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Instruments�\�Gravity�\�Gravity Meters�\�
Name: |
Torsion Balance |
Date: |
1902 |
Item Number: |
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Donor: |
Norm Domenico |
Type: |
N/A |
� |
Manufacturer:
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N/A |
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Description:
This instrument, called a torsion balance, measures the earth's natural
gravity field. Constructed in 1902 in Germany, it is the predecessor of
torsion balances used extensively in the early stages of petroleum exploration
in the 1920's and 30's.
What does it measure?
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 (a thick accumulation of rock layers), 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.
How was it used?
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. Since only two or three instruments
were available, 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.
How does it work?
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.
(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.
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