Chap. 12] MISCELLANEOUS GEOPHYSICAL METHODS 929 



surface subsidence. Surface subsidence occurs, first, as a natural process 

 in consequence of the leaching of salt beds, domes and other rocks or 

 formations which may be removed by the action of subsurface waters. 

 More frequently ground subsidence is encountered above underground 

 workings, particularly coal mines; sometimes it results from the excava- 

 tion of subway tunnels in cities; it has been observed in oil fields as a 

 consequence of the removal of oil, gas, salt water, and sand from wells. 

 An example is the Goose Creek field in Texas in which the ground subsi- 

 dence over the center was as much as 3.25 feet in 8 years.'^ Subsidence 

 may be due also to the removal of artesian water from large basins and the 

 pumping dry of sands and sandy clays in the process of subway or building 

 excavation. It may, finally, result from the sanding up and accumulation 

 of sediments in large reservoirs, although the latter is a slow process requir- 

 ing most delicate means of observation. 



Next in accuracy to the simple levehng devices discussed above are 

 mechanical gauges. They come equipped with more or less intricate lever 

 arrangements and with dials for reading the displacement, or with a pen 

 or stylus for continuous recording. The latter are useful in the surveillance 

 of active faults, shear zones and fissures, mine roof subsidence, and tunnel 

 movements. For it is only by the continuous and systematic study of the 

 time variation of such displacements that we can hope to predict roof and 

 wall failure underground and possibly the occurrence of tectonic earth- 

 quakes. An instructive example has been published by Landsberg^^ show- 

 ing that the rate of roof subsidence changed in a definite manner (ap- 

 proximately in inverse proportion to the distance of the pillar retreat line) 

 until a cave-in occurred. 



For attaining the ultimate objective, that is, the ability to predict the 

 time at which a roof or wall is likely to fail or a fault is likely to slip, it 

 would be necessary to study not only the time variation of the relative 

 displacements along mine walls or faults, but to record simultaneously the 

 variation of as many other factors as may be suspected of accumulating 

 tensions and contributing to such failures. In addition to processes under 

 human control, such as the removal of rock, and shocks produced by blast- 

 ing, the following phenomena should be observed continually: natural 

 earthquakes, rock bursts, variations in barometric pressure, variations in 

 moisture, and possibly the bodily tides produced by sun and moon (see 

 page 164). Most of these phenomena may be recorded by a single instru- 

 ment and may possibly be combined with the displacement record by using 

 a gauge that is sensitive to both displacement and vibration. It should be 

 added that rock bursts and roof and wall displacements may be mutually 



92 W. T. Thorn, A.I.M.E. Tech. Publ. No. 17, 9 pp. (Sept., 1927). 

 "A.I.M.E. Tech. Publ. No. 685, 5 (Feb., 1936). 



