gravity field determination and characteristics retrospective and prospective gravimetry has had a long history using pendulums torsion balances and static spring gravimeters relative accuracy adequate for many geophysical problems was already attained by 1900 but it took another half century to build readily portable gravimeters calibration and datum definition remained problems until the 1970s when free fall absolute gravimeters were developed that now have a precision of 10 3 mgal the problems of geographic inaccessibility and field party costs notably in areas of greatest tectonic interest are now being overcome by airborne gravimetry that has already achieved accuracies of 1 3 mgal with resolutions of 10 to 20 km satellite techniques are the best way to determine the long wavelength variations of the gravity field the resolution of the models has steadily improved with the number of satellites and the precision of the observations the best current model includes tracking data from more than 30 satellites satellite altimetry and surface gravimetry and has a resolution of about 290 km harmonic degree 70 with the most recent improvements coming from doppler orbitography and radiopositioning integrated by satellite doris tracking of the spot 2 satellite and satellite laser ranging slr doris and global positioning system gps tracking of the topex poseidon satellite meanwhile radar altimetry has become the dominant technique to infer the marine geoid with a resolution of tens of kilometers or shorter similarly the gravity fields of the moon venus and mars have been determined to harmonic degrees 70 75 and 50 respectively although tracking limitations result in variations of spatial resolution modeling earth s gravity field from the abundance of precise data has become an increasingly complex task with which the development of computer capacity has kept pace contemporary solutions now entail about 10 000 parameters half of them for effects other than the fixed gravity field of earth temporal variations arising from tides have long been well modeled and nontidal changes are now being identified the improvement in gravitational models engendered corresponding advances in geophysical interpretation isostatic models were refined and expanded to account for regional thermal and tectonic histories interpretation of the long wavelength gravity field determined by satellite techniques has been mainly in terms of plate tectonics as a manifestation of mantle convection gravity has been significant in inferring that there must be a large increase in viscosity with depth most strongly from the apparent slow sinking of subducted slabs the prospects for increasing accuracy and resolution in the determination of earth s gravity field rest primarily with the development of new measurement systems airborne gravimetry is taking promising new steps using gps but significant global model improvement awaits a dedicated satellite gravimetry system and future satellite altimeter missions will do more for ocean dynamics studies than geoid improvement advances in interpretation will occur through the development of other data such as seismic tomography and larger scale computer modeling of tectonics and convection

avo and velocity analysis velocity analysis using semblance estimates velocities based on a constant amplitude model for seismograms and does not take amplitude variation with offset avo into account in the presence of avo the constant amplitude model becomes inaccurate particularly for events which exhibit polarity reversals an avo sensitive velocity analysis procedure which is a generalization of the traditional semblance method can be devised by giving an offset dependence to the modeled seismograms incorporating avo into velocity analysis requires additional parameters to describe the reflectivity this results in reduced velocity precision by introducing a regularization term which provides a controlled suppression of the contributions due to avo effects we describe an avo sensitive velocity analysis algorithm that properly deals with events exhibiting polarity reversals or large amplitude variation with offset

tidal parameters for an inelastic earth abstract the gravimetric factors for an elliptical uniformly rotating earth are computed according to a new definition with respect to wahr s results and considering the mantle inelasticity the love numbers h and k are defined in the elliptical earth case to correspond to the love spherical definition they become latitude dependent the tidal parameters discussed in this paper are computed for an inelastic earth s mantle the numerical results show an increase of 0 4 of the gravimetric factors with respect to the wahr s values the love numbers h and k increase by 1 4 to 3 with respect to the elastic case

azimuthal variation in avo response for fractured gas sands natural fractures in reservoirs play an important role in determining fluid flow during production and hence the density and orientation of fractures is of great interest in the presence of aligned vertical fractures the reflection amplitude at finite offset varies with azimuth the effect of natural fractures on the azimuthal avo response from a gas sandstone reservoir encased within shale is investigated a simple expression for the difference in p wave reflection coefficient from the top of the reservoir parallel and perpendicular to the strike of the fractures is obtained in terms of the normal and tangential compliances z n and z t of the fractures this expression is valid for small anisotropy and material contrasts and is compared with the results of numerical modelling for a given value of z t the azimuthal variation in reflection coefficient at moderate offsets is found to increase with decreasing z n z t for gas filled open fractures z n z t 1 but a lower ratio of z n z t may result from the presence of cement or clay within the fractures or from the presence of a fluid with non zero bulk modulus for z n z t 1 and moderate offsets the variation with offset of the reflection coefficient from the top of the fractured unit is dominated by the contrast in poisson s ratio between the gas sand and the overlying shale the effect of fractures only becoming noticeable as the critical angle for the unfractured sandstone is approached however for reflections from the base of the fractured unit the variation in reflection amplitude with azimuth is much greater at conventional seismic offsets than for the reflection from the top azimuthal variations in the strength of the reflection from the top of the reservoir depend only on the variation in reflection coefficient whereas the raypath is also a function of azimuth for reflections from the base of the fractured unit leading to stronger more visible variations of avo with azimuth it follows that an azimuthal variation in avo due to fractures in the overburden may be misinterpreted as due to the presence of aligned fractures in the reservoir

inversion of reflection seismograms by differential semblance analysis algorithm structure and synthetic examples1 seismograms predicted from acoustic or elastic earth models depend very non linearly on the long wavelength components of velocity this sensitive dependence demands the use of special variational principles in waveform based inversion algorithms the differential semblance variational principle is well suited to velocity inversion by gradient methods since its objective function is smooth and convex over a large range of velocity models an extension of the adjoint state technique yields an accurate estimate of the differential semblance gradient non linear conjugate gradient iteration is quite successful in locating the global differential semblance minimum which is near the ordinary least squares global minimum when coherent data noise is small several examples based on the 2d primaries only acoustic model illustrate features of the method and its performance

gravity estimation from stage a satellite to satellite tracking mission an analysis was performed to determine the accuracy of estimating gravity anomalies using satellite to satellite tracking between the space transportation system nasa s space shuttle and the constellation of gps satellites stage shuttle gps tracking for anomalous gravitation estimation second time derivatives of the observed ranges obtained from carrier phase measurements between the low and high satellites provide in situ line of sight components of gravitation as long as on board accelerometers are used to correct for the nongravitational signals absolute nondifferential single difference and double difference modes of tracking the gps satellites were considered in the analysis which showed that the last yields the highest gravity estimation accuracy with instrumentation already or soon to be available it is anticipated that the gravitational acceleration at altitude can be measured to an accuracy of about 0 3 mgal 1 mgal 10 5 m s2 1 g for an integration time of 60 s double difference acceleration data collected by the shuttle at 300 km altitude could depending on mission duration and data processing techniques yield estimates of 2 mean gravity anomalies on the earth s surface to an accuracy of about 5 mgal accuracy in 1 mean anomalies of about 4 5 mgal would be achievable using a free flyer at 160 km altitude and assuming a potentially feasible measurement accuracy of 0 1 mgal 30 s integration time

the offset midpoint traveltime pyramid of p waves in homogeneous orthorhombic media abstractthe offset midpoint traveltime pyramid describes the diffraction traveltime of a point diffractor in homogeneous media we have developed an analytic approximation for the p wave offset midpoint traveltime pyramid for homogeneous orthorhombic media in this approximation a perturbation method and the shanks transform were implemented to derive the analytic expressions for the horizontal slowness components of p waves in orthorhombic media numerical examples were shown to analyze the proposed traveltime pyramid formula and determined its accuracy and the application in calculating migration isochrones and reflection traveltime the proposed offset midpoint traveltime formula is useful for kirchhoff prestack time migration and migration velocity analysis for orthorhombic media

sr and nd isotope compositions of atmospheric mineral dust at the summit of mt sefuri north kyushu southwest japan a marker of the dust provenance and seasonal variability abstract we analyzed rb sr nd isotope ratios of mineral dust in total aerosol load collected with rainwater continuously from 1998 to 2006 at the summit of mt sefuri northern kyushu southwestern japan during this period the total mass of the dust generally increased in late winter peaked in early spring and then decreased 87sr 86sr in atmospheric mineral dust varied from 0 7096 to 0 7180 and end 0 chur from 19 9 to 3 5 during heavy deposition periods the dust had high 87sr 86sr isotope ratios and low to middle end 0 chur values respectively these compositions are comparable to those of sand and loess in arid areas of northeast china takla makan and western beijing such particles were transported by westerlies from those areas to northern kyushu in winter and spring in summer and autumn the isotopic compositions of the dust varied greatly however during light deposition periods the sr isotope composition was low in these seasons the contributions to the dust from japanese soils and volcanic ash transported by southern winds were relatively larger than in winter and spring because of decreased mineral dust particle transport from the continent nevertheless fine sandy desert particles and loess in general accounted for most mineral dust deposition in northern kyushu year round even in summer local soils derived from weathered granite and volcanic ash were minor components only the net mass of water insoluble inorganic matter in the collected mineral dust fluctuated from year to year deposition on mt sefuri was relatively large before 2001 decreased from 2002 to 2005 and increased greatly in spring 2006 these year to year differences probably reflected changes in the strength of the westerlies and in climate conditions in the arid source areas

seismic characterization of multiple fracture sets does orthotropy suffice as geophysicists rely increasingly on effective media theories to characterize naturally fractured reservoirs they become more and more interested in evaluating the accuracy of different theories estimating their limits of applicability and assessing their usefulness for practice with this in mind we compare two popular seismological theories of hudson and schoenberg with the theory of kachanov developed in the context of mechanics of materials by performing finite element simulations of effective media for models that contain several sets of nonintersecting circular vertical fractures embedded in otherwise isotropic host rock we examine the accuracy of these theories our numeric study reveals that predictions of both the first and second order hudson s theories are typically inferior to those of others especially when the fractures are dry while on average the theories of schoenberg and kachanov fit finite element computations with comparable accuracy the latter appears to be more useful

a case study of azimuthal avo analysis with anisotropic spreading correction geomechanical properties of tight low porosity reservoirs are largely governed by natural fracture networks hence reliable estimation of fracture density and orientation is extremely important for cost effective hydraulic completion and hydrocarbon production direct information about fracturing can be obtained using borehole methods such as image log analysis which provide estimates of fracture counts and orientations on various scales the main shortcoming of borehole measurements is that they are sensitive only to formation properties in the immediate vicinity of the well in some cases the spatial distribution of fractures can be inferred from fault maps obtained by coherence analysis of surface seismic data however the correlation between areas of high fracture density and fault locations is not always straightforward

miocene to pliocene changes in south african hydrology and vegetation in relation to the expansion of c4 plants the miocene expansion of c4 plants mainly tropical grasses between 8 and 4 million years ma remains an enigma since regional differences in the timing of the expansion rules out decreased co2 pco2 as a dominant forcing e g tipple and pagani 2007 the early origins of terrestrial c4 photosynthesis annu rev earth planet sci 35 435 461 other environmental factors such as low latitude aridity and seasonality have been proposed to explain the low tree versus grass ratio found in savannahs and tropical grasslands of the world but conclusive evidence is missing here we use pollen and stable carbon 13c and hydrogen d isotope ratios of terrestrial plant wax from a south atlantic sediment core odp site 1085 to reconstruct miocene to pliocene changes of vegetation and rainfall regime of western southern africa our results reveal changes in the relative amount of precipitation and indicate a shift of the main moisture source from the atlantic to the indian ocean during the onset of a major aridification 8 ma ago we emphasize the importance of declining precipitation during the expansion of c4 and cam mainly succulent vegetation in south africa we suggest that the c4 plant expansion resulted from an increased equator pole temperature gradient caused by the initiation of strong atlantic meridional overturning circulation following the shoaling of the central american seaway during the late miocene

from chandler wobble to free oscillations comparison of cryogenic gravimeters and other instruments in a wide period range abstract the cryogenic gravimeter was developed to overcome the mechanical deficiencies of spring gravimeters the stability of the instrument permits the study of long term gravity variations for the first time short term especially periodical gravity variations have been well observed with spring gravimeters this investigation tries to determine up to what level and frequency the various types of instrument have equivalent quality side by side runs of two cryogenic instruments and comparisons with relative spring gravimeters and long period seismometers should help to answer this question from the intercomparison of two cryogenic gravimeters in bad homburg germany tt40 and tt60 and in miami usa tt40 and ucsd sg b one can see that in general the gravity signals coincide within several nanometres per second squared but also intervals are noted where the differences are 10 30 nm s 2 over a timespan of 1 3 months usually known instrumental problems are the cause for these discrepancies data analysis as well as comparisons with absolute gravity measurements in parallel demonstrate that the drift of the meters is monotonic and in general has exponential characteristics this allows investigations in the long period tide band as well as in the range of long term gravity variations e g the gravitational effects of polar motion water table and air pressure variations at the other end of the spectrum in the period range from 15 min to 1 day a side by side run of the cryogenic gravimeter sg 102 and the spring gravimeter lcr et 19 in the black forest observatory in schiltach germany shows that the noise spectrum of the spring gravimeter is slightly lower than that of the cryogenic one analysis of the free oscillations of the earth from several large earthquakes shows that this spring gravimeter and modern seismometers are at least of equal quality data sets recorded with cryogenic gravimeters open the possibility of investigating gravity variations in a wide frequency range however especially for investigations in the high frequency band 1 c day 1 and above one should also make use of well maintained conventional spring gravimeters and possibly modern force balance seismometers which may have at least the same or better quality

global charts of ocean tide loading effects global ocean tide loading charts of the radial displacement the potential divided by g gravity acceleration and the gravity effect have been computed using the 11 constituents m2 s2 n2 k2 k1 o1 p1 q1 mf mm ssa of schwiderski s tidal model these new charts have a resolution of 1 1 on the continents as well as on the oceanic area a description of farrell s convolution method to compute the loading effects is given and an estimate of the numerical errors leads to the conclusion that these global charts have a precision better than 2 5 independent of the accuracy of schwiderski s maps the current approximation of the loading effects by a proportionality relation with the local oceanic tides is also compared with farrell s convolution method departures of several centimeters systematically appear in particular over the continental shelves we then show that the maps of the oceanic tides deduced from satellite altimetry could be corrected for the loading effect by an iterative computational procedure based on our algorithm of farrell s convolution

velocity analysis using weighted semblance increasing the resolution of semblance based velocity spectra or semblance spectra is useful for estimating normal moveout velocities as increased resolution can help to distinguish peaks in the spectra the resolution of semblance spectra depends on the sensitivity of semblance to changes in velocity by weighting terms in the semblance calculation that are more sensitive to changes in velocity we can increase resolution our implementation of weighted semblance is a straightforward extension of conventional semblance somewhat surprisingly we increase resolution by choosing an offset dependent weighting function that minimizes semblance we test our method on synthetic and field data and our tests confirm that weighted semblance provides higher resolution than conventional semblance

estimation of fracture parameters from reflection seismic data part iii fractured models with monoclinic symmetry existing geophysical and geological data indicate that orthorhombic media with a horizontal symmetry plane should be rather common for naturally fractured reservoirs here we consider two orthorhombic models one that contains parallel vertical fractures embedded in a transversely isotropic background with a vertical symmetry axis vti medium and the other formed by two orthogonal sets of rotationally invariant vertical fractures in a purely isotropic host rock using the linear slip theory we obtain simple analytic expressions for the anisotropic coefficients of effective orthorhombic media under the assumptions of weak anisotropy of the background medium for the first model and small compliances of the fractures all effective anisotropic parameters reduce to the sum of the background values and the parameters associated with each fracture set for the model with a single fracture system this result allows us to eliminate the influence of the vti background by evaluating the differences between t

fracture characterization of deep tight gas sands using azimuthal velocity and avo seismic data in saudi arabia complex spatial distribution of fractures in a reservoir greatly affects the flow of fluids including hydrocarbons and can significantly increase reservoir porosity and permeability this reservoir quality enhancement is especially important for tight reservoirs where low primary matrix porosity and or permeability would deem the play noncommercial therefore fracture characterization is an important part of reservoir development and management in particular for the type of deep and tight gas reservoirs west of the ghawar field in saudi arabia

a search for the slichter modes in superconducting gravimeter records using a new method summary a detection of the slichter modes is attempted using a new method that focuses on the use of the spectra of individual superconducting gravimeter sg records to test if the latitudinal and longitudinal dependence of amplitudes and phases of the slichter modes are satisfied in a statistical point of view the new method allows considerations of lower peaks in the spectra as possible candidates of the slichter modes as the new method searches for the three slichter modes individually it does not rely on the theory of splitting which remains controversial and is still valid even if not all of the three slichter modes are excited to the observable level a loose splitting rule is used to check if a set of candidates of the prograde axial and retrograde modes may constitute the whole slichter modes the attempted detection is made using three sets of sg records selected from the global geodynamics project ggp network based on criteria including data spans geological locations and the quality of the gravity time series the selected three data sets are the simultaneous records of five sgs located in canberra matsushiro strasbourg sutherland and vienna the data spans are 30 17 and 12 months with the later two data sets after the 2001 peruvian earthquake of magnitude 8 4 that might have excited the slichter modes to larger amplitudes our results suggest two possible scenarios that are still inconclusive the first is a candidate of the prograde mode with a frequency around 4 485 cycle per day cpd that is present in all three data sets the second is a combination of a prograde and an axial modes with frequencies around 7 098 and 7 546 cpd that are present in the two data sets after the peruvian earthquake

observations of pkikp pcp amplitude ratios and implications for earth structure at the boundaries of the liquid core 1 mining of the database produced by seismic array stations of the international monitoring system has resulted in hundreds of new observations of precritical reflections from earth s inner core here we present short period amplitude ratios between these pkikp phases and corresponding reflections pcp from the core mantle boundary cmb pkikp pcp amplitude ratios are one of the few seismic observations that directly constrain the change in s velocity and density across the inner core outer core boundary we measure the amplitude ratios from optimally tuned array beams and use a bootstrap technique to estimate observational uncertainties we also use a model resampling technique to estimate uncertainties associated with the background reference model the optimal models have s velocities at the top of the inner core less than about 2 5 km s and density jumps across the inner core outer core boundary of less than about 0 45 g cm3 these values can be reconciled with the higher estimates from normal mode constraints if strong radial gradients are present in the top of the inner core or if the outermost inner core has material properties distinct from the bulk of the inner core we also find evidence for an ultralow velocity zone in the lowermost mantle beneath the eastern coast of australia a unique best fitting model cannot be determined but an example that is consistent with the data has a reduction in p velocity of 10 a reduction in s velocity of 35 and an increase in density of 20 this type of structure could be explained by the presence of partial melt and increased iron content just above the cmb

p wave and s wave azimuthal anisotropy at a naturally fractured gas reservoir bluebell altamont field utah reflection p and s wave data were used in an investigation to determine the relative merits and strengths of these two data sets to characterize a naturally fractured gas reservoir in the tertiary upper green river formation the objective is to evaluate the viability of p wave seismic to detect the presence of gas filled fractures estimate fracture density and orientation and compare the results with estimates obtained from the s wave data the p wave response to vertical fractures must be evaluated at different source receiver azimuths travelpaths relative to fracture strike two perpendicular lines of multicomponent reflection data were acquired approximately parallel and normal to the dominant strike of upper green river fractures as obtained from outcrop core analysis and borehole image logs the p wave amplitude response is extracted from prestack amplitude variation with offset avo analysis which is compared to isotropic model avo responses of gas sand versus brine sand in the upper green

large scale global surface mass variations inferred from gps measurements of load induced deformation 1 detrended crustal deformation measurements from a global network of up to 200 continuous global positioning system gps tracking sites are inverted for low degree and order n m 6 spherical harmonic series of surface mass variations an independent geophysical model of the atmosphere oceans and water and snow on land is also used to guide the spherical harmonic truncation and to evaluate aliasing errors in the results inversion uncertainties decrease significantly as spatial coverage and data quality improve with time especially in the last few years consequently gps inverted seasonal geocenter and low degree zonal harmonics converge to those determined by measurements from satellite laser ranging slr significant n 1 and non zonal variations have also been found

constraining core undertones using stacked ida gravity records a search is made for oscillatory gravity signals possibly associated with core undertones in the frequency range between the diurnal and semidiurnal tidal bands ida tidal data recorded during 6 month intervals following four large earthquakes were used and the data from different stations were stacked according to spherical harmonic surface amplitude patterns with angular order l 4 the detection level of a signal in the diurnal semidiurnal frequency band was roughly 10 20 ngal but the only signal detected was a 9 ngal signal of 9 54 hr period whose origin remains unexplained

performance of wielandt streckeisen sts 1 seismometers in the tidal domain preliminary results abstract the aim of this work is to show that sts 1 seismometers are able to provide a signal with a very long period vertical component which can be used for earth tide studies provided a number of recording conditions are improved we use data from the geoscope network for bng bangui central african republic mbo m bour senegal crzf crozet taaf terres australes et antarctiques frandc aises indian ocean ech echery france and paf port aux frandc ais kerguelen taaf indian ocean stations operated by the ecole et observatoire de physique du globe de strasbourg eopgs and institut frandc ais de recherche scientifique pour le de veloppement en coope ration orstom these data are analysed with a least squares adjustment method to fit the earth tidal waves and the results for these stations are compared special attention is paid to the comparison of results from ech with those from the superconducting gravimeter of strasbourg j 9 for several stations used in this study crzf mbo and ech the results are promising but they are disappointing for others paf and bng we did not find any clear reason for such a difference in quality we propose some improvements that will enable researchers to achieve a better recording of this long period signal and obtain higher quality data allowing gravimetric studies from global seismological networks

anisotropic geometrical spreading correction for wide azimuth p wave reflections compensation for geometrical spreading along a raypath is one of the key steps in avo amplitude variation with offset analysis in particular for wide azimuth surveys here we propose an efficient methodology to correct long spread wide azimuth reflection data for geometrical spreading in stratified azimuthally anisotropic media the p wave geometrical spreading factor is expressed through the reflection traveltime described by a nonhyperbolic moveout equation that has the same form as in vti transversely isotropic with a vertical symmetry axis media the adapted vti equation is parameterized by the normal moveout nmo ellipse and the azimuthally varying anellipticity parameter to estimate the moveout parameters we apply a 3d nonhyperbolic semblance algorithm of vasconcelos and tsvankin that operates simultaneously with traces at all offsets and azimuths the estimated moveout parameters are used as the input in our geometrical spreading computation numerical tests for models composed of orthorhombic layers with strong depthvarying velocity anisotropy confirm the high accuracy of our travetime fitting procedure and therefore of the geometricalspreading correction because our algorithm is based entirely on the kinematics of reflection arrivals it can be incorporated readily into the processing flow of azimuthal avo analysis in combination with the nonhyperbolic moveout inversion we apply our method to wide azimuth p wave data collected at the weyburn field in canada the geometrical spreading factor for the reflection from the top of the fractured reservoir is clearly influenced by azimuthal anisotropy in the overburden which should cause distortions in the azimuthal avo attributes this case study confirms that the azimuthal variation of the geometrical spreading factor often is comparable to or exceeds that of the reflection coefficient

fracture detection using land 3d seismic data from the yellow river delta china over the past 10 years there has been a consistent increase in using 3d p wave data to characterize fractures which is critical for ensuring economic oil and gas production in tight formations of otherwise low permeability here we present a case study of fracture detection using 3d p wave seismic data from the yellow river delta in east china the target formation is a naturally fractured mud rock reservoir the field site has been in production for more than 10 years however virgin pressure wells are still being drilled the purpose of the survey was the remote identification for future well planning of zones of high fracture density that are residual oil charged a major aspect of this study is to compare the different seismic attributes and different analysis techniques on a common data set for fracture detection our aim is to understand the merits of these different techniques and to establish some basic guidelines for fracture detection using p wave data if we assume the fracture population consists of predominantly one major orientation the azimuthal variation of p wave seismic attributes such as traveltime stacking velocity reflected wave amplitudes impedance etc can be approximately described by an ellipse the long axis of the ellipse indicates the fracture orientation and the relative ratio of the long to short axes of this ellipse is proportional to the fracture density or intensity of the rock concerned as we know at least three data points are required to define an ellipse in azimuthal planes thus fracture orientation and intensity maps can be built from 3d p wave data if there is sufficient azimuthal coverage we shall call this technique azimuthal attribute analysis or the 3a technique in the practical application of the 3a technique two methods are often employed to extract the fracture information full azimuth surface fitting and narrow azimuth

3 d moveout inversion in azimuthally anisotropic media with lateral velocity variation theory and a case study reflection moveout recorded over an azimuthally anisotropic medium e g caused by vertical or dipping fractures varies with the azimuth of the source receiver line normal moveout nmo velocity responsible for the reflection traveltimes on conventional length spreads forms an elliptical curve in the horizontal plane while this result remains valid in the presence of arbitrary anisotropy and heterogeneity the inversion of the nmo ellipse for the medium parameters has been discussed so far only for horizontally homogeneous models above a horizontal or dipping reflector here we develop an analytic moveout correction for weak lateral velocity variation in horizontally layered azimuthally anisotropic media the correction term is proportional to the curvature of the zero offset traveltime surface at the common midpoint and therefore can be estimated from surface seismic data after the influence of lateral velocity variation on the effective nmo ellipses has been stripped the generalized dix equat

noise levels of superconducting gravimeters updated comparison and time stability abstract since the beginning of the global geodynamics project ggp in 1997 the number of superconducting gravimeters sgs has increased to reach 30 operating sites today data from this network allow a comparison of the noise levels of the different contributing stations the knowledge of the noise levels at each site is important in any combination of data to determine global earth parameters for example the stacking of the data in the search for elusive signals like the gravity variations associated with the translational mode of the inner core we use a standardized procedure based on the computation of the residual power spectral densities psds over a quiet time period in order to evaluate the combined instrument plus site noise in the long period seismic band 0 3 mhz 1 mhz the experience at strasbourg france has shown some improvements from the tt70 t005 full size instrument to the c026 compact model in terms of noise reduction while the most recent observatory sg types osg044 at bad homburg germany and osg052 at sutherland south africa do not show any further improvement with respect to the compact models respectively cd30 and cd037 operating at the same stations at black forest observatory bfo in germany the experience of the dual sphere osg with a lower sphere heavier than usual has shown that the instrumental and site conditions make this station the least noisy one at frequencies larger than 0 1 mhz the noise analysis using the longest time series available has shown that the noise level at these sites is mostly stable within 1 over the years the comparison with some seismological noise models shows that the best sg sites are less noisy than long period seismometers below 1 mhz however the noise level of the best sgs is still at the limit of detection of the subseismic translational mode of the inner core

azimuthal avo analysis with anisotropic spreading correction a synthetic study analysis of prestack amplitude variation with offset and azimuth often called azimuthal avo analysis or avaz is one of the most effective tools for seismic characterization of fractures and in situ stress field the main advantage of amplitude methods compared to traveltime inversion is their high vertical resolution that makes avo analysis applicable to relatively thin reservoirs also body wave amplitudes are highly sensitive to seismic anisotropy and in particular to azimuthal velocity variations associated with vertical fracture systems and nonhydrostatic stresses

using avo for fracture detection analytic basis and practical solutions tight low porosity reservoirs can produce significant amounts of hydrocarbons if fracture networks provide the necessary permeability characterization of naturally fractured reservoirs using surface seismic data is an important exploration problem that has attracted much attention in the literature tle for example had an entire issue august 1996 on this subject while the data and modeling in existing publications demonstrate that fracturing causes significant azimuthal variation in recorded seismic signatures the problem is how to infer fracture parameters from these data

measuring seismic normal modes with the gwr c021 superconducting gravimeter abstract in the framework of the global geodynamic project cgp that plans to study earth s normal modes with superconducting gravimeters sg we present a study of the c021 sg located in membach belgium in the seismic normal modes band periods shorter than 1 h our results are based on the spectra associated with the irian jaya february 17 1996 m w 7 9 and the baleny island march 25 1998 m w 8 1 earthquakes we demonstrate that the signal to noise ratio snr of the sg can be improved by a factor of 4 below 1 2 mhz using local barometric records zurn w widmer r 1995 on noise reduction in vertical seismic records below 2 mhz using local barometric pressure geophys res lett 22 3537 3540 a side by side comparison of a long period sts 1 v seismometer with the sg c021 confirms the low noise level of this latter instrument and demonstrates its ability to measure seismic normal modes several suggestions are given about the noise sources that could affect sgs in the seismic frequency band

mean gravity anomalies from a combination of apollo ats 6 and geos 3 ats 6 sst tracking campaigns the satellite to satellite tracking sst technique has been used to measure the earth s gravity field data from the apollo soyuz and geos 3 missions obtained at greatly differing altitudes have been used in combination to determine 5 and 10 equal area mean gravity anomalies in the south atlantic and indian oceans and over the african continent the apollo ats 6 sst data consist of 47 passes of data taken from the 230 km apollo altitude while the geos 3 ats 6 data set consists of 45 passes taken from 840 km altitude through a combination of the two data sets with the geos 3 sst data providing stability for the solution at the longer wavelengths excellent 5 at 3 mgal 10 1 mgal results were obtained for this largely unsurveyed region these results have been compared to an altimetric surface at comparable wavelengths providing good agreement 1 mgal for 10 blocks and 5 mgal for 5 blocks between these independent measuring techniques the terrestrial data over the african continent is shown to differ significantly rms difference of 8 mgals with a maximum difference of 23 mgals from a comparison of 10 geos 3 sst derived blocks a simulation has also been performed to assess the data requirements aliasing and noise characteristics of a low spacecraft being tracked in a high low sst configuration the results of the simulation were employed in the design of the sst data reduction procedures

on reduction of long period horizontal seismic noise using local barometric pressure summary tilt from atmospheric loading has long been known to be the major source of long period horizontal seismic noise we try to quantify these effects for seismic data from the black forest observatory bfo which is known to be a very quiet station experimental transfer functions between local barometric pressure and horizontal seismic noise are estimated for two long time series by standard methods two simple analytical physical models are developed the local deformation model ldm and the acoustic gravity wave model twm subsequently these models with only two free parameters are fit using least squares to the observed seismic noise for time series of widely differing lengths the results are variable sometimes rather dramatic variance reductions are obtained and sometimes the reduction is hardly significant the method produces the best results when barometrically induced noise is high the resulting admittances for the ldm are compared to finite element calculations since the methods are simple and can result in conspicuous reductions in noise we provide one more reason for installing barometers at even the best broad band seismic stations

non hyperbolic moveout inversion of wide azimuth p wave data for orthorhombic media the azimuthally varying non hyperbolic moveout of p waves in orthorhombic media can provide valuable information for characterization of fractured reservoirs and seismic processing here we present a technique to invert long spread wide azimuth p wave data for the orientation of the vertical symmetry planes and five key moveout parameters the symmetry plane nmo velocities v 1 nmo and v 2 nmo and the anellipticity parameters 1 2 and 3 the inversion algorithm is based on a coherence operator that computes the semblance for the full range of offsets and azimuths using a generalized version of the alkhalifah tsvankin non hyperbolic moveout equation the moveout equation provides a close approximation to the reflection traveltimes in layered anisotropic media with a uniform orientation of the vertical symmetry planes numerical tests on noise contaminated data for a single orthorhombic layer show that the best constrained parameters are the azimuth of one of the symmetry planes and the velocities v 1 nmo and v 2 nmo while the resolution in 1 and 2 is somewhat compromised by the trade off between the quadratic and quartic moveout terms the largest uncertainty is observed in the parameter 3 which influences only long spread moveout in off symmetry directions for stratified orthorhombic models with depth dependent symmetry plane azimuths the moveout equation has to be modified by allowing the orientation of the effective nmo ellipse to differ from the principal azimuthal direction of the effective quartic moveout term the algorithm was successfully tested on wide azimuth p wave reflections recorded at the weyburn field in canada taking azimuthal anisotropy into account increased the semblance values for most long offset reflection events in the overburden which indicates that fracturing is not limited to the reservoir level the inverted symmetry plane directions are close to the azimuths of the off trend fracture sets determined from borehole data and shear wave splitting analysis the effective moveout parameters estimated by our algorithm provide input for p wave time imaging and geometrical spreading correction in layered orthorhombic media

on the resolution of density within the earth roughly 30 years have passed since the last publication of a linear resolution calculation of density inside the earth since that time the data set of free oscillation degnerate frequencies has been completely re estimated taking into account the biassing effects of splitting and coupling due to 3d structure this paper presents a new resolution analysis based on the new data and focuses on two particular issues 1 the density jump at the inner core boundary which is important in discussions of the maintenance of the geodynamo and 2 a possible density excess in the lowermost mantle which might be indicative of a hot abyssal layer we find that the density jump at the inner core boundary is 0 82 0 18 mg m 3 which is significantly larger than previously thought we also find little support for an excess density in the lowermost mantle though an increase

constraints on upper mantle discontinuities from observations of long period reflected and converted phases stacked images combining over 5 years of long period global digital seismograph network data reveal many phases associated with reflections and conversions from upper mantle discontinuities these images show travel time and amplitude relative to a reference phase which is aligned and normalized on all seismograms prior to stacking results obtained for p s ss and pp reference arrivals resolve numerous phases from discontinuities near 410 and 660 km while some of the stacks also show evidence for a weaker discontinuity near 520 km phases of particular interest include p and sh multiples resulting from topside reflections precursors to ss from underside reflections and p to sv converted phases these phases can be clearly seen both in the waveform stacks and in cross correlation analysis of individual seismograms travel times for these arrivals are converted to discontinuity depths relative to velocities in the preliminary reference earth model and empirical corrections are applied for the effects of lateral velocity variations in the upper mantle average apparent depths to the discontinuities for the different phases agree to within 3 km for the 410 km discontinuity to within 4 km for the 520 km discontinuity and to within 8 km for the 660 km discontinuity the best global averages are obtained from the ss precursor data which indicate discontinuities at 415 519 and 659 km discontinuity depths obtained from the p to sv converted phases at over 35 individual seismic stations exhibit variations of less than 20 km apparent depths to the 660 km discontinuity consistently show greater variability than depths to the 410 km discontinuity supporting recent laboratory results which indicate that the clapeyron slope for the 660 km discontinuity is significantly larger in magnitude than the slope for the 410 km discontinuity precursors to ss seen between 110 and 180 are particularly useful for mapping possible lateral variations in discontinuity depths since each arrival can be associated with a single underside reflection point apparent discontinuity depths computed from ss precursors for different tectonic regions agree to within about 5 km the ss precursors have especially good coverage near the subducting slabs in the northwest pacific analysis of apparent discontinuity depths in this area suggests the presence of a broad 1000 to 1500 km wide region near the slab in which the 660 km discontinuity is depressed by about 20 km measuring absolute amplitudes for these phases is difficult due to the large corrections required to compensate for the effects of incoherent stacking relative amplitude analysis suggests that the p and s wave impedance changes at 410 km are about 0 8 1 1 times the size of the changes at 660 km and that the contrasts at 520 km are between 0 3 and 0 6 of the changes at 410 km

high resolution analysis of the gravest seismic normal modes after the 2004 mw 9 sumatra earthquake using superconducting gravimeter data 1 the mw 9 sumatra andaman earthquake on december 26 2004 has strongly excited the low frequency seismic modes and is a unique opportunity to improve the frequency and damping measurements of the gravest seismic modes the precise estimation of the frequency splitting of the seismic modes below 1 mhz is needed to improve 1d density models of the earth from this event 2s1 is observed for the first time without any stacking on both sg superconducting gravimeter and seismic records we report here analyses of sg data obtained from 11 sites the spectra of sg records clearly show the splitting of 0s2 into 5 completely resolved singlets and the splitting of 0s3 into 7 singlets at individual stations the present results demonstrate that sgs provide high quality data for a precise analysis of the low frequency seismic modes

observation of seismic core modes from a superconducting gravimeter record abstract there have been few reliable reports of observation of seismic core modes in spite of their geophysical importance in this study we analyze low frequency seismic signals from a superconducting gravimeter scg located at the esashi earth tides station japan to make observations of core modes spectral analysis of an scg record from the great macquarie ridge earthquake which occurred during the period of our test runs of a scg by means of the sompi method has yielded well resolved signals which are most likely to be the core modes 2s2 and 6s2 close investigation of the analysis results however has revealed that there may be systematic bias in their q estimates due to some methodological problems our tentative conclusion is that the q values of the core modes observed here are relatively low probably not exceeding 1000 and also that their amplitudes are larger than theoretically predicted from a spherically symmetric earth model at any rate the present results demonstrate that the scg also has excellent potential as a long period seismometer

fracture detection in a carbonate reservoir using a variety of seismic methods we combine various methods to estimate fracture orientation in a carbonate reservoir located in southwest venezuela the methods we apply include the 2 d rotation analysis of 2 d p s data along three different azimuths amplitude variation with offset avo of 2 d p wave data along the same three azimuths normal moveout nmo analysis of the same 2 d data and both 3 d azimuthal avo and nmo analysis of 3 d p wave data recorded in the same field the results of all methods are compared against measures of fracture orientation obtained from formation microscanner logs recorded at four different locations in the field regional and local measures of maximum horizontal stress and the alignment of the major faults that cross the field p s data yield fracture orientations that follow the regional trend of the maximum horizontal stress and are consistent with fracture orientations measured in the wells around the carbonate reservoir azimuthal avo analysis yields a similar regional trend as that obtained from the p s data but the resolution is lower local variations in fracture orientation derived from 3 d avo show good correlation with local structural changes in contrast due to the influence of a variety of factors including azimuthal anisotropy and lateral heterogeneity in the overburden azimuthal nmo analysis over the 3 d p wave data yields different orientations compared to those obtained by other methods it is too early to say which particular method is more appropriate and reliable for fracture characterization the answer will depend on factors that range from local geological conditions to additional costs for acquiring new information

fortnightly earth rotation ocean tides and mantle anelasticity summary this study of the fortnightly mf tide comprises three main topics 1 a new determination of the fortnightly component of polar motion and length of day lod from a multidecade time series of observed space geodetic data 2 the use of the polar motion determination as one constraint in the development of a hydrodynamic ocean model of the mf tide and 3 the use of these results to place new constraints on mantle anelasticity at the mf tidal period our model of the mf ocean tide assimilates more than 14 years of altimeter data from the topex poseidon and jason 1 satellites because the mf altimetric signal to noise ratio is very small it is critical that altimeter data not be overweighted the polar motion data plus tidegauge data and independent altimeter data give useful additional information with only the polar motion putting constraints on tidal current velocities the resulting ocean tide model plus the dominant elastic body tide leaves a small residual in observed lod caused by mantle anelasticity the inferred effective tidal q of the anelastic body tide is 90 and is in line with a frequency dependence with in the range 0 2 0 3

first observation of 2s1 and study of the splitting of the football mode 0s2 after the june 2001 peru earthquake of magnitude 8 4 the recent peru earthquake with a moment magnitude mw 8 4 on june 23 2001 presents a good opportunity for the observation of the seismic gravest normal modes for the longest periods below 1 mhz superconducting gravimeters sgs have been shown to be less noisy than long period seismometers a stack based on the spatial and temporal properties of the modes of degree one in a spherical harmonic development applied to 164 h of common records at the cantley canberra strasbourg sutherland and vienna sg stations has led to the first clear observation of the 2s1 long period seismic modes with a central frequency close to 0 4 mhz moreover the spectrum of the sg record at the strasbourg station confirms the benefit sgs can provide to normal mode seismology with the clear observation of the splitting of the fundamental mode 0s2 into 5 fairly well resolved singlets with only one record

estimation of interval anisotropy parameters using velocity independent layer stripping moveout analysis of long spread p wave data is widely used toestimatethekeytime processingparameter inlayeredtransversely isotropic media with a vertical symmetry axis vti inversion for interval values however suffers from instability caused by the trade off between the effective moveout parameters and by subsequent error amplification during dix type layer stripping we propose an alternative approach to nonhyperbolic moveout inversion based on the velocity independent layerstripping vils method of dewangan and tsvankin also we developthe3dversionofvilsandapplyittointervalparameter estimation in orthorhombic media using wide azimuth longspreaddata iftheoverburdenislaterallyhomogeneousandhasa horizontal symmetry plane vils produces the exact interval traveltime offset function in the target layer without knowledge of the velocity field hence dix type differentiation of moveout parameters used in existing techniques is replaced by the much morestablelayerstrippingofreflectiontraveltimes theinterval traveltimes are then inverted for the moveout parameters using the single layer nonhyperbolic moveout equation the superior accuracy and stability of the algorithm are illustrated on raytraced synthetic data for typical vti and orthorhombic models even small correlated noise in reflection traveltimes causes substantial distortions in the interval values computed by conventional dix type differentiation in contrast the output ofvils is insensitive to mild correlated traveltime errors the algorithm is also tested on wide azimuth p wave reflection data recorded above a fractured reservoir at rulison field in colorado the interval moveout parameters estimated by vils in the shale layer above the reservoir are more plausible and less influenced by noisethanthoseobtainedbythedix typemethod

fracture characterization using converted waves we present a method for inversion of fracture compliance matrix components from wide azimuth noisy synthetic ps reflection data and quantitatively show that reflection amplitude variations with offset and azimuth for converted ps waves are more informative than p waves for fracture characterization we consider monoclinic symmetry for fractured reservoir parameters chosen from woodford shale which can be formed by two or more sets of vertical fractures embedded in a vertically transverse isotropic background components of effective fracture compliance matrices for a medium with monoclinic symmetry are related to the characteristics of the fractured medium monte carlo simulation results show that inversion of ps reflection data is more robust than that of pp reflection data to uncertainties in our a priori knowledge vertically transverse isotropic parameters of unfractured rock than pp reflection data we also show that while inversion of pp reflections is sensitive to contrasts in elastic properties of upper and lower media inversion of ps reflections is robust with respect to such contrasts

generalized dix equation and analytic treatment of normal moveout velocity for anisotropic media despite the complexity of wave propagation in anisotropic media reflection moveout on conventional common midpoint cmp spreads is usually well described by the normal moveout nmo velocity defined in the zero offset limit in their recent work grechka and tsvankin showed that the azimuthal variation of nmo velocity around a fixed cmp location generally has an elliptical form i e plotting the nmo velocity in each azimuthal direction produces an ellipse and is determined by the spatial derivatives of the slowness vector evaluated at the cmp location this formalism is used here to develop exact solutions for the nmo velocity in anisotropic media of arbitrary symmetry for the model of a single homogeneous layer above a dipping reflector we obtain an explicit nmo expression valid for all pure modes and any orientation of the cmp line with respect to the reflector strike the contribution of anisotropy to nmo velocity is contained in the slowness components of the zero offset ray along with the derivatives of the vertical slowness with respect to the horizontal slownesses quantities that can be found in a straightforward way from the christoffel equation if the medium above a dipping reflector is horizontally stratified the effective nmo velocity is determined through a dix type average of the matrices responsible for the interval nmo ellipses in the individual layers this generalized dix equation provides an analytic basis for moveout inversion in vertically inhomogeneous arbitrarily anisotropic media for models with a throughgoing vertical symmetry plane i e if the dip plane of the reflector coincides with a symmetry plane of the overburden the semi axes of the nmo ellipse are found by the more conventional rms averaging of the interval nmo velocities in the dip and strike directions modelling of normal moveout in general heterogeneous anisotropic media requires dynamic ray tracing of only one zero offset ray remarkably the expressions for geometrical spreading along the zero offset ray contain all the components necessary to build the nmo ellipse this method is orders of magnitude faster than multi azimuth multi offset ray tracing and therefore can be used efficiently in traveltime inversion and in devising fast dip moveout dmo processing algorithms for anisotropic media this technique becomes especially efficient if the model consists of homogeneous layers or blocks separated by smooth interfaces the high accuracy of our nmo expressions is illustrated by comparison with ray traced reflection traveltimes in piecewise homogeneous azimuthally anisotropic models we also apply the generalized dix equation to field data collected over a fractured reservoir and show that p wave moveout can be used to find the depth dependent fracture orientation and to evaluate the magnitude of azimuthal anisotropy

noise levels of superconducting gravimeters at seismic frequencies summary until recently superconducting gravimeters sgs have been used principally in tidal studies periods 6 24 hr due to their high sensitivity and low drift rates this paper considers the performance of these instruments as long period seismometers particularly in the normal mode band periods 1 54 min to judge their suitability in providing useful information to seismology it is important to determine their noise characteristics compared to other established instruments such as spring gravimeters we compare several continuously recording instruments the sgs in esashi japan wuhan china strasbourg france and cantley canada and the spring gravimeter et 19 and seismometer sts 1 at the black forest observatory bfo germany we also include non permanent instruments the sg102 at bfo as well as the et 18 in metsahovi finland the ve quietest days out of the available records are stacked to obtain the power spectral density of the noise in the frequency band 0 05 20 mhz 50 s to 6 hr our reference is the new low noise model designed for seismometers only at the bfo site were there several instruments that could be compared even so in order to obtain the best individual data for each instrument the records selected were not simultaneous the noise characteristics of the diierent instrument site combinations are compared leading to conclusions about site selection instrument modi cations and the recent potential of sgs to contribute to seismic normal mode studies we refer to our previous work on the seismic noise magnitude a summary statistic derived from the power spectral density which has been used to rank the performance of instrument site combinations

velocity analysis in the presence of amplitude variation conventional semblance velocity analysis is equivalent to modeling prestack seismic data with events that have hyperbolic moveout but no amplitude variation with offset avo as a result of its assumption that amplitude is independent of offset this method might be expected to perform poorly for events with strong avo especially for events with polarity reversals at large offset such as reflections from tops of some class 1 and class 2 sands we find that substantial amplitude variation and even phase change with offset do not compromise the conventional semblance measure greatly polarity reversal however causes conventional semblance to fail the semblance method can be extended to take into account data with events that have amplitude variation expressed by avo intercept and gradient i e the shuey approximation however because of the extra degrees of freedom introduced in avo sensitive semblance resolution of the estimated velocities is decreased this is because the data can be modeled acc

three dimensional seismic models of the earth s mantle accurate models of the distribution of elastic heterogeneity in the earth s mantle are important in many areas of geophysics the purpose of this paper is to characterize and compare quantitatively a set of recent three dimensional models of the elastic structure of the earth to assess their similarities and differences and to analyze their fit to one class of data in order to highlight fruitful directions for future research the aspherical models considered are the following m84c woodhouse and dziewonski 1984 lo2 56 dziewonski 1984 mdlsh tanimoto 1990a sh 10c 17 masters et al 1992 and s12 wm13 su et al 1994 through much of the discussion m84c and lo2 56 are combined into a single whole mantle model m84c lo2 56 the fit of each model to previously tabulated even degree normal mode structure coefficients taken from smith and masters 1989a and ritzwoller et al 1988 for multiplets along the normal mode fundamental and first second and fifth overtone branches is also presented rather than concentrating on detailed comparisons of specific features of the models analyses of these models are general and statistical in nature in particular we focus on a comparison of the amplitude and the radial and geographical distribution of heterogeneity in each model and how variations in each affect the fit to the normal mode observations in general the results of the comparisons between the models are encouraging especially with respect to the geographical distribution of heterogeneity and in the fit to the normal mode data sensitive to the upper mantle and lowermost lower mantle there remain however significant discrepancies in amplitude and in the radial distribution of heterogeneity especially near the top of the upper mantle and near the top of the lower mantle the confident use of these models to constrain compositional and dynamical information about the mantle will await the resolution of these discrepancies the factors that may be responsible for the differences in the models and or for the misfit between the observed and predicted normal mode data are divided into two types intrinsic or procedural and extrinsic or structural we discuss only three extrinsic factors at length here including errors in the reference crustal models unmodeled topography on discontinuities in the interior of the mantle and errors in the assumed relationships between shear s and compressional p heterogeneity

the search for the slichter mode comparison of noise levels of superconducting gravimeters and investigation of a stacking method a worldwide network of superconducting gravimeters sgs is presently running under the coordination of the global geodynamics project ggp sgs have proved in the past to be suited for the study of the long period seismic and sub seismic modes the present paper provides some evidence of these results through the analysis of the noise levels at ggp stations in the seismic and sub seismic frequency bands and the study of the 2001 mw 8 4 peru earthquake the knowledge of the noise level at each station is important in a number of studies that combine the data of several stations to determine global earth parameters one example is the stacking of the data in the search for the gravity variations associated with the translational mode of the solid inner core slichter mode synthetic signals are first used to estimate the possible detection of this degree one mode with sgs and a stacking method is tested this method is then applied to the gravity residuals from the ggp records after the peruvian earthquake

the tidal displacement field at earth s surface determined using global gps observations 1 we investigate the 3 d tidal displacement field on earth s surface recorded globally by 456 continuous global positioning system gps stations of igs spanning 1996 2011 for eight principal diurnal and semidiurnal tidal constituents in phase and quadrature amplitudes of the residual tidal displacements after removal of an a priori body tide model are estimated using the precise point positioning ppp technique on the daily gps data the resultant daily estimates are combined to derive final estimates for each tide at each station the results are compared with the predictions of eight recent global ocean tide models separately for coastal 307 and inland 149 stations we show that gps can provide tidal displacement estimates accurate to the level of 0 12 mm horizontal and 0 24 mm vertical for the lunar only constituents m2 n2 o1 and q1 and less favorably for solar related tidal constituents s2 k2 k1 and p1 although improved by ambiguity resolution most recent ocean tide models fit the gps estimates equally well on the global scale but do not agree well between them in certain coastal areas especially for the vertical displacements suggesting the existence of model uncertainties near shallow seas the tidal residuals for the inland stations after removing both body tides and ocean tidal loading otl furthermore show clear continental scale spatial coherence implying deficiencies of the a priori body tide modeling in catching lateral heterogeneity in elastic as well as inelastic properties in the earth s deep interior we assert that the gps tidal displacement estimates now achieve sufficient accuracy to potentially provide constraints on the earth s structure

the effects of the atmospheric pressure changes on seismic signals or how to improve the quality of a station seismic investigations are mainly limited by seismic noise two microbarometers have been installed in the seismic vault of two different geoscope stations one at ssb and the other at tam all vertical components and most of the horizontal components show a significant correlation with pressure in order to correct the seismic signals from the atmospheric pressure noise a transfer function between the pressure data and the seismic data is inverted results show that after correction the noise levels reached on the horizontal components are similar between the two stations and the vertical components display noise levels below the low noise model as defined by peterson 1993 this technique reduces part of the noise and allows detection of small earthquakes and a better extraction of normal modes the analysis of the lowest normal modes of the earth excited by the m s 8 2 macquarie island earthquake is given to illustrate the perspectives of the method

constraints on density and shear velocity contrasts at the inner core boundary summary the density jump icb at the inner core boundary icb is an important constraint on the dynamics and history of the earth s core two types of seismological data sensitive to icb have been studied since the 1970s free oscillation eigenfrequencies and the amplitudes of core reflected phases pkikp pcp the preliminary reference earth model prem of dziewonski anderson based largely on normal mode data has a relatively low value of icb 0 60 g cm 3 whereas most studies based on pkikp pcp amplitude ratios find significantly larger values sometimes in excess of 1 0 g cm 3 it has been argued that because pkikp is rarely observed in the distance range considered 10 70 the latter type of measurement provides only upper bounds on icb we have analysed 10 yr of high quality global broad band data accumulated since the work of shearer masters we systematically analysed over 4500 seismograms from intermediate deep events depth 70 km and nuclear explosions in the distance range 10 70 the data were filtered in the bandpass 0 7 3 hz we performed rigorous data selection and identified five pairs of very clear quality a and 15 possible quality a pkikp and pcp arrivals in addition 58 records showed no pkikp but a clear pcp together we obtain a much less dispersed data set than previously available with the quality a data at the lower end of the ensemble of amplitude ratios versus distance we combine our high quality measurements with two measurements from the literature that fall within our rigorous selection criteria and obtain estimates of icb in the range 0 6 0 9 g cm 3 and icb in the range 2 3 km s 1 our estimate of icb is in agreement with a recent re evaluation of normal mode data thus reconciling results from body wave and mode studies and providing a tighter constraint on icb for geodynamicists our study also provides evidence for a shear velocity gradient at the top of the inner core

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

avo sensitive semblance analysis for wide azimuth data conventional semblance based moveout analysis models prestack reflection data with events that have hyperbolic move out and no amplitude variation with offset avo substantial amplitude variation and even phase change with offset do not significantly compromise the semblance operator however polarity reversal associated with a change in the sign of the reflection coefficient may cause conventional semblance to fail an existing modification of the semblance operator that takes amplitude variations into account so called ak semblance is limited to narrow azimuth data and cannot handle nonhyperbolic moveout at large offsets we employ a 3d nonhyperbolic moveout inversion algorithm to extend the ak semblance method to wide azimuth data recorded on long spreads to preserve velocity resolu tion the ratio k of the avo gradient and intercept is kept constant within each semblance window in the presence of azimuthal anisotropy however the parameter k has to be azimuthally dependent synthetic tests con

seismic anisotropy in exploration and reservoir characterization an overview recent advances in parameter estimation and seismic processing have allowed incorporation of anisotropic models into a wide range of seismic methods in particular vertical and tilted transverse isotropy are currently treated as an integral part of velocity fields employed in prestack depth migration algorithms especially those based on the wave equation we briefly review the state of the art in modeling processing and inversion of seismic data for anisotropic media topics include optimal parameterization body wave modeling methods p wave velocity analysis and imaging processing in the p domain anisotropy estimation from vertical seismic profiling vsp surveys moveout inversion of wide azimuth data amplitude variation with offset avo analysis processing and applications of shear and mode converted waves and fracture characterization when outlining future trends in anisotropy studies we emphasize that continued progress in data acquisition technology is likely to spur transition from t

comparative study of superconducting gravimeters and broadband seismometers sts 1 z in seismic and subseismic frequency bands abstract superconducting gravimeters and broadband seismometers vertical component both measure gravity but whereas the former are most sensitive to very long period signals gravity tides with periods longer than 6 h the latter are designed for recording the seismic band elastic normal modes with periods shorter than 1 h we investigate here the behaviour of each type of instrument in the spectral band where it is not generally used more precisely we compare the french superconducting gravimeter located at station j9 near strasbourg and the vertical component of an sts 1 seismometer located in a mine at echery ech in the vosges about 70 km away two different frequency bands are considered the seismic band frequencies between 0 2 and 1 667 mhz for the study of normal modes after the bolivian earthquake of 9 june 1994 and the subseismic band frequencies lower than 0 2 mhz including the study of gravity tides the analysis of fourier amplitude spectra and power spectral densities shows the obvious result that the broadband seismometer is more sensitive than the superconducting gravimeter in the seismic band because of a lower noise level whereas the reverse is true in the subseismic band the poorer quality of the gravimeter record in the seismic band is probably due to site effects sediments vs bedrock rather than of instrumental origin in contrast the higher noise level of the seismometer in the subseimic band is probably due to the temperature response of the instrument it is expected that operating the sts 1 isothermally or recording on site temperature changes for correction will considerably improve its signal to noise ratio in the subseimic band in comparison with recent mean models of high and low seismic background noise levels both instruments nevertheless indicate low noise levels at all frequencies

obtaining earth surface gravity disturbances from a gps based high low satellite to satellite tracking experiment b a high low satellite to satellite tracking measurement process involving a single low orbiter 300 km altitude and the gps constellation 20 189 km altitude has been proposed by jekeli upadhyay 1990 to estimate earth surface gravity disturbances this paper examines the potentiality of such a mission by initially conducting an error analysis based on signal to noise ratios of the spectra of intersatellite line of sight los accelerations possessing an assumed white noise process using a 60 day mission duration los acceleration data rates ranging from 1 to 60s and data noise levels from 0 1 to 0 3mgal corresponding errors of commission and omission propagating into the spectrum of a desired set of 1d and or 2d mean earth surface vertical gravity disturbances residual to harmonic degree and order n m 6 were obtained the resulting total estimated errors in a global average sense of the 1d mean disturbances ranged from 6 to 12 mgal and for 2d values the range was 1 5 to 6 mgal the lower error bounds corresponding to a 1 s data rate and a 0 1 mgal data noise level an optimal least squares collocation simulation was next carried out the simulated los accelerations reflected all gravimetric perturbations due to an adopted true gravity field extending to n m 180 a downward continuation approach was used in conjunction with singular value decomposition techniques to predict 1d mean disturbances over two separate regions on the earth s surface for a 10d by 10d region possessing a fairly tranquil local gravity field the overall rms of 100 true predicted mean differences using the aforementioned los data rates and noise levels ranged from 5 to 9 mgal and was only 1 mgal when the data were noise free for a more turbulent 15d by 20d region the rms of 300 differences was 1 5 mgal using perfect data and was in the 6 to 11 mgal range using noisy data again the lower error bounds corresponded to 1 s and 0 1 mgal data rate and noise levels if the low satellite takes on a polar orbit one could therefore obtain respectable 1d mean disturbances and highly reliable 2d mean values over all previously inaccessible earth surface areas

stacking global gps verticals and horizontals to solve for the fortnightly and monthly body tides implications for mantle anelasticity the availability of long term position measurements from permanent gps stations distributed around the earth makes it feasible to extract small globally coherent long period geophysical signals from the data using 11 years of daily vertical and horizontal positions from over 600 permanent gps stations worldwide we solve for the amplitude and phase of the fortnightly and monthly body tides by stacking the surface displacements against spherical harmonics and isolating the fortnightly and monthly signals in the stacks we use our solutions to help constrain the earth s anelastic properties which are not well understood within the tidal frequency band our error estimates include the effects of the following 1 random noise across the fortnightly and monthly tidal bands 2 fortnightly and monthly ocean tide model errors 3 errors in the diurnal and semidiurnal ocean tide models that alias into the fortnightly and monthly frequency bands 4 errors in the solid earth green s functions used to compute ocean tidal loading corrections and 5 leakage from gps draconitic signals at periods close to the fortnightly and monthly frequencies the mantle anelasticity coefficients we infer from our solutions are consistent with a frequency dependence of mantle q with in the range 0 1 0 3

an analysis of avo inversion for postcritical offsets in hti media abstractazimuthal variations of wavefield characteristics such as traveltime or reflection amplitude play an important role in the identification of fractured media a transversely isotropic medium with a horizontal symmetry axis hti medium is the simplest azimuthally anisotropic model typically used to describe one set of vertical fractures there exist many techniques in industry to recover anisotropic parameters based on moveout equations and linearized reflection coefficients using such a model however most of the methods have limitations in defining properties of the fractures due to linearizations and physical approximations used in their development thus azimuthal analysis of traveltimes based on normal moveout ellipses recovers a maximum of three medium parameters instead of the required five linearizations made in plane wave reflection coefficients pwrcs limit the amplitude versus offset avo analysis to small incident angles and weak contrast interfaces inversion based on azimuthal

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

on noise reduction in vertical seismic records below 2 mhz using local barometric pressure we propose a simple method for the reduction of noise in vertical seismic recordings at frequencies below 2 mhz the method consists in subtracting the locally recorded scaled atmospheric pressure signal from the gravity record the scale factor is frequency independent and can be linearly estimated from the data while such a pressure correction was previously only used in the analysis of tidal records it is equally efficient at the low frequency end of the normal mode band and we use gravity recordings of the big 1994 shikotan and bolivian quakes to demonstrate this point our examples also show that this correction permits to achieve noise levels well below the new low noise model nlnm peterson 1993 the success of the correction also sheds some light on the physical processes responsible for the generation of low frequency seismic noise at quiet observatories