7 



GRAVITATIONAL METHODS 



I. INTRODUCTION 



Gravitational exploration falls in the category of ''direct" geo- 

 physical procedures by which physical forces are measured at the earth's 

 surface without application of an artificial extraneous field. The field of 

 gravitation is present everywhere and at all times; it is due to the funda- 

 mental property of all matter to have mass. Since all masses, regardless 

 of size, exert an attraction upon one another, any method designed to 

 measure the gravitational field will invariably determine the influence of 

 all masses within range and, therefore, lack the depth control possessed 

 by the seismic and the electrical methods. Consequently, direct methods 

 of interpretation (that is, determinations of the depths, dimensions, and 

 physical properties of geologic bodies from surface indications) are rare 

 and applicable only where essentially one single mass produces the gravi- 

 tational anomaly. 



The application of gravitational exploration methods is dependent on 

 the existence of differences in density between geologic bodies and their 

 surroundings. Because of the vertical differentiation of the earth's crust 

 in regard to density (due to the general increase of density with depth 

 and the effect of structural movements which have uplifted deeper and 

 denser portions and have placed them in the same level as younger and 

 lighter formations) there occur changes of density in horizontal direction 

 which are essential for the successful application of gravitational methods. 

 For the interpretation of gravity data, it is fortunate that densities remain 

 constant for considerable distances in formations which had their origin 

 in large depositional basins. 



Although gravitational exploration is concerned with one field of force 

 only, a number of characteristic parameters exist which lend themselves 

 readily to accurate observation. The magnitude of the gravity vector is 

 determined by measuring the oscillation period of a pendulum or the 

 deflection of a mass suspended from a spring (gravimeter). It is not 

 feasible to measure the absolute direction of the gravity vector in space. 



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