and the desired resolution, accuracy, depth, and areal extent. Data 

 are interpreted with the aid of computers, and computerized oper- 

 ations are becoming routine. 



The geophysical survey is of no practical use if the structure dis- 

 covered under water cannot be returned to, located, and occupied for 

 drilling and development. The location of potential and actual drill- 

 ing sites is becoming progressively more dependent on geodetic posi- 

 tioning techniques as the oil industry moves farther offshore. The 

 fluid nature of the surface element— the superjacent water— com- 

 pounds this difficulty. To solve this problem, several methods have 

 been developed to use satellites directly as control for surveys at sea.^^ 



DRILLING 



A major element in the exploration for petroleum is the exploratory 

 drilling that follows the geophysical work. This drilling determines 

 the economic value of the discovery. A structure that has all the ele- 

 ments requisite for the accumulation of hydrocarbons may, when 

 drilled, produce a dry hole ; hence the old saying among oilmen that 

 "oil is where you find it." 



Offshore drilling equipment is a direct descendent of equipment 

 for land drilling, with adaptation to the marine environment. The 

 basic rig (including derrick, kelley, rotary table, blowout preventor, 

 drill pipe, bit, and casing) is used from several types of platforms. 

 There are fixed platforms constructed on piles driven securely into 

 the ocean floor. Most fixed platforms have a maximum efficiency in 

 waters less than 300 feet deep, with more recent designs reaching 

 600 feet. A second stage of evolution is the semi-fixed platform 

 designed to rest on the sea floor while drilling; when drilling is 

 finished, the platform is refloated and moved to another site. The 

 design that employs the jack-up concept is the most versatile and 

 most common among offshore platforms. The third type is the floating 

 platform. Floating platforms are far less costly for exploratory drill- 

 ing than the fixed or semi-fi^ed platforms. They also have the added 

 advantage of capability in much deeper water. The basic concept is 

 that of a conventional rig in the center of a ship or barge modified to 

 perform the task. Recent drill vessels can operate without being 

 anchored, by dynamic positioning over the drill site. 



Although exploratory drilling has been conducted mostly in depths 

 less than 1,000 feet, and completion of producing wells in less than 

 300 feet, further advances in both technologies are imminent. A 

 wildcat for a conmiercial well was recently drilled from a drill barge 

 off Santa Barbara, California, for Humble Oil Company (Tract 322) 



58 Alton B. Moody. "Geodesy and oil exploration." In "Papers from the Technical Con- 

 ference of the American Society of Photbgrammetry — American Congress on Surveying 

 and Mapping, October 7-10, 1970." (Denver, Colorado), pages 301-312. Moody states that 

 as operations extended seaward beyond the range of visual observations, shore-based radio 

 systems were pressed into service, such as shoran, Raydist, lorac, and Decca, which are 

 still widely used. These, however, suffer from a number of limitations, including propaga- 

 ition problems, geometry, limited range, logistics, and political difficulties. Errors vary from 

 500 meters to 100 meters (roughly 1,700 to 330 feet) in locating a position. The most 

 sophisticated system thus far developed is now in operational use by the Western Geo- 

 physical Company of America. It consists of a satellite receiver, Doppler sonar equip- 

 ment, an inertial navigator, and various ancillary equipment. Post analysis of data per- 

 mits determination of the position of the vessel during the survey to an accuracy of about 

 150 feet on the continental shelf, and with an error about 10 per cent larger in deeper 

 water. Thus, the system exceeds the accuracy requirements specified for oil exploration 

 at sea. 



(See also: Alton B. Moody and W. A. Knox. "Geodetic position-finding at sea and the 

 search for oil." Surveying and Mapping (December 1970), pages 581—591.) 



