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at the U.S. Dept. of
Energy's Ames Laboratory, Ames, IA
Expedited Site Characterization: Technologies Used
MICROGRAVITY SURVEY
Non Technical Description
While the earth's gravitational force is often stated as an overall average, it
really varies from one spot to the next, depending on what lies below the
surface and other important factors. A microgravity instrument measures the
earth's gravity on a small scale. Set on a plate, the instrument is placed on
the surface and moved in straight lines across the sample area.
The instrument consists of very sensitive springs on which a weight is placed. By observing
the slight variation in spring compression as the instrument moves over the
surface, researchers can measure the specific gravity for each point location.
Gravity measurements reveal changes in the earth's gravitational field caused by
local changes in the density of the soil and rock. For instance, a change in
gravity would be detected if you moved from a subsurface region containing rock
to that with a void space. Microgravity measurements characterize detailed
localized geologic conditions (such as bedrock channels, caves, karst,
abandoned tunnels and mines). Data can be interpreted to:
- estimate depth size and the nature of an anomaly.
- evaluate geologic structure.
- estimate change in porosity.
- indicate material type.
- detect fractures and cavities.
How it was used in St. Louis, MO
The objective of microgravity survey work performed in St. Louis was to map
karst features in limestone, because solution cavities and channels are conduits
for groundwater flow and contaminant migration. Why Gravity? Because changes in
density can be determined from gravity surveys. For example, note these density
differences:
-
Massive limestone: 2.6 - 2.8 g/cm3
- Air Void: 0.0 g/cm3
- Water filled void: 1.0 g/cm3
- Debris filled void: 1.6 g/cm3
Technical Description
GRAVITY
Gravity measures the Earth's gravitational field = 980 x 103 milligals = 980 x
106 microgals
Gravity anomalies are local changes in the Earth's gravitational field
Objective: observe changes in gravity of 5 microgals.
Corrections to be applied:
- Drift and Tidal = 20 - 50 microgals
- Latitude Adjustment (40 degrees) = less than 1.5 microgals
- Elevation Adjustment (free air correction)= 310 microgal/m = 3/1 microgal/cm
- Accuracy of elevation measurement required: plus or minus 0.5 cm
- Bouguer Correction = 25 micro gals/m = 0.25 microgals/cm
GRAVITY ANOMALIES
-
Gravity anomalies depend on: Dimensions of cavities, Depth of top, Density
contrast.
CONTRACTOR: Northeast Research, Inc. - Farmington, CT
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Disclaimer
Last Modified: 1 January 2002 by dave eckels
Expedited Site Characterization: etd/technologies/projects/esc/technologies/microgravity.html