Ch. 14] ENGINEERING SIGNIFICANCES 257 



bacteria; algae; 87 species of diatoms (Taber, 1943a); bones of at 

 least 20 species of large mammals, represented by tens of thousands 

 of specimens (Taber, 1943a; Wilkerson, 1932); numerous species of 

 rodents; and a few species of mollusks, sponges, and insects (Taber, 

 1943a). Permafrost in Siberia has been a storehouse for Pleistocene 

 mammals (TolmachofT, 1929). 



Permafrost upsets many readings taken by geophysicists in deter- 

 mining the internal constitution of the earth. Velocities of seismic 

 waves, for instance, are materially increased by frozen ground con- 

 taining much ice and may result in considerable errors in determina- 

 tions of depths. Although the actual increases are not definitely 

 known, they probably fall within the range of 1,000 to 8,000 feet per 

 second. Unfortunately the lower contact of permafrost causes, with 

 present equipment, no satisfactory reflections or refractions. Seismic 

 methods cannot be used to determine the thickness or variability of 

 the zone distorting the seismic waves. Difficulties in drilling, pre- 

 paring the explosive charges, checking the ground waves, and getting 

 interpretable effects are augmented in permafrost areas. 



Electrical methods, particularly the resistivity methods, have given 

 promise of solving some of the difficulties in determining the extent and 

 thickness of permafrost (Enenstein, 1947; Swartz and Shepard, 1946; 

 Muller, 1945; and Joestings, 1941). Generally resistivities of frozen 

 silt and gravel are several thousand ohms higher than comparable un- 

 frozen materials and may be 20 to 120 times as high (Swartz and 

 Shepard, 1946; Joestings, 1941). However, it is well-known that the 

 type of material is less important than the amount of unfrozen ground 

 water and dissolved salts within the material. Even in frozen ground 

 these factors are so variable that resistivity data can be interpreted 

 with reliability only in the hands of experienced men and generally 

 only in areas where some positive checks can be made through drilling. 



Sumgin and Petrovsky (1947) discuss a new radio-wave technique 

 used where permafrost is below — 5° C. 



ENGINEERING SIGNIFICANCES 



In Alaska during World War II the difficulties encountered by our 

 armed forces in obtaining permanent water supplies and in construct- 

 ing runways, roads, and buildings in permafrost areas focused atten- 

 tion on permafrost as no other difficulties could have (Wilson, 1948; 

 Jaillite, 1947; Barnes, 1946; Taber, 1943b). Only then did most of 

 us realize that in Russia similar difficulties with railroads, roads, 

 bridges, houses, and factories had impeded colonization and develop- 



