138 * Marine Minerals: Exploring Our New Ocean Frontier 



ured be incorporated into larger data sets, so that 

 data at different scales are simultaneously available 

 to investigators. 



Gravity Methods 



Like magnetic methods, the aim of gravity meth- 

 ods is to locate anomalies caused by changes in 

 physical properties of rocks. *^ The anomalies sought 

 are variations in the Earth's gravitational field re- 

 sulting from differences in density of rocks in the 

 crust — the difference between the normal or ex- 

 pected gravity at a given point and the measured 

 gravity. The instrument used for conducting total 

 field gravity surveys is a gravimeter, v^^hich is a well- 

 tested and proven instrument. Techniques for con- 

 ducting gradiometric surveys are being developed 

 by the Department of Defense, although these will 

 be used for classified defense projects and will not 

 be available for public use.^° 



The end product of a gravity survey is usually 

 a contoured anomaly map, showing a plane view 

 or cross-section. The form in which gravity, as well 

 as magnetic, data is presented differs from that for 

 seismic data in that the fields observed are integra- 

 tions of contributions from all depths rather than 

 a distinct record of information at various depths. 

 Geophysicists use such anomaly characteristics as 

 amplitude, shape, and gradient to deduce the loca- 

 tion and form of the structure that produces the 

 gravity disturbance.^' For example, low-density 

 features such as salt domes, sedimentary infill in 

 basins, and granite appear as gravity "lows" be- 

 cause they are not as dense as basalt and ore bod- 

 ies, which appear as gravity "highs." Interpreta- 

 tion of gravity data, however, is generally not 

 straightforward, as there are usually many possi- 

 ble explanations for any given anomaly. Usually, 

 gravity data are acquired and analyzed together 

 with seismic, magnetic, and other data, each con- 

 tributing different information about the sub-bot- 

 tom geological framework. 



Since variations in terrain affect the force of grav- 

 ity, terrain corrections must be applied to gravity 



*'Sharma, Geophysical Methods in Geology, p. 88. 



^°J. Brozena, Naval Research Laboratory, OTA Workshop on Tech- 

 nologies for Surveying and Exploring the Exclusive Economic Zone, 

 Washington, DC, June 10, 1986. 



^'Sharma, Geophysical Methods in Geology, p. 131. 



data to produce an accurate picture of the struc- 

 ture and physical properties of rocks. Bathymetric 

 data are used for this purpose; however, terrain cor- 

 rections using existing bathymetry data are rela- 

 tively crude. Terrain corrections using data pro- 

 duced by swath mapping techniques provide a 

 much improved adjustment. 



Like magnetic data, the acquisition of gravity 

 data may be from satellite, aircraft, or ship. The 

 way to measure the broadest scale of gravity is from 

 a satellite. SEAS AT, for instance, has provided 

 very broad-scale measurements of the geoid (sur- 

 face of constant gravitational potential) for all the 

 world's oceans. To date, almost all gravity cover- 

 age of the EEZ has been acquired by ship-borne 

 gravimeters. Gravimetry technology and interpre- 

 tation techniques are now considered mature for 

 ship-borne systems. However, the quality of ship- 

 based gravity data more than 10 years old is poor. 

 Airborne gravimetry is relatively new, and tech- 

 nology for airborne gravity surveys (both total field 

 and gravity gradient types) is still being refined. 

 As airborne gravity technology is further developed, 

 it can be expected that this much faster and more 

 economical method of gathering data will be used. 



Of all the techniques useful for hard mineral 

 reconnaissance, however, gravity techniques are 

 probably the least useful. This is because it is very 

 difficult to determine variations in structure for 

 shallow features (e.g., 200 meters or less). Shallow 

 material is all about the same density, and excess 

 noise reduces resolution. Gravity techniques are 

 used primarily for investigating intermediate-to- 

 deep structures — the structure of the basement and 

 the transition between continental and oceanic 

 crust. Many of these structures are of interest to 

 the oil industry. Although large faults, basins, or 

 seamounts may be detected with air- or ship-borne 

 gravimeters, it is unlikely that shallow placer de- 

 posits also could be located using this technique. 



USGS has published gravity maps of the Atlan- 

 tic coast, the Gulf of Mexico, central and south- 

 ern offshore California, the Gulf of Alaska, and the 

 Bering Sea. However, little of the EEZ has been 

 mapped in detail, and coverage is very spotty. For 

 example, port areas appear to be well-surveyed, but 

 density of track lines decreases quickly with distance 

 from port. Oil companies have done the most grav- 



