1050 Subsurface Geologic Methods 



Volcanic "float," telephone lines, pipe lines, fences, railroads, and other 

 factors that disturb the conventional ground survey are rendered ineffective 

 at higher elevations. Surveys are commonly flown at about 1,000 feet 

 above the ground, this elevation largely eliminating near-surface effects 

 and allowing an ample safety margin for the aircraft. 



Of particular importance in mining surveys are measurements made 

 with the Schmidt-type horizontal-component field balance. This instru- 

 ment is similar in design to the vertical-component field balance with the 

 exception that the magnetic system is placed in a vertical rather than 

 a horizontal position. By using both vertical and horizontal data, vectors 

 may be plotted that indicate quite accurately the location of the disturbing 

 body. This technique is extremely valuable in underground surveys. (See 

 fig. 551.) 



Magnetic Interpretation 



I Interpretation of magnetic data is generally empirical. The trained 

 interpreter coordinates the known local and regional geology with the 

 magnetic map, and, drawing on his previous experience in similar areas, 

 outlines trend lines or areas of possible structural disturbance. In areas 

 where experience with magnetic data may be lacking, he frequently re- 

 quests experimental surveys over known geologic features and utilizes the 

 results for interpretative control. Another mode of attack requires the 

 computation of anomalies for various geometric shapes and comparison 

 with observed results. 



Primarily, magnetic surveys in oil exploration are utilized to indicate 

 large regional geologic trends and the depth to the basement complex. 

 In new areas particularly, a knowledge of the thickness of the sedi- 

 mentary section is of vital importance in the search for oil accumulation. 

 The areal extent and magnitude of magnetic anomalies depend upon 

 several factors, among which are depth, size and shape, and polarization 

 or intensity of magnetization. The sharpness of magnetic anomalies de- 

 pends almost entirely on the depth alone, comparatively sharp anomalies 

 originating at shallow depth and broad anomalies at greater depths. 

 By assuming certain simple geometric shapes, rules are available for es- 

 timating the depths to magnetized material from the extent of their 

 surface magnetic effects. ^^ Depth rules are simple to apply and often 

 give important clues to the geologic structure. It should be understood 

 that they do not give conclusive information, and they should be applied 

 with reservations. In regions where the approximate depth to the base- 

 ment complex is known, application of the depth rules may aid in dis- 

 tinguishing intrabasement variations from actual basement uplift. 



Where the disturbing geologic body is roughly circular and the 

 section small compared with the depth extent, the lines of vertical in- 

 tensity will be nearly circles or short ellipses, and only a positive anomaly 



" Nettleton, L. L., Geophysical Prospecting for Oil, p, 224, New York, McGraw-Hill Book Co., Inc., 

 1940. 



