637 



Figure 6. Free Air Gravity Contour Data m the Gulf of Mexico 



'"-^J 



This figure shows a contour plot 

 of the gravity field of a section of the 

 Gulf of Mexico at a resolution of one 

 arc minute. 



The area shown includes in the right 

 a section of the thickly sedimented 

 Mississippi fan. and in the left center 

 the broken area of the Sigsbee 

 Escarpment (both viewed through 

 the "lens" of the gravity field). 



The gridded data used for construct- 

 ing this plot over water were derived 

 from measurements along ship tracks 

 spaced to sample the gravity high 

 properly. 



The entire Gulf of Mexico survey 

 required 34.638 miles of ship track 

 and produced 162,836 original data 

 points. Considerable processing was 

 necessary to ensure data uniformity 

 and to eliminate errors, and then to 

 produce a uniformly sampled 

 gridded data set. 



Gravity data over the land area were 

 acquired from other sources. 



gravity point observations with the latitude, longitude, 

 observation time, free air anomaly, and gravity value, supported 

 with over a dozen tables containing survey, data processing, 

 and statistical information. The observations table consists of 

 over .38 x 10' rows of data which can be sorted by cruise and 

 date-time group or geographic coordinates. 



Information is retrieved from the database by both interactive 

 and batch processing. There are Navy plans to replace this 

 database with a relational database and then load the data into 

 the Integrated Database Management System (Section III.C) as 

 the Gravity Core Database. 



b. Accessibility 



The Navy's ship board gravity data are normally classified 

 because they reveal locations and dates of the ship tracks 

 that might inadvertently release information about opera- 

 tional interests. 



c. Scientific Utility 



In the presence of a seamount the enhanced gravitational 

 attraction of the seamount will cause the water overhead to pile 

 up above the seamount, resulting in an anomaly in the geoid. A 

 satellite altimeter can thus be used to map the seafloor from 

 space in the same way a shipbome gravimeter can be used to 

 construct a map of the underlying topography (Figure 7), 



Unfortunately, the actual situation is not quite as simple as the 

 foregoing would make it seem. Since late last century it has 

 been known that mountains often have light "roots" that allow 

 the mountain to be buoyed up in the fluid mantle in much the 

 same way as an iceberg's keel supports the portion of the body 

 rising above water (Archimedes' Principle). Viewed from far 

 away, the dense mountain is compensated by the light root and 

 the overall gravity anomaly is zero. Because a gravimeter on 

 the ocean's surface is relatively close to the seamount. the 



