SEISMIC METHODS 907 



it operates in an almost perfect physical medium where the velocity is 

 practically constant and the attenuation is relatively low. The high velocity 

 of radio frequency energy in the atmosphere (300,000,000 meters per 

 second) makes possible the almost instantaneous observation of a tre- 

 mendous number of reflections. In fact one of the principal disadvantages 

 of typical radar, when applied to accurate survey work, is that too much 

 data are generally obtained. This presents a difficulty and sometimes an 

 uncertainty in the recognition of reflections from specific marker targets. 

 Exploration geophysicists have been employing radar for surveying 

 offshore locations along the Gulf Coast. Most of these radar systems use 

 what is known as the p.p.i. (plan position indicator) type of presentation. 

 The antenna is slowly rotated, thus directing the sending and receiving 

 of the radio beam. The instantaneous direction of this antenna is syn- 

 chronized with that of the outward radial sweep on the cathode ray tube 

 used for observation. This cathode ray beam sweeps from the center of 

 the cathode ray tube toward its circumference at a uniform rate, and the 

 beam is blanked out on returning from the circumference to the center. 

 Echoes from objects reflecting radio waves appear as bright spots on the 

 radial trace of the cathode ray beam, so that the net result is the produc- 

 tion of a crude map of all the echoes received with the radar station 

 located in the center of the cathode ray tube screen. Superimposed on 

 this small scale map are range or distance markers which appear as con- 

 centric circles with the radar station at their center. The radii of these 

 circles represent distance from the radar station, and thus the distance 

 to any point on the map may be read directly. Such a system would be 

 ideal if only echoes from the desired land markers appeared on the cathode 

 ray tube screen. Unfortunately, echoes from many water waves, f land 

 masses, structures and desired targets all appear on the screen as a con- 

 fusing clutter. Furthermore, as the ship carrying the radar moves along 

 its course, some targets cease to give reflections and others appear on the 

 screen, so that often it is a difficult problem to locate land marks with the 

 certainty needed for horizontal survey control. To avoid this dilemma 

 certain land markers are often flagged with artificial reflectors, thus making 

 up a type of triangulation shore network. This arrangement is helpful in 

 most cases, but generally there are so many reflections presented on the 

 screen that a unique solution is seldom possible for determining accurate 

 locations. The navigation radar is excellent for the purpose for which it 

 was designed, that is, for detecting and locating other vessels and obstacles 

 in fog or darkness and for determining position relative to land markers 

 on shore. The operational range is limited to line-of -sight, which may be 

 up to 15 miles, depending upon terrain conditions and height of station. 

 Within this distance, an accuracy of ± 25 to 100 feet may be obtained. 

 This system is most satisfactory for ship to near-shore use. 



t W. F. Gerdes and R. C. Johnson, "Industrial Radar for Hurricane Tracking," Science, Vol. 

 110, Oct. 7, 1949. 



