How can marble cavities and karstic features be detected in ground investigation?

Marble is metamorphic rock derived from limestone and is dissolves in slightly acidic water to form cavities (partly filled with debris). It poses great problem for construction of tall buildings which requires the seating of firm foundation.

One of the way to identify marble cavities and karstic features is to employ a combination of rotary drilling and micro-gravity method. Micro-gravity method involves the measurement of minute variations in gravitational pull of the Earth and interpretation of the presence of cavities from them. The principle of the technique is to locate areas of contrasting density in the sub-surface. As a cavity represents a lower density when compared with its surrounding soils, the subsequent small reduction in the pull of the Earth’s gravity is observed over the cavity.

This question is taken from book named – A Self Learning Manual – Mastering Different Fields of Civil Engineering Works (VC-Q-A-Method) by Vincent T. H. CHU.