154 WORZEL AND HARRISON [CHAP. 9 



survey. The surveys complement one another, each providing information 

 helpful in the interpretation of the others, and it is important to survey interest- 

 ing areas in as many ways as possible. In particular, gravity, magnetic and 

 topographic surveys can very conveniently be made concurrently. 



Two definite pieces of information can be learned from the anomaly field 

 ])roduced by a buried mass. It is possible to assign a maximum permissible 

 depth to the mass and to estimate the total mass excess or deficit. It is further 

 possible to calculate the exact distribution of mass, if this mass can be assumed 

 to be a surface distribution on a horizontal plane at known depth. Otherwise 

 the interpretive process consists of computing the attraction of trial models and 

 modifying, accepting or rejecting the model on the basis of the fit with the 

 observed anomalies. These interpretive methods are described in more detail in 

 many of the standard geophysical texts (see Garland, 1956). 



3. Gravity Observations and Geological Interpretations 



Early studies of the seas were hampered by a lack of data and a concentration 

 of the data over anomalous features. The important main conclusion was 

 drawn that the ocean basins were closely in isostatic equilibrium. The geo- 

 logical conclusion that the Mohorovicic discontinuity (crust-mantle interface) 

 lay at a depth of 10 to 15 km below sea-level (depending on the choice of 

 density differential between the crust and the mantle) was not drawn until 

 after the early measurements of seismic refraction work indicated the interface 

 at these depths (see Chapter 7). 



Worzel and Shurbet (1955) derived a "standard" section from the comparison 

 of six sea stations with seven land stations. Seismic, gravity, elevation and 

 water-depth data were available for each station. They concluded that a column 

 composed of 5 km of water of density 1.03, 1 km of sediment of average density 

 2.30, 4.5 km of crustal rocks of average density 2.84 and a mantle of average 

 density 3.27 beneath the crustal rocks was most representative of the average 

 condition in typical ocean basins ; a column composed of 33 km of crustal rocks 

 of average density 2.84 overlying a mantle of average density 3.27 was most 

 representative of the average conditions of continents. These two columns are 

 in isostatic equilibrium. 



The oceans cover about 70% of the surface of the Earth. There are, however, 

 many features in the oceans that cannot be called typically oceanic such as 

 continental margins, islands, seamounts, mid-ocean ridges, etc. Worzel and 

 Talwani (1959) studied the 4214 pendulum-gravity stations at sea to determine 

 how well the International Formula of gravity fitted the observations. Sea 

 observations in typically oceanic areas are especially good for such a study, as, 

 first, the observations are made directly on the geoid to be studied and, secondly, 

 there are no anomalous masses near the point of observation. 



Measurements within the area of inttuence of the features clearly not typically 

 oceanic were eliminated from consideration. The criteria for ehminating ob- 

 servations were determined from gravity sections across such features (see, 



