developing new technology for your exploration activities...
Gravity Meter (often called a “gravimeter”)
Measurements of minute ( 1:200,000,000 ) changes in the gravitational field over a region can indicate the location of:
-
hydrocarbon deposits, such as oil and gas
-
minerals
-
underground tunnels
-
new tunnels, through the use of differential gravity measurements
-
collapsed mines and tunnels, refilled by earth (as shown below)
The FG5 absolute gravimeter and the CG-5 AutoGRAV gravimeter
The heart of the gravimeter mechanical unit operates within a hard vacuum. A corner-cube reflector drops in free-fall a distance of 20 cm, its position detected both by an Iodine stabilized HeNe gas laser (wavelength = 632.9913982 nm) in an interferometer configuration and by a photocell that accelerates an elevator ahead of its downward path, removing stray remaining molecules from interfering with its descent. After its 200 millisecond journey the elevator catches the reflector and transports it back to its starting point, from where it is ready for another drop. The whole cycle has a duration of roughly 1 second. Electronic circuitry sequences the move, timing the zero-crossings of the 631,920 interference fringes using a precision Cesium Cs-133 atomic clock with a resonant frequency of 9,192,631,770 Hz (the International standard definition of one second) to sub-nanosecond accuracy. The absolute value of gravity is then calculated from a least mean squared fitting (including correction terms) to the precisely timed zero-crossings.
The uncertainty in the wavelength of a standards grade HeNe laser stabilised on the i (a13) component of the 11-5 R(127) hyperfine transition of the 127I2 Iodine molecule is 2.1E-11. The error in the precision Cesium atomic clock is around 1E-12.
www.colorado.edu/YeLabs/PDFfiles/YoonApplPhysB2001.pdf
- Accuracy: 2uGal (observed agreement between FG5 instruments)
- Precision: 15uGal/sqrt(Hz) at a quiet site:
- about 1.0uGal after 225 cycles (elapsed time = 225 seconds = 3.75 minutes)
- about 0.1uGal after 22,500 cycles (elapsed time = 22,500 seconds = 6.25 hours)
Note that the gravitational field strength decreases by about 3uGal with a 10 mm increase in height.
- from www.lacosteromberg.com/images/fg5schem.jpg
|
Example of an absolute gravity meter: the Scintrex CG-5 |
The Scintrex CG-5 from www.scintrexltd.com has a resolution of 1 uGal, and a standard deviation of less than 5 uGal. The “Gal” unit is named after “Galileo”, and equals 1 cm/s2.
The gravitational constant = g = 980.665 Gal.
The accuracy of the FG-5 shown on the topof the opposite page is 2.1 uGal, equivalent to 6 mm change in altitude. Sander Geophysics Limited report a resolution for their airborne AIRGrav system from 150 … 300 uGal. To put these figures in context, the earth’s gravitational field strength is 980,600,000 uGal. The major challenge is to have an extremely low vibration level in the aircraft to enable accurate microgravity measurements. An alternative approach is to measure the gradient in the Earth’s gravitational field using a completely different and expensive instrument, called a “gradiometer”.
|
Ability to infer sub-surface structure from accurate gravity measurements |
Microgravity data (A) and model (B) showing anomalous area. Areas of low gravity anomalies (100 to 175 µGals) were identified in five of the survey lines. The anomalous areas are centred within the collapse area and are aligned with the strike of the mine. An example of the data along one of the profile lines, is shown above.
- from www.fhwa.dot.gov/mine/ky0316.htm (link no longer exists)
|
The Superconducting gravity meter |
© Barnard Microsystems Limited 2006 - 2008