248 



EXPLORATION GEOPHYSICS 



Comparisons of Gravity and Magnetic Methods (Continued) 



Item 

 Basic variation (Cont'd) 



Absolute determinations 



Underlying physical 

 property of rocks and 

 formations 



The force itself 



Effects of terrain 

 and/or topography 



Cost of equipment 

 Personnel requirements 



Speed of field work 



Applicability 



Gravity 

 Change in north-south direc- 

 tion only is of significance 



Difficult to measure total 

 gravity with high accuracy 



Density. Very easy to meas- 

 ure, direct correlation with 

 mineralogical and geological 

 character 



Attraction always. There is 

 no repulsion, negative grav- 

 ity or polarity 



Important critically and 

 affects accuracy in torsion 

 balance work. Of less im- 

 portance in pendulum and 

 gravimeter operations. 



In general rather costly 



Requires field crew of sev- 

 eral men 



Slow in pendulum and tor- 

 sion balance ; a few stations 

 per day, due to nature of the 

 measurements. Rapid in 

 gravimeter surveys, 20 to 30 

 stations per day 



In general limited to geologic 

 studies for petroleum explo- 

 ration. Some mining appli- 

 cations 



Magnetic 

 Change in north-south direc- 

 tion and in east-west direc- 

 tion must be considered 



Comparatively simple to 

 measure the magnetic ele- 

 ments 



Susceptibility. and permeabil- 

 ity not readily determinable. 

 Subtle relations with geo- 

 logic conditions 



Attraction or repulsion. Po- 

 larity of subsurface bodies 

 and magnets must be con- 

 sidered 



Not too important, but diffi- 

 cult to evaluate 



Cost moderate 



One man can operate efifec- 

 tively, two men efficiently 



Rapid ; high output per unit 

 per field day, averaging 25 to 

 40 stations per day 



Applicable to both petroleum 

 and mining problems 



Fundamental Principles and Phenomena 



The Force of Gravity. — Gravity may be defined as the mutual attract- 

 ive force betvv^een masses tending to draw them tovv^ard each other. The 

 word "gravity" comes from the Latin ''gravis/' meaning heavy. The law 

 of universal gravitation, first clearly stated by Sir Isaac Newton, is 

 expressed by the following familiar equation : 



F = -G 



mi W2 



(1) 



F = the force of attraction in dynes between two masses, lUi and 111^, being 

 proportional to their product and inversely proportional to the square of 

 their distance apart (r) in cm. multiplied by a constant G. The minus sign 

 indicates that the direction of F is back toward mi along r. When lUi = Wg 

 = r = 1, |F| = G. The best value for the constant of gravitationf, is G = 



t P. R. Heyl, "A Redetermination of the Constant of Gravitation," Bureau of Standards 

 Journal of Research, Vol. 5, Dec, 1930. 



