886 MISCELLANEOUS GEOPHYSICAL METHODS [Cuav. 12 



cation as a chemical method results from the fact that certain characteristic 

 chemical compounds are the object of separation and quantitative detec- 

 tion, whereas their inclusion among geophysical methods is justified on 

 the grounds that they constitute a definite prospecting method and that 

 procedures of detection and analysis are physico-chemical if not entirely 

 physical. 



A. Macroscopic and Microscopic Methods 



Soil analysis, in principle, is a refinement of the observation methods 

 used for some time by the geologist in his quest for indications of sub- 

 surface oil accumulations. Although these indications have always been 

 present and some of them were well known to the ancients, their signifi- 

 cance was then not appreciated*^ and thus the vast subterranean reservoirs 

 of oil and gas remained untapped up to the middle of the past century. 



Surface indications of oil deposits may be of a direct nature, for example, 

 emanations of hydrocarbons in the form of oil seeps, gas exhalations, oil 

 impregnations, asphalt deposits, earth wax (ozokerite), iridescent oil films, 

 or gas bubbles in water. Indirect indications are: shows of characteristic 

 inorganic compounds, such as h.ydrogen sulfide ("sour" dirt, "sour" 

 waters), sulfur bacteria, brines, and bromine and iodine waters; character- 

 istic vegetation; or signs of mechanical displacements associated with gas 

 emanations, such as mud "volcanoes," gas mounds, sandstone dikes, brec- 

 ciated clay dikes, and mud flows. Careful observations of these phe- 

 nomena have led to considerable success in recent years. In the Gulf 

 coast salt dome province alone, 75 out of 219 domes discovered prior to 

 Februar>', 1936, were located b}^ the application of such "macroscopic" 

 detection methods. 



These methods have the disadvantage, however, that the indications are 

 not always unique. For instance, methane may be formed by decaying 

 vegetation in swamp}^ areas (and in coal and lignite seams), and salt 



"Examples: the oil seeps near Cuba in southwest New York, used by the 

 Seneca Indians for medicinal purposes; the St. Quirinus spring on Tegern Lake in 

 Bavaria, known to the monks since the fifteenth century; the asphalt deposits near 

 the Dead Sea (Genesis 14 : 10) and between Babylon and Nineveh; those at Apsheron, 

 at Sakhalin, and in Trinidad (Sir Walter Raleigh, 1595); and, further, the "eternal 

 fires" of the Chimaira in Lycia, as described by Herodotus about 450 B.C., and the 

 fires of Burma, Mesopotamia, and Baku, the latter well known from the fire-worship- 

 ping Zoroastrians or Parsis. 



'^ Plutarch relates that the Macedonian warriors of .\le.\ander the Great found, 

 in 328 B.C., oil oozing out of the rock on the banks of the Oxus (Amu-Darya) River 

 (near Bukhara, I'.S.S.R.) and were much surprised since in the vicinity "no olive 

 trees" were in evidence. 



36 G. Sawtelle, A.A.P.G. Bull. 20 (6), 728 (June, 1936). Another table in the 

 same article gives .35 out of 141 domes. 



