Chap. 5] GEOPHYSICAL METHODS IN MINING 53 



field, a classification somewhat different from that customary in economic 

 geology will be adopted. Petroleum, natural gas, asphalt, and related 

 bitumina will be excluded, and the remainder will be classed as follows: 

 (a) coal, (6) sulfur, (c) salt, (d) nitrates, phosphates, potash, (e) building 

 and road materials, (/) abrasives, (g) materials for various industrial uses, 

 (h) gems and precious stones. 



A. Coal, Including Anthracite and Lignite 



In this group only anthracite and possibly lignite offer possibilities of 

 direct geophysical location; virtually all other types of coal are amenable 

 to indirect prospecting only. Anthracite may be located directly by self- 

 potential measurements or resistivity or other electrical methods. Some 

 anthracitic coals are conductive while some other varieties act as insu- 

 lators.^ 



In stratigraphic and structural investigations of coal deposits, various 

 methods are applicable. For general reconnaissance, the magnetic method 

 may be useful if the carboniferous strata are conformable with basement 

 topography. For the mapping of regional Paleozoic structure, extensive 

 pendulum surveys were undertaken at one time in northern Germany. 

 For detail, electrical resistivity methods have been used. Examples are 

 the Carboniferous syncline of Villanueva de Minas and Villanueva del Rio 

 in Spain and the Saar coal basin. Both refraction and reflection-seismic 

 methods can render valuable service in the determination of the structure 

 of Carboniferous areas, as demonstrated in Spain, in Silesia, and in West- 

 phalia. Under favorable conditions structural studies of the Carbonif- 

 erous have been made by means of the torsion balance and the gravimeter. 



Various geophysical methods have been tried on lignite deposits. The 

 success of electrical methods appears to depend greatly on the local stra- 

 tigraphy and the water content of the lignite. Seismic-refraction methods 

 have possibilities for determining the thickness of lignite beds. However, 

 structural and stratigraphic investigations may be expected to be more 

 successful as exemplified by Schlumberger's electrical surveys in the de- 

 partment of Landes, France, and by Edge-Laby's and Seblatnigg's torsion 

 balance work on faults associated with lignite deposits in Australia and 

 Germany respectively. 



B. Sulfur 



Geophysics has been applied on a large scale to the indirect location of 

 sulfur deposits found in the cap rocks of salt domes on the Gulf coast. 



' For details and bibliography see C. A. H., "Geophysics in the Non-Metallic 

 Field," A.I.M.E. Geophysical Prospecting, 54&-576 (1934). 



