancing seismic based exploration in carbonate successions

velocities of compressional and shear waves in limestones carbonate rocks are important hydrocarbon reservoir rocks with complex textures and petrophysical properties porosity and permeability mainly resulting from various diagenetic processes compaction dissolution precipitation cementation etc these complexities make prediction of reservoir characteristics e g porosity and permeability from their seismic properties very difficult to explore the relationship between the seismic petrophysical and geological properties ultrasonic compressional and shear wave velocity measurements were made under a simulated in situ condition of pressure 50 mpa hydrostatic effective pressure at frequencies of approximately 0 85 mhz and 0 7 mhz respectively using a pulse echo method the measurements were made both in vacuum dry and fully saturated conditions in oolitic limestones of the great oolite formation of southern england some of the rocks were fully saturated with oil the acoustic measurements were supplemented by porosity and permeability measurements petrological and pore geometry studies of resin impregnated polished thin sections x ray diffraction analyses and scanning electron microscope studies to investigate submicroscopic textures and micropores it is shown that the compressional and shear wave velocities vp and vs respectively decrease with increasing porosity and that vp decreases approximately twice as fast as vs the systematic differences in pore structures e g the aspect ratio of the limestones produce large residuals in the velocity versus porosity relationship it is demonstrated that the velocity versus porosity relationship can be improved by removing the pore structure dependent variations from the residuals the introduction of water into the pore space decreases the shear moduli of the rocks by about 2 gpa suggesting that there exists a fluid matrix interaction at grain contacts which reduces the rigidity the predicted biot gassmann velocity values are greater than the measured velocity values due to the rock fluid interaction this is not accounted for in the biot gassmann velocity models and velocity dispersion due to a local flow mechanism the velocities predicted by the raymer and time average relationships overestimated the measured velocities even more than the biot model

estimation of velocity and attenuation coefficient maps from crosswell seismic data using a classic traveltime tomography and an attenuation tomography based on the centroid frequency shift method velocity and attenuation coefficient tomograms are computed from two crosswell seismic data because of the limited frequency band of the data and the difficulties of picking the first transmitted events velocity and attenuation coefficients are assumed independent of frequency during the processing meaning that the attenuation linearly increases with frequency this reduces the possibility of using a theoretical model to interpret the results moreover attenuation results include both scattering and viscous attenuation nevertheless the attenuation coefficient tomograms confirm some of the features seen in the velocity tomograms the sand zones show a higher attenuation although the shale zones and the seals of the reservoirs show a lower attenuation with a kramers kronig relation for the dispersion formula based on frequency independent attenuation coefficient the smooth part of the m

hydraulic and acoustic properties as a function of porosity in fontainebleau sandstone laboratory measurements have been made of the permeability k free porosity l compressional velocities vp or ve and compressional attenuations qp or qe in fontainebleau sandstone over a continuous range of porosities from 3 to 28 this large variation was achieved without any composition change fontainebleau sandstone is made of fine quartz grains with regular grain size 250 m permeability was measured with a falling head permeameter velocities and attenuations were obtained either through an ultrasonic experiment for frequencies around 500 khz or through a resonant bar technique experiment for frequencies around 5 khz and in both cases with varying water saturation the results show an excellent correlation between permeability k and total porosity for all our samples for low porosities 3 to 9 permeability in millidarcies is 2 75 10 5 7 33 while for high porosities 9 to 28 permeability k in millidarcies is given by 0 303 3 05 the correlation is also excellent between free porosity and total porosity on the other hand the correlation between acoustic properties and total porosity is not as clear as for hydraulic properties whatever the frequency 500 khz or 5 khz or the water saturation on the average velocity decreases and attenuation roughly increases with increasing total porosity velocity and attenuation values are related to the variation of grain contact structure and two samples with the same porosity and permeability may exhibit different velocities and attenuations the clear correlation between hydraulic properties and porosity is related to constant grain size while the lack of correlation for acoustic properties emphasizes the importance of the microstructure

the squirt flow mechanism macroscopic description we introduce a poroelasticity model that incorporates the two most important mechanisms of solid fluid interaction in rocks the biot mechanism and the squirt flow mechanism this combined biot squirt bisq model relates compressional velocity and attenuation to the elastic constants of the drained skeleton and of the solid phase porosity permeability saturation fluid viscosity and compressibility and the characteristic squirt flow length squirt flow length is a fundamental rock property that does not depend on frequency fluid viscosity or compressibility and is determined experimentally we find that the viscoelastic response of many sandstones is dominated by the squirt flow component of the bisq mechanism and that the viscoelastic properties of these rocks can be expressed through a single dimensionless parameter omega r 2 kappa where omega is angular frequency r is the characteristic squirt flow length and kappa is hydraulic diffusivity the biot mechanism alone does not give an adequate explanation of the observed velocity dispersion and attenuation and the viscoelastic behavior of many sandstones

fluid transfer in anorthosites and related rocks from rogaland southwest norway evidence from stable isotopes abstract c and h contents and c h and o isotopic compositions have been measured in sixteen samples of the ana sira massif type anorthosite rogaland southwest norway and in 30 surrounding rocks including the metamorphic envelope and magmatic intrusions related to the anorthosite emplacement oxygen isotopic compositions of the ana sira anorthositic massif 18 o 6 1 0 3 and the tellnes orebody 18 o 5 2 0 2 are consistent with a pure mantle origin and show no evidence for exchange with hydrothermal fluids the associated rocks norites to charnockites have 18o values 5 2 0 9 similar to uncontaminated mantle derived magmas the h2o content of the rocks magmatic or metamorphic is lower than 0 54 suggesting the preservation of original granulite facies conditions during cooling and uplift due to their cumulate origin the anorthosites are very poor in carbon bulk c the carbon distribution observed at the contact between later intrusions and anorthosites as well as gneisses indicates that some co2 has fluxed from the later magmas over a distance of a few hundred metres this suggests only minimal lateral propagation of the fluids by diffusion or infiltration tentative carbon budgets suggest that most of the volatiles were transferred upwards into the overlying continental crust

an experimental investigation of factors influencing compressional and shear wave velocities and attenuations in tight gas sandstones results are presented for compressional and shear velocities and attenuations in fully brine saturated tight gas cores with porosities from 3 to 11 9 percent and clay contents from 1 to 38 percent the influence of porosity clay content frequency and stress on velocities and attenuations were examined using the amplitude spectra of p and s waves in the frequency domain attenuations of samples were obtained using the spectral ratio method for a few selected samples the attenuations were also measured using the length correlation method and these results were compared with the spectral ratio results in tight gas sandstones the attenuations obtained were 2 to 5 times greater than the attenuation obtained for berea sandstone in general the presence of clay softens the rock grain contacts causing smaller values of compressional vp and shear vs velocities as the clay content increases however the vp vs ratio was found to increase with clay content compressional and shear wave amplitude spectra

geochemical constraints on the genesis of the tellnes ilmenite deposit southwest norway the ilmenite norite of the tellnes fe ti deposit in the aana sira massif type anorthosite is connected to a main dike which varies in composition from monzonorite to quartz mangerite fine grained satellite dikes also occur in the vicinity of the main dike a number of rock samples have been analyzed for 36 major and trace elements an rb sr whole rock isochron 12 samples gives an age of 930 ma initial sr isotope compositions of the ilmenite noritc the main dike and the satellite dikes are 0 7050 0 7069 and 0 7100 respectively thus precluding a comagmatic origin without invoking contamination the evolution of the main dike can be accounted for by a closed system fractional crystallization of a monzonoritic magma that is without progressive contamination which gave rise to a series of liquids prior to intrusion mass balance calculation permits a two stage process in which noritic cumulates were subtracted trace element behavior is also consistent with such a model the satellite dikes represent undifferentiated monzonorites with trace elements and isotopic ratios implying a source material quite different from that of the main dike monzonorite the orebody is made up of ilmenite norite and small amounts of more plagioclase rich rocks which plot on linear trends in variation diagrams it is interpreted as a noritic cumulate which was enriched in dense oxide rich magma before being emplaced as a crystal mush the parental magma was noritic in composition unsaturated in apatite and probably derived from the crystallization of the enclosing massif anorthosite it is not precluded either that it could have given rise to the main dike monzonoritic liquid after being contaminated by lower crustal material nevertheless as an alternative it is not possible to reject two distinct parental magmas for the orebody and the main dike they would have evolved in two separated magma chambers before the tapping of these chambers and simultaneous injection of their contents in the dike the main dike illustrates the process by which a quartz mangerite can derive from a monzonoritic magma and gives support to a comagmatic origin of charnockitic and monzonoritic rocks

attenuation of p and s waves in limestones ultrasonic compressional and shear wave attenuation measurements have been made on 40 centimetre sized samples of water and oil saturated oolitic limestones at 50 mpa effective hydrostatic pressure confining pressure minus pore fluid pressure at frequencies of about 0 85 mhz and 0 7 mhz respectively using the pulse echo method the mineralogy porosity permeability and the distribution of the pore types of each sample were determined using a combination of optical and scanning electron microscopy a helium porosimeter and a nitrogen permeameter the limestones contain a complex porosity system consisting of interparticle macropores dimensions up to 300 microns and micropores dimensions 5 10 microns within the ooids the calcite cement and the mud matrix ultrasonic attenuation reaches a maximum value in those limestones in which the dual porosity system is most fully developed indicating that the squirt flow mechanism which has previously been shown to occur in shaley sandstones also operates in the limestones it is argued that the larger scale dual porosity systems present in limestones in situ could similarly cause seismic attenuation at the frequencies of field seismic surveys through the operation of the squirt flow mechanism

evidence for correlation of ultrasonic attenuation and fluid permeability in very low porosity water saturated rocks the measured amplitude a of ultrasonic pulses in intact and fractured samples of water saturated gabbro and granite is observed to decrease as the permeability increases according to the proportionality a 1 2 this relation is predicted by biot s theory of elastic waves in fluid saturated porous media and therefore suggests that biot s attenuation mechanism may play a significant role in low porosity materials at ultrasonic frequencies the evidence is not conclusive the limited data set studied here is also consistent with correlations of the form a e where 0 2 e 0 6 nevertheless the observed correlations may still provide a means for monitoring changes in permeability of low porosity underground repositories of radioactive waste

acoustic properties of carbonates effects of rock texture and implications for fluid substitution more than 250 plugs from outcrops and three nearby boreholes in an undisturbed reef of miocene tortonian age were quantitatively analyzed for texture mineralogy and acoustic properties we measured the p and s waves of carbonate rocks under dry humidified and brine saturated conditions at 10 mpa effective pressure with an ultrasonic pulse transmission technique 1 mhz the data set was compared with an extensive database of petrophysical measurements of a variety of rock types encountered in carbonate sedimentary sequences two major textural groups were distinguished on the basis of trends in plots of compressional wave velocity versus poisson s ratio a specific ratio of p wave over s wave velocity in granular rocks the framework of depositional grains is the main medium for acoustic wave propagation in crystalline rocks this medium is provided by a framework of interlocking crystals formed during diagenesis rock textures are connected to primary depositionalparameters and a diagenetic ov

post granulite facies fluid infiltration in the adirondack mountains granulite facies lithologies from the adirondack mountains of new york contain alteration assemblages composed dominantly of calcite chlorite sericite these assemblages document fluid infiltration at middle to upper crustal levels cathodoluminescence of samples from the marcy anorthosite massif indicates that the late fluid infiltration is more widespread than initially indicated by transmitted light petrography alone samples that appear unaltered in transmitted light show extensive anastomosing veins of calcite 0 05 mm wide along grain boundaries in crosscutting fractures and along mineral cleavages the presence of the retrograde calcite documents paleopermeability in crystalline rocks and is related to the formation of high density co sub 2 rich fluid inclusions recognition of this process has important implications for studies of granulite genesis and the geophysical properties of the crust

estimation of source parameters quality factor qs and site characteristics using accelerograms uttarakhand himalaya region abstract a study of source path and site characteristics was conducted for the uttarakhand himalaya region using accelerogram data from 15 earthquakes m l 3 5 these earthquakes were recorded at the 16 station accelerograph network operated by the indian institute of technology roorkee during 2005 2011 the average seismic moment m 0 of the studied earthquakes ranges between 1 20 10 22 and 1 02 10 24 dyn cm and the average moment magnitude m w is between 4 0 and 5 3 the estimated corner frequency f c varies from 1 1 to 3 3 hz radius of rupture r d from 0 5 to 1 4 km and stress drop from 6 to 172 bars indicating continuous seismic energy release in the uttarakhand region the interdependence between estimated source parameters is shown by determining scaling laws for the studied region the constant q 0 of the shear wave quality factor q s q 0 f n varies between 40 and 300 and exponent n varies between 0 85 and 1 5 providing an average relation of q s 174 f 1 27 however least square fitting of the observed data set in the frequency range 0 1 20 hz gives q s as 159 f 1 16 the value of q s demonstrates that the region is heterogeneous seismically active and attenuative the high frequency spectral fall off factor varies from 1 3 to 2 1 and the upper crustal attenuation factor from 0 023 to 0 07 s at different sites with an average of 0 044 s the site response characteristics are estimated by horizontal to vertical spectral ratio hvsr and generalized inversion ginv techniques results obtained from both the techniques show 1 1 correspondence the site amplification factor varies between 2 3 and 9 4 using hvsr and between 2 6 and 10 9 using ginv among different stations the predominant frequency ranges from 1 3 to 8 3 hz with hvsr and from 1 3 to and 9 0 hz with ginv online material tables of source path and site parameters with associated errors at different sites for the earthquakes recorded from december 2005 to march 2011 figures represent accelerogram records source spectra matching and site amplification patterns

application of the bisq model to clay squirt flow in reservoir sandstones 1 an understanding of the variations of compressional wave velocity and attenuation in sandstones as a function of pore fluid properties is important for the identification and monitoring of hydrocarbon reservoirs in order for seismic inversion to be well constrained a rock physics model that describes accurately the observed behavior is required a number of such models have been proposed but have not been extensively tested against laboratory data the unified biot and squirt flow model was applied to data obtained from ultrasonic experiments conducted on a wide range of sandstone types when saturated with different pore fluids with viscosities ranging from 0 3 to 1000 cp this model was seen to match well all the observed velocity and attenuation values as opposed to the biot model which was only successful for clean sandstones saturated with low viscosity fluids this has led to the conclusion that a squirt flow mechanism is important at the grain scale in addition an increase in the dynamic uniaxial strain modulus of the dry frame with increasing pore fluid viscosity was observed in the absence of free gas or grain contact microcracks the laboratory data were collected at 60 mpa effective pressure this can be linked to decreasing local flow and decreasing relaxation in the soft pore space of the sandstone the extent of which is in turn connected to the clay content and pore structure of the sandstone these results imply that at ultrasonic frequencies the squirt flow is due to the presence of clay

attenuation of p and s waves as a method of distinguishing gas and condensate from oil and water compressional and shear wave attenuation data were calculated from sonic waveforms in three wells the results show that at similar porosities and at in situ conditions gas and condensate sandstone reservoirs exhibit higher p wave attenuation lower p wave quality factor q p than either fully water or partially oil water saturated sandstones however s wave attenuation and quality factor q s do not show such a strong dependence on pore fluids furthermore q p q s 1 indicates presence of gas or condensate while q p q s i indicates full water or oil water saturation these field data are consistent with laboratory data and theoretical predictions by various researchers the crossovers of p swave attenuation and q p q s quality factors proved useful for distinguishing gas and condensate from oil and water reservoirs they may be used in conjunction with vp vs and other logs as an aid in formation evaluation for the detection of gas condensate and oil several case studies in the literature report high p wave energy absorption at seismic frequencies below gas and condensate pools this suggests that it is not unreasonable to assume that this method could also be used in seismic exploration as an aid for distinguishing gas and condensate from oil and water formations

sonic to ultrasonic q of sandstones and limestones laboratory measurements at in situ pressures laboratory measurements of the attenuation and velocity dispersion of compressional and shear waves at appropriate frequencies pressures and temperatures can aid interpretation of seismic and well log surveys as well as indicate absorption mechanisms in rocks construction and calibration of resonant bar equipment was used to measure velocities and attenuations of standing shear and extensional waves in copper jacketed right cylinders of rocks formula in length formula in diameter in the sonic frequency range and at differential pressures up to formula we also measured ultrasonic velocities and attenuations of compressional and shear waves in formula diameter samples of the rocks at identical pressures extensional mode velocities determined from the resonant bar are systematically too low yielding unreliable poisson s ratios poisson s ratios determined from the ultrasonic data are frequency corrected and used to calculate thesonic frequency compressional wave velocities and attenuations from the shear and extensional mode data we calculate the bulk modulus loss the accuracies of attenuation data expressed as formula where q is the quality factor are formula for compressional and shear waves at ultrasonic frequency formula for shear waves and formula for compressional waves at sonic frequency example sonic frequency data show that the energy absorption in a limestone is small formula greater than 200 and stress independent and is primarily due to poroelasticity whereas that in the two sandstones is variable in magnitude formula ranges from less than 50 to greater than 300 at reservoir pressures and arises from a combination of poroelasticity and viscoelasticity a graph of compressional wave attenuation versus compressional wave velocity at reservoir pressures differentiates high permeability formula formula brine saturated sandstones from low permeability formula formula sandstones and shales

a petrologic and fluid inclusion study of charnockites from the lutzow holm bay region east antarctica evidence for fluid rich metamorphism in the lower crust abstract the proterozoic terrain of lutzow holm bay in east antarctica comprises an amphibolite to granulite facies progression from north to south exposing an oblique section of the lower crust the granulite terrain is represented dominantly by orthopyroxene bearing anhydrous granulites charnockites garnet and biotite bearing gneisses leptynites and garnet sillimanite and graphite bearing aluminous metapelites khondalites thus it is petrographically identical with its gondwanian counterparts in sri lanka and southern india charnockite formation is characterized by the consumption of garnet biotite and quartz to produce orthopyroxene alkali feldspar and plagioclase under isothermal conditions and at reduced water activities a number of mineral reaction textures including orthopyroxene plagioclase symplectites around clinopyroxene relics in intermediate charnockites and spinel rims between adjacent garnet and sillimanite in khondalites suggest near isothermal conditions of equilibration mineral phase equilibria barometry traverse across the granulite terrain indicates an increase in pressure from c 5 kbar in the north to c 8 9 kbar in the south the temperature gradient of c 100 c is less pronounced fluid inclusion studies indicate abundant co 2 trapped within the charnockite minerals the systematic variation in fluid densities with a garnet feldspar quartz succession within individual samples argues against post metamorphic fluid infiltration and fits a scenario of mineral equilibration during near isothermal uplift following a clockwise pressure temperature time path along the pressure gradient the fluid densities show an increase from c 0 98 to c 1 10 g cm 3 this strengthens other evidence of a synmetamorphic entrapment of co 2 preliminary carbon isotope measurements of co 2 extracted from inclusions yield 13 c values of c 5 to 6 suggesting a sub continental mantle source the gondwanan lutzow holm bay terrain had undergone substantial extension during the late proterozoic with displacement along megafaults and emplacement of several alkaline subalkaline granite and syenite plutons and also k basalts it is therefore envisaged that the abundant co 2 in the charnockites could have been derived from an attenuated continental lithosphere and transferred through magmatic conduits

relationships among compressional wave attenuation porosity clay content and permeability in sandstones anelastic attenuation is the process by which rocks convert compressional waves into heat and thereby modify the amplitude and phase of the waves understanding the causes of compressional wave attenuation is important in the acquisition processing and interpretation of high resolution seismic data and in deducing the physical properties of rocks from seismic data we have measured the attenuation coefficients of compressional waves in 42 sandstones at a confining pressure of 40 mpa equivalent to a depth of burial of about 1 5 km in a frequency range from 0 5 to 1 5 mhz the compressional wave measurements were made using a pulse echo method in which the sample 5 cm diameter 1 8 cm to 3 5 cm long was sandwiched between perspex lucite buffer rods inside the high pressure rig the attenuation of the sample was estimated from the logarithmic spectral ratio of the signals corrected for beam spreading reflected from the top and base of the sample the results show that for these samples compressio

oxygen isotope fractionation in quartz albite anorthite and calcite laboratory measurements of equilibrium oxygen isotope fractionation in quartz albite anorthite and calcite have been carried out by anhydrous exchange between silicates and calcite at temperatures of 600 c and above exchange in these systems is as rapid as exchange between silicates and water fractionation factors can be summarized in the following equations qc 0 38 106t 2 abc 0 57 106t 2 anc 1 59 106t 2 from which the silicate pair fractionations are readily obtained these results are compared with published theoretical estimates as well as data derived from hydrothermal experiments some significant differences are found in particular it is difficult to reconcile all of the hydrothermal data either with theoretical calculations or with the present experimental data the new experiments provide an internally consistent set of fractionation factors suitable for isotopic thermometry and for test of disequilibrium in natural systems

dynamic poroelasticity a unified model with the squirt and the biot mechanisms the velocities and attenuation of seismic and acoustic waves in rocks with fluids are affected by the two most important modes of fluid solid interaction 1 the biot mechanism where the fluid is forced to participate in the solid s motion by viscous friction and inertial coupling and 2 the squirt flow mechanism where the fluid is squeezed out of thin pores deformed by a passing wave traditionally both modes have been modeled separately with the biot mechanism treated in a macroscopic framework and the squirt flow examined at the individual pore level we offer a model which treats both mechanisms as coupled processes and relates p velocity and attenuation to macroscopic parameters the biot poroelastic constants porosity permeability fluid com pressibility and viscosity and a newly introduced microscale parameter a characteristic squirt flow length the latter is referred to as a fundamental rock property that can be determined experimentally we show that the squirt flow mechanism dominates the biot mechanism and is responsible for measured large velocity dispersion and attenuation values the model directly relates p velocity and attenuation to measurable rock and fluid properties therefore it allows one to realistically interpret velocity dispersion and or attenuation in terms of fluid properties changes e g viscosity during thermal enhanced oil recovery eor or to link seismic measurements to reservoir properties as an example of the latter transformation we relate permeability to attenuation and achieve good qualitative correlation with experimental data

squirt flow in fully saturated rocks we estimate velocity frequency dispersion and attenuation in fully saturated rocks by employing the squirt flow mechanism of solid fluid interaction in this model pore fluid is squeezed from thin soft cracks into the surrounding large pores information about the compliance of these soft cracks at low confining pressures is extracted from high pressure velocity data the frequency dependence of squirt induced pressure in the soft cracks is linked with the porosity and permeability of the soft pore space and the characteristic squirt flow length these unknown parameters are combined into one expression that is assumed to be a fundamental rock property that does not depend on frequency the appropriate value of this expression for a given rock can be found by matching our theoretical predictions with the experimental measurements of attenuation or velocity the low frequency velocity limits as given by our model are identical to those predicted by gassmann s formula the high frequency limits may sign

magma influx and mixing in the bjerkreim sokndal layered intrusion south norway evidence from the boundary between two megacyclic units at storeknuten abstract the bjerkreim sokndal layered intrusion belongs to the proterozoic anorthositic province in the rogaland area of southern norway the northwestern part of the intrusion comprises a ca 6 km thick layered series made up of megacyclic units mcu arranged in a syncline each megacyclic unit reflects the influx of fresh magma into the chamber the boundary between megacyclic units iii and iv has been studied in detail at storeknuten on the southern flank of the syncline the megacyclic units can be subdivided into a series of cumulate stratigraphic zones the interval from the top of zone iiie to the base of zone ivd is exposed in the storeknuten area modally layered plagioclase hypersthene ilmenite magnetite augite apatite cumulates belonging to zone iiie are overlain by 30 m of massive plagioclase rich rocks commonly containing ilmenite and or hypersthene constituting zone iva the entry of cumulus olivine defines the base of zone ivb dominantly plagioclase olivine ilmenite cumulates which is about 100 m thick many of the olivines are partly or completely replaced by ca poor pyroxene fe ti oxide symplectites this massive leucotroctolitic zone is overlain by modally layered laminated plagioclase hypersthene ilmenite cumulates of zone ivc the successive entry of magnetite apatite accompanied by ca rich pyroxene and inverted pigeonite defines zones ivd e and f respectively the entry of k feldspar accompanied by fe rich olivine defines the base of a jotunitic transition zone which passes upwards into mangerites and quartz mangerites there is a compositional regression through zone iva the upper part of zone iiie has ca poor pyroxene with about en68 plagioclase with an44 48 and a sr isotope ratio of about 0 7062 while the base of zone ivb has olivine with fo75 together with en78 an53 and 0 7050 respectively similar reversals are shown by the minor element compositions of plagioclase and fe ti oxides sr isotope ratios increase systematically up through zone ivb reaching 0 7058 in zone ivd while an and sr in plagioclase and ni and cr in fe ti oxides decrease olivine compositions vary unsystematically and are believed to have changed their fe mg ratios as a result of trapped liquid shift the magma residing in the chamber when the influx at the base of megacyclic unit iv took place was compositionally zoned and assimilation of gneissic country rock at the roof had resulted in the sr isotope ratio increasing up through the magma column the new magma had a sr isotope ratio of about 0 7050 while the resident magma had a ratio of 0 7062 at the floor increasing upwards the new magma mixed with the basal layer s of the compositionally zoned resident magma and crystallization of this hybrid magma during influx and mixing produced the compositional regression in zone iva when magma influx ceased olivine bearing rocks began to crystallize at the base of zone ivb the leucotroctolites at the base of this zone are the most primitive rocks in the entire intrusion the systematic increase in sr isotope ratios up through zone ivb resulted from progressive mixing between new and resident magma this mixing either took place during magma influx or by the progressive mixing of overlying resident magma layers during crystallization calculations based on geochemical modelling the thickness of cumulate stratigraphy repeated and sr isotope ratios indicate that the new magma influx had a thickness of 350 500 m in the storeknuten section and that the leucotroctolites of zone ivb represent about 20 30 crystallization of this influx

a theory of compressional wave attenuation in noncohesive sediments published reviews indicate that attenuation coefficients of compressional waves in noncohesive water saturated sediments vary linearly with frequency biot s theory which accounts for attenuation in terms of the viscous interaction between the solid particles and pore fluid predicts in its presently published form variation proportional to f2 at low frequencies and f1 2 at high frequencies a modification of biot s theory which incorporates a distribution of pore sizes is presented and shown to give excellent agreement with new and published attenuation data in the frequency range 10 khz to 2 25 mhz in particular a linear variation of attenuation with frequency is predicted in that range

metamorphism of reduced granulites in low co 2 vapour free environment pervasive flooding of co2 has been proposed as the cause of granulite facies metamorphism that is capable of producing many distinctive characteristics of the deep continental crust reduced water activity orthopyroxene bearing assemblages depletion of large ion lithophile lil elements and dehydration1 6 the calculations presented here of c o h fluid composition for conditions of granulite facies metamorphism show that oxygen fugacity f o2 estimates from many terranes are sufficiently low that the addition of co2 rich fluid causes graphite to precipitate for values of pressure p and temperature t common to granulites and with f o2 slightly below the quartz fayalite magnetite buffer qfm the addition of co2 sufficient to grow 10 vol orthopyroxene requires the precipitation of 1 5 vol graphite as 0 1 vol graphite is readily recognizable but is not reported in most low f o2 granulites these rocks have not been flooded by co2 and low f h2o is probably due to extraction of a magma or recrystallization of an already dry rock7 8 granulite terranes may thus result from a combination of these three processes and the dominance of any one cannot now be demonstrated on a regional basis

in situ investigation of seismic body wave attenuation in heterogeneous media field experiments have been carried out to study the nature and magnitude of seismic wave attenuation for a variety of lithologies in each survey two three component sets of geophones with wall clamping mechanisms were lowered down boreholes and signals originating from surface compressional and shear wave sources were recorded the data collected were corrected for spherical divergence and analyzed to determine intrinsic attenuation for situations of anomalous wavefront expansion and in cases where multiple reflection losses may be significant corrections were supplied by a synthetic seismogram programme to improve the estimates of intrinsic attenuation values of attenuation were obtained for pure sandstone sandstone marl sequences and fissured unfissured chalk sequences these formations were all near surface and relatively porous significant differences in the relative values of compressional and shear wave attenuation in the various lithologies were noted in particular compressional absorption in the fissured zones of chalk was twice the absorption associated with the seismic velocities and the absorption of shear waves fluctuated much less a large contrast in p wave attenuation was also observed between pure bunter sandstone and the sandstone marl sequence absorption was over three times as small in the former a smaller contrast was noted for shear attenuation the results obtained suggest that for the relatively homogeneous formations such as pure sandstone and unfissured chalk shear absorption was dominant over bulk absorption in contrast bulk absorption was larger than shear absorption for other formations e g the fissured chalk sequences and a partially saturated chalk section attenuation was found to be approximately proportional to frequency in all experiments