An internally damped inertial platform for marine gravimetry and a test case in the South China Sea

To dampen periodic off-levelling motions within an inertial platform while undergoing horizontal accelerations of the same period and to achieve a levelling accuracy of a few tens of arcseconds with that system, an internally damped inertial platform for a marine scalar gravity system was the developed. Methods for attenuating horizontal acceleration and reducing off-levelling error by a satisfactory gyro-levelling loop, which are fundamental to the internally damped inertial platform, were designed and implemented. In addition, phase delays are introduced by the levelling loop. The resulting off-levelling gravity errors were analyzed and modeled. A series of tests on a motion simulator were performed in laboratory for a variety of simulated sea conditions. We found that the motion of the platform is a function of the amplitude and period of the simulated ship motions and ranges between 10 and 40 arcseconds. In addition, the phase lag between platform motion and ship motion is not constant but ranges 180°–270°, depending on the period and amplitude of the motion. Then, the platform, on which a gravimeter was mounted, was installed on the R/V Shiyan 2 to conduct a gravity survey in the South China Sea. Despite rough sea conditions, it was shown that in short periods of 2–30 s, the off-levelling angle was less than 30 arcseconds, and the phase lagged the horizontal acceleration by 230°–260°. From a repeated survey line and intersecting survey points, the estimated errors of gravity measurements were between 1.3 and 1.7 mGal. The marine measurements results were compared with those of satellite altimetry data and show a mean value of 0.5 mGal in a standard deviation of 1.5 mGal.