GRAVITATIONAL METHODS
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 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.
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After the year 1938, the gravity-meter almost completely superseded the torsion balance and should be credited with practically all of the subsequent gravimetric discoveries.
The Eötvös Unit.-The gradient of gravity as measured by a torsion balance is defined as the rate of change of the force of gravity per centimeter, in a horizontal direction. It is measured in Eötvös units, (named for the Hungarian physicist, Roland von Eötvös) ; symbol E, 1 E = 1 • 10-° dynes per cm. Maximum gradient is implied in speaking of the gradient, or the dg/ds.
A gradient of gravity of 1 E indicates an increase in the intensity of gravity per centimeter (in a horizontal direction) in the amount of 1 • 10- dynes.
This is a rather small quantity. An Eötvös unit is about 1 millionth of a millionth (10-12) of the aver- age value of the force of gravity (~ 10+3 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 • 10-12 grams. This weight is in the order of the magnitude of the forces measured by the Eötvös torsion balance. Relation of Gravity Gradient to Subsurface Geology.-The gradient gravity, as was seen from the Ag”o curve for the anticline or granite + V.
G. Gabriel, “Probable Discovery Rates in the Gulf Coast Area,” Oil Weekly, Vol. 95, No.
GRAVITATIONAL METHODS
About 1 foot of the core is removed from the cylinder and put into a container, or on a small sheet of canvas.
The bottom of the hole is leveled, and small rocks removed. This material is added to the core, and the total material removed from the hole is weighed.
The volume of the hole is measured by putting into it a thin cloth sack of slightly larger diameter and of cylindrical shape.
The sack is filled to the level of the original ground surface with glass beads or small marbles (about ¼-inch diameter) or other constant-volume, light-weight material.
The volume of the material required to fill the sack is then measured, in cubic centimeters.
The density of the surface layer is calculated by dividing the weight (grams) by the volume (cubic centimeters).
At the risk of generalization it may be said that most sedimentary formations have been deposited in fairly large basins and their density remains substantially constant, if undisturbed, for considerable distances laterally.
This makes the assumption of uniform density of beds and the continuity of density contact surfaces, or surfaces of density contrast, valid to a useful degree, giving a basis for interpreting gravity results.
It is not implied that lateral changes in density may not give rise to gravity anomalies, for such are of record. However, they are the exception rather than the rule. In fact, if the density of the underlying materials is not reasonably uniform (as may be the case in glacial drift, which changes character very rapidly in short distances and in which boulders of various sizes may be present) the measured values of gravity gradient and curvature may be erratic and entirely useless.