SECT. 1] GRAVITY AT SEA 



the average value of which, over a 5-min interval, is 



141 



X\ K{d —a) —p{h —ho)i —mgl 



kV 



K 



(4) 



the same as for a deflection in the absence of the wave motion. 



In practice one computes the gravity value from the dial setting of the 

 instrument and adds to it the correction read from the axial position of the 

 record. The 5-min averaging time is imposed by the time constant of the com- 

 plete mechanical-electrical system. Graf (1958) gives more details about the 

 construction and testing of the instrument. 



The first tests of the Graf Sea Gravimeter were made in August, 1955, on 

 Lake Starnberg (Graf, 1956) using a gimbal mounting. Later tests were made 



DlfFEKEMCE BETWEEN METERS 104 



& 102 0.\ RUN 7 



980150 



^80050- 



Dl STANCE 



150 



Fig. 4. Comparison of observed gravity on eight rvms over as nearly the same course as 

 possible and the differences observed by two meters on the same stable platform on 

 the same run. 



in the Adriatic Sea in May, 1956 (Graf, 1957). Comparisons were made with 

 the Vening Meinesz pendulum apparatus on board U.S.S. Becuna in August, 



1956 (Worzel and Graf, 1957). Of the 59 observations 3 had large discrepancies, 

 17 fell within the range of 3-9 mgal and 39 within the range 0-3 mgal, showing 

 a close approach to a statistical distribution. The first successful surface-ship 

 gravity observations were made with this instrument mounted on a gyro- 

 stabilized platform on an I.G.Y. cruise on U.S.S. Compass Island in November, 



1957 {I.G.Y. Bull., 1958). More complete comparisons, as well as a number of 

 continuous profiles, were reported by Worzel (1959). Seven continuous profiles, 

 150 nautical miles long, agreed with themselves and with previous submarine 

 gravity measurements within a spread of 20 mgal (Fig. 4). The spread is 

 partially caused by navigation errors, and partially by the inability to recover 

 the same course and jDositions. At three other crossings of former submarine 

 profiles differences of the order of 6 mgal were observed. The continuous track 

 of U.S.S. Compass Island passed within 5 miles of 1 1 other submarine gravity 



