1042 Subsurface Geologic Methods 



Subsurface maps are carried on as many horizons as are convenient. In 

 many areas resort must be made to dip shooting and the contouring of 

 so-called phantom horizons. In other regions three-dimensional dip shoot- 

 ing is practiced, the final map being expressed in terms of strike and dip 

 symbols. Various combinations of the foregoing seismic techniques are 

 often practiced, depending upon the particular area involved. Then upon 

 completion of the seismograph survey and evaluation of all data bearing 

 on the area, recommendations are made by the geological and geophysical 

 departments for the location of the test well. 



If such a program as outlined above is followed, it is obvious that 

 the credit for a discovery does not belong to any individual method, as 

 would probably be indicated in an analysis of exploratory effort. Cofl&n ^^ 

 cites an example involving the Richardson and South Ellinwood fields in 

 Barton County, Kansas, where credit for the discoveries was given the 

 core drill and reflection seismograph. One year prior to the completion 

 of the discovery wells, on the basis of a magnetically anomalous trend 

 supported by limited gravity work, 50,000 acres were leased at a cost of 

 ten cents an acre. After seismic and core-drill work on anomalous areas, 

 the discovery wells were located. 



Cost of Geophysical Surveys 



When discussing costs it must be borne in mind that wide variations 

 are bound to occur, physiographic conditions probably affecting the ulti- 

 mate cost to the greatest degree. Other variables are climatic conditions, 

 costs of materials and equipment, and salaries of personnel. The average 

 cost of a magnetic survey, either air-borne or on the ground, with a density 

 of 48 stations to the township will be approximately 1| cents an acre 

 under average conditions. For a gravity survey with a station density of 

 85 stations to the township the costs will average about five cents an acre 

 under normal conditions. Costs of seismograph surveys show the greatest 

 variation, as the previously mentioned factors influence not only the 

 cost but also the type of survey conducted. For instance, a reconnaissance 

 survey with shot points located a mile apart might cost an average of 25 

 cents an acre, whereas a continuous survey in the same area might average 

 two dollars an acre. It follows then that the amount and accuracy of 

 subsurface information varies more or less with the cost of obtaining the 

 information. 



Magnetic Prospecting 



Perhaps the oldest application of geophysics to prospecting for min- 

 eral deposits and buried geologic structure was the use of magnetic meth- 

 ods by the Swedes in 1640. During the three centuries that have elapsed 

 since its inception, the magnetometer has proved to be a valuable explora- 

 tory tool. The discovery of several oil fields may be directly attributed 

 to the magnetometer. In a score of others the magnetic results recom- 



" Coffin, R. C, op. cit., p. 2016. 



