Citation

Omnes G. Transp. Res. Rec. 1976; 581: 42-51.

Copyright

(Copyright © 1976, Transportation Research Board, National Research Council, National Academy of Sciences USA, Publisher SAGE Publishing)

DOI

unavailable

PMID

unavailable

Abstract

Microgravity surveys may be defined as surveys in which the accuracy of measurements is better than 0.005 mgal (0.5 mu m/sq s) and the spacing between stations is less than 100 m. The development of microgravity as a tool for detecting cavities was favored because of local European conditions, particularly the numerous old underground quarries in and near the cities. A method was needed that could be used on roads still open to traffic, among or within buildings, and in gardens without causing any damage. Moreover, gravity measurements are not affected by the buried metal pipes that disturb resistivity measurements considerably. The productivity of microgravity crews was greatly increased by the introduction in 1968 of the Microgal gravity meter. Before the end of 1968, good standard gravity meters were used but stations had to be surveyed up to 3 times to obtain required accuracy. Significant anomalies of 0.020 mgal (0.2 mu m/sq s) or a little less with an accuracy better than 0.004 mgal (0.04 mu m/sq s) can now be studied. Microgravity has been successfully applied to the detection of serval types of cavities. The amplitude of the anomaly due to a cavity frequently has been observed to be more than twice the size of the anomaly calculated on the basis of the dimensions of the cavity. A microgravity survey gives significant results only if the behavior of the gravimeter is observed continuously, the leveling of stations is carried out with an accuracy of better than 1 cm, and all calculations and corrections are adapted to the type of problem. A microgravity test carried out at Golden Hill near Boulder, Colorado, on an adit dug in a granite hill showed that the 6 by 8-ft (1.8 by 2.4-m) adit was detected at a depth greater than 20 ft (6.1 m).