Continuous Gravity Observations Research Articles

A geophysical investigation of the Peru-Chile Trench

Twenty-six profiles of topography, free-air gravity anomaly, and total intensity magnetic anomaly across the Peru-Chile Trench are presented and discussed. The trench is obliterated as a topographic feature south of about 40°S, but a well-defined negative gravity belt and seismic reflection profiles indicate that the trench continues as a structural feature at least into the Drake Passage. The trench can be broadly divided into two main provinces, the main sediment-free province which extends from 8°S to 32°S and the sedimentary province which extends from 33°S to 57°S. Three other minor provinces are also defined. A free-air gravity anomaly map of the offshore area of the entire west coast of South America is presented. A secondary negative gravity belt is found that roughly parallels the prominent negative gravity belt associated with the trench. This secondary belt is thought to be a reflection of the Bolivar Geosyncline which lies very near the present coast of Colombia, Ecuador, and Peru. Studies incorporating seismic refraction and reflection measurements and gravity measurements indicate a pronounced crustal thinning beneath the offshore flank of the trench. Crustal thicknesses beneath the axis of the trench appear to be normal for the transition region of an ocean-continent margin. Seismic refraction measurements and continuous gravity observations are in close mutual agreement. It is suggested that the trench may originate by high angle normal faulting near the base of the continental slope accompanied by a downward flexure of the crust further offshore. The gravity data indicate that there are no radical variations in the crustal structure along the entire length of the western continental margin of South America. The magnetic anomalies cannot be correlated over large distances. The magnetic signature is considerably smoother over the shoreward flank of the trench than over the offshore flank and the trench axis roughly defines the boundary. This points to a sharp contrast in the magnetic state of the material underlying the two flanks of the trench. The relatively large amplitudes of some observed magnetic anomalies strongly suggest that they are not produced primarily by induced magnetization of bodies in the crust. In certain areas the observed anomalies correlate well with the basement topography. There is good evidence that some of these anomalies are caused by remanent magnetization of magnitude many times larger than the predicted induced magnetization and in a direction opposite to the earth's present field.

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.