1118 Subsurface Geologic Methods 



Questions 



1. May the "divining rod" which has been used in prospecting for 

 water, petroleum, and minerals be classified as a geophysical method? 

 Discuss. 



2. Which general class of geophysical methods lacks the ability to 

 render unique geological solutions? Discuss reasons for the ambi- 

 guity. In which general category would geothermal measurements be 

 classified? 



3. List several economic minerals which may be located directly by 

 geophysical methods and indicate the proper instrument for each 

 mineral. 



4. Outline a complete exploration program for a remote region in 

 Brazil where little is known of the regional geology. 



5. Show distortion of a magnetic field by the insertion of paramagnetic, 

 nonmagnetic, and diamagnetic bodies. 



6. By reference to the typical daily diurnal curves in figure 549, plan 

 a magnetic survey using only one field instrument so that maximum 

 error in correcting diurnal variation by assuming linear diurnal 

 variation will always be less than five gammas. Although base 

 checks should be made at least every two hours, large errors may 

 occur at certain portions of the day unless the survey is carefully 

 planned. 



7. By inspection of figure 549, indicate source of error in using pub- 

 lished daily magnetic variation curves issued by government observa- 

 tories at Tucson, Arizona, and Cheltenham, Maryland. 



8. For a Schmidt type compensated magnetic system operating in a 

 vertical field of 0.5 oersteds, calculate the pole strength of the 

 magnets (assume both blades equal) if the system is to be in equilib- 

 rium. Assume that the gravitational force is equal to 980 dynes and 

 is acting on a mass of 5 grams at center of gravity which is located 

 one centimeter from axis of rotation. Distance between magnetic 

 poles is seven centimeters. 



9. Calculate the force of attraction in dynes between two spheres sep- 

 arated a distance of 100 centimeters between centers if the spheres 

 consist of lead (mass = 50 grams, density = 5.6) and aluminum (mass 

 = 100 grams, density = 2.4) . 



10. List factors which influence the force of gravity on the earth. 



11. Using depth rules for a sphere, calculate depth to the top of the 

 disturbing mass in figure 565. 



12. Using depth rules for the horizontal cylinder, calculate depth to top 

 of disturbing mass in figure 571. 



13. Calculate the critical angle ic if velocity of Vi is 2000 feet per second 

 and V2 is 6000 feet per second. See figure 576. 



14. Discuss differences between correlation and dip methods of seismic 



