Abstract
In 1958, Prof. I. Tsubokawa proposed a gravity meter with three strings intersecting at right angles to one another as shown in Fig. 1. The principle of measurement is as follows [1]. Let the components of gravity along three strings be g1, g2 and g3, and the frequencies of the strings 1, 2 and 3 be f1, f2 and f3 respectively, then
g2=g12+g22+g32
g1=a1f12, g2=a2f22, g3=a3f32}(1)
where a1, a2 and a3 are the coefficients which depend on the dimentions, linear density and elasticity of the strings and the weight. Let the respective angles between the strings be α12, α23 and α31, where α12=90°-ε12, α23=90°-ε23 and α31=90°-ε31.
g=√a12f14+a22f24+a32f34+2a1a2f12f22sinε12+2a2a3f22f32sinε23+2a3a1f32f12sinε31 (2)
We assume construction accuracies of gravity meter as follows;
1) ε12, ε23, ε31<1×10-3
2) a1, a2 and a3 are equal in the accuracy of 1×10-3
By integration Eq. (2) from time O to T, the value of the gravity difference between the starting and observation stations Δg can be expressed as
The suffix 0 corresponds to the starting station and s the observation station. In this equation, gh is the horizontal acceleration due to the ship's movement and the last term is the Eötvös correction, in which V is the speed of the ship in knot, ψlatitude and A the azimuth of the course. Gravity survey was carried out on the sea around Japan during the period from April to June in 1969 on the board "Hakuhop-maru" belonging to the Ocean Research Institute, the University of Tokyo. The ships' tracks are shown in Fig. 4 with profile No. of gravity anomaly. The plofiles of free air and Bouguer anomalies are shown in P. 1-17 with bottom topography.
