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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.

A four-year geophysical data acquisition programme on the East Greenland shelf was completed in 1982. This includes 63 000 km of aeromagnetic data, 7800 km of multichannel reflection seismic data, 50 sonobuoy refraction seismic profiles and 7800 km of marine gravity data.

A sub-bottom profiler survey of the Andaman Sea was conducted as part of the marine geophysical program of the U. S. Coast and Geodetic Survey during the International Indian Ocean Expedition. The survey lines were run at right angles to the predominantly north-south tectonic lineations of the island arc system. A 20,000-joule sparker, energized every 4 sec, was used as a sound source and was towed about 100 m behind the ship. A 20-hydrophone array received the reflected signals, which were recorded both on a paper strip chart and magnetic tape. The sub-bottom profiler sections and the marine gravity and magnetic measurements augmented knowledge of the geology of the island arc system; the linear structural belts on land were traced through the Andaman Sea by geophysical methods. The structural development of the island arc system from east to west can be traced, based on available continental and marine data. It was possible to delineate the major segments of the island arc system through a distance of 600 nautical mi (1,110 km) in the Andaman Sea, specifically, the foredeep, outer sedimentary island arc, interdeep, inner volcanic arc (and rift valley), and backdeep.

DURING May and June 1970, the Institute of Geological Sciences (IGS) undertook a seismic reflexion profiling (Sparker) and gravity survey in the Moray Firth along a network of lines spaced approximately 10 km apart. The Bouguer anomaly gravity map shown in Fig. 1 includes the results of the marine gravity survey, land survey results compiled from unpublished IGS data, some unpublished results made available by Drs W. Bullerwell and J. Phemister, and results of a survey published by McGregor and Wilson1. A previous survey using a sea bed gravity meter has been reported by Collette2, whose published map showed a similar pattern of anomalies. The wide and irregular spacing of the stations prevented satisfactory interpretation, however.

Offshore reconnaissance seismic sparker surveys shot during the mid-sixties in the Bonaparte Gulf Basin on the Northwest Australian Shelf, outlined a number of diapiric features in the southern part of the Basin. Early aeromagnetic and marine gravity programmes were run on too wide a grid to give clear indications of the nature of the intrusives. Later, more sophisticated seismic equipment was used to map the plugs and it became possible to date the age of the movement of the intrusives in several cases. A wildcat well, Sandpiper No. 1 was subsequently drilled on the crest of one of the features; it entered massive salt at a fairly shallow depth, as did another well, Pelican Island No. 1, a hundred miles to the south. Hypotheses involving salt migration toward the Basin margins are put forward to explain the Wickham Gravity High, uplift at the SW Basin Margin Fault and the Petrel Structure. The salt is dated as “probably Devonian” with salt flowage probably continuous from at least Upper Permian ...

A color relief image of the marine gravity field from SEASAT altimeter measurements of the topography of the ocean surface is now available through the National Geophysical Data Center (NGDC) of the National Oceanic and Atmospheric Administration. This image, prepared by William F. Haxby (Lamont-Doherty Geological Observatory of Columbia University, Palisades, N.Y.), has been published by NGDC for the Office of Naval Research, which was the principal sponsor of the effort leading to the development of the image. The U.S. Geological Survey, National Mapping Division, printed the map.

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.