Subsurface Methods as Applied in Geophysics 1061 



The constant y is independent of the composition of the masses in- 

 volved and has a numerical value depending on the units used. If the mass 

 is given in grams, the distance in centimeters, and the force in dynes, the 

 constant y is equal to 66.67X10'^ c.g.s. units. 



The familiar force known as the "weight of an object" is due to the 

 gravitational attraction of the earth on the object in question. Gravity, a 

 force directed approximately toward the center of the earth, may be quan- 

 titatively expressed as a force per unit mass with units of dynes per gram, 

 or the equivalent acceleration may be used with units of centimeters per 

 second per second. The approximate value of gravity so expressed is 980 

 dynes per gram or 980 centimeters per second per second. Geophysicists 

 commonly speak of gravitational acceleration in terms of gals, named 

 after the famous renaissance scientist Galileo. The approximate value of 

 gravity may thus also be expressed as 980 gals. 



The force of gravity is everywhere present over the surface of the 

 earth, its intensity at any one position being affected not only by matter 

 located in the vicinity, but by all mass in the earth, on the earth, and in 

 the solar system surrounding the earth. In gravity surveys, depending 

 upon the nature of the survey, corrections may be made for irregularities 

 in the distribution of matter beneath the surface of the earth (isostatic 

 correction) and on the surface of the earth (cartographic or terrain cor- 

 rection), as well as for the relative positions of the sun and the moon 

 (tidal correction) . The corrections may be applied in addition to the 

 corrections for the normal variations of gravity with latitude, elevation, 

 and surface density, which are always required. 



Gravity exploration depends on the existence of density contrasts 

 between geologic bodies and the surrounding materials in a horizontal 

 direction. Density contrasts, of course, exist between individual geologic 

 horizons as we progress toward the center of the earth. It is only when 

 a denser or lighter material is uplifted or intruded into the normal coun- 

 try rock that an anomaly is observed. Structures involving rocks of the 

 same density will not produce a gravity anomaly. (See fig. 559.) 



The force of gravity is defined as the rate of change of the gravita- 

 tional potential in the vertical direction. Gravity, being a vector quantity, 

 has both magnitude and direction. If it were not for surface and near- 

 surface variations in the distribution of the masses that go to make up 

 the outer shell of our earth, the gravity vector would be directed toward 

 the center of the earth, assuming the earth to be a perfect sphere. Inhomo- 

 geneities due to either topography or variations in mass distribution be- 

 neath the surface cause the gravity vector to be deflected in the direction 

 of the excess mass. (See fig. 560.) For example, near an igneous plug 

 intruded into sedimentary rocks we should expect the gravity vector to 

 be deflected toward the denser material. The magnitude of the vector 

 increases as we approach the plug. The total force of gravity at a point 

 near such an inhomogeneity would be the vectorial sum of the normal 



