Torsion Balance Eotvos Instrument 2

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Item Torsion Balance Eotvos Instrument
Item code
Manufacturer Oertling
Donor Geophysical Resource Center
Location Taos
Serial number 212
Taxonomy GSH498

The torsion balance derives its name from the two parallel weighted beams, each suspended by a wire at its center, that sense variations in the earth's gravity. A weight is attached to one end of a beam and another identical weight is suspended below the other end. The positions of the weights on the other beam are reversed. The suspended weights are contained in two lower cylinders. The difference in the gravitational force on these weights creates a torque causing each beam to rotate. The amount of torque that is created give information about the local value of gravity. To obtain the gravity information, from the instrument readings made at each of the three orientations of the beams requires the solution of five complicated equations. Results give the horizontal gravity gradient (how the value of gravity changes as one moves laterally across the surface) and the change in curvature of the gravity equipotential surface (the surface along which gravity is constant; mean sea level is an equipotential surface). The accuracy is to one-half billionth of the total gravity value. In the late 1930's, 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 changes 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. This instrument, measures the earth's natural gravity field. Constructed in 1902 in Germany, the predecessor of torsion balances used extensively in the early stages of petroleum exploration in 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 differences is that between rock salt and adjoining sedimentary rock layers. In many sedimentary basins salt has penetrated the rock layers to form salt domes. Porous rock layers that are pierced and bent upward by the rising salt are excellent traps for petroleum and natural gas. The torsion balance proved very effective in locating salt domes and the wells drilled off the flanks of the domes often proved productive of oil and gas. Until its demise in 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. A total of four hours were required at each measurement location or station! In reconnaissance surveys, the spacing between stations was from one-quarter to one-half mile. several days were required for the completion of a survey covering just a few square miles. Another disadvantage was the extreme sensitivity of the measurements to surrounding terrain which made use of the instrument in rugged areas impractical.