Engineering Methods

Additional Information

Gravity

The gravity method is used by engineers and earth scientists while working n difficult settings where other, more rapid and economical methods cannot be employed.  It is an unobtrusive method capable of performing in environmentally sensitive areas such as inside buildings and parking garages.  Most sites requiring gravity surveys – often as the only resort - involve mapping voids, underground wash-outs, sink holes and other cavities.  Gravity has been used successfully when nothing else worked to find old UST’s that were abandoned prior to construction where building foundations had been placed on top of them. To   measure such small spatial differences in the gravitational pull of the earth, an extremely sensitive instrument – the Gravimeter – is required to gravitational anomalies to be less than one ten-millionth of the earth’s total gravitational field.   is used 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 with the computed models being presented as contour maps.

Common engineering and environmental applications include:

• locating subsurface caverns, tunnels, voids, sinkholes
• locating abandoned UST’s beneath buildings and foundations
• mapping bedrock topography beneath landfills
• profiling lateral lithologic changes
• detecting fault