24 EXPLORATION GEOPHYSICS 



mining exploration is the preliminary use of a magnetic survey to outline 

 the plan location of certain magnetic dikes or ore-bearing mineral veins, 

 followed by an electrical survey to determine the occurrence, location, and 

 depth of sulphide mineralization in the veins, or associated with the dikes. 

 A similar example in petroleum exploration is the common procedure of 

 supplementing preliminary magnetic or gravimetric results by detailed 

 seismic surveys. 



Outline of an Exploration Program 



A general exploration program can, with minor revisions, be applied to 

 a wide variety of exploratory problems. The initial step for such a program 

 in any given area, whether for ore deposits, petroleum, or other resources, 

 should be a preliminary study of available geological literature pertaining 

 to the region and to the specific area. This will be concerned principally 

 with regional geology, manner of occurrence of ore or petroleum deposits, 

 and the history of their discovery and exploitation in this and adjacent areas. 



The second step is the securing of a temporary legal control and right- 

 of-way on the land to be investigated. This may be of various forms. In 

 mining, the common form is the bond and lease with option to purchase. In 

 oil prospecting, however, the geophysical work occasionally is done with 

 nothing but right-of-way permission to trespass on the property or permits 

 to drill shot holes. Lease-holds may be obtained during or after completion 

 of the survey and the discovery of favorable prospects. 



The third step in this general program is the preliminary geological field 

 study, undertaken for two purposes : ( 1 ) to determine to what extent geo- 

 physical methods will be required to secure information necessary for 

 economical development of the property, and (2) to provide information 

 which will determine the choice of geophysical methods and aid in subse- 

 quent interpretation of the geophysical data. Since the geophysical survey 

 is to be conducted to gather geophysical data pertaining to matters such as 

 structure and the location and depth of certain rock formations, it is essen- 

 tial that all available geological information from rock outcrops, drill holes, 

 pits, and excavations, be correlated and appraised in advance of the more 

 costly geophysical survey. By this means the geophysical survey may be 

 planned to the best advantage, and the time and money spent may be mate- 

 rially reduced. In many cases, when diagnostic surface conditions prevail, 

 the geological study alone will indicate the character and amount of geo- 

 physical work needed to complete the desired geological picture. 



The preparation of preliminary maps is an important part of this third 

 step in the exploration program. Topographic maps are of value for several 

 reasons. (1) They aid in planning any exploration program and in inter- 

 preting surface geology. (2) They provide the topographic control essential 

 to the interpretation of geophysical data obtained from areas of appreciable 

 surface relief. (3) Topographic relief frequently is related to structure 

 and is a further aid, therefore, in accomplishing the purpose of the survey : 



