According to the filter process of the gravity meter system, the gravity data has a time lag of 180 seconds between the measurement and its
output. After adjustment of this lag time, the following corrections and calculations were performed.

(1) Drift correction

D = ((Vge-Vgs)-(Age-Ags))/(Te-Ts)

D: Drift value (mGal/day)

Vgs: The shipboard gravity at the start of the cruise (mGal)

Vge: The shipboard gravity at the
end of the cruise (mGal)

Ags: The absolute gravity at the shipboard sensor position at the start of the cruise (mGal)

Age:
The absolute gravity at the shipboard sensor position at the end of the cruise (mGal)

Ts: The start time of the cruise (day)

Te: The end time of the cruise (day)

(2) Eoetvoes correction

E = 7.503*S*cos(φ)*sin(α)+0.004154*S^2

E: Eoetvoes correction (mGal)

S: Ground speed of the ship (knot)

φ: Latitude (radian)

α: Course of the
ship (radian、measured clockwise from the north)

Reference: Blakely,R.J., Potential theory in gravity & magnetic applications, Cambridge University Press,New York,441pp,1995

* The navigation data such as S,φand αare the 4-min average values. Before average processing, following data were removed from
each dataset. If the number of data used for a 4-min average calculation did not include more than 50% of good data, the processed average
value was considered as a missing value.

⋅ Time error (inversion of time, continuation of same timestamps)

⋅ Ship
speed exceeding 20knot

⋅ Course of the ship except 0-360°

(3) Calculation of the absolute gravity

G = Ags+(Vg-Vgs)-D*(T-Ts)+E+H*β

G: The absolute gravity at sea surface (mGal)

Ags: The absolute gravity at the shipboard sensor position at the start of the
cruise (mGal)

Vgs: The shipboard gravity at the start of the cruise (mGal)

Vg: The shipboard gravity at the measurement
time (mGal)

D: Drift value (mGal/day)

Ts: The start time of the cruise (day)

T: The measurement time (day)

E: Eoetvoes correction (mGal)

H: Height from sea surface of the shipboard sensor position (m)

β: Free-air gradient
0.3086 (mGal/m)

(4) Calculation of the Free-air anomaly

Gf = G -γ+δ

Gf: The Free-air anomaly (mGal)

G: Absolute gravity at sea surface (mGal)

γ: Normal gravity(mGal)

*The normal
gravity formula of the Geodetic Reference System 1980

γ=978032.67715(1+0.0052790414sin^2φ+0.0000232718sin^4φ+0.0000001262sin^6φ+0.0000000007*sin^8φ)

δ:
Atmospheric correction at sea surface

δ=0.87-0.0000965*0(mGal)

(5) Output of the data

Time (UTC)

Latitude (degree)

Longitude (degree)

Processed absolute gravity at sea surface (mGal)

Free-air
anomaly (mGal)