Subterranean gravity and other deep hole geophysics

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Abstract
The early history of the determination of the Newtonian gravitational constant, G, was closely linked with the developments of geodesy and gravity surveying. The current search for non‐Newtonian effects that may provide an experimental guide to unification theories has led to our retracing some of this history. Modern geophysical techniques and facilities, using especially mines and deep ocean probes, permit absolute measurements of G for distance scales up to a few kilometers. Although the accuracy of the very long range determinations cannot equal that of the best laboratory measurements, they are crucial to assessment of the possibility of a scale dependence of G. Preliminary data give values of G on a scale 100–1000 m biased about 1% higher than the laboratory value. Possibilities of systematic ierror compel us to question this apparently significant bias but it provides the incentive for better controlled large scale experiments. Several are in progress or under development. A particular difficulty concerns the measurement of in situ density. Even for hard rock, release from overburden pressure causes microcracks and pores to open. Natural pore closure is effective only with deep burial and for this reason there are advantages in deep instrument placement for several geophysical studies.

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