Torsion-Balance Principles as Applied by the Original Eotvos Torsion Balance

Abstract

With the original type of torsion balance three to four stations a day can be taken with a crew of three men and two light trucks with a large instrument, or two men and one truck with the new smaller instrument. The distance between stations depends upon the type of the survey and the conditions of terrain and structure. The observed gradient must be corrected for terrain effects. The principle of interpretation of torsion-balance results is that the gravity gradient is toward the heavier mass and is the greater the heavier the mass, the nearer the heavy mass is to the surface, and the steeper its flank. In the case of a dome such as a salt dome, heavier than the country rock, the gradient is toward the center of the dome, is zero on top of the dome, and is opposite in d rection on opposite sides of the dome. If the dome is lighter than the country rock, the gradient is away from the dome. The gradient existing above such a dome is illustrated by the results of the surveys of two salt domes in the Gulf Coast. In the case of an anticline the gradient is toward the axis. In the case of a fault, the gradient reverses across the fault, but differs in intensity on the up- and down-throw sides of the fault. The gradient relations above a series of hypothetical faults and folds are illustrated by the results of a torsion-balance survey in Texas. The irregular gradient conditions caused by irregular subsurface structure are illustrated by a torsion-balance survey in the vicinity of the Kellum lease in the Powell field, Texas.