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In recent years, the analysis of satellite gravity data is used as a powerful tool for geologic mapping. This study is based on various filtered maps of Bouguer gravity anomaly data of the Southwes...

A high-resolution gravimetric geoid model for the Kingdom of Saudi Arabia area was determined. A data set of 459,848 land gravity, 80,632 shipborne marine gravity data, DTU17 altimetry gravity mode...

Incremental improvements to the Arctic Gravity Project (AGP) grid have accumulated through the steady acquisition of marine gravity anomaly data in the Arctic Ocean and other data sets.The explosio...

The resolutions of current global altimetric gravity models and mean sea surface models are around 12 km wavelength resolving 6 km features, and for many years it has been difficult to improve the resolution further in a systematic way. For both Jason 1 and 2, a Geodetic Mission (GM) has been carried out as a part of the Extension-of-Life phase. The GM for Jason-1 lasted 406 days. The GM for Jason-2 was planned to provide ground-tracks with a systematic spacing of 4 km after 2 years and potentially 2 km after 4 years. Unfortunately, the satellite ceased operation in October 2019 after 2 years of Geodetic Mission but still provided a fantastic dataset for high resolution gravity recovery. We highlight the improvement to the gravity field which has been derived from the 2 years GM. When an Extension-of-Life phase is conducted, the satellite instruments will be old. Particularly Jason-2 suffered from several safe-holds and instrument outages during the GM. This leads to systematic gaps in the data-coverage and degrades the quality of the derived gravity field. For the first time, the Jason-2 GM was “rewound” to mitigate the effect of the outages, and we evaluate the effect of “mission rewind” on gravity. With the recent successful launch of Sentinel-6 Michael Freilich (S6-MF, formerly Jason CS), we investigate the possibility creating an altimetric dataset with 2 km track spacing as this would lead to fundamental increase in the spatial resolution of global altimetric gravity fields. We investigate the effect of bisecting the ground-tracks of existing GM to create a mesh with twice the resolution rather than starting all over with a new GM. The idea explores the unique opportunity to inject Jason-3 GM into the same orbital plane as used for Jason-2 GM but bisecting the existing Jason-2 tracks. This way, the already 2-years Jason-2 GM could be used to create a 2 km grid after only 2 years of Jason-3 GM, rather than starting all over with a new GM for Jason-3.

Publisher Copyright: © 2021 The Author(s). Published by Informa UK Limited, trading as Taylor & Francis Group. Copyright: Copyright 2021 Elsevier B.V., All rights reserved. Traditionally, geoid models have been validated using GNSS-levelling benchmarks on land only. As such benchmarks cannot be established offshore, marine areas of geoid models must be evaluated in a different way. In this research, we present a marine GNSS/gravity campaign where existing geoid models were validated at sea areas by GNSS measurements in combination with sea surface models. Additionally, a new geoid model, calculated using the newly collected marine gravity data, was validated. The campaign was carried out with the marine geology research catamaran Geomari (operated by the Geological Survey of Finland), which sailed back and forth the eastern part of the Finnish territorial waters of the Gulf of Finland during the early summer of 2018. From the GNSS and sea surface data we were able to obtain geoid heights at sea areas with an accuracy of a few centimetres. When the GNSS derived geoid heights are compared with geoid heights from the geoid models differences between the respective models are seen in the most eastern and southern parts of the campaign area. The new gravity data changed the geoid model heights by up to 15 cm in areas of sparse/non-existing gravity data.

In this study, we present a comparative analysis between two types of gravity data used in geophysical applications: satellite altimeter-derived gravity and sea-bottom gravity.It is largely known that the marine gravity field derived from satellite altimetry in coastal areas is generally biased by signals back-scattered from the nearby land. As a result, the derived gravity anomalies are mostly unreliable for geophysical and geological interpretations of near-shore environments.To quantify the errors generated by the land-reflected signals and to verify the goodness of the geologic models inferred from gravity, we compared two different altimetry models with sea-bottom gravity measurements acquired along the Italian coasts from the early 50s to the late 80s.We focused on the Gulf of Manfredonia, located in the SE sector of the Adriatic Sea, where: (i) two different sea-bottom gravity surveys have been conducted over the years, (ii) the bathymetry is particularly flat, and (iii) seismic data revealed a prominent carbonate ridge covered by hundreds of meters of Oligocene-Quaternary sediments.Gravity field derivatives have been used to enhance both: (i) deep geological contacts, and (ii) coastal noise. The analyses outlined a “ringing-noise effect” which causes the altimeter signal degradation up to 17 km from the coast.Differences between the observed gravity and the gravity calculated from a geological model constrained by seismic, showed that all datasets register approximately the same patterns, associated with the Gondola Fault Zone, a major structural discontinuity traversing roughly E-W the investigated area.This study highlights the importance of implementing gravity anomalies derived from satellite-altimetry with high-resolution near-shore data, such as the sea-bottom gravity measurements available around the Italian coasts. Such analysis may have significant applications in studying the link between onshore and offshore geological structures in transitional areas.

The Surface Water and Ocean Topography (SWOT) altimeter mission can measure high-resolution wide-swath sea surface heights (SSHs) that may greatly improve the current accuracy and spatial resolution of marine gravity from nadir-looking altimeters. To investigate the potential of SWOT in recovering high-quality marine gravity and how SWOT observation errors should be treated to optimize the accuracy of gravity anomaly from SWOT observations, we create high-wavenumber SSH components from multi-beam depths in the northern South China Sea (SCS) and simulate SWOT SSH errors. To cross-validate gravity signals and avoid gravity errors from SWOT, we use two computational methods (inverse Vening-Meinesz formula, IVM and inverse Stokes' integral, ISM) and recommend separate optimal data processing strategies when using geoid gradients (GGs) and geoid heights (GHs) for gravity recovery. The use of GGs (for IVM) effectively eliminates systematic errors in gravity derivation. If GHs (for ISM) are used in gravity recovery, the tilt in wide-swath SSHs should be removed before gravity computation, and the recovered gravity must be filtered (post-processed) to avoid artificial gravity signals due to the SSH errors. Our assessments using mgal-accuracy shipborne gravity anomalies in the northern SCS show that multiple-cycle SWOT observations can deliver high-quality marine gravity anomalies. IVM is more robust than ISM in resisting random and systematic errors in SWOT. Our processing strategies can be used for the gravity validation of SSHs from SWOT's fast-sampling and science phases.

Global gravity models are being developed according to new data sets available from satellite gravity missions and terrestrial/marine gravity data which are provided by different countries. Some countries do not provide all their available data and the global gravity models have many vague computational methods. Therefore, the models need to be evaluated locally before using. It is generally understood that the accuracy of global gravity models is enough for local (civil, mining, construction, etc.) projects, however, our results in Iran show that the differences between synthesized values and observation data reach up to ∼300 mGal for gravity anomalies and ∼2 m for geoid heights. Even by applying the residual topographical correction to synthetized gravity anomalies, the differences are still notable. The accuracy of global gravity models for predicting marine gravity anomalies is also investigated in Persian Gulf and the results show differences of ∼140 mGal in coastal areas. The results of evaluating selected global gravity models in Iran indicate that the EIGEN-6C4 achieves the lowest RMS for estimating the geoid heights. EGM08 predicts the closest results to terrestrial gravity anomalies. DIR-R5 GOCE satellite-only model estimates the low-frequency part of gravity field more accurately. The best prediction of marine gravity anomalies is also achieved by EGM08.

Since 2010 several new satellite altimetry missions have commenced delivering altimetry-derived gravity data with a global offshore coverage and with a quality in many regions nearing that of ship-borne gravity observations. This is resulting in greatly improved global offshore high resolution gravity fields. The DTU13 and Sandwell and Smith’s v23.1 grids of altimetric gravity anomalies from offshore Western Australia are compared with ship-track gravity anomalies computed from the Geoscience Australia marine gravity database. The standard deviation of the difference between the ship-borne gravity data and the satellite altimetric gravity data is 3.1mGal for the DTU13 data and 3.3 mGal for Sandwell and Smith’s v23.1 grid. In water depths less than 20m we observed significant differences between ship-borne gravity and altimetric gravity. Over the sampled wavelengths, the DTU13 altimetric gravity data appears to have the best resemblance to the reference marine gravity data, exhibiting the overall least difference amplitude over most wavelengths.

Gravity data have been playing an important role in marine exploration and research. However, obtaining gravity data over an extensive marine area is expensive and inefficient. In reality, marine gravity anomalies are usually calculated from satellite altimetry data. Over the years, numerous methods have been presented for achieving this purpose, most of which are time-consuming due to the integral calculation over a global region and the singularity problem. This paper proposes a fast method for the calculation of marine gravity anomalies. The proposed method introduces a novel scheme to solve the singularity problem and implements the parallel technique based on a graphics processing unit (GPU) for fast calculation. The details for the implementation of the proposed method are described, and it is tested using the geoid height undulation from the Earth Gravitational Model 2008 (EGM2008). The accuracy of the presented method is evaluated by comparing it with marine shipboard gravity data. Its efficiency is demonstrated through comparison with the conventional sequential method. The tests demonstrate that the proposed method can be employed for accurately calculating marine gravity anomalies and provides an advantage on computational efficiency.

Based on laser gyros and quartz flexure accelerometers, Beijing Institute of Aerospace Control Devices has developed the prototype of Laser Strapdown Airborne Gravimeter (LSAG). The hardware improvements and the design of post-processing software are presented, and field test results of the system on the aircraft and oceanic vessel during 2013 and 2014 are given. Field test results showed that the internal accord accuracy after level adjustment was about 1mGal, demonstrating that LSAG has the abilities to implement airborne gravity surveys, and has the potential for marine gravity surveys.

The experiment on tying the relative gravity measurements at sea to the values of the gravitational field model is described. It is shown that under the appropriate conditions, tying the marine measurements to the model can be applied instead of tying them to land stations.

In the continental gravity compilations (1986–1995), available marine gravity surveys were patchy and resulted in an ever growing reliance on satellite altimeter-derived gravity data. The early satellite work by Haxby (Lamont Doherty Geological Observatory) encouraged Getech to get involved in its own processing of these data using repeat mission data. In the second half of the 1990s, after the main continental compilations had been completed, Geosat and European remote-sensing (ERS-1) geodetic-mission data became available with combined track spacing of 3 to 4 km at the equator.

The variation of a marine gravity anomaly reference map is one of the important factors that affect the location accuracy of INS/Gravity integrated navigation systems in underwater navigation. In this study, based on marine gravity anomaly reference maps, new characteristic parameters of the gravity anomaly were constructed. Those characteristic values were calculated for 13 zones (105°–145° E, 0°–40° N) in the Western Pacific area, and simulation experiments of gravity matching-aided navigation were run. The influence of gravity variations on the accuracy of gravity matching-aided navigation was analyzed, and location accuracy of gravity matching in different zones was determined. Studies indicate that the new parameters may better characterize the marine gravity anomaly. Given the precision of current gravimeters and the resolution and accuracy of reference maps, the location accuracy of gravity matching in China’s Western Pacific area is ~1.0–4.0 nautical miles (n miles). In particular, accuracy in regions around the South China Sea and Sulu Sea was the highest, better than 1.5 n miles. The gravity characteristic parameters identified herein and characteristic values calculated in various zones provide a reference for the selection of navigation area and planning of sailing routes under conditions requiring certain navigational accuracy.

The precise point positioning (PPP) technology is applied to an airborne gravity survey. By analyzing the advantages and disadvantages of several velocity and acceleration measurement methods and in combination with an actual marine gravity survey, the position difference method is confirmed to be a useful survey method for velocity and acceleration. Finally, the practicability of using PPP in airborne marine gravity survey is verified by measured data.

Fifteen years (2000–2015) of altimetric data from ERS2, ENVISAT, SARAL and GEOSAT-FOLLOW-ON satellites are optimally combined with in situ marine gravity observations employing the spectral Multiple Input/Multiple Output System Theory (MIMOST) for Dynamic Ocean Topography (DOT) estimation. The spectral behavior of the method is investigated by assimilating low frequency information from GOCE-derived geopotential models in a test area of the Eastern Mediterranean Sea. The frequency content of the reference field used in the reductions of the original observations and its effect to the DOT approximation is studied. The evolution of the annual DOT is validated against oceanographic information of recently available circulation models in the area under study. The effect of the reference field used in data reductions to the geostrophic circulation of the Eastern Mediterranean Sea is analyzed and remarks on the combination of gravity, altimetry and pure oceanographic data are outlined.

The experience of areal gravity surveying in the open ocean with the correction of measurements by the EGM2008 gravity field model is described. It is concluded that correction of the gravimeter measurements by the model data is admissible and reasonable.

The MAGELLAN-2 package developed at the Russian Academy of Sciences’ Institute for Physics of the Earth (IPE) for use in a Windows environment is a development of the MAGELLAN-1 version and is oriented toward modern gravimeters and the growing demand for accuracy and detail in measuring the gravity forces at sea and in the air.

Satellite radar altimetry has been monitoring the earth’s oceans from space for several decades. However, only the GEOSAT and ERS-1 geodetic mission data recorded more than a decade ago provide altimetry with adequate spatial coverage to derive a high-resolution marine gravity field. The original geodetic mission data suffer from degradation in quality and coverage close to the coast and in Polar Regions as well as the occasionally wrongly retracking of these, even in the open ocean. In order to improve the quality of these geodetic mission data and to derive a new improved global marine gravity field called DNSC08GRA, a new double retracking technique for analyzing the waveform data has been developed. Multiple retracking allows the system to retrack more data to increase the spatial coverage of the data. Subsequently, a second retracking run is used to enhance the SSH determination by using information from the first fitting to inform the second set of retrackers about smoothly varying sea state parameters. The development of the new global marine gravity field DNSC08GRA is described in this paper. Besides application of new retracking techniques the radar altimetry has been processed using EGM2008 as reference and augmented with ArcGP gravity data and laser altimetry from ICESat to close the Polar gap. DNSC08GRA is seen to perform significantly better than previous global marine gravity field like KMS02. The improvement in accuracy is better than 20% in general, but in coastal regions, the improvement is in many places of the order of 40–50% compared to older global marine gravity field KMS02.

Much of the ship-track marine gravity data in the Australian national gravity database must not be relied upon because several large (>900 mGal) biases exist in them. These biases were detected and cross-validated through comparisons with marine gravity anomalies derived from re-tracked multi-mission satellite altimetry and a recent satellite-only global geopotential model derived from the Gravity Recovery And Climate Experiment (GRACE). This shows the need to carefully screen ship-track gravity data to ensure that they have been crossover-adjusted before they are relied upon in any Earth-science study.

Over the oceans, data from altimetry are currently the only input data for the latest global geoid models such as EGM08 or EIGEN6C. Over the Mediterranean sea, satellite altimetry does not give good results for gravity models. In particular because of the high ocean variability and in the vicinity of the coast in some areas. A marine gravity data compilation and screening was conducted as part of an international project GEOMED2 for calculating the geoid of the Mediterranean sea. In this paper, marine gravity data and their processing are described. The shipborne gravity data are validated and an estimation of the error is done. Then the cleaned data are used to validate the Global Geoid models (GGm).

Several new satellite altimetry missions have started delivering data that will seriously improve global high resolution gravity fields. The impact of these new geodetic mission data is significant with the quality of the altimetric gravity field nearing that of marine gravity observations in many regions. We demonstrate the quality of the new satellite altimetry data by comparison to high quality marine gravity data over the Capel and Faust basins, offshore Eastern Australia. An improvement of 10% is seen in the region in comparison with vintage altimetric gravity fields. Over the sampled wavelengths, the DTU13 gravity data appears to have the best resemblance to the reference marine gravity data, exhibiting the overall least difference amplitude over most wavelengths.

The effect of sea surface height (SSH) variability is one of the primary factors that limit the accuracy and resolution of altimeter-derived gravity values. We propose a method to estimate the influence of variation of the sea surface height on the accuracy of satellite-derived gravity by simulation technique, with a case study around Indonesian waters. Wederived an Indonesian marine gravity map using the Geosat-geodetic mission (GM). Since most of the area studied is located around coastal and shallow areas, the measurement of SSH of this area is less accurate. To obtain a distribution of SSH variability over the study area, Topex/Poseidon (T/P) data were first processed and assessed. Processing 52 cycles of the Topex/Poseidon data, the root mean square (RMS) of SSH variability for each cycle was found to vary from 1 to 179 cm. Further, for the purpose of estimating the accuracy of altimeter-derived gravity, we derived several levels of Gaussian noise, computed simulation data by adding the Gaussian nois...

New high‐resolution multicomponent gravity gradient data now becoming commercially available yield important subsurface information when properly modeled. Gravity gradients represent minute variations in the 3-D gravity field. Txx represents the gradient in the x direction of the x component of the gravity (Gx). Tyy represents the gradient in the y direction of the y component of the gravity (Gy). Tyz represents the gradient in the y direction of the vertical component of the gravity (Gz). Tzz represents the vertical gradient of the vertical component of the gravity (Gz), and so forth. The measured gravity gradients thus provide a more detailed picture of the subsurface by reflecting the edges, shape, and approximate depth of dominant mass anomalies.

One-year average satellite altimetry data from the Exact Repeat Missions (ERM) of GEOSAT have been used to determine marine gravity disturbances in the Labrador Sea region using the inverse Hotine approach with FFT techniques. The derived satellite gravity information has been compared to shipboard gravity as well as gravity information derived by least-squares collocation (LSC), GEMT3 and OSU91A geopotential models in the Orphan Knoll area. The RMS and mean differences between satellite and shipboard gravity disturbances are about 8.0 and 2.8 mGal, respectively. There is no significantly difference between the results obtained using FFT and LSC.

The tide-free release of the EGM2008 combined global geopotential model and its pre-release PGM2007A are compared with Australian land and marine gravity observations, co-located GPS-levelling on the [admittedly problematic] Australian Height Datum, astrogeodetic deflections of the vertical, and the AUSGeoid98 regional gravimetric quasigeoid model. The results show that we cannot legitimately claim to truly validate EGM2008. Instead, EGM2008 confirms already-known problems with the Australian data, as well as revealing some previously unknown problems. If one wants to claim validation, then EGM2008 is validated because it can confirm the errors in our regional data. Simply, EGM2008 is a good model over Australia.

A new gravimetric geoid (OCTAS07v2) is generated using Stokes’ formula with gravity data as input. As local gravity data, a combination of land gravity data, new and old airborne gravity data, and adjusted marine gravity data has been used. All marine gravity data has been error screened and quality assured by removing dubious data and adjusting the data when necessary. Voids in the gravity data distribution were patched with gravity data from satellite altimetry. The OCTAS07v2 geoid was estimated using the remove-compute-restore technique. The long-wavelength signal of the local gravity data was reduced using a Wong-Gore modified Stokes’ function. The long-wavelength part was represented by a global gravity field model based on GRACE data. The OCTAS07v2 geoid model was combined with the OCTAS07_MSS model to create a synthetic Mean Dynamic Topography (MDT) model. In comparison with the OCCAM MDT, our new synthetic MDT model gave a std. dev. of the residuals of 11 cm. A comparison to the main northern North Atlantic currents show many similar features

An improved geoid, called ADBVE2006 (Autorita di Bacino di Venezia 2006), has been calculated for the north-east of Italy using new land and marine gravity data together with new high resolution multibeam bathymetric data. A standard processing procedure has been applied to gravity data in order to compute the Free Air and the Bouguer anomalies. The gravity reference datum is IGSN71. The long wavelenghth part of the gravity field has been modelled by EGM96. The computation has been carried out by the “remove-restore” spectral technique (Stokes’ approach) using the software GRAVSOFT. The goal of the calculated model is to improve the geoid estimation in the coastal areas, where the calculated geoids usually suffer from the lack of gravimetric data as well as a good bathymetry

With the development of new underwater navigation technology, the passive navigation based on geophysical information is more and more realized by people, such as gravity, magnetism, topography. Gravity aided navigation is the localization technology of combination the stored gravity map and the gravity data measured in real time. One of the key technologies of Gravity aided navigation is to construct a high accuracy gravity map to meet the demands of navigation. The main gravity measurement manners are satellite altimetry, airborne gravity measurement and ship gravity measurement. These different ways have their own characteristic and adapt area, reflecting the different frequency gravity information. In order to construct a high accuracy gravity map, it is necessary to combine three kind data for the whole description of gravity. Thus, how to effective make a combination of these gravity information is important. In this paper, the basic idea of gravity aided navigation is briefly introduced, based on the above, combination method is researched, and the simulation on the method are performed to evaluate the effect of the algorithm. It is a new try for constructing high accuracy marine gravity map.

A series of KMS high resolution near global gravity fields from altimetry have been released during the past 7 years. This presentation focuses on describing the improvements leading to the release of the new KMS2002 field.

An airborne gravity survey has been carried out 1998–99 to cover the ice-covered parts of the seas around northern and north-eastern Greenland. The aerogravity survey has been done by a Danish-Norwegian aerogravity system setup, based on a Lacoste and Romberg “S” gravimeter. A Twin-Otter aircraft has been used, capable of low and slow flights, yielding airborne gravity measurements of relatively high accuracy and resolution. Crossover adjustments and comparisons to independent marine gravity data indicate accuracies of 2 mGal r.m.s., at a resolution of 6–7 km. This kind of accuracy level corresponds to relative geoid errors around 10 cm across the coastal region.

Gravity field determination in ice covered regions of the polar marine areas is possible by using remote sensing techniques, such as satellite altimetry. An approach using standard altimetry products (ocean products, OPR) was developed and successfully applied in the Weddell Sea region, Antarctica. Comparisons to marine gravity data clearly demonstrate good agreement between both data sets.

Various recent marine altimetric gravity fields based on geodetic mission altimetry are compared with ship borne gravity in four regions around Greenland. This is done over different states of the ocean surface ranging from ice free open ocean to permanently ice covered ocean. Distinguishing between results obtained over these different states is important because, usually, the accuracy of the altimetric products decreases with increasing coverage of ice and proximity to coast.

Gravity gradiometry has the potential to become a powerful exploration tool with enhanced resolution relative to classic marine gravity. The improved resolution of gravity gradiometry for exploration applications is equivalent to the improvement 3-D seismics provided over 2-D seismics. Over the last 2 years, gravity gradiometry acquired over exploration targets has been acquired with the Bell Aerospace gravity Gradiometry Survey System (GSS). The GSS is a formerly classified gravity sensing system that contains the world's only moving base gravity gradiometer. The system measures both gravitational acceleration and gravity gradients, yielding six measurements that define the local gravity field and its gradients in three dimension. This paper will present an overview of gravity gradiometry applications illustrated with actual data from the Gulf of Mexico. Exploration examples of high-resolution gravity gradiometry use include enhancement of seismic interpretations, identification of sub-salt sediment fairways and exact location of salt body boundaries.

Geoscience Australia recently revised the corrections applied to the Australian National Gravity Database (ANGD) and switched from applying the simple Bouguer correction to the observed gravity values in its database to applying the more accurate gravity curvature (Bullard B) correction. This change is a straightforward procedure in the case of land-based gravity surveys. However, due to the inherent non-linearity of the Bullard B correction, the original formula for the gravity curvature correction is not applicable to observed data from gravity surveys, which involve layers of different materials, as is the case in marine or airborne gravity surveys. Here we present an extension of the closed-form solution for the Bullard B correction, which allows its proper application in any gravity survey setting. In particular, we present formulae to correctly apply the Bullard B correction to observed gravity data from airborne and marine gravity surveys.

Satellite altimetry data have been processed by treating sea-surface topography measurements from each satellite track as being analogous to seismic reflection traces. We utilised the mature technology developed for manipulating and processing seismic data to sort the large volume (4.5 Gbytes) of altimetry data, convert to gravity and optimise its signal-to-noise characteristics. A wide range of pre-stack processes were tested with the most effective described below.

The topographic surface of the sea can be determined by subtracting the geoid from the geometric height of the stationary sea surface. This latter can be obtained from altimetry measurements; the former can be derived from marine gravity measurements, treated in the form of gravity disturbances.

High-resolution maps of the marine gravity field can be produced for oil and gas exploration. Ship and airborne gravity data collected by oil companies in the preliminary stages of a survey may be combined with existing public-domain satellite altimetry data, greatly increasing the resolution potential of the latter, and providing a regional gravity field for emplacement of the exploration data. The combination of altimeter data with land and ship gravity data is an iterative process, designed to seamlessly match the data sets. The method is quick and easy to implement, and avoids laborious crossover adjustments of the ship data.

Over the last 6 months, a dramatic increase in satellite altimetry coverage of the oceans has led to great improvements in the resolution of the marine gravity field. The 35-day repeat orbit of the ERS-1 altimeter has doubled spatial coverage globally, and recently declassified Geosat altimeter data have provided extremely dense coverage south of 30'S. Early results from these new data (see Eos, March 24, 1992) were presented at the AGU 1992 Fall Meeting in San Francisco. To provide better access to these large and diverse data sets, a global gravity grid has been constructed using satellite altimeter profiles from ERS-1, Geosat, and Seasat.

Part of the gravity data collected by the U.S.C. & G.S. Ship “Oceanographer” from the continental margin west of India has been analysed. The analyses reveal a deep-lying (>8 km) crustal fracture which trends parallel to the continental margin (NW-SE) above 180 nautical miles west of Bombay.

A description is given of the development, during the previous fifteen years, of gravity surveying methods in marshlands and marine areas. The methods described include the use of marshbuggies, weasels, helicopters, tripods, diving bells and remote control instruments. Special emphasis is placed on remote control instruments and a brief description of their construction is given. Finally, the corrections involved in marine gravity surveying are discussed, and the costs of the several methods are compared.

A very fast technique involving Fourier transformation can find the gravity or magnetic anomaly of an irregular crustal model as observed on a plane above the material. It is shown how the method can be used to invert the magnetic field data to obtain a magnetization model, but the model so obtained is not unique. The normal restrictions placed on the magnetization models lead to a family of solutions with one degree of freedom.

Geophysicists at the Atlantic Geoscience Centre have recently completed a major overhaul and compilation of marine gravity and magnetic data collected off the east coast of Canada, and have enhanced these observations with other information collected over adjacent land and sea areas: aeromagnetic data, land gravity measurements, gravity values derived from satellite altimetry, bathymetry, and depth to basement. The resulting data sets have been organized for efficient handling and storage. Used in conjunction with recently implemented tools for rapid data manipulation and display, the data bases are a valuable resource for scientific research. This paper describes the general procedures followed in implementing the data bases, and presents some sample display products.

The theoretical basis is outlined for a quantitative relationship between two-dimensional bathymetry and free air gravity in stable ocean basins. The relationship, designed for use as an algorithm to predict gravity in areas where only bathymetry is known, is based on flexure theory for thin elastic plates and on the expected evolution with seafloor age of the isostatic response of oceanic lithosphere. The relationship is tested against bathymetric and gravity data in the central Pacific. (Author)

The paper presents an original method of soil analysis using elasto plastic finite elements. The method is a deterministic approach of the fatigue problems which consist in correlating cyclic laboratory test results with magnitude and duration of wave loadings. The loadings used for the soil analysis are determined by a separate statistical study. The analysis presented aims at a sound evaluation of the safety criteria of gravity platform foundations taking into account the fatigue effects.

Correct procedures for the reduction of marine gravity data for the purpose of geophysical modeling, as established by previous authors (e.g., Bomford, 1962), include addition of a correction term necessitated by the undulation of the geoid surface on which gravity is measured. The variation of gravity that is due to the undulation of the geoid is known as the indirect effect. Although this correction is generally small and has not been commonly applied in marine geophysical modeling, there are cases in which it becomes significant. For regional studies in which the long-wavelength components of gravity (λ > 500 km) are analyzed, adjustments must be made to remove the indirect effect and avoid the possibility of serious misinterpretation. For local studies in which shorter-wavelength components are considered, gravity values may be satisfactorily approximated while neglecting this small gravity correction.