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B. GEOLOGY AND GEOPHYSICS 



1. MARINE GRAVriY 



a. Data Description 



Gravity is the force that attracts bodies to Earth.' In fact the 

 acceleration of gravity over the surface of the Earth is not a 

 constant from location to location. Rotation flattens the Earth's 

 shape near the poles to approximately that of an oblate spheroid, 

 thus reducing surface gravity at the equator. Over the past 30 

 years, as the need for precision in the positioning and navigation 

 of space vehicles and other platforms increased, it became 

 increasingly important to account for the slightest gravity 

 variations. Thus, not only the general overall shape of the Earth 

 (oblate spheroid) was needed to drive the gravity field, but ever 

 smaller topographic features and geographic structures (which 

 can cause gravity changes sufficient to affect critical 

 instrumentation) had to be mapped. 



The Earth' s shape has been measured with increasing accuracy 

 ever since the development of earth-orbiting artificial 

 satellites. In the 1980s the Navy measured the equipotential 

 surface of the oceans, the geoid. very accurately using a satellite 

 altimeter aboard Geosat (Section II. B. 3). The geoid differs 

 significantly from the reference ellipsoid. Not surprisingly, it 

 is possible to derive the gravity field from the geoid: in 

 particular, the product of the gravity field and the geoid anomaly 

 in meters (i.e.. the difference between the geoid and the reference 

 ellipsoid) is equal to the anomaly in the gravitational potential. 

 The Navy's accurate measurement of the geoid thus allows a 

 global inference of the associated gravity field. 



In dedicated Navy surveys, gravity measurements were made at 

 sea with oceanographic ships tracing closely spaced tracks 

 sufficient to provide scientific quality measurements. Lacoste 

 andRomberg Air- SeaMeters were used from 1966 to 1983. and 

 in 1969 Bell Aerospace BGM-3 and BGM-5 gravimeters were 

 inu-oduced. In addition tocollecting the data, scientists measured 

 gravity variations and conducted vertical deflection studies (the 

 difference between the vertical, as indicated by the plumb line 

 of the instrument, and the theoretical perpendicular to the 

 Earth) to obtain true positions. This concept is illustrated in 



Figure 5 . They also computed gravity anomalies (the difference 

 between observed and theoretical values) that can help geologists 

 locate mineral and oil deposits and can be applied to geophysical 

 studies of the Earth. 



Figures. Graviti' Variations and Vertical 



^„._. - - - -^ 



Vte geoid dtjjf-i Sii;n:ficarid\ jtrn] ihe rejeiLiiLC . 



surprisingly, il is pus^ibU to dinve the Earth's giu^-i. JiL.J ,. -'in .. , .c . .. 



knowledge of the g^oid. 



Depicted schematically here are the reference ellipsoid and the perturbed 



shape of the geoid due to the masses of oceans and mountains. As shov.-n. 



the different shapes of these ftv-o surfaces result in the norinal directions 



being different. 



The product of the gravity field and the geoid anomaly in meters (i.e.. the 



difference between the geoid and the reference ellipsoid) is equal to the 



anomaly in the gravitational potential. 



Survey data are digitized at one minute (of time) intervals and 

 indexed by geographic position. These digitized data are called 

 "point data." Representing gravity at one minute (of position) 

 intervals in a survey area is considered the best estimate of 

 gravity at an indexed point. These computed data are called 

 "point-average gravity anomalies." NAVOCEANO collects 

 gravity data in support of required fleet activities, which are 

 usually classified. Figure 6 depicts an example of high- 

 resolution gravity data from the Gulf of Mexico. - A study of the 

 data in the Gulf of Mexico provided an opportunity to consuuct 

 an unclassified one minute point-average gridded data set. 

 which is illustrative of the classified data covering much of the 

 world's oceans 



From these at-sea measurements the point observations of 

 gravity were processed andorganizedintoarelational database. 

 This Navy Point Gravity Database consists of a table of marine 



' The gravity field results in an acceleration near the surface of =9.81 m/s- so that a mass of I kg weighs one Newton (N). A gravitational unit, the gal. 

 was named in honor of Galileo, who first syslemalically measured the acceleration of gravity (gal = 10' m/s'). 



- The gravity data shown in Figure 6 as extending over land were taken from an earlier land-based survey and inlegrated with the NAVOCEANO 

 shipboard measurements to form a single field. 



