Continuous Gravity Measurements Research Articles

Crustal structure of the mid-ocean ridges: 2. Computed model from gravity and seismic refraction data

Seismic results have demonstrated that the crust under the mid-ocean ridges is not thicker than under the ocean basins. Seismic results also show that the velocity in the mantle immediately under the ridge flanks is normal but that it is low under the axial zones, indicating an ‘anomalous mantle.’ The free-air gravity anomaly over the entire ridge is close to 0 (within ±50 mgal), and consequently the gradients in the Bouguer anomaly are steep (sometimes approaching 1 mgal/km). In order for gravity and seismic results to agree, the anomalous mantle, which provides compensation in the axial zone of the ridge, is postulated to extend to the flanks below the normal mantle observed by seismic refraction measurements. A model computed on the basis of continuous gravity measurements over the mid-Atlantic ridge and the seismic refraction results given in part 1 is presented. Alternative models that provide compensation at greater depth under the ridge flanks are ruled out by the steep gradients in the Bouguer anomaly. It is shown that a similar model can explain the seismic and gravity data obtained over the east Pacific rise but that a deeper compensation is not impossible because of the smaller gradients in the topography and Bouguer anomaly curves. A lack of correlation between the sharp east Pacific rise heat-flow anomalies and the magnetic and gravity anomalies is also noted.

SURFACE‐SHIP GRAVITY MEASUREMENTS IN THE NORTH SEA*

AbstractModern marine gravimeters enable us to measure gravity at sea with a mean error of 2 to 5 mgals, depending on sea conditions and navigational accuracy.Good results could be obtained, even in rough seas, by means of Graf's ASKANIA Sea Gravimeter mounted on an ANSCHÜTZ gyrostabilized platform.When a DECCA Navigator System is used, the gravity measured at the points of intersection agrees within 5 mgals at wind‐force Beaufort 4 and vertical accelerations of about 60.000 mgals.The southern North Sea has been surveyed with VFS “GAUSS”, the research vessel of the German Hydrographic Institute, Hamburg. Since 1959 about 4.000 miles of continuous gravity measurements combined with simultaneous magnetic measurements have been carried out in this area. Here nearly all remote‐control gravimeter stations of the Netherlands Geodetic Commission were passed once or several times. Hence they could be used as basic points. However, there are a few cases in which the Dutch data could not be confirmed.The results are given as a free air isogam map covering the area between the meridian of Greenwich and 8° east, and between 54° and 56° latitude, with decreasing density of the measurements from east to west.

Continuous gravity measurements on a surface ship with the Graf Sea gravimeter

Gravity measurements were made aboard the surface vessel U.S.S. Compass Island (EAG 153) with the Graf Sea Gravimeter mounted on a stabilized platform. Repeated observations along a line of gravity measurements previously made in submarines showed discrepancies of the order of the navigational variations. Many additional measurements agree satisfactorily with nearby previous submarine measurements. It was demonstrated that the swell is the principal limitation to the measurements on Compass Island. When the vertical heave becomes too large, the boom of the instrument strikes the walls of its case, making observations impossible. On a five-week cruise to the Mediterranean during March and April, 1958, observations were made 50 per cent of the time in the Atlantic and 90 per cent of the time in the Mediterranean. The problem of measuring the Eotvos correction is more severe on a surface ship than on a submarine because of the larger effects from winds and currents to which the surface ship is subjected. This problem and the problem of locating the observation lines are the most serious difficulties encountered in making gravity measurements at sea. The value of continuous gravity observations is demonstrated by examples of anomalies observed over seamounts, across the Mid-Atlantic Ridge, over the Canary Basin, in the Tyrrhenian Sea, and through the Strait of Gibraltar. Continuous observations reduce the chance of computational errors, prevent overlooking unanticipated features, provide a more representative gravity anomaly for an area, and provide more definitive data for the curve fitting that is required for geological interpretation of anomalies. The Graf Sea Gravimeter in conjunction with a stable platform initiates a new era in the measurement of gravity at sea. Areas which are too hazardous for submarine observations can now be readily investigated.