36 METHODS OF GEOPHYSICAL EXPLORATION (Chap. 2 



detector which may be of the hot filament type or which may indicate 

 the amount of carbon dioxide hberated on combustion of the hydro- 

 carbons contained in the air. These detectors do not differentiate be- 

 tween methane and the heavier hydrocarbons. In soil analysis methods, 

 on the other hand, the soil samples are degassed and the gases passed 

 through an analytical apparatus, where their constituents are separated 

 by low-temperature distillation and combustion into methane, ethane, 

 and the heavier hydrocarbons. Soil samples may also be extracted by 

 suitable solvents, and their content in socalled pseudohydrocarbons (soil 

 waxes, and the like) may be determined. Several of the known oil fields 

 have been found to be characterized by a corona or halo pattern of the 

 significant heavy hydrocarbons. The pseudohydrocarbons and methane 

 appear to follow a similar, though less regular, trend, methane being 

 possibly least significant, since its distribution may be affected by near- 

 surface decomposition processes. 



Dynamic ground tests, strain gauging, vibration tests. Building sites 

 may be tested by setting them into forced vibration and measuring their 

 amplitudes at any desired points by vibration recorders as functions of 

 frequency, thereby determining their natural frequency and damping. 

 Experience appears to indicate that the former frequently coincide with 

 their predominant response frequency to earthquake vibrations. Hence, 

 any structure to be built on such ground should be so designed that reso- 

 nance between structure and ground is avoided. 



Strain gauging is a comparatively new field of geophysics. Its purpose 

 is the measurement of small displacements between adjacent portions of 

 ground in underground openings such as tunnels, mine shafts, and the 

 like. Continuous recording of strains in subsurface openings and of their 

 fluctuations with time makes it possible to predict roof and wall failures 

 and to guard against them. 



The purpose of vibration tests is to determine the magnitude and char- 

 acteristics of vibrations set up in buildings by traffic, mine operations, 

 blasting, and so on, by one or more vibration recorders set up at various 

 distances from the source of the disturbance or in various stories of the 

 building. Results of such tests have played no unimportant part in 

 settling disputes arising from damage claims in connection with industrial 

 operations. 



Acoustic methods. Acoustic methods are concerned with the trans- 

 mission and reception of audible sounds in the air, in water, and in the 

 ground. Their purpose is communication, position finding, sound rang- 

 ing, direction finding, echo sounding, and, indirectly, the collection of data 

 for noise prevention. For communication or signaling, transmitters of 

 suitable frequency and directional characteristic are required, depending 



