accomplished in two ways: with a group of pendulums, by determining 

 the variations in the periods of swinging pendulums of known lengths 

 at different locations; or by means of a spring-type gravimeter, by 

 measuring the variations in the length of a weighted spring at different 

 locations. 



B. EXISTING INSTRUMENTS 



The status of instruments pertaining to gravity surveys may be 

 considered good insofar as the needs of the Hydrographic Office are 

 concerned. The several types of instruments available or currently 

 under development are discussed below. 



1. Airborne gravity instruments 



Some laboratory research and a few field trials have been carried 

 out toward the development of an airborne gravimeter. Lundberg 

 Explorations, Ltd. of Toronto, Canada and Pick Laboratories of 

 Saratoga, California have experimented with gradiometer-type instru- 

 ments. These instruments involve systems that use two masses 

 suspended vertically, one above the other, to permit observation 

 of a ratio or a derivative of the vertical gradient of gravity. This 

 principle appears to be applicable to exploration surveying, but it 

 complicates the data processing for geodetic use. Neither of these 

 companies has produced a working instrument to date. 



Several tests have been made with existing gravimeter s in a balloon 

 and aircraft. The Air Force Cambridge Research Center (AFCRC) made 

 the first feasibility test using a LaCoste and Romberg shipboard meter 

 aboard a KC135 (jet tanker) at Edwards AFB, California in November 

 1958. Flights made at 20,000 and 30,000 feet proved that such an instru- 

 ment would operate under these conditions. Three Companies: LaCoste 

 and Romberg, Fair child Aerial Surveys, and Gravity Meter Exploration 

 Co., known as FLAGS, then conducted a similar test aboard a B-17 

 aircraft flying at 12,000 feet. Since the completion of these initial tests, 

 AFCRC has been continuing research and development to determine the 

 correlation between airborne results and ground data, to improve 

 methods for reading the meter at high speeds, and to provide means 

 for rapid calculation of data. FLAGS currently is planning a test 

 aboard a helicopter which, if successful, should contribute greatly to 

 the solution of the problem of correlating air and surface data. 



2. Shallow water gravimeters 



Shallow water gravimeters are required on surveys made to 



XIII- 2 



