Abstract
Abstract This paper reports a finding that marine gravity data around the Japanese Islands are inconsistent with nearby land gravity data. The comparison between land and marine gravity was made possible by the gravity measurements from the air using a helicopter-borne gravimeter (SEGAWA Model) developed by the present authors. The ground/sea truth gravity anomaly can be checked against the gravity from the air, though it is not free air gravity anomaly but gravity disturbance. The newly-developed airborne gravimeter first manufactured in 1998 shows a good performance with a 1–2 mgal average repeatability of measurement under a 90-knot flight speed. Thus we have found disagreements between ground truth and sea truth gravity anomalies on the basis of airborne gravity data in the areas in Japan where we have so far made measurements. Among them we will report the case in the area from Saitama and Ibaraki offshore to the Kashima-Nada Sea, p]Japan. Our conclusion about this area is that the past marine gravity data obtained by surface ship gravimeters involve systematic errors of more than 10 mgals. This kind of inconsistency between marine and land gravity is expected to be found in a number of other areas around the Japanese Islands.
Highlights
Practical measurements of gravity at sea were initiated by Vening Meinesz (1929, 1941)
The marine gravity measurements were made in most part of the sea in the world by many ships and operators
This map is contoured with the interval of 2 mgal using gravity data compiled by Geological Survey of Japan (GSJ)
Summary
Practical measurements of gravity at sea were initiated by Vening Meinesz (1929, 1941). The gravity disturbance on the Normal Ellipsoid δghg for the ground/sea truth data, on the other hand, can be expressed by δghg = ghg + 0.3086hg = ghg − γe + 0.3086hg where hg is the height of geoid. In order to convert ground/sea truth gravity disturbance on the Normal Ellipsoid to that on the helicopter flight level, we have to apply the inverse. This map is contoured with the interval of 2 mgal using gravity data compiled by GSJ. First we adjust the bias of airborne gravity data within the land area so that they exactly agree with the upward-continued ground truth gravity disturbance. Discrepancies of measurements among ‘heliborne’, HOD and GSJ are obvious
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