The Measurement of Gravity at Sea.

Abstract

271From a physical point of view there might not be an objection to this assumption." T h e determination of the depth of the "20° discontinuity," as so far made,t rests on the assumption that the velocity does not increase strongly down to the discontinuity and would therefore no longer be valid.T h e different behaviour of P and S in the neighbourhood of 20°, if real, might be due to the fact that "gradual" change of structure should be interpreted in relation to the nature of the incident wave. T h e expectation seems to be that a change is more apt to be gradual relative to P than relative to S , and, contrary to this, P seems to behave as if there was a discontinuitj.near 20°, whereas S does not.Summary.-It has been shown that the slow gradual increase of velocity which is likely to occur in uniform sedimentary layers gives rise to straight time-curves such as are usually obtained in prospecting. When depth is determined from straight time-curves on the assumption that the velocity is constant within the layers, a good approximation to actual depths is obtained provided the increase of the velocity in the layers is small and the separate layers are not very deep.If the velocity increases strongly within a layer we may again obtain straight time-curves, but if such time-curves are interpreted on the assuniption of constant velocity within separate layers, serious departures from true conditions will be involved. It may be possible to detect a strong gradual increase of velocity by means of amplitude observations, but depth determination remains uncertain.

Measurement of gravity at sea and in the air

“…From the 1920's Vening-Meinesz pendulums have been used in submarines to measure gravity [¾ening Meinesz, 1929[¾ening Meinesz, , 1932[¾ening Meinesz, , 1941Browne, 1937;Browne andCooper, 1950, 1952;Worzel et al, 1955;Harrison, 1960]. For many years they were the only instruments used for measuring gravity in deep water.…”

“…where g -gravity. This equation is known as the Browne correction [Browne, 1937] or the horizontal acceleration correction, and it has been found to be adequate for the motions encountered in submerged submarines. For motions occurring on a surface ship, LaCoste and Harrison [1961a] have made a more comprehensive analysis in which simultaneous horizontal and vertical accelerations were considered.…”

Gravity observations at sea in USS Conger, Cruise III

“…Ewing [1950, 1952] and Worzel, Shurbet, axxd Ewing [1955ab] have described modifications of the basic apparatus and associated equipment to improve techniques of observation and data reduction. A long-period pendulum ap paratus designed by Vening Meinesz [1941] was used to observe the horizontal accelerations [Browne, 1937].…”

“…chronometer rate, tilt of the swinging plane, and amplitude to give values of observed gravity at the depth of submergence. These values were corrected to the sea surface, for the second order effects of acceleration [Browne, 1937], and for the Eotvos effect to give values of observed gravity at the sea surface. Each value is the average value of observed gravity over a horizontal distance of about 1 mile, since each observation requires about 30 minutes while the submarine is traveling at about two knots.…”

Gravity observations at sea in USS Corsair

“…Computatims-Pendulum periods read from the records were corrected for temperature, air density, isochronism, chronometer rate, tilt of the swinging plane, and amplitude. Gravity values at the depth of submergence were corrected for the second order effects of acceleration [Browne, 1937] Temperature and density coefficients-The coeffi . cients listed below for pendulums 4, 5, and 6, are discussed by Ewing, Worzel, and Shurbet [in press ).…”