Instruments \ Gravity \ 

Description:
The instrument before you, called a torsion balance, measures the earth's natural gravity field. Constructed in 1902 in Germany, it is the progenitor of torsion balances used extensively in the early stages of petroleum exploration during the 1920's and 30's. The earth's gravity field is distorted by variations in the density of subsurface rock layers. One of the largest density difference, and, thus, one most easily detected by gravity-measuring instruments, is that between salt and the adjoining sedimentary rock layers. In many sedimentary basins (that is, an accumulation of rock layers) salt has penetrated the rock layers to form salt domes. Porous rock layers pierced and bent upward by the rising salt are excellent traps for fluids I the rock layers which are halted in their natural upward movement by the adjoining nonporous salt. As would be expected, the torsion balance proved very effective in locating salt domes. Subsequent wells drilled off the flanks of the domes often proved productive of oil and gas accompanied by prodigious amounts of water.

Up to its demise in about 1938, the torsion balance contributed exclusively or partially to the discovery of 79 oil fields in the Gulf coastal region of Texas and Louisiana where it was principally applied in oil exploration. It is estimated that these fields contained petroleum reserves of approximately one billion barrels. Unfortunately, torsion balance field surveys were laborious and expensive. The instrument had to be housed in an insulated portable hut, to reduce temperature variations, and mounted on an aluminum baseplate for stability. Readings were required at three orientations of the instrument, at 120 degree intervals, with one orientation repeated. After each rotation one hour was required for the beams to stabilize before readings could be made. Thus, a total of four hours were required at each station. In reconnaissance surveys the spacing between stations was from one-quarter to one-half mile. Since only two or three instruments were available, considerable time was required for the completion of a survey covering several square miles. Another disadvantage was the extreme sensitivity of the measurements to surrounding terrain which made use of the instrument in rugged areas impractical.

The torsion balance derives its name from the gravity measuring element which consists of two parallel weighted beams, each suspended by a wire at its center. A weight is attached to one end of a beam and another identical weight is suspended below the other end. Position of weights on the other beam are reversed. The suspended weights are contained in the two lower cylinders. The difference in the gravitational force on these weights creates a torque causing each beam to rotate. Derivation of pertinent gravity values from readings made at each of three orientations of the beams requires solution of five complicated equations. Results give the horizontal gravity gradient (that is, the horizontal rate of change of the gravitational force) and the change in curvature of the gravity equipotential surface (that is, the surface along which the total gravitational force is constant). The accuracy is one-half billionth of the total gravity value.

The torsion balance was replaced by a much simpler gravity-measuring device called a gravimeter. Its measuring element consists simply of a weight suspended from a spring. Variation in the vertical component of the gravitational force is determined by measuring the consequential variation in the length of the spring. Whereas, only four or five stations could be occupied daily by the torsion balance, 50 or more stations could be occupied by the gravimeter. Several gravimeters of different designs are exhibited in the museum.

For more information, see the article:

The SEG Museum's torsion balance, The Leading Edge, June, 1994, p. 683-686.