10 



METHODS OF GEOPHYSICAL EXPLORATION 



[Chap. 2 



variations of gravity forces per unit horizontal distance, also known as 

 "gradients" of gravity are determined. Since the gravitational effects of 

 geologic bodies are proportional to the contrast in density between them 

 and their surroundings, gravity methods are particularly suitable for the 

 location of structures in stratiJGied formations. As there is generally an 

 increase of density with depth, the uplift of deeper formations will result 

 in placing formations of greater density in the same horizontal level as 

 lighter and younger formations. 



Pendulum methods. It is well known that a 

 pendulum may be used to determine not only time 

 but gravity as well. Gravity pendulums are kept 

 as constant as possible in length so that variations 

 in period indicate changes in gravity only. To 

 obtain the necessary accuracy, the pendulum period 

 must be determined to within 1/10,000,000 of a 

 second. By using an inverted pendulum (Lejay- 

 Holweck type, see Fig. 2-1), the sensitivity of the 

 period to gravity variations may be increased 1000 

 to 2000 times. 



The most common method for securing the 

 necessary accuracy in pendulum observations is 

 the "coincidence" or beat method whereby the 

 gravity pendulum is compared with a chronometer 

 or another pendulum of nearly equal period. If 

 the interval between successive coincidences of the 

 two time pieces is measured with an accuracy of 1 

 millisecond, gravity is determined with an accuracy 

 of 1 milligal.- Comparisons of field pendulums 

 with the reference time piece are usually made 

 by electrical wire or radio transmission. Correc- 

 tions are applied on observed periods for the 

 "rate" of the comparison time piece, air temperature and density, pendu- 

 lum amplitude and flexure of the support. 



Gravimeters. Pendulum, or "dynamic" methods of measuring gravity 

 have been superseded recently by "static" or "gravimeter" methods in 

 which gravity is compared with an elastic spring force. Mechanically 

 simplest are the Threlfall and Pollock instrument (in which a thin hori- 

 zontal quartz bar is suspended from a horizontal torsion wire), the Hartley 

 gravimeter (containing a horizontal, hinged beam suspended from two 

 helical springs), the Lindblad-Malmquist and the Askania gravimeters in 

 which the masses are suspended directly from a spring or springs, with 



Fig. 2-L Lejay-Hol- 

 weck pendulum (sche- 

 matic). P, Pin; Q, 

 quartz rod; E, elinvar 

 spring; D, diaphragm; 

 A, arresting device. 



* "Gal" (after Galileo), acceleration unit of 1 centimeter per second squared. 



