Chap. 2] METHODS OF GEOPHYSICAL EXPLORATION 11 



arrangements for electrical or similar means of magnifying the displace- 

 ment. An increase in mechanical sensitivity may be attained by providing 

 "astatizing" mechanisms which involve the application of a labilizing force 

 nearly equal and opposite to the elastic restoring force. Examples are 

 the Ising gravimeter, in which a vertical quartz rod is suspended in in- 

 verted position from a taut horizontal quartz fiber, gravimeters using 

 bifilar and trifilar suspensions, the Truman-Humble gravimeters and the 

 Thyssen gravimeter (Figs. 2-2 and 2-3), astatized by a rigidly attached 

 inverted pendulum. 



Corrections on gravity values observed with pendulums and gravimeters. 

 The following corrections must be applied on relative gravity values: (1) 

 a correction for normal variations of gravity (planetary effect); (2) ter- 

 rain correction; (3) free-air and Bouguer (elevation) correction. The 

 planetary correction is due to the variation of 

 gravity with latitude. The effect of terrain is 

 calculated from elevations along radial lines and 

 concentric circles around the station. Elevation 

 is allowed for by a reduction to sea level (free- 

 air correction) to which the influence of the rocks 

 between station and sea level is added (Bouguer 

 reduction). 



Interpretation of gravity anomalies. Gravity 

 anomalies may be represented by contours (iso- 

 gams) or profiles in connection with geologic sec- 

 tions. Their interpretation is largely qualitative , ^^j x ' ' '^jTV^^ ^ u 

 '^ . lowered to ground through 



and is given in terms of structural highs and lows floor of passenger car 

 or presence or absence of heavier or lighter bodies (Thyssen). 

 (see Fig. 2-3). 



If some information is available about the subsurface section and di- 

 mensions and nature of geologic bodies to be expected, more quantitative 

 interpretation methods may be applied by calculating their attraction 

 and by varying the assumptions regarding dimensions, shape, differences 

 in density, and depth until a reasonable agreement between field curves 

 and theoretical curves is obtained. This method of interpretation is of a 

 trial and error nature and generally referred to as indirect interpretation. 



The Eotvos torsion balance. Contrarily to the beam in an ordinary 

 balance, the beam in a torsion balance revolves in a horizontal plane and 

 is deflected from a position corresponding to the torsionless condition of 

 the suspension wire by the unbalance of horizontal forces acting on it. 

 For the sake of illustration, two types of torsion balance beams may be 

 distinguished. In the beam of the first type (see Fig. 2-4a) two masses 

 are at the ends of the beam and at the same level. The beam is deflected 

 by forces resulting from horizontal differences of the horizontal compo- 



