The field is sampled at two- or four- second intervals at the 

 discretion of the operator. The time necessary for each field reading 

 varies but is in the order of 1.4 seconds. 



The XN-4901 is designed for both analog and digital data output. 

 The analog output, as is currently used, appears on a Varian Gil 

 or G11A strip chart recorder. 



E. LAND MEASUREMENTS 



As a part of regular hydrographic surveys, Hydrographic Office 

 personnel also occupy land magnetic stations in the course of estab- 

 lishing a base triangulation net. Instruments currently available for 

 this purpose are Ruska field magnetometers. These instruments are 

 quite satisfactory for the establishment of repeat magnetic stations. 

 The observations, however, are quite time-consuming. 



F. SPECIAL PURPOSE SURVEYS 



Surveys to determine the magnetic environment of an area are 

 required for various applications. Factors required are: (1) Spatial 

 distribution of magnetic anomalies, (2) characteristics of magnetic 

 temporal variations, and (3) propagation geometry of induced magnetic 

 fields. The Hydrographic Office currently is engaged in surveys which 

 fulfill a portion of these requirements. 



G. CONCLUSIONS 



1. Airborne geomagnetic surveys 



In general, the instrumentation for airborne geomagnetic surveys 

 can be considered reliable and accurate. Present capabilities for navi- 

 gation and flight control impose limitations on the realizable accuracy 

 of the survey. Improvements to the magnetometer equipment alone 

 cannot be expected to yield substantially increased accuracy. However, 

 certain aspects of the present systems can and should be improved 

 in order to achieve greater flexibility, economy of effort, and reliabil- 

 ity. 



2. Marine geomagnetic surveys 



The present marine geomagnetic survey equipment is standing 

 up well under steady operations. The weak points in the system have 

 been in the towing gear, both in the fish and the cable. However, 

 recent advances show good promise of alleviating these difficulties. 



XIV- 8 



