Earth Planets Space, Vol. 57 (No. 4), pp. 243-252, 2005
Jiro Segawa1, Masao Komazawa2, K. Vijay Kumar1, Eiji Nakayama3, E. John Joseph4, Shigekazu Kusumoto1, Ken-ei Onodera5, and Yuki Kuroishi6
1School of Marine Science and Technology, Tokai University, 3-20-1 Shimizu-Orido, Shizuoka 424-8610, Japan
2Geological Survey of Japan, National Institute of Advanced Industrial Science and Technology, 1-1-1 C#7 Higashi, Tsukuba, Ibaraki 305-8567, Japan
3AeroGRAV, 1-44-5-206 Bessho, Hachioji, Tokyo 192-0363, Japan
4Geology and Geophysics, School of Earth and Environmental Sciences, the University of Adelaide, SA 5005, Australia
5Hydrographic and Oceanographic Department, Japan Coast Guard, 5-3-1 Tsukiji, Chuo-ku, Tokyo 104-0045, Japan
6Geographical Survey Institute, 1 Kitasato, Tsukuba, Ibaraki 305-0811, Japan
(Received July 9, 2004; Revised February 28, 2005; Accepted March 1, 2005)
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, 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.
Key words: Inconsistency between marine and land gravity, helicopter-borne gravimeter.