18 EXPLORATION GEOPHYSICS 



Since that time more than 2,000,000 acres of submerged land off the coast 

 of Louisiana have been leased, and eleven fields discovered. Early in 1949 

 about 30 active operations were under way, with 17 reflection seismographs 

 in use. Some exploration was being done beyond the 27-marine mile 

 boundary claimed by the state. It has been shown by seismic work that 

 salt domes similar to those found on land extend at least this distance from 

 the coast. Gravity data assembled by the U. S. Navy indicate the existence 

 of salt domes as far as 75 miles from shore. In such cases the mechanical 

 difficulties in exploring and drilling in these deeper waters must be added 

 to the hazards caused by sea and weather conditions. The water in the open 

 Gulf is often rough, and hurricanes may force the withdrawal of a crew. 



However, since off-shore work indicates a density pattern similar to 

 that found in the coastal land belt, as well as a thicker sedimentary section, 

 it is believed that the total oil reserve in this area will exceed that on land. 

 To meet the challenge of this assumption new methods and equipment will 

 be developed to speed exploration, drilling, and production in this difficult 

 and perhaps rewarding area. It is estimated that $70,000,000 per year is 

 currently being spent on exploration in the tidelands area alone. 



An interesting and popular reconnaissance technique has developed in 

 the last few years as an outgrowth of submarine detection methods. In 

 this method, a magnetometer is carried by an aircraft. It traces a continu- 

 ous profile of relative magnetic intensity and gives a generalized picture 

 of the area rather than the discrete information collected by ground sur- 

 veys. It has the obvious advantages of large coverage and extreme rapidity. 

 For example, an airborne magnetometer, carried by a plane flying at an 

 altitude of 300 to 1,500 feet with a speed of about 125 miles per hour, can 

 furnish data on an area of 1,000 to 10,000 square miles per month. f More 

 than a quarter of a million square miles have been mapped by the airborne 

 magnetometer since its first preliminary runs in 1941. This method will 

 undoubtedly continue to grow in popularity, and will result in the recon- 

 naissance mapping of extremely large areas over both land and water. 



Exploration geophysics will continue to move forward. Increasing appli- 

 cations will be found for both instruments and fundamental techniques. 

 Many of these developments will be employed in other lines of commercial 

 endeavor and for military purposes. For instance, applications of seismic 

 methods are being made to buildings and foundations, dam sites, etc., to 

 determine their natural period of vibration as an aid in minimizing possible 

 earthquake damage. 



t Homer Tensen and E. F. Peterson, "Prospecting from the Air," Scientific American, Vol. 178, 

 No. 1, pp. 84-26, January, 1948. 



