Subsurface Methods as Applied in Geophysics 1067 



Tidal Effects — Corrections for the attraction of the celestial bodies 

 are generally not applied to gravity data, unless unusual accuracy is 

 required or special field methods demand such corrections. The maximum 

 amplitude of tidal effects is about 0.3 mg., and the change occurs with a 

 period of about 24 hours. Normal operational procedure, in which the 

 gravity meter returns to a check station every two hours, automatically 

 compensates for these effects as instrumental "drift." 



Gravity Instruments 



The history of gravity prospecting in the United States records the 

 use of the torsion balance, the pendulum, and the gravity meter. Al- 

 though the gravity meter does not have the resolution powers of the 

 torsion balance, its greater speed, portability, and consequent economy 

 of operation have offset the advantages offered by the torsion balance. 

 Gravity explorations conducted in rugged, mountainous terrain in the 

 past several years with the gravity meter would have been almost impos- 

 sible with the torsion balance because of the extreme sensitivity of the 

 balance to the disturbing influence of terrain irregularities and near-sur- 

 face density variations. Mott-Smith ^- has compared the influence of mass 

 irregularities on the torsion balance and gravity meter and concluded 

 that shallow density variations would make torsion-balance data practic- 

 ally worthless. The discussion of gravity methods is, therefore, more or 

 less restricted to the gravity meter. As can be seen from figure 546, there 

 are about 105 gravity-meter crews in operation in this country today. 



A gravity meter is essentially an accurate weighing device in which 

 the gravitational force on a mass is offset by the tension in a spring. 

 Gravity meters may be either direct-reading devices, in which the final 

 resting place of the mass is observed (Mott-Smith meter), or they may 

 be null-type instruments, in which the mass is returned to a given posi- 

 tion on the scale (LaCoste-Romberg meter) . In null-type instruments 

 the restoring force required to bring the mass to its zero position is read 

 from an appropriate indicator and converted to gravity units. 



The extreme sensitivity of the gravity meter may be appreciated 

 from the following illustration. If a scale could be built that would 

 accommodate 3,000 tons of coal (corresponding to the force of gravity on 

 the earth) with a sensitivity such that the addition or subtraction of one 

 ounce of coal would be measurable (corresponding to about 0.01 milli- 

 gal) , we should have a scale of comparable accuracy to the modern gravity 

 meter. In order to attain such a high degree of accuracy, instruments 

 must be carefully constructed of materials with the lowest temperature 

 coefi&cients and further protected from thermal variations by enclosure 

 in a thermostatically controlled "oven." 



Modern instruments are light in weight (25-45 pounds) and readily 

 portable. Manufacturers are now offering portable meters enclosed in 



^Mott-Smith, L. M., Gravitational Surveying with the Gravity Meter: Geophysics, vol. 2, no. 1, pp. 

 30-32, Jan. 1937. 



