934 MISCELLANEOUS GEOPHYSICAL METHODS [Chap. 12 



able information on strain distribution in structures can be obtained by 

 photoelastic studies. However, the fact that only relatively thin sections 

 can be used limits the validity of the conclusions to a single plane. If 

 actual conditions are to be duplicated in three dimensions, it is necessary 

 to resort to reduced scale models and test them with miniature strain 

 gauges placed at suitable points. Attention must be given to model scale 

 factors, and the elastic properties of the model material must be scaled 

 down in keeping with the requisite dimensional relations. 



In concluding this section on strain gauging, it should be pointed out 

 that a strain gauge will indicate merely the variation of stress or strain 

 with time and not the absolute stress that may be present in the member 

 to be tested. To a certain extent this difficulty may be circumvented by 

 imbedding strain gauges uito a structure in the process of construction, or 

 by relieving the stress at suitable points after the installation of the gauges. 

 There is a possibility of obtaining absolute pressures in rocks (in situ) by 

 measuring velocities of elastic waves. As was shown in Chapter 9 (see 

 page 474), the elastic modulus and therefore the elastic wave speed of 

 porous rocks change with pressure. The variation is not linear; it is fairly 

 large for small pressures but decreases for larger pressures as a limiting 

 value is approached. It is assumed that the* pores are first closed up by 

 the lower pressures, after which the stress begms to work on the mineral 

 grains themselves. Hence, the variation of elastic modulus with pressure 

 is different for every type of rock and is dependent on porosity, moisture, 

 crystalline structure, and anisotropy. The variation of elastic wave speed 

 with pressure for various types of rocks has been determined recently by 

 L. Obert.^"^ At reasonably shallow depths, where the pressure is not so 

 great that the fiat part of the pressure-velocity curve is approached, 

 stresses in underground workings may therefore be determinable by seismi c 

 velocity observations, 



IV. ACOUSTIC METHODS 



Acoustic methods are included here in the discussion of geophysical 

 exploration since, by definition, geophysics is concerned with the three 

 acoustic transmission media: the earth, the water, and the atmosphere. 

 Although we are inclined to associate the transmission of sound with the 

 latter only, sound passes with equal and often greater ease through the 

 media of water and solid ground. Transmission of infra-acoustic frequency 

 earthquake waves of natural or artificial origin is usually referred to as 

 seismic wave propagation, whereas sound transmission through air and 

 water, ranging in frequency from single impulses to supersonics, as well 



i«» U. S. Bur. Mines Rep. of Invest. No. 3444, April, 1939. 



