Determination Internal Layer Gravity of Ocean Based on Surface Shallow Layer

  Objectives  Classical physics geodetic mainly focuses on the solution of the geodetic boundary value problem, which is usually given by the theorems of Stokes and Molodensky. Solving the internal gravity field of the ocean is a problem of solving the internal potential of the earth, to which classical theoretical method cannot be applied. In order to solve this limitation, the concept of shallow seawater is introduced based on the surface shallow method to calculate the internal ocean gravity at different depths.

  Methods  Using high-resolution data of DTU21 mean sea surface and EGM2008, combined with Newton's integration, spherical harmonic analysis, and spherical harmonic comprehensive theoretical methods, the internal ocean gravity field in the western Pacific region ((131.5°±0.25°) E, (19.5°±0.25°) N) was determined. The number of iterations of spherical harmonic analysis was determined, and the relationship between the depth of shallow seawater and the integral area radius was analyzed.

  Results and Conclusions  An optimal integral area radius of 1° was determined for 100 m, 500 m and 1 000 m and the depth of 1.5° for 4 000 m. The accuracy of the shallow seawater method to remove-restore the ocean surface gravity is evaluated and the root mean square error is 0.13 mGal at a depth of 100 m, 0.61 mGal at a depth of 500 m, 1.21 mGal at a depth of 1 000 m, and 3.93 mGal at a depth of 4 000 m. Compared to surface gravity, gravity of the inner layer of the ocean increases by 22.11 mGal at 100 m depth, 110.50 mGal at 500 m depth, 220.87 mGal at 1 000 m depth, and 877.31 mGal at 4 000 m depth.