eastern mediterranean gravity potential and the hellenic subduction process tornographic images of the mediterranean upper mantle p wave velocity structure have been used to analyse the gravity potential in the vicinity of the hellenic subduction zone the velocity anomalies are assumed to be proportional to density variations according to birch s law the effect of the topography on the geoid in the region is also calculated the results indicate that the upper mantle geoid signal probably has significant amplitudes of several metres but it correlates poorly with the observed geoid the geoid calculated from topography correlates well but has an amplitude that is too large in comparison with the observed geoid the results show that an improved understanding of the hellenic subduction zone geoid requires refinement of the moho topography so that the effect of this topography can be separated from the upper mantle signature

study of the high frequency spectrum of the anomalous gravity potential the analysis of the high frequency spectrum of the anomalous gravity potential is important for a number of current and emerging technologies in geodesy and geophysics among them is the precise determination of deflections of the vertical by inertial techniques the development of data processing methods in airborne gradiometry and the signal extraction methods for airborne gravimetry the spectral information required can be obtained by analyzing dense gravity data by fast fourier transform techniques an attempt has been made to use available data files in canada for this purpose the data consisted of a rather uniform set of 5 arc min 5 arc min point anomalies in most of the continental areas of canada and four sets of point gravity anomalies with an average data spacing of about 1 km in areas of limited extent the analysis showed that the gravity spectrum has to be resolved to about degree and order 7000 i e to a point spacing of about 3 km to resolve the gravity anomalies to 1 mgal thus a similar data spacing would be required in airborne gradiometry and gravimetry in order to get an equivalent resolution the derived spectrum was used to determine suitable degree variance models and covariance functions for the above applications the spectrum estimated from the 5 arc min 5 arc min gravity anomalies covering the nonmountainous areas of canada indicates that the degree variances of the anomalous potential decay as l 3 6 this means that a number of currently used covariance models do not agree with the results of this analysis a discussion on the choice of a more adequate model concludes the paper

gravity field of the southeast central pacific from geosat exact repeat mission data efficient techniques for mapping the marine gravity field with geosat exact repeat mission erm data are demonstrated geosat has completed more than fifty two 17 day global erm repeat cycles and has collected roughly 50 million observations of geoid undulations or more precisely sea surface topography because the global ground track repeats every 17 days the erm observations are highly redundant by averaging the repeat cycles very precise profiles of alongtrack geoid undulations are obtained for a region of the southeast central pacific without any crossing arc orbit adjustments these profiles are shown to be repeatable to within 10 cm over their entire 3000km length i e to three parts in 108 this southeast central pacific region contains a portion of the east pacific rise as well as some cross grain gravity field lineations and is therefore of considerable geophysical interest in addition to profiles of alongtrack deflections of the vertical regularly gridded gravity anomalies are derived for this region and portrayed in color the procedure used for recovering gravity has two stages in stage 1 alongtrack deflections of vertical are derived by fitting cubic splines to alongtrack heights computing alongtrack slopes at standardized latitudes and then averaging slopes for coincident passes in stage 2 gridded band limited gravity anomalies are recovered from deflections by using fast fourier transform techniques to accomplish the necessary inverse vening meinesz transformation high precision gem t1 orbits are used and crossover adjustments are not performed the operational navy astronautics group orbits are shown to have systematic errors which may be as large as 10 m

differential opening of the central and south atlantic oceans and the opening of the west african rift system abstract plate tectonic studies of the development of the central and south atlantic oceans using seasat and geosat altimeter and magnetic anomaly isochron data now provide quantitative models of seafloor spreading through time such models enable an initial assessment of the differential opening between these two oceanic basins to be determined the equatorial atlantic is an integral part of this oceanic rifting process allowing stresses arising from the differential opening to be dissipated into both the caribbean and africa along its northern and southern boundaries respectively the tectonic model for the west african rift system based on geological and geophysical studies shows a series of strike slip fault zones diverging into africa from the gulf of guinea and dissipating their shear movement into the development of extensional basins orientated perpendicular to these faults zones the development of the west african rift system was contemporaneous with the early opening of the south atlantic continued to develop well after the final breakup of south america from africa and did not cease until the late cretaceous when there was a major phase of basin inversion and deformation santonian 80 ma deformation across the benue trough nigeria is broadly contemporaneous with dextral shear reactivation of the central african fracture system which in turn resulted in renewed extension in the sudan basins during the late cretaceous and early tertiary this paper illustrates the close linkage in both time and space between the history of the african rift basins and the opening of the atlantic both exhibit distinct phases of evolution with the rift basins developing in direct response to the differential opening between the central and south atlantic in order to dissipate stresses generated by this opening the mesozoic tectonic model proposed is therefore one of an intimate interaction between oceanic and continental tectonics

seafloor tectonic fabric from satellite altimetry abstract ocean floor structures with horizontal scales of 10 to a few hundred kilometers and vertical scales of 100 m or more generate sea surface gravity anomalies observable with satellite altimetry prior to 1990 altimeter data resolved only tectonic lineaments some seamounts and some aspects of mid ocean ridge structure new altimeter data available since mid 1995 resolve 10 km scale structures over nearly all the world s oceans these data are the basis of new global bathymetric maps and have been interpreted as exhibiting complexities in the sea floor spreading process including ridge jumps propagating rifts and variations in magma supply this chapter reviews the satellite altimetry technique and its resolution of tectonic structures gives examples of intriguing tectonic phenomena and shows that structures as small as abyssal hills are partially resolved a new result obtained here is that the amplitude of the fine scale 10 80 km roughness of old ocean floor is spreading rate dependent i

an estimate of equatorial gravity from terrestrial and satellite data equatorial gravity has been estimated from terrestrial gravity data and from satellite implied data the terrestrial estimate was based on the analysis of 43271 1 1 mean free air gravity anomalies the value estimated was 978032 35 0 5 mgals this value may be slightly 0 to 1 5 mgals too low because of the neglect of the downward continuation of surface free air anomalies to the geoid an alternate estimate based on the comparison of terrestrial anomalies and altimeter derived anomalies yields 978032 6 mgals equatorial gravity from space data depends on gm from lageos and lunar laser data and on a new estimate 6378136 2 0 5 m of the equatorial radius based on doppler tracking station positions and a new spherical harmonic expansion of the earth s gravity field to degree 360 the resultant equatorial gravity is 978032 78 0 2 mgals the difference between the two terrestrial estimates is 0 4 and 0 2 mgals or 0 4 ppm or 0 2 ppm the agreement is within the accuracy of the two separate estimates these comparisons can put constraints on parameters of non newtonian force models

gravity map of brazil 1 representation of free air and bouguer anomalies gravity data of brazil were processed to obtain continental scale free air and bouguer anomaly maps for a large segment of the south american plate gravity measurements are referred to the international gravity standardization net 1971 and the anomalies are calculated using geodetic reference system 1967 and 2 67 g cm 3 for bouguer reduction in the calculation of the gravity anomalies information used included geopotential model topography geoidal heights and astrogeodetic deviation of the vertical depending on the availability and distribution of land gravity data various regions have been classified as complete incomplete or empty in complete regions the gravity field is represented by 0 5 0 5 mean free air and bouguer anomalies which were computed from observed gravity data only in incomplete regions mean anomalies in empty 0 5 0 5 blocks were computed from nearest known mean anomalies by least squares collocation regions totally devoided of gravity data had the gravity field calculated by least squares collocation using all available geodetic data the gravity map obtained using this approach allows to identify the major gravity anomalies with dimensions greater than 100 km such anomalies are mainly caused by intracrustal density variations of geological and tectonic interest

a geopotential model from satellite tracking altimeter and surface gravity data gem t3 an improved model of earth s gravitational field gem t3 has been developed from a combination of satellite tracking satellite altimeter and surface gravimetric data gem t3 provides a significant improvement in the modeling of the gravity field at half wavelengths of 400 km and longer this model complete to degree and order 50 yields more accurate satellite orbits and an improved geoid representation than previous goddard earth models gem t3 uses altimeter data from geos 3 1975 1976 seasat 1978 and geosat 1986 1987 tracking information used in the solution includes more than 1300 arcs of data encompassing 31 different satellites the recovery of the long wavelength components of the solution relies mostly on highly precise satellite laser ranging slr data but also includes tranet doppier optical and satellite to satellite tracking acquired between the ats 6 and geos 3 satellites the main advances over gem t2 beyond the inclusion of altimeter and surface gravity information which is essential for the resolution of the shorter wavelength geoid are some improved tracking data analysis approaches and additional slr data although the use of altimeter data has greatly enhanced the modeling of the ocean geoid between 65 n and 60 s latitudes in gem t3 the lack of accurate detailed surface gravimetry leaves poor geoid resolution over many continental regions of great tectonic interest e g himalayas andes estimates of polar motion tracking station coordinates and long wavelength ocean tidal terms were also made accounting for 6330 parameters gem t3 has undergone error calibration using a technique based on subset solutions to produce reliable error estimates the calibration is based on the condition that the expected mean square deviation of a subset gravity solution from the full set values is predicted by the solutions error covariances data weights are iteratively adjusted until this condition for the error calibration is satisfied in addition gravity field tests were performed on strong satellite data sets withheld from the solution thereby ensuring their independence in these tests the performance of the subset models on the withheld observations is compared to error projections based on their calibrated error covariances these results demonstrate that orbit accuracy projections are reliable for new satellites which were not included in gem t3

gravity model improvement for seasat analysis of the data from the seasat altimeter a 10 cm precision instrument requires accurate knowledge of the radial position of the spacecraft for oceanographic applications such as the study of tides and ocean circulation as well as for geodetic applications initial orbit determinations using the best available geodetic and force models gave radial errors approaching 5 m the dominant source of this radial error was the gravity field model improvements in the gravity modeling were made by successively incorporating the seasat laser and unified s band ground tracking data the geos 3 altimeter data and finally the seasat altimeter data with the most recent model a set of spherical harmonic coefficients complete to degree and order 36 seasat ephemerides have been computed for an orbital arc length of 12 days with an rms radial accuracy of about 70 cm

the determination of geoid undulations and gravity anomalies from seasat altimeter data the global seasat altimeter data set has been edited and adjusted using a crossing arc procedure fixing one long arc to provide control in doing this a set of 168 master arcs were formed from repeat and near repeat arcs the adjustment carried out by d rowlands was done in a global net and four regional areas the average crossover discrepancy before the adjustment was 1 5 m while after the adjustment solving for a bias and trend it was 28 cm the examination of the crossover discrepancies by area indicated larger than normal discrepancies in certain areas such as the hudson bay the mediterranean and the north and baltic seas some of these discrepancies are caused by invalid tide values applied to the altimeter data in these areas the adjusted seasat sea surface heights were used to predict point values at 1 1 intersections in 10 different geographic regions for comparison with corresponding geos 3 values a mean difference geos 3 minus seasat of 1 3 0 67 m was found the mean difference being caused by different ellipsoid parameters the adjusted data were also used to determine mean surface heights and gravity anomalies in 37 9051 1 and 1178 5 equal area blocks with some of these values being on land these estimations were done using least squares collocation using covariances functions scaled to each 5 region the average predicted standard deviation of the 1 1 anomalies was 5 1 mgal and 2 7 mgal for the 5 blocks seasat values were compared to the geos 3 values finding a difference of 7 8 mgal and 0 87 m for the 1 1 data and 2 2 mgal and 0 76 m for the 5 data comparisons with terrestrial data showed no significant difference between the geos 3 seasat implied anomalies although a number of significant discrepancies have been resolved with the seasat data

a preliminary model for geosat altimeter data errors noise power spectral densities psds are derived from the analysis of geosat altimeter data from segments of nearly collinear repeat tracks the difference time series which are analyzed using autoregressive ar modeling techniques contain primarily time varying oceanographic signals and altimeter noise since geoid content is canceled by the differencing process an average noise psd obtained from eight independent repeat track psds is fitted closely by a first order markov model with a white noise floor the geosat average noise psd is compared with analogous spectra derived earlier for geos 3 and seasat in general the geosat noise psd is the same as the seasat noise spectrum at wavelengths greater than about 100 km but has a lower noise level at shorter wavelengths

the seasat altimeter mean sea surface model gridding techniques are used to combine an 18 day set of seasat altimeter data and two precisely computed seasat ephemerides in order to arrive at global contour maps of the mean sea surface topography the altimeter data have an rms agreement of 111 cm with the ss3 mean sea surface computed by means of the pgs s3 ephemerides and of 70 cm with the ss4 mean surface derived from the pgs s4 ephemerides while comparisons with the gem 10b 1 x 1 deg gravimetric geoid have yielded rms differences of 2 8 m those with a global mean sea surface derived from geos 3 altimeter data show rms differences of 1 3 m and 1 1 m for the cases of the ss3 and ss4 surfaces respectively an ss4 mean sea surface topograph is featured among the study findings presented further improvements in the representation of mean sea surface topography are expected with the development of more accurate gravity models for orbit computation

surface heating and patchiness in the coastal ocean off central california during a wind relaxation event the field work was funded by the u s office of naval research onr coasta i sciences program code i 122cs as part of the coastal transition zone project s r r re ceived additional support from direct research funding at the u s naval postgraduate school nps the portion of this work carried out at the jet propulsion laboratory california institute of technology was sponsored by onr and the national aeronautics and space administration model simulatiotis were conducted by the oceanic planetary boundary layer laboratory sponsored by onr and funded by nps the boom probe was designed and built by the oceanography department support staff at the naval postgraduate school paul jessen assisted with the data collection and processing the satellite image was provided by toby garfield and the nps interactive digital environmental analysis idea laboratory

amplitude and horizontal structure of a large diurnal sea surface warming event during the coastal ocean dynamics experiment abstract intense diurnal warming of the ocean surface was observed in april 1982 off california using a combination of mooring hydrographic and satellite infrared and satellite pigment measurements the event corresponded to a spatial and temooral minimum of the wind stress the diurnal surface temperature amplitude exceeded 6 6 c locally despite a 490 nm optical depth of 20 m suggesting that phytoplankton was not responsible for the shallow heat trapping coherent horizontal temperature streaks at least 50 km long and 4 8 km wide formedduring the subsequent erosion of the shallow warm layers it is hypothesized that thcfr scale was set by planetary boundary layer circulations

oceanographic and geophysical applications of satellite altimetry since the last quadrennial report new satellite altimeter data have not been generally available nevertheless important progress has been made from analyses of geos 3 and seasat data the marine gravity field has been determined to an extraordinary resolution and accuracy and important solid earth geophysical interpretations made in the area of oceanography statistical estimates of the variability of dynamic topography have been published methods for incorporating altimeter data into numerical models have been developed and applied and tentative but interesting determinations of the mean circulation have been made in addition altimeter waveform analysis has made it possible to use altimeter data to determine elevations of major ice fields finally important new insights into the nature of satellite ephemeris error and its influence on altimetrically derived sea surface topography have been achieved

gravity fields of the southern ocean from geosat data in august 1990 the u s navy declassified all geodetic mission gm radar altimeter data acquired by the geosat satellite over oceanic regions south of 60 s we have used these gm data in conjunction with the unclassified lower resolution geosat exact repeat mission erm altimeter data to construct high resolution gravity fields on a 5 km grid covering the annular region of the southern ocean which lies between 60 s and 72 s and encircles antarctica during the gm a complete mapping of the marine geoid between 72 and 72 n was accomplished the gm produced more densely spaced ground tracks typically 2 or 3 km at 60 s than those of either the erm or seasat consequently we were able to use the gm data to map the marine gravity field at a higher resolution than was previously possible using satellite altimeter data this paper describes the techniques we used to derive these gravity fields and image them these techniques involve 1 computing along track sea surface height slopes 2 gridding of these ascending and descending slopes 3 converting the slopes to conventional deflections of the vertical 4 transforming the deflections to gravity anomalies in the frequency domain and 5 imaging the resulting images of the marine gravity field reveal much that is new about the seafloor and the tectonic fabric of the southern ocean a region which includes large expanses of seafloor that have never been surveyed by ships

what controls seasonal variations of the diurnal cycle of sea surface temperature in the eastern tropical indian ocean abstractrecent in situ buoy observations revealed interesting seasonal features of the diurnal sea surface temperature cycle dsst in the eastern tropical indian ocean composite analysis shows that areas away from the equator exhibit stronger seasonal variations of dsst while weaker seasonal variations appear near the equator the most interesting characteristic is the distinctive contrast of the seasonal variations of dsst between the bay of bengal bob and the region south of the equator particularly around 12 s while the range of dsst is weakest in the bob during boreal summer it has its largest range around 12 s in austral summer furthermore bob dsst exhibits two peaks that occur during the monsoon transitions march april and october whereas dsst south of the equator shows only a single peak in its annual cycle using a one dimensional oceanic mixed layer model the authors examined the cause of the distinctive annual cycles of dsst north and south of the equator two parallel experiment

high resolution mean sea surface computed with altimeter data of ers 1 geodetic mission and topex poseidon the remote sensing satellite ers 1 launched in 1991 to study the earth s environment was placed on a geodetic 168 day repeat orbit between 1994 april and 1995 march to map through altimetric measurements the gravity field over the whole oceanic domain with a resolution of 8 km at the equator in both along track and cross track directions we have analysed the precise altimeter data of the geodetic mission and by also using one year of topex poseidon altimeter data we have computed a global high resolution mean sea surface the various steps involved in pre processing the ers 1 data consisted of correcting the data for environmental factors editing and reducing through crossover analyses the radial orbit error which directly affects sea surface height measurements for this purpose we adjusted sinusoids at 1 and 2 cycle rev 1 along the ers 1 profiles in order to minimize crossover differences between ers 1 and yearly averaged topex poseidon profiles in effect the orbit of topex poseidon is very accurately determined within 2 3 cm for the radial component so topex poseidon altimeter profiles can serve as a reference to reduce the ers 1 radial orbit error the ers 1 residual orbit error was further reduced through a second crossover analysis between all ascending and descending profiles of the geodetic mission the along track ers 1 and topex poseidon data were then interpolated over the whole oceanic domain on a regular grid of 1 16 1 16 size the mapping of the gridded sea surface heights reveals the very fine structure of the marine geoid up until now unknown at a global scale this new data set will be most useful for marine geophysical and tectonic investigations

marine tectonics processes at mid ocean ridges the use of multibeam echo sounders and various side looking sonar systems has greatly increased our knowledge and understanding of mid ocean ridge mor processes since the last iugg review on mid ocean ridge research macdonald 1983 the extensive use of swath mapping tools such as sea beam deep tow seamarc i and ii the application of multichannel seismic reflection techniques to the detection of axial magma chambers and the use of satellite data in the study of oceanic lithosphere have rejuvenated marine tectonics and have generated and stimulated a large number of important contributions this review will focus on what we feel were the major developments in marine tectonics with an emphasis on mid ocean ridges in the last four years certain topics such as marine seismicity and satellite studies are only briefly mentionned for reference and will be covered in greater detail in other reviews because of the exponential growth in the number of publications related to mors during 1982 1986 only a few references pertinent to each of the major research topics will be quoted in the text as examples but the reader will find an extensive bibliography at the end of the text references published prior to 1982 were added only where appropriate

gravity anomalies and sea surface heights derived from a combined geos 3 seasat altimeter data set gravity anomalies and sea surface heights have been computed on a 0 125 grid in the ocean areas from a combined geos 3 seasat altimeter data set the basic estimation procedure used least squares collocation where model covariance functions are tailored to individual areas through altimeter residual variance scaling preliminary tests led to production prediction procedures using a reference model defined by a set of potential coefficients complete to degree 180 comparisons of the predicted anomalies with ship derived values showed agreement varying from 9 to 30 mgal no correction to the altimeter implied sea surface heights was made for sea surface topography effects the maximum anomaly predicted is 396 mgal near hawaii and the most negative anomaly is 361 mgal over the puerto rican trench the actual resolution of the predicted quantities was estimated to be about 0 19 based on a power spectrum analysis the resolution is limited by our data selection process which uses 300 data points from a thinned altimeter data set for the prediction of the 0 125 in a 3 3 block with one data selection and matrix inversion the predicted data are used to compute 104 potential coefficient spectrums using flat earth approximations developed by forsberg the spectra were classified in terms of smooth mild or rough areas the great majority 78 of the spectra were in the smooth classification good agreement was found from spectra computed from topographically reduced land data and the mean ocean data up to degree 800 we found that the spectra decayed as l 3 6 as compared to the decay of l 3 implied by the kaula model the gridded data was used to compute 1 1 and 0 5 0 5 mean values the 1 mean anomalies were compared to terrestrial data where an rms difference of 7 mgal was found in comparing 10 139 values these new values have allowed us to identify 1 mean anomalies based on terrestrial estimates that are in substantial error

an experiment to invert seasat altimetry for the mediterranean and black sea mean surfaces summary the high accuracy mapping of the mean sea surface mss from satellite altimetry requires efficiency flexibility and mathematical transparency from the data analysis this paper argues that so does the least squares inverse method prior to the data inversion models of covariance functions are constructed for the expected residual mean sea surface relative to a starting map and for the various error sources in the data including the radial orbit errors instrument noise and sea height variability the unique optimum solution is then restored from the data in a single step analysis with formal error estimates and possibly local a posteriori covariances resolving kernels numerical experiments are performed to obtain the mediterranean and black sea mean surfaces by an inversion of the seasat altimeter data in our present computational environment the constraint raised by the cost of a run leads us to degrade the rigorous data analysis strategy owing to these limitations and to the assumed covariance models and data coverage the mss is mapped with an accuracy of 12121x1 for wavelengths greater than 335 km the supplementary error related to shorter scales is 70 cm the inverse mss solution is compared with another model computed at the bureau gravimetrique international in a classical way crossing arc analysis plus data filtering from geos3 and seasat then neglecting the dynamic heights of the sea circulation we invert the seasat data set to map the equivalent free air gravity anomalies the cross covariance used is consistently derived from the a priori power spectrum of the mss the gravity anomalies are recovered with an accuracy of 1 mgal moreover a large error of 25 mgal is expected from the smoothing of the shorter scales an external comparison made with the gravimetric map of torge weber wenzel 1983 broadly agrees with our formal error estimates several prospective specifications concerning the covariance models and the numerical procedure are drawn from these preliminary experiments and comparisons

lee region of gran canaria the mountainous canary islancls present obstacles to the trade wincls ancl to the canary current flowing equatorwarc past them in situ observations of hydrographic properties ancl surface wincls south of gran canaria together with advancecl very high resolution racliometer ancl synthetic aperture racial images e uring 2 weeks in summer 1995 are analyzecl a cyclonic edcly shecl l rom the west of the islancl c1riftecl southwestwarcl at 5 s j while the southeast coast was approachecl by an upwelling filament originating off nw africa a wincllee region bounclecl by intense horizontal shear lines hacl a weak return islandwarcl wincl in its center the lee formee a triangulai c1iurnally varying war111 water 001 with two sea surface temperature maxima separatecl by lower temperatures below the return wincl shallow temperature stratification occurrecl behincl the islancl in contrast to the l1niform surface mixecllayer in exposecl regions upwelling ancl clownwelling of 10 20 111 were indicatecl on the cyclonic ancl anticyclonic sicles of the lee region in the sar la u lines of strong current shear along a temperature front between the cyclonic eclcly the upwelling filament were iclentifiable i iowever the racial images were clominatecl atmospheric phenomena including mountain lee wave paclcets winclrows and wincl lines estimation of the wincl field l rom the sar backscatter intensity revealed ex structure and intensification on the eclges of the warm lee

gravity anomaly map of north america the gravity anomaly map of north america is the product of a 12 year international effort to compile critically edit and merge gravity anomaly data on a continental and global scale this color pixel map printed on four quadrant sheets at a scale of 1 5 000 000 and including a fifth sheet showing a color index map with data references is the first such map at this large scale to include several hundreds of thousands of precise surface data of the united states canada mexico and central america as well as other high quality surface data from neighboring continental and oceanic areas the map which shows bouguer gravity anomalies on land and free air gravity anomalies over oceans is remarkable for its detail sixty six colors or shadings have been used in a carefully conceived nonlinear scheme to show anomalies at a 5 or 10 mgal interval over a dynamic range from about 300 mgal to 130 mgal

oceanwide gravity anomalies from geos 3 seasat and geosat altimeter data three kinds of satellite altimeter data have been combined along with 5 5 bathymetric data to calculate a 0 125 ocean wide gridded set of 2 3 106 free air gravity anomalies the procedure used was least squares collocation that yields the predicted anomaly and standard deviation the value of including the bathymetric data was shown in a test around the dowd seamount where the root mean square rms difference between ship gravity measurements decreased from 40 mgal to 20 mgal when the bathymetry was included comparisons between the predicted anomalies and ship gravity data is described in three cases in the banda sea the rms differences were 20 mgal for two lines in the south atlantic rms differences over lines of 2000 km in length were 7 mgal for cruise data in the antarctica region the discrepancies were 12 mgal comparisons of anomalies derived from the geosat geodetic mission data by marks and mcadoo 1992 with ship data gave differences of 6 mgal showing the value of the much denser geosat geodetic mission altimeter data

3 d modelling of seamount topography from satellite altimetry we develop a complete set of algorithms to perform 3d modelling of seamount bathymery from satellite altimetry the first stage of the data processing consists in gridding the geoid to account for the long wavelength errors geoid heights are first bias adjusted at cross overs then a collocation on a regular grid is performed accounting for the altimeter errors in a second stage geoid heights are converted into bathymetry no simplifying assumption on the shape and location of the bathymetry highs is necessary bathymetric uncertainties due to the data sampling and the parameters of the mechanical and crustal models are evaluated

source of the australasian tektite strewn field a possible off shore impact site although there is a preponderance of evidence that tektites were formed by asteroid impacts on the earth no source crater has been found for the largest and youngest of the strewn fields the australasian strewn field we have examined a combined seasat geos 3 altimeter data set of sea surface heights in the northern portion of the australasian strewn field for negative gravity anomalies on the continental shelf and slope which might be related to the source crater for these tektites a large negative anomaly which we call the qui nhon slope anomaly is a sea surface depression of approximately 1 5 meters over an area of 100 km diameter centered at 13 47 n latitude and 110 37 e longitude it corresponds to a gravity anomaly of about 50 mgal a surface ship gravimeter track across most of the structure confirms the satellite measurements this site is on the continental slope in 1 to 3 km of water a sea floor topographic depression of more than 200 meters occurs in the southeast quadrant of the anomaly the center of the structure is 175 km offshore and is less than 300 km from the muong nong substrewn field and the prolific dalat site perhaps the largest concentration of tektites within the australasian field due to its location size and magnitude we propose this anomaly may be due to the impact structure that produced the australasian strewn field

altimetry ship gravimetry and the general circulation of the north atlantic gravity accelerations estimated from satellite altimetric mean sea surfaces seasat and geos 3 are compared to ship gravity measurements ship gravity are closer to an estimate based on least squares collocation orbit perturbations altimetry and terrestrial gravity than to an estimate based on fourier transforms orbit perturbations and altimetry only both altimetric estimates yield a smoothed picture of the geostrophic component of sea surface currents in the north atlantic when gravity acceleration data from only nine cruises are subtracted from the altimetric gravity