240 ANNUAL REPORT SMITHSONIAN INSTITUTION, 19 40 



geophysics is meant any physical method of investigating the sub- 

 surface structure of the earth, and applied geophysics in this sense 

 is a product of very recent years. 



The methods used are based on gravimetric, seismic, or electrical 

 principles, each of which has advantages by certain circumstances. 

 The gravity method was the first to be developed, and an extremely 

 sensitive torsion balance, invented by the Hungarian scientist Baron 

 von Eotvos, was first used for oil finding by Dr. H. de Bockh, 

 also a Hungarian. During the last few years a new type of gravity 

 instrument, called the gravimeter, has been extensively used, and 

 although the physical principles of the instrument are considerably 

 different, the final result for practical purposes is similar. These 

 gravity instruments measure minute differences of the force of grav- 

 ity throughout a given area, and it has been found that a large anti- 

 cline, the core of which has rocks of higher specific gravity than its 

 flanks, causes at the surface a slight increase in the value of gravity 

 as compared to its surroundings. The effect may range from a few 

 milligals to perhaps as much as 50 in the case of a major unit, but 

 as a milligal is 0.001 cm./sec.^, or approximately one-millionth part 

 of the total force of gravity at sea level, it will be readily appreciated 

 that the instruments used are most highly sensitive and their manipu- 

 lation is a matter of extreme care and skill. An inverted gravity 

 result is obtained in the case of salt domes where the salt cofe of the 

 dome is of a lighter gravity than the surroundings, and in this case 

 the target that is required takes the form of a gravity minimum. 



The geophysical method which is now perhaps the most usual is 

 based on seismic principles. This is a type of echo sounding, but sub- 

 surface conditions are naturally much more complicated than is the 

 case with the marine echo sounder. A small explosion is fired in a 

 shot hole of 50 or so feet depth and the return vibrations, reflected 

 from various harder rock groups at depth, are registered by groups 

 of seismometers. The results are correlated and depicted in the form 

 of contour maps which show the structural attitude of the various 

 reflecting horizons. In some cases however the composition of the 

 rocks is not suitable for giving satisfactory reflections and it is nec- 

 essary to use the refraction method. Its use requires a much larger 

 charge of explosive, which in exceptional circumstances may be as 

 much as 1 to 2 tons of gelignite, and the seismometer instruments are 

 placed much farther from the shot point than is the case with the 

 reflection method, the distance being as great as 10 to 15 miles. 



In other cases certain electrical methods of surveying are used, 

 although the conditions suitable for their employment are less fre- 

 quent than with the methods already described. The electrical 

 method measures the relative resistivity and conductivity of the var- 

 ious groups of rocks and provides a contour map of their structure. 



