150 



WORZEL AND HARRISON 



[chap. 9 



should be recorded directly in digital form on punched paper tape and com- 

 puted on a high-speed computer using numerical rather than analogue filters. 



F. Reduction of Observations and Methods of Interpretation 



The instruments described above yield a gravity value on the moving ship 

 or submarine. Coriolis force due to the vessel's motion on the rotating earth 

 alters the observed value of gravity and means that a correction — the Eotvos 

 correction — must be applied to the observed value to reduce it to that A\hich 

 would have been measured on a stationary vessel. The correction to be added 

 to the observed gravity value is given (Worzel, 1959) by the formula : 



hg = 1 ASl S sin C cos (p + {S^ 1 24:0. S). 



G is the true course made good, rp the latitude and S the shij)'s speed in knots. 



0ENSITY=2.67 



250 mgal 



200 



150 



- 100 



50 



-50 



-\ 

 I 



T^TTTTTiTnTTTTTrTTTlTm 



j_ 



2 

 3 



4 



5 km 



50 



100 



150 km 



Fig. 11. Gravity-anomaly profiles across Seamount Jasper. (After Harrison and Brisbin, 

 1959.) 



Measured gravity corrected for the Eotvos effect is next reduced to sea-level. 

 A shipboard measurement is taken close to sea-level and only a small correction 

 for the free-air gradient is needed. The value of gravity at the latitude of 

 observation according to the International Formula is subtracted from this 

 finally corrected observed value. This difference is known as the free-air 

 anomaly by analogy with the free-air anomaly on land. In fact the correction 

 to sea-level for a sea observation is trivial, and it is not necessary to make any 



