avo and devonian reef exploration difficulties and possibilities although avo has proven useful in hydrocarbon exploration in shallow gas sands there are few examples of its application to carbonate exploration this paper presents a conjecture that avo can help differentiate between shale and porous dolomite in devonian reefs a critical problem in canada where oil production from nisku reefs in the devonian formation accounts for a large part of industry activity devonian formations have generated 60 of canadian oil production and 25 of gas production

ava analysis and interpretation of a carbonate reservoir northwest java basin indonesia a detailed analysis is performed of amplitude variation with angle ava observations in six common midpoint gathers with reflection points that are over and beside carbonate reefs in the parigi formation in the northwest java basin both empirical analysis and full wavefield modeling of the ava data suggest that the presence of gas affects ava by reducing the bulk density of the reservoir decreasing of the overburden vp vs ratio and by local attenuation caused by gas sieving through the overlying sediments the slopes of ava curves for reflections from the top of the parigi are negative for brine saturation and strongly positive for gas saturation

avo as an exploration tool gulf of mexico case studies and examples over the past decade amplitude variation with offset avo has evolved from a relatively new technology to increasingly mature applications particularly successful results have been achieved both offshore and onshore in the us gulf coast careful processing detailed petrophysical modeling from complete log suites and avo analyses closely calibrated to measured properties of target reservoir sands have demonstrated effectiveness however routine application of avo has not been without problems and misapplication has led to disappointing results

a 100 kyr periodicity in the flux of extraterrestrial 3he to the sea floor most of the helium 3 in oceanic sediments comes from interplanetary dust particles idps and can therefore be used to infer the accretion rate of dust to the earth through time 3he records from slowly accumulating pelagic clays indicate that the accretion rate varies considerably over millions of years probably owing to cometary and asteroidal break up events muller and macdonald have proposed that periodic changes in this accretion rate due to a previously unrecognized 100 kyr periodicity in the earth s orbital inclination might account for the prominence of this frequency in climate records of the past million years here we report variations in the 3 he flux to the sea floor that support this idea we find that the flux recorded in rapidly accumulating quaternary sediments from the mid atlantic ridge oscillates with a period of about 100 kyr we cannot yet say however whether the 100 kyr climate cycle is a consequence of a cause of or an effect independent of these periodic changes

cosmogenic nuclide enhancement via deposition from long period comets as a test of the younger dryas impact hypothesis abstract we explore the idea that detectable excursions in 26 al may arise from direct deposition by any bolide and excursions in 14 c and 10 be abundances in the atmosphere may result from long period comet impacts this is very different from the usual processes of production by cosmic rays within earth s atmosphere long period comets experience greatly increased cosmic ray flux beyond the protection of the sun s magnetic field we report the computed amount of 14 c 10 be and 26 al present on long period comets as a function of comet mass we find that the amount of nuclide mass on large long period comets entering the earth s atmosphere may be sufficient for creating anomalies in the records of 14 c and 10 be from past impacts in particular the estimated mass of the proposed younger dryas comet is consistent with its having deposited sufficient isotopes to account for recorded 14 c and 10 be increases at that time the 26 al 10 be ratio is much larger in extraterrestrial objects than in the atmosphere and so we note that measuring this ratio in ice cores is a suitable definitive test for the younger dryas impact hypothesis even if the hypothetical bolide is not a long period comet and or did not contribute to the 14 c and 10 be increases

26al 10be systematics in deep sea sediments we report the development of a naoh leaching technique to extract the authigenic al and be in marine sediments which should allow the sedimentary signals of cosmogenic 26al and 10be to be more effectively studied as geochemical geophysical tracers the technique has been applied to the study of 26al and 10be in opal rich sediments from the north pacific the 26al 27al and 10be 9be ratios of 3 10 14 and 1 10 7 leached from the sediments are higher than those in the total sediments by factors of 3 10 and 2 3 respectively the authigenic 10be 9be ratios are similar to those in deep waters at the study site pointing to the potential usefulness of the ratios in paleoceanographic studies the fractions of total be and al in the sediment studied that are of authigenic origin average about 40 and 17 respectively estimated 26al 27a1 ratios in detrital sediments are 10 15 low enough to indicate that the source of 26al in the ocean mainly comes from atmospheric production in the study area the deposition flux of 26al is comparable to its atmospheric supply however the deposition flux of 10be is about threefold higher than the atmospheric input signifying lateral transport of 10be from the open ocean to this area of relatively high particle flux the 26al 10be ratio being insensitive to the oceanic influx of lithogenic particles may serve as a valuable proxy for paleoproductivity both the authigenic and total sediment 26al 27al and 10be 9be ratios decreased significantly at 6 ka which could reflect an enhanced input of windblown lithogenic particles to the north pacific while the enhanced dust input did not significantly alter the deep water 9be concentration it contributed to the observed increase of authigenic 9be flux to the sediment

astronomically controlled cycles in the lower tertiary of gubbio italy abstract a grouping of marl limestone beds in the tertiary of gubbio is interpreted in terms of sedimentary cycles two methods of recording are used subjective mapping of cycle boundaries in the field and power spectral analysis of bed thickness measurements both result in cycle thicknesses of between 30 and 70 cm the cycle record is compared with the known magnetic stratigraphy and this gives an absolute time estimate of 100 kyr for the cycle it therefore seems likely that the earth s eccentricity period is the ultimate cause of the cycle

postdepositional losses of methane sulfonate nitrate and chloride at the european project for ice coring in antarctica deep drilling site in dronning maud land antarctica nitrate and typically 51 20 of msawere lost while for chloride no significant depletion could be observed in firn older than one year assuming a first order exponential decay rate the characteristic e folding time for msais 6 4 3 years and 19 6 years for nitrate it turns out that for nitrate and msathe typical mean concentrations representative for the last 100 years were reached after 5 4 and 6 5 years respectively indicating that beneath a depth of around 1 2 1 4 m postdepositional losses can be neglected in the area of investigation only msaconcentrations and postdepositional losses showed a distinct dependence on snow accumulation rate consequently msa concentrations archived at this site should be significantly dependent on the variability of annual snow accumulation and we recommend a corresponding correction with a simple approach we estimated the partial pressure of the free acids msa hno3 and hcl on the basis of henry s law assuming that ionic impurities of the bulk ice matrix are localized in a quasi brine layer qbl in contrast to measurements this approach predicts a nearly complete loss of msa n o3 and cl index terms 0322 atmospheric composition and structure constituent sources and sinks 0330 atmospheric composition and structure geochemical cycles 0365 atmospheric composition and structure troposphere composition and chemistry 1827 hydrology glaciology 1863 1863 hydrology snow and ice 1827 keywords postdepositional loss snow chemistry methane sulfonate

inversion and interpretation of a 3d seismic data set from the ouachita mountains oklahoma acoustic impedance inversion ai and simultaneous angle dependent inversion sadi of a 3d seismic data set characterize reservoirs of mississippian morrowan age in the triangle zone of the frontal ouachita mountains oklahoma acoustic impedance of the near angle seismic data images the 3d spatial distributions of wapanucka limestone and cromwell sandstone lame parameters density and and sections are derived from the p wave and s wave impedance ip and is sections produced by the sadi lithology is identified from the gamma logs and the vp vs and are interpreted in terms of a hydrocarbon distribution pattern the ip is used to identify high ip regions that are consistent with high sand shale ratio the estimated impedances and derived lame parameter sections are consistent with the interpretation that parts of the wapanucka limestone and cromwell sandstone contain potential gas reservoirs in fault bounded compartments the cromwell sandstone contains the main inferred

authigenic 10be 9be ratios and 10be fluxes 230thxs normalized in central baffin bay sediments during the last glacial cycle paleoenvironmental implications abstract authigenic 10 be 9 be ratios and 10 be fluxes reconstructed using the 230 th xs normalization proxies of the cosmogenic radionuclide 10 be production rate in the atmosphere have been measured in a sedimentary core from baffin bay north atlantic spanning the last 136 ka bp the normalization applied on the exchangeable authigenic 10 be concentrations using the authigenic 9 be isotope and 230 th xs methods yield equivalent results strongly correlated with sedimentological parameters grain size and mineralogy lower authigenic beryllium be concentrations and 10 be 9 be ratios are associated with coarse grained carbonate rich layers while higher authigenic be values are related to fine grained felspar rich sediments this variability is due to i sediment composition control over beryllium scavenging efficiency and ii glacial history that contributed to modify the 10 be concentration in baffin bay by input and boundary scavenging condition changes most paleo denudation rates inferred from the 10 be 9 be ratio vary weakly around 220 76 tons km 2 yr 1 0 09 0 03 mm yr 1 corresponding to relatively steady weathering fluxes over the last glacial cycle except for six brief intervals characterized by sharp increases of the denudation rate these intervals are related to ice surging episodes coeval with heinrich events and the last deglaciation period an average freshwater flux of 180 6 km 3 yr 1 0 006 sv consistent with recent models has been calculated in order to sustain glacially derived 10 be inputs into baffin bay it is concluded that in such environments the authigenic 10 be measured mainly depends on climatic effects related to the glacial dynamics which masks the 10 be production variation modulated by geomagnetic field changes altogether these results challenge the simple interpretation of 10 be concentration variation as a proxy of interglacial glacial interstadial stadial cycles in arctic and sub arctic regions they rather suggest the effect of higher frequency paleoclimatic changes and local glacial dynamics on 10 be signature

10 be evidence for the matuyama brunhes geomagnetic reversal in the epica dome c ice core every few hundred thousand years earth s north and south magnetic poles swap places for the first time a record of one of these geomagnetic reversals has been found in an ice core because the strength of the field is very low during a reversal the intensity of cosmic rays in the earth s atmosphere is increased this leads to an increase in the quantity of 10be which like 14c is formed by cosmic rays in the atmosphere deposited in the ice it is this 10be enhancement in the ice that has been detected an ice core drilled at dome c antarctica is the oldest ice core so far retrieved1 on the basis of ice flow modelling and a comparison between the deuterium signal in the ice with climate records from marine sediment cores the ice at a depth of 3 190 m in the dome c core is believed to have been deposited around 800 000 years ago2 offering a rare opportunity to study climatic and environmental conditions over this time period however an independent determination of this age is important because the deuterium profile below a depth of 3 190 m depth does not show the expected correlation with the marine record2 here we present evidence for enhanced 10be deposition in the ice at 3 160 3 170 m which we interpret as a result of the low dipole field strength during the matuyama brunhes geomagnetic reversal which occurred about 780 000 years ago if correct this provides a crucial tie point between ice cores marine cores and a radiometric timescale

preservation of miocene glacier ice in east antarctica antarctic climate during the pliocene has been the subject of considerable debate one view holds that during part of the pliocene east antarctica was largely free of glacier ice and that vegetation survived on the coastal mountains1a 4 an alternative viewpoint argues for the development of a stable polar ice sheet by the middle miocene which has persisted since then5a 10 here we report the discovery of buried glacier ice in beacon valley east antarctica which appears to have survived for at least 8 1 million years we have dated the ice by 40ar 39ar analysis of volcanic ash in the thin overlying glacial till which we argue has undergone little if any reworking isotope and crystal fabric analyses of the ice show that it was derived from an ice sheet we suggest that stable polar conditions must have persisted in this region for at least 8 1 million years for this ice to have avoided sublimation

direct detection using avo central graben north sea an amplitude versus offset avo study was undertaken for lines ecl 1 and 2 and well a of the united kingdom north sea to determine if variations in fluid saturation can be detected in eocene sand mounds these features are a major play in this area the eocene sands contain gas and oil in well a elastic modeling using the well data was done to match the seismic common midpoint cmp gather target zone fluid saturations in the model were then altered to investigate changes in avo and stacked trace response the model and field data were processed using the same processing steps and parameters seismic offset dependent amplitude stack sodas a color display system was used to display the avo results for the field and model data it was found that gas oil and water sands could be distinguished from each other though full and partial gas saturation were indistinguishable clay content porosity and multiple reflections also had important effects on the avo response on the basis of the hydrocarbon

untangling the palaeocene climatic rhythm an astronomically calibrated early palaeocene magnetostratigraphy and biostratigraphy at zumaia basque basin northern spain abstract the magnetostratigraphy of a 54 m long section above the cretaceous tertiary boundary at the sea cliff section of zumaia in the basque basin northern spain has been established the section encompasses the entire danian and the lower part of the selandian stages as indicated by calcareous plankton biostratigraphy the studied interval consists of hemi pelagic limestone marl alternations in the form of couplets and bundles which range from centimetre decimetre to metre scale respectively and a few thin bedded calcareous turbidites the magnetostratigraphy based on samples from about 200 stratigraphic levels allows the identification of six reversal boundaries from chron c29r to c26r at a bed level the spatial or temporal evolution of periodicities from a lithologically coded series is studied with the continuous wavelet transform technique a preliminary age model based on the standard ck95 gpts indicates that the basic lithologic carbonate marl couplet corresponds to the 19 23 kyr precession cycle 21 31 cm cycle in the depth domain and that a bundle cycle usually groups of four to six basic couplets with global periodicity centred at 1 22 m corresponds to the 110 kyr eccentricity cycle we have tuned the bundle cycles to the va03 r7 eccentricity orbital solution astrophys j 592 2003 620 630 following an initial match of a node of the 2 4 ma eccentricity modulatory cycle in the target time series to particularly carbonate rich bundles from the upper part of the zumaia section that displays significant power of a 4 4 m period cycle corresponding to the 404 kyr eccentricity cycle consistency between lithologic patterns and characteristics in the eccentricity target is reasonably met although the 404 kyr eccentricity cycle is not persistent throughout the tuning however appears robust as it brings the age of the k t boundary at 65 8 ma it is argued that a sea level signal tectonically driven is superimposed on the climatic forcing at the milankovitch band masking the full expression of the low frequency astronomical periods we provide a cycle tuned duration for all intervening early palaeocene polarity chrons and estimate relative ages for bioevents the cycle tuned chronology indicates that the ck95 gpts overestimates the duration of chrons c28 and c27 by 20 and 26 respectively our data may prove useful to better constrain early palaeocene biostratigraphy of calcareous plankton and in the redefinition of the boundary between the danian and selandian stages

eight glacial cycles from an antarctic ice core the antarctic vostok ice core provided compelling evidence of the nature of climate and of climate feedbacks over the past 420 000 years marine records suggest that the amplitude of climate variability was smaller before that time but such records are often poorly resolved moreover it is not possible to infer the abundance of greenhouse gases in the atmosphere from marine records here we report the recovery of a deep ice core from dome c antarctica that provides a climate record for the past 740 000 years for the four most recent glacial cycles the data agree well with the record from vostok the earlier period between 740 000 and 430 000 years ago was characterized by less pronounced warmth in interglacial periods in antarctica but a higher proportion of each cycle was spent in the warm mode the transition from glacial to interglacial conditions about 430 000 years ago termination v resembles the transition into the present interglacial period in terms of the magnitude of change in temperatures and greenhouse gases but there are significant differences in the patterns of change the interglacial stage following termination v was exceptionally long 28 000 years compared to for example the 12 000 years recorded so far in the present interglacial period given the similarities between this earlier warm period and today our results may imply that without human intervention a climate similar to the present one would extend well into the future

seismic impedance inversion and interpretation of a gas carbonate reservoir in the alberta foothills western canada acoustic and simultaneous elastic impedance inversions of a 2d land seismic data set are performed to characterize a carbonate reservoir of mississippian age in the turner valley formation in the rocky mountain foothills of western canada the inversions produce p wave and s wave impedance sections ip and is respectively from which lame parameter density and sections are derived the ip data provide a separation between the clastics and carbonates the data provide an estimate of porosity distribution within the dolomitized limestone target deviations from baseline curves for water saturated carbonates of versus porosity versus porosity and is versus ip are interpreted as indicators of gas potential these indicators all provide similar spatial patterns of areas of high gas potential and are consistent with the gas occurence observed in a well

challenges in constraining ages of fluvial terraces in the vienna basin austria using combined isochron burial and pirir225 luminescence dating abstract burial dating with terrestrial cosmogenic nuclides and luminescence dating techniques have become two powerful tools to temporally constrain quaternary deposits a combination of both methods at the same geological setting has rarely been realized to date although their viable time frames overlap by several tens of thousands of years when middle pleistocene sediments with depositional ages ranging around ca 120 ka to ca 300 ka are targeted both methods are employed but come towards their lower and upper limits respectively a combined dating approach can be worthwhile at this age range and allows not only exploring the edges of both methods but holds the opportunity to do a cross check of results at an age spectrum where both dating techniques are at risk to become fuzzy here we present a case study where numerical ages of two middle pleistocene terraces located in the vienna basin were generated by combining burial and luminescence dating a variety of processes such as changing sediment input rates erosion and tectonics controlled the formation of fluvial terraces in the basin and shaped its complex modern surface age correlation of the evolved mosaic of blocks and dislocated sediment bodies is challenging and requires quantitative geochronological information in order to establish a coherent terrace stratigraphy luminescence and burial samples originating from two fluvial terraces the lower gaenserndorf terrace gdt and the higher schlosshof terrace sht were analyzed and evaluated luminescence and burial ages at the gdt site are in good agreement and suggest a depositional age of 140 170 ka bracketed by pirir225 luminescence ages ranging from 120 10 ka to 260 30 ka luminescence samples at the sht site are in saturation but provide minimum ages which are coherent with the burial dating result of 340 170 ka the new numerical ages indicate that the vertical offset between the gdt site and the sht site was not purely caused by fault activity but suggest two independent episodes of sediment accumulation besides providing new insights into the stratigraphic and morphological configuration of the central vienna basin area the cosmogenic nuclide data set is compelling from a methodological point of view at the gdt site several samples exhibited 26al 10be nuclide ratios exceeding the surface production ratio of 6 75 even though affected samples were excluded from burial age calculations a detailed investigation on possible scenarios which could have caused an upwards shift of 26al 10be ratios was carried out

implications of thin layers for amplitude variation with offset avo studies amplitude variation with offset avo or amplitude variation with angle ava analyses of seismic reflection data are becoming increasingly popular in the exploration industry ostrander 1984 pichin and mitchell 1991 mazzotti and mirri 1991 and also in scientific studies of the earth s crust juhlin 1990 in the exploration industry avo analyses are particularly suitable for the detection and mapping of gas zones since reservoirs often consist of shale with high poisson s ratio high vp vs overlying gas bearing sands with low poisson s ratio low vp vs if the gas sand has lower impedance than the overlying shale the magnitude of the reflection coefficient will increase with increasing angle of incidence or offset other combinations of rock types will also show a similar increase in magnitude such as shale over hard limestone but the sign of the reflection coefficient will be positive in most of these cases therefore if the polarity of the reflection can be determined to be negative and

signal analysis of cyclicity in maastrichtian pelagic chalks from the danish north sea abstract low field bulk magnetic susceptibility has been determined on maastrichtian chalk samples from a drill core from the dan field in the danish north sea fast fourier transformations fft have been used to detect possible cycles in the magnetic susceptibility data power spectra from the complete section and from sub sections of the magnetic susceptibility reveal two cyclicities of ca 0 4 cycles m and ca 1 7 cycles m which are present on a 90 confidence level signal analysis of natural gamma ray wire line log data supports these findings sedimentation rate estimates place the cycles in the milankovitch frequency band artificial time series are used to study the applicability of the fft to identifying cyclicity in chalks expected geological distortions e g hiati and sedimentation rate variations are introduced into the time series to investigate the response of the frequency spectra different methods of handling missing data intervals are also examined

the oldest ice on earth in beacon valley antarctica new evidence from surface exposure dating abstract beacon valley antarctica contains unique remnants of glacier ice underneath a till layer covering the valley floor to constrain the age and evolution of this important indicator of antarctic paleoclimate we analyzed two dolerite erratics from the till surface and one from within the ice for cosmogenic helium and neon a conservative minimum exposure age of the older surface sample is 2 3 ma but taking into account erosion the true exposure age of this boulder is likely to be considerably higher the buried sample contains more than 20 times less cosmogenic noble gases than the old surface sample although its current shielding would imply only a three times lower production rate this indicates that the ice level has slowly been lowered by sublimation at the rate of a few m ma the high exposure age of the surface sample as well as the very low sublimation rate of the relict ice both support the conclusion that the remnant ice in beacon valley was deposited many million years ago sugden et al nature 376 1995 412 414 and has never been thinner than at present in addition we found that cosmogenic helium and neon are released quantitatively from pyroxene at temperatures of 1000 c respectively

target oriented avo inversion of data from valhall and hod fields a stratigraphic elastic inversion scheme has been applied to a real seismic data set from valhall and hod fields in the north sea avo inversion shows discontinuous high porosity zones that can be interpreted as evidence for stratigraphic traps in the upper chalk interval this was tested by exploration drilling after avo processing was completed and hydrocarbons were found

cosmogenic 26al in the atmosphere and the prospect of a 26al 10be chronometer to date old ice abstract cosmogenic radionuclides in the one million year half life range offer unique possibilities for age determinations in geophysics in measurements where the radioactive decay is being utilized as a clock uncertainties in age determinations may be reduced if the ratio of two radioisotopes with different half lives can be used as a chronometer in this work we investigate the atomic ratio of atmospheric 26al t1 2 0 717 ma to 10be t1 2 1 386 ma measured with accelerator mass spectrometry ams and its potential as a chronometer for dating old ice the 26al 10be ratio decreases with an effective half life of t1 2 26al 10be 1 49 ma for its application as a chronometer the atmospheric 26al 10be ratio has to be well characterized however the properties of atmospheric 26al have been understood only poorly so far at the vera ams facility of the university of vienna a first systematic study of the global variations of the 26al 10be ratio in the atmosphere and in surface firn has been carried out and pilot measurements of the 26al 10be ratio in deep antarctic ice have been performed our results indicate that this ratio is globally constant to within 5 in the atmosphere and in surface firn with a mean value of 1 89 10 3 the data also suggest that non atmospheric sources of 26al such as extraterrestrial in situ produced or re suspended 26al do not contribute significantly to the observed 26al 10be ratio in addition atmospheric mixing seems to exert only a minor influence in a first application of the method 26al 10be ratios were measured in chips collected in connection with the drilling of the lowest part of an ice core 2250 to 2760 m in dronning maud land antarctica surprisingly variable 26al 10be ratios ranging between 0 5 and up to 2 times the atmospheric ratio were found at different locations in this deep ice core while the cause for the ratios higher than atmospheric remains unexplained so far the ratios lower than atmospheric may be caused by radioactive decay allowing a first dating attempt using the 26al 10be ratio thus at an ice depth of 2760 m an approximate date of 6 7 2 6 105 years was established

rock lithology and porosity determination from shear and compressional wave velocity data examined in this study are previously published laboratory shear s and compressional p wave velocity measurements on water saturated sandstone calcareous sandstone dolomite and limestone cores as well as laboratory porosity measurements on the sandstone and limestone cores sandstone and limestone porosities range from 092 to 299 and from 006 to 229 respectively differential pressure was varied from 500 to 6000 psi corresponding to approximate burial depths from 290 to 3460 m respectively sandstone limestone and dolomite are effectively separated by poisson s ratio or equivalently by the ratio of p to s wave velocity separation of sandstone and limestone appears to result from the difference in of the matrix material namely quartz 056 and calcite 316 respectively an empirical function 1 v a b was fit by regression analysis to sandstone and limestone velocity vs and vp versus porosity values at each differential pressure in this equation a and b are const

trace based and geostatistical inversion of 3 d seismic data for thin sand delineation an application in san jorge basin argentina argentina s san jorge basin which straddles the southern provinces of chubut and santa cruz in the heart of patagonia has been producing hydrocarbons since 1907 the region currently accounts for 32 of the country s production the existence and early geologic evolution of the basin is due to the same rift process responsible for the opening of the atlantic ocean in early jurassic times extensive direct faulting and local erosion accompanied the rift evolution thereby facilitating the accumulation of chiefly nonmarine terrigenous sediments well into the early cretaceous at this point andean tectonism became a major source of pyroclastic concentration in the sedimentary column it was also responsible for pervasive batholitic intrusions clastic deposition in the hydrocarbon producing zone is characterized by thick shale laminations of lacustrine and floodplain origin interspersed with much thinner and laterally sparse sand bodies the hydrocarbon reservoirs

extra terrestrial influx rates of cosmogenic isotopes and platinum group elements realizable geochemical effects we present estimates of realizable influx of selected cosmogenic isotopes 3he 10be 26al and 14c and platinum group elements ir os and re on the earth via influx of meteoroids we define realizable as the particulate fraction of mass influx which is included in standard geochemical analyses of terrestrial sediments this is the surviving mass fraction of size 0 5 1 cm which gets mixed with terrestrial samples and is analyzed in normal terrestrial assays of sediments larger surviving meteoroid fragments of the order of cm to m or larger obviously belong to the non realizable flux category since i their distribution in terrestrial samples would be very patchy resulting in a highly variable density matrix and ii they would also generally except in wide diameter cores be excluded from sediment cores we estimate the realizable influx of meteoritic particles based on a recent model describing production of smaller size fragments arising during the atmospheric entry of meteoroids implicit in this work is the assumption that the secondary fragments are not subject to much heating and therefore most of the initial 3he and noble gases would be preserved in the secondary fragments since after break up they are not subjected to much heating under this assumption production of small size fragments in the ablation process constitutes a safe landing mechanism for parts of the meteoroid in this paper we show that the meteoritic ablation fragmentation process produces a significant flux of 3he platinum group metals and cosmogenic 26al in fact measurements of these isotopes in ocean sediments should allow a reasonable estimate of temporal variations in the flux of meteoroids of 50 cm to 5 m radii which produce most of the secondary fragments in the size range 1 cm we would like to state here that as pointed out in our previous work stratospheric collections would be biased towards collection of the primary incident extra terrestrial particles whereas terrestrial accumulations representing large area time accumulations as in the case of ocean sediments would efficiently sample the fragmented particles

critical porosity a key to relating physical properties to porosity in rocks discovering accurate relations between porosity and seismic wave velocities in porous rocks has been an important topic of rock physics for decades such relations if they actually exist are critical when seismic and or sonic data are to be used to infer porosity and porosity related properties e g permeability strength saturation in situ these inferences became especially important in recent years with the rapid development of seismic and sonic methods which are applied to the characterization of heterogeneous reservoirs and recovery monitoring

bedrock incision rock uplift and threshold hillslopes in the northwestern himalayas the topography of tectonically active mountain ranges reflects a poorly understood competition between bedrock uplift and erosion dating of abandoned river cut surfaces in the northwestern himalayas reveals that the indus river incises through the bedrock at extremely high rates 2 12 mm yr 1 in the surrounding mountains the average angles of hillslopes are steep and essentially independent of erosion rate suggesting control by a common threshold process in this rapidly deforming region an equilibrium is maintained between bedrock uplift and river incision with landsliding allowing hillslopes to adjust efficiently to rapid river down cutting

limited dechlorination of sea salt aerosols during the last glacial period evidence from the european project for ice coring in antarctica epica dome c ice core 1 chloride cl and sodium na in ice cores originate mainly from sea salt and one would thus expect the cl na ratio to reflect the seawater ratio however at dome c a low accumulation site in east antarctica this is not the case in present day snow instead a cl excess relative to na is observed in surface snow and within a few meters depth the cl concentration decreases and the cl na ratio becomes significantly lower than the seawater ratio aerosol studies at coastal antarctic sites have shown that the reaction of sea salt aerosols with nitric and sulphuric acid leads to the formation of hcl that eventually escapes the sea salt aerosol the observed decrease in cl concentrations in the uppermost snow layers is due to reemission of hcl from the snow postdepositional loss of hcl depends among other factors on the accumulation rate at the site with lower accumulation rates leading to larger losses during the last glacial maximum lgm the cl na ratio is relatively stable and close to the seawater ratio despite the even lower accumulation rate during that time the likely explanation for this conflicting observation is that high levels of dust neutralized nitric and sulphuric acids during the lgm which in turn reduced the formation of hcl from sea salt aerosol with less or no hcl formed postdepositional loss would be prevented keeping the cl na ratio close to that of sea water

a long marine history of carbon cycle modulation by orbital climatic changes pacing of the marine carbon cycle by orbital forcing during the pliocene and pleistocene ice ages past 2 5 million years myr is well known as older deep sea sediment records are being studied at greater temporal resolution it is becoming clear that similar fluctuations in the marine carbon system have occurred throughout the late mesozoic and tertiary despite the absence of large continental ice sheets over much of this time variations in both the organic and the calcium carbonate components of the marine carbon system seem to have varied cyclically in response to climate forcing and carbon and carbonate time series appear to accurately characterize the frequency spectrum of ancient climatic change for the past 35 myr much of the variance in carbonate content carries the polar signal of obliquity 41 000 years 41 kyr forcing over the past 125 myr there is evidence from marine sediments of the continued role of precessional 21 kyr climatic cycles repeat patterns of sedimentation at about 100 400 and 2 400 kyr the modulation periods of precession persistently enter into marine carbon cycle records as well these patterns suggest a nonlinear response of climate and or the sedimentation of organic carbon and carbonates to precessional orbital perturbations nonlinear responses of the carbon system may help to amplify relatively weak orbital insolation anomalies into more significant climatic perturbations through positive feedback effects nonlinearities in the carbon cycle may have transformed orbital climatic cycles into long wavelength features on time scales comparable to the residence times of carbon and nutrient elements in the ocean

porosity identification using amplitude variations with offset examples from south sumatra abstract this paper describes the use of a relatively new exploration technique in the modeling and interpretation of seismic data the technique holds promise for identification of primary lithology and of local lithological facies variations its underlying principle is that any single reflection within a cdp gather will show an amplitude variation between near and far offset traces given certain physical properties of rocks across the reflecting boundary stacking velocity information above the reflector and the cdp geometry it is possible to model the observed amplitude variation such modeling can result in useful information on lithology two examples will be shown of the application of this technique in the detection of porosity in miocene reefal buildups in south sumatra

30 000 years of cosmic dust in antarctic ice polar ice provides an archive for the influx of cosmic dust here we present a high resolution glacial to interglacial record of cosmic dust using helium isotope analysis of the european project for ice coring in antarctica epica ice core drilled in dronning maud land we obtained a relatively constant 3he flux over the past 30 000 years this finding excludes 3he as a pacemaker of late pleistocene glacial cycles rather it supports 3he as a constant flux parameter in paleoclimatic studies a last glacial to holocene shift of the 4he non sea salt ca2 ratio appears to indicate a glacial to interglacial change in the terrestrial dust source

integrated geological and geophysical interpretation case study and lame rock parameter extractions using avo analysis on the blackfoot 3c 3d seismic data southern alberta canada blackfoot field southeast of calgary alberta canada has produced oil and gas from a glauconitic compound incised valley system the glauconitic compound incised valley has three cycles of incision and valley fill lower lithic and upper incised valleys the upper and lower incised valleys are the main reservoirs the geophysical interpretation of compressional pp seismic data resulted in the definition of the compound valley extent and in the mapping of the upper and lower incised valleys a stratigraphic well log template was built using the most significant lithological information and well logs to integrate both geological and geophysical interpretations the well log cross sections and corresponding depth converted seismic were superimposed furthermore a detailed geological facies interpretation of the upper and lower incised valleys was undertaken and incorporated a good correlation was found between the interpreted geological facies and the seismic data response information about the natu

volcanic and solar activity and atmospheric circulation influences on cosmogenic 10be fallout at vostok and concordia antarctica over the last 60years abstract the cosmogenic nuclide beryllium 10 10be recovered from ice cores is often used to study solar activity on long timescales however the 10be signal is also influenced by factors other than the sun to identify and quantify various contributions to the 10be signal two antarctic snow records from the vostok and concordia sites spanning the last 60 years were studied at a sub annual resolution three factors that contribute to the 10be signal were identified first a significant period of approximately 11 yr that can be associated with the modulation of 10be production by solar activity was detected in both records the solar imprint constitutes 20 35 of the variance within the total signal the 11 yr 10be snow component was attenuated by a factor of 0 5 and was delayed by 1 4 yr compared to the 10be production expected within the polar atmosphere the result could be interpreted as the composite response of a stratospheric 10be reservoir with an 11 yr modulation that was attenuated and delayed with respect to 10be polar production and to a tropospheric 10be reservoir with an 11 yr modulation that was not attenuated or delayed then peaks in 10be concentrations that were 66 and 35 higher than the average concentration were observed during the stratospheric volcanic eruptions of agung in 1963 and pinatubo in 1991 respectively in light of these new results published 10be ice core records could be reinterpreted because spikes in 10be concentration appear at the time of several stratospheric events the data indicate that stratospheric volcanic eruptions can impact 10be transport and deposition as a result of the roles played by the sedimentation of sulfate aerosols and the formation and rapid settling of polar stratospheric clouds psc also an interannual variability of 4 yr was determined in both 10be records corresponding to 26 of the variance within the signal at vostok as with species of marine origin sodium this 4 yr variability is interpreted as a tropospheric modulation the 4 yr variability could be associated with atmospheric circulation associated with the coupled southern ocean ocean atmosphere system the results presented here from sites within the high antarctic plateau open new possibilities for ice core dating over the last few centuries and for the reconstruction of past solar activity in relation to climate

avo correction for scalar waves in the case of a thinly layered reflector overburden the reflection response of a seismic target is significantly affected by a thinly layered overburden which creates velocity anisotropy and a transmission loss by scattering attenuation these effects must be taken into account when imaging a target reflector and when inverting reflection coefficients describing scalar wave i e acoustic wave or sh wave propagation through a stack of thin layers by equivalent medium theory provides a simple generalized o doherty anstey formula this formulation is defined by a few statistical parameters that depend on the 1 d random fluctuations of the reflector overburden the formula has been combined with well known target oriented and amplitude preserving migration inversion algorithms and amplitude variation with offset avo analysis procedures the application of these combined procedures is demonstrated for sh waves in an elastic thinly layered medium these techniques offer a suitable tool to compensate for the thin layer influence on traveltimes and amplitud

carbonate porosity from s p traveltime ratios a twenty mile s wave seismic line was acquired on the northeastern shelf of the anadarko basin by the 1977 1978 conoco p wave s wave group shoot the resolution of the s wave section is at least as good as the resolution of a coincident p wave section acquired in 1975 the top and bottom of a collection of paleozoic carbonates called the hunton group can be identified on both sets of data p and s traveltimes and traveltime ratios delineate lateral variations in the thickness and average porosity of the hunton where the pore space is saturated with liquid a systematic increase in average porosity from less than 2 percent to about 11 percent correlates with an increase in the s p traveltime ratio from 1 84 to 1 96 the ratio decreases to values between 1 65 and 1 80 at two places along the line probably indicating that the pore space is partially gas saturated at these locations the data are consistent with the seismic response of a petrophysical model of the hunton in which a majority of the pore shapes are elongated rather than round

estimation of completeness of stratigraphical sections using empirical data and theoretical models the completeness of a stratigraphical section is the fraction of time intervals of some specified length t that have left a record a record is left when some sediment is deposited during the interval and is not subsequently eroded a complete section contains no hiatuses longer than t the completeness of a section varies with t and its accumulation rate varies with the length of the time span over which it is measured plots of measured accumulation rate against time span are an empirical means of estimating completeness simple theoretical models help extrapolate meagre data and identify bias completeness at time scale t can depend upon three general properties of the accumulation history the age of the section the long term net accumulation rate drift and the unsteadiness of the sedimentation rate the way unsteadiness is measured depends upon the kinds of fluctuations that can be recognized in the accumulation rate to describe random fluctuations a standard deviation of rates is sufficient one dimensional brownian motion is a model of random fluctuations that explains many aspects of stratigraphical completeness regular periodic fluctuations in accumulation rate may be described in terms of wavelength and amplitude completeness is an increasing function of t drift and in the periodic case the wavelength of fluctuations it decreases with increasing standard deviation of accumulation rate and the amplitude of periodic fluctuations the completeness and thickness of stratigraphical sections are weakly positively associated

elastic behaviour of north sea chalk a well log study we present two different elastic models for respectively cemented and uncemented north sea chalk well log data we find that low biot coefficients correlate with anomalously low cementation factors from resistivity measurements at low porosity and we interpret this as an indication of cementation in contrast higher biot coefficients and correspondingly higher cementation factors characterize uncemented chalk for the same low porosity accordingly the poisson s ratio porosity relationship for cemented chalk is different from that of uncemented chalk we have tested the application of the self consistent approximation which here represents the unrelaxed scenario where the pore spaces of the rock are assumed to be isolated and the gassmann theory which assumes that pore spaces are connected as tools for predicting the effect of hydrocarbons from the elastic properties of brine saturated north sea reservoir chalk in the acoustic impedance poisson s ratio plane we forecast variations in porosity and hydrocarbon saturation from their influence on the elastic behaviour of the chalk the gassmann model and the self consistent approximation give roughly similar predictions of the effect of fluid on acoustic impedance and poisson s ratio but we find that the high frequency self consistent approach gives a somewhat smaller predicted fluid saturation effect on poisson s ratio than the low frequency gassmann model the gassmann prediction for the near and potentially invaded zone corresponds more closely to logging data than the gassmann prediction for the far virgin zone we thus conclude that the gassmann approach predicts hydrocarbons accurately in chalk in the sonic frequency domain but the fluid effects as recorded by the acoustic tool are significantly affected by invasion of mud filtrate the amplitude versus angle ava response for the general north sea sequence of shale overlying chalk is predicted as a function of porosity and pore fill the ava response of both cemented and uncemented chalk generally shows a declining reflectivity coefficient versus offset and a decreasing normal incidence reflectivity with increasing porosity however for the uncemented model a phase reversal will appear at a relatively lower porosity compared to the cemented model

avo study of a gas hydrate deposit offshore costa rica amplitude variation with offset avo analysis and waveform inversion are techniques used to determine qualitative or quantitative information on gas hydrates and free gas in sediments however the quantitative contribution of gas hydrates to the acoustic impedance contrast observed at the bottom simulating reflector and the reliability of quantitative avo analyses are still topics of discussion in this study common midpoint gathers from multichannel wide angle reflection seismic data acquired offshore costa rica have been processed to preserve true amplitude information at the bottom simulating reflector for a quantitative avo analysis incorporating angles of incidence of up to 60 corrections were applied for effects that significantly alter the observed amplitudes such as the source directivity avo and rock physics modelling indicate that free gas immediately beneath the gas hydrate stability zone can be detected and low concentrations can be quantified from avo analysis whereas the offset dependent reflectivity is not sensitive to gas hydrate concentrations of less than about 10 at the base of the gas hydrate stability zone bulk free gas saturations up to 5 have been determined from the reflection seismic data assuming a homogeneous distribution of free gas in the sediment assuming a patchy distribution of free gas increases the estimated concentrations up to 14 there is a patchy occurrence of bottom simulating reflectors south east of the nicoya peninsula on the continental margin offshore costa rica avo analysis indicates that this phenomenon is related to the local presence of free gas beneath the gas hydrate stability zone probably related to a focused vertical fluid flow in areas without bottom simulating reflectors the results indicate that no free gas is present

amplitude versus offset variations in gas sands seismic reflections from gas sands exhibit a wide range of amplitude versus offset avo characteristics the two factors that most strongly determine the avo behavior of a gas sand reflection are the normal incidence reflection coefficient r0 and the contrast in poisson s ratio at the reflector of these two factors r0 is the least constrained based on their avo characteristics gas sand reflectors can be grouped into three classes defined in terms of r0 at the top of the gas sand class 1 gas sands have higher impedance than the encasing shale with relatively large positive values for r0 class 2 gas sands have nearly the same impedance as the encasing shale and are characterized by values of r0 near zero class 3 sands have lower impedance than the encasing shale with negative large magnitude values for r0 each of these sand classes has a distinct avo characteristic an example of a gas sand from each of the three classes is presented in the paper the class 1 example involves a hartshorn channel s

eocene magnetic stratigraphy at gubbio italy and its implications for paleogene geochronology the sequence of polarity zones established by alvarez and others for the cretaceous and paleocene in the scaglia formation in the bottaccione gorge at gubbio has now been extended through the uppermost paleocene into the upper eocene globigerinatheka semiinvoluta zone anomaly 18 the relationships between the standard paleogene foraminiferal zones and the sea floor anomalies are defined with the exception of the mid eocene zones of morozovella lehneri orbulinoides beckmanni and the base of the truncorota loides rohri zone which are obscured by inadequate preservation and a fault gap the base of the eocene lies above anomaly 25 the lower to middle eocene contact base lutetian lies in the upper part of anomaly 22 the base of the bartonian lies below the base of anomaly 18 and the base of the priabonian upper eocene lies within anomaly 18 radiometric ages appear to diverge systematically from a geochronology based on steady sea floor spreading rates suggesting that spreading may have been as much as a factor of 3 slower in the paleocene and the late eocene than in the early and middle eocene

recognition of cyclicity in the petrophysical properties of a maastrichtian pelagic chalk oil field reservoir from the danish north sea subdivision of the maastrichtian chalk from the danish north sea has proven difficult because of its homogenous composition which is caused by a fairly stable depositional environment nevertheless the chalk petrophysical properties show cyclic behavior that potentially can be used for subdivision petrophysical properties of a 23 m long succession of maastrichtian chalk from the dan field were examined for cyclicity by spectral analysis magnetic susceptibility values measured on plug samples together with the neutron porosity and natural gamma radiation well log data were analyzed both as complete data sets and in subsets to survey the general existence of cycles in the chalk all three properties contain consistent cycles at approximately 2 m wavelength as well as additional frequencies where the relative dominance of the different frequencies differs cross spectral analyses of subsets of the petrophysical parameters provide substantial evidence for the coexistence of the approximately 2 m cycles in the maastrichtian chalk furthermore the magnetic susceptibility carries spectral evidence for a higher frequency because most power spectra of the subsets from the magnetic susceptibility have a spectral peak in the 1 8 2 0 cycles m interval i e approximately 0 5 m cycle the cycles can be interpreted as resulting from the eccentricity 94 5 k y and the precession 22 5 k y of the milankovitch cycles assuming a sedimentation rate of 2 6 cm k y the existence and the understanding of cycles at a submeter scale yield possibilities for an advantageously precise subdivision of chalk reservoirs and correlation between wells begin page 2004

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

exposure history of shergottites abstract the cosmogenic nuclides 36 cl t 1 2 3 0 10 5 years 26 al 7 05 10 5 years 10 be 1 6 10 6 years and 53 mn 3 7 10 6 years were measured in three shergottites alha 77005 eeta 79001 and shergotty by accelerator mass spectrometry and neutron activation the cosmogenic nuclide data constrain the history and origin of shergottites alha 77005 was ejected from its parent body about 2 7 my ago and was exposed to cosmic rays as a body of 5 6 cm radius solar cosmic ray produced 26 a1 and 53 mn were first clearly observed in this meteorite the exposure age of eeta 79001 was 0 6 my and the preatmospheric radius was about 15 cm the exposure age of shergotty was calculated to be 2 2 my and the preatmospheric size was about 10 15 cm in radius the terrestrial age of alha 77005 was 0 2 my and that of eeta 79001 was less than 0 06 my all cosmogenic nuclide data indicate that the three shergottites were never exposed to cosmic rays in their parent bodies whether that parent body was mars or an asteroid all shergottites were ejected from greater than a 3 m depth in their parent bodies the ejection of eeta 79001 was a different event from that of the other shergottites

paleogene magnetic stratigraphy in umbrian pelagic carbonate rocks the contessa sections gubbio the umbrian sequence of pelagic carbonate rocks provides an opportunity for precise correlation between paleogene biostratigraphy and geomagnetic polarity history the red to pink paleocene to middle eocene scaglia rossa limestone the varicolored upper eocene scaglia variegata limestone and the gray green oligocene scaglia cinerea marlstone form a 250 m thick continuous exposure in the contessa valley near gubbio italy magnetostratigraphic lithostratigraphic and biostratigraphic investigations in three sections covering the entire paleogene have confirmed and dated the geomagnetic reversal sequence for most of this period the ferromagnetic mineral in the scaglia cinerea is magnetite and af demagnetization to peak fields of 20 mt is sufficient to define the characteristic remanent magnetization the scaglia variegata and pink scaglia rossa samples contain an additional hematite component which is very pronounced in dark red paleocene scaglia rossa samples changes of magnetic mineralogy take place in these limestones during heating especially above 500 c however thermal demagnetization is effective in isolating the characteristic remanence vectors which form almost antipodal clusters of directions representing normal and reversed polarities the directions are rotated counterclockwise partly due to post oligocene tectonism paleontological zonations of planktic foraminifera and calcareous nannofossils were hampered by poor preservation but the major epoch and stage 29 as in the gubbio bottaccione section paleocene eocene just above anomaly 25 eocene oligocene between anomalies 13 and 15 oligocene miocene just below anomaly 6c these correlations require slight modifications to previous conclusions on paleogene sea floor spreading rates

26al 10be and u th isotopes in blake outer ridge sediments implications for past changes in boundary scavenging we have investigated the distributions of 26 al 10 be u th isotopes and stable 27 al and 9 be in core ch88 11p 30 40pn 74 41pw 3337 m from the blake outer ridge bor in the western north atlantic an area that is characterized by high terrigenous input results show authigenic 26 al 27 al of 6 2 8u10 314 atom atom authigenic 10 be 9 be of 0 5 2 6u10 38 atom atom 6 0 2 1 8u10 6 atoms g of 26 al and 5 3 15 1u10 8 atoms g of 10 be in the core over the last glacial interglacial cycle these values as well as total 230 thex 232 th and 10 be 9 be ratios were all at their minima during the last glacial maximum lgm reflecting intensified terrestrial influx to the area aluminosilicate material clays in this influx preferentially scavenged 230 th over the two other particle reactive nuclides 26 al and 10 be such that their boundary scavenging during lgm followed the order 230 ths 10 bes 26 al as opposed to 10 bev 230 ths 26 al in the holocene they appear at variance with the order 10 bes 26 alw 230 th as would be predicted from the relative particle reactivities of the three species these scavenging characteristics point to 10 be 26 al as a potentially more suitable proxy for paleoproductivity than 10 be 230 thex mass balance considerations for 26 al and 10 be show a five fold lgm to holocene increase in deep water circulation in the study area with little change in ocean productivity except during the deglaciation when it increased noticeably fl 2001 elsevier science b v all rights reserved

effects of transverse isotropy on p wave avo for gas sands velocity anisotropy should be taken into account when analyzing the amplitude variation with offset avo response of gas sands encased in shales the anisotropic effects on the avo of gas sands in transversely isotropic ti media are reviewed reflection coefficients in ti media are computed using a planewave formula based on ray theory we present results of modeling special cases of exploration interest having positive reflectivity near zero reflectivity and negative reflectivity the avo reflectivity in anisotropic media can be decomposed into two parts one for isotropy and the other for anisotropy zero offset reflectivity and poisson s ratio contrast are the most significant parameters for the isotropic component while the difference between shale and gas sand is the most important factor for the anisotropic component for typical values of tl anisotropy in shale positive and e both difference and e difference e amplify avo effects for small angles of incidence plays a

wave propagation effects on amplitude variation with offset measurements a modeling study wave propagation effects can significantly affect amplitude variation with offset avo measurements these effects include spreading losses transmission losses interbed multiples surface multiple reflections p sv mode converted waves and inelastic attenuation examination of prestack elastic synthetic seismograms suggests that spreading losses and the transmission losses plus compressional interbed multiples are manifest mainly as a time and offset effect on the primary reflections the surface related multiples and the p sv mode converted waves interfere with prestack amplitudes inducing distortions in the avo pattern such distortions cause large variances in avo model fitting prestack viscoelastic synthetic seismograms also suggest that inelastic attenuation further complicates the avo response because of the offset and time variant amplitude decay effects and the phase change due to dispersion together all these effects severely alter avo behavior and result in serious errors in avo parameter

temporal variability of accumulation at neumayer station antarctica from stake array measurements and a regional atmospheric model in this study we investigate the ability of the regional atmospheric model racmo to represent temporal variations of antarctic accumulation using weekly stake array measurements from neumayer station the model uses ecmwf reanalyses data to force the atmospheric variables at the lateral boundaries of the model domain accumulation is defined as precipitation minus sublimation generally the model represents the synoptic situations that lead to precipitation reasonably well the amounts of accumulation are however usually lower in the model than in the measurements it cannot be distinguished whether the model underestimates precipitation or whether this effect is due to the redistribution of snow by the wind which is not taken into account in the model but affects the accumulation at the measuring site significant differences between model and measurements also occur in cases of net ablation due to wind erosion or when accumulation was due to snowdrift from southwest without precipitation observed

three term taylor series for t2 x2 curves of p and s waves over layered transversely isotropic ground the arrival time curve of a reflection from a horizontal interface beneath a homogeneous isotropic layer is a hyperbola in the x t domain if the subsurface is one dimensionally inhomogeneous horizontally layered or if some or all of the layers are transversely isotropic with vertical axis of symmetry the statement is no longer strictly true though the arrival time curves are still hyperbola like in the case of transverse isotropy however classical interpretation of these curves fails interval velocities calculated from t 2 x 2 curves do not always approximate vertical velocities and therefore cannot be used to calculate depths of reflectors to study the relationship between velocities calculated from t 2 x 2 curves and the true velocities of a transversely isotropic layer we approximate t 2 x 2 curves over a vertically inhomogeneous transversely isotropic medium by a three term taylor series and calculate expressions for these terms as a function of the elastic parameters it is shown that both inhomogeneity and transverse isotropy affect slope and curvature of t 2 x 2 curves for p waves the effect of transverse isotropy is that the t 2 x 2 curves are convex upwards for sv waves the curves are convex downwards for sh waves transverse isotropy has no effect on curvature

in situ10be 26al exposure ages at meteor crater arizona a new method of dating the surface exposure of rocks from in situ production of 10be and 26al has been applied to determine the age of meteor crater arizona a lower bound on the crater age of 49 200 1 700 years has been obtained by this method

some effects of velocity variation on avo and its interpretation in recent years interest has increased in the interpretation of the amplitude variation of reflected signals as a function of offset avo a more meaningful relationship for interpreting reflection coefficients at the target horizon is amplitude variation with incident angle ava the challenge is to convert from avo to ava the effects of velocity variation in the overburden on amplitude variation with offset avo and on the final inversion of avo data into velocity density and poisson s ratio can be significant examples are given here for subsurface medium with a vertical velocity gradient range of 0 2s 1 to 0 8s 1 when the medium is treated as homogeneous in the conversion from avo to ava this velocity variation causes significant errors about 10 percent in both the gradient of ava and in the normal incident reflection coefficient such errors produce errors of similar magnitude in the inversion of ava data into the elastic parameters of velocity poisson s ratio and density the errors dep