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GRAVIMETRY

METHOD

The gravity method is a passive, non-destructive geophysical technique involving the precise measurement of the Earth’s gravitational field at specific locations on the Earth’s surface. The careful processing and imaging of these measurements provides the detection of  gravity changes due to lateral variation in subsurface density. The method can be used to detect density variation on all scales; from microgravity surveying for natural or man-made voids to prospect scale surveying for ore deposits, to space-borne surveying to characterise the shape of the Earth’s core. Geo Alps specialises in ground based gravity surveying from microgravity (10s of m) to regional (100s of km) scales. The gravity technique is based on measuring localised variations in the Earth’s gravitational field which are caused by materials of different densities. The presence of an anomalously high (or low) density body in the subsurface causes a localised high (or low) anomaly in the measured gravitational field. The gravity effects described can be extremely small, however modern instrumentation and exhaustive data processing techniques enable detection of both geological and artificial structures or voids. The Gravity method  is used by Geo Alps while working in difficult settings where other more rapid and economical methods cannot be employed. It is a method capable of performing in environmentally sensitive areas such as inside buildings and parking garages. Most of the sites requiring gravity surveys, often as the only resort, involve mapping voids, underground wash-outs, sink holes and other cavities

PROCEDURE

To measure such small spatial differences in the gravitational pull of the earth is required  an extremely sensitive instrument: the Gravimeter. It is used to detect gravitational anomalies less than one ten-millionth of the total gravitational field of the earth. This instrument does not measure the absolute value of the pull of gravity  but measures spatial differences in the gravity pull – essentially relative gravity. If the gravity field over an air-filled void (i.e. cavern, tunnel or UST) is measured and the void is close enough to the ground surface then a decrease in the gravitational pull across the void will be measured. Anomalies , such as voids, and the distribution of subsurface materials are determined from gravity data using non linear least squares inversion of the data. The computed models are presented as contour maps.

APPLICATIONS

Typical targets for high resolution ‘microgravity’ surveys include (i) low density targets such as caves, fault zones, mine-workings and basements, and (ii) high density targets such as dykes, shallow rock, buried walls etc.. It is also possible to carry out regional gravity surveys targeting hydrocarbon traps or groundwater resources for local supply or geothermal energy. Gravity has been successfully used, when nothing else worked, to find old USTs before the placing of foundations on top of them.

  • Archaeology
  • Utility mapping
  • Structure inspection
  • Sedimentary cover
  • Geological structures
  • Cavity detection
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