GRAVITATIONAL METHODS 285 



some base station where the value of gravity is known, such as at Washington, D.C. 

 This assumes that no change has taken place in the pendulum between readings at the 

 different stations. 



TORSION BALANCE METHOD OF 

 GRAVITY PROSPECTING 



The torsion balance measures the gravity gradient in a horizontal 

 plane and the curvature quantity or horizontal directive tendency 

 (H.D.T.) directly. 



This instrument was introduced into the United States in 1922 and 

 from that time until about 1940 was very widely used in reconnaissance 

 and detailed gravimetric work in the American oil industry. At the peaks 

 of its popularity, from 1928 to 1930 and from 1934 to 1936 inclusive, not 

 less than 125 instruments were engaged in exploration for oil each year. 



Partial or total credit is given to the torsion balance for the discovery 

 in the Gulf Coast area of seventy-nine oil fields and salt domes up to the 

 beginning of 1938. t The addition in oil reserves which should be credited 

 entirely to the torsion balance probably amounts to 1,027,500,000 barrels. 

 (The pendulum and gravimeter should be given credit in the same area for 

 the discovery of 108,000,000 barrels of oil, during that same period.) After 

 the year 1938, the gravity-meter almost completely superseded the torsion 

 balance and should be credited with practically all of the subsequent gravi- 

 metric discoveries. 



The Eotvos Unit. — The gradient of gravity as measured by a tor- 

 sion balance is defined as tJw rate of change of the force of gravity per 

 centimeter, in a horizontal direction. It is measured in Eotvos units, (named 

 for the Hungarian physicist, Roland von Eotvos) ; symbol E, \ E = \ • 

 10~^ dynes per cm. Maximum gradient is implied in speaking of the 

 gradient, or the dg/ds of Figure 131. A gradient of gravity of 1 £ indicates 

 an increase in the intensity of gravity per centimeter (in a horizontal direc- 

 tion) in the amount of 1 • 10~^ dynes. This is a rather small quantity. 



An Eotvos unit is about 1 millionth of a millionth (10~^^) of the aver- 

 age value of the force of gravity ('— ^ 10+^ dynes). Such a small quantity 

 is difficult to comprehend. As an example, it has been calculated that if a 

 piece of wire weighing 1 gram were stretched out to encircle the earth 25 

 times and 1 centimeter of the stretched wire were cut, the segment would 

 weigh 1 • 10"^- grams. This weight is in the order of the magnitude of 

 the forces measured by the Eotvos torsion balance. 



Relation of Gravity Gradient to Subsurface Geology. — The gradient 

 of gravity, as was seen from the i^g"o curve for the anticline or granite 



t V. G. Gabriel, "Probable Discovery Rates in the Gulf Coast Area," Oil Weekly, Vol. 96, No. 

 1, Sept. 11, 1939. 



