GRAVITATIONAL METHODS 371 



considered for precise work, as it may in unseasoned common steel springs 

 cause an increase in length of twenty parts in ten million per hour. Elastic 

 lag inhibits the rapid return of a spring to its original length after a dis- 

 placement. The gradual return of a spring to its original condition de- 

 pends on the amount of displacement and the length of time it has been 

 displaced. Elastic lag, if present to an appreciable extent, impairs the ac- 

 curacy because its effects are unpredictable. 



The limitations of the gravimeter are due almost exclusively to the 

 limitations of available spring materials. In addition to the preceding 

 problems there are numerous secondary ones. The calibration of the in- 

 strument must not be affected by jars or vibration such as encountered in 

 average field operations. Also, the damping of the elastic system should 

 be as nearly aperiodic as possible, consistent with sensitivity. Unless this 

 is done the system will be affected unduly by disturbances, such as might 

 be caused by a passing automobile which sets the system into oscillations 

 that persist for an extended period of time. Under these conditions, obser- 

 vation would be impossible if disturbances are frequent. Average vibra- 

 tion of the ground usually will not cause appreciable unrest of a properly 

 designed elastic system ; however, near the ocean, the pounding of the 

 waves does produce a quite noticeable unrest of the elastic system. 



Classifications of Gravimeters 



The gravimeters used in geophysical prospecting may be classified into 

 two types : static and astatic instruments. The simplest type of static grav- 

 imeter comprises a heavy mass M rigidly attached to an elastic spring. 

 Some form of amplification system is employed, often optical, whereby 

 slight displacements of the mass may be measured. The operative tech- 

 nique may be summarized briefly as follows. The equilibrium extension, 

 or compression, of the spring supporting the weight is determined at a 

 base station, and the reading recorded. The meter is then moved to one 

 of the field stations and the equilibrium extension, or compression, at this 

 station is recorded. A comparison of the two readings, after making the 

 necessary corrections, gives the relative gravity of the field station as com- 

 pared to the base station. 



Astatic gravimeters employ an auxiliary restoring force opposite in 

 sign and approximately equal in magnitude to the elastic restoring force. 

 In general, therefore, the mass M or bob of an astatic gravimeter is sub- 

 ject to three reactions: (1) the pull of gravity, (2) the elastic restoring 

 force of a spring, and (3) an astatizing or labilizing force which acts in 

 the same direction as gravity and has a magnitude nearly, but not quite, 

 the same as the elastic restoring force of the spring. 



For small displacements, the force due to gravity tending to displace 

 the mass is A(g — go) where g is the gravity value at the point of reading 

 and go is the value at a base station ; the elastic force is —Bx, where x is the 

 displacement of the mass from the position occupied at the base station ; and 



