fill the gaps between ship operations and land-based surveys and in 

 areas such as lagoons or inland lakes. Several commercially produced 

 gravimeters for shallow water work currently provide the accuracy 

 and stability necessary for all foreseeable requirements. Their prices 

 range from $30,000 to $35,000. These instruments are operated by 

 remote control from surface ships to depths as much as 200 fathoms 

 and have an attainable accuracy of 0.1 milligal. 



These meters, in general, weigh approximately 250 to 350 pounds 

 and require the use of two cables with their corresponding winches 

 during operations. One cable is used only for lowering and raising 

 the meter, and the other is a waterproof multi conduct or cable which 

 can support only its own weight at 200 fathoms. 



Recent developments in this type of meter have produced a much 

 smaller meter that weighs only about 90 pounds. This meter is self 

 leveling, a feature which greatly simplifies the observing procedure 

 and reduces the size of the required multiconductor. As a result, it 

 has been possible to develop a single-cable system in which the 

 conductor cable also supports the meter. 



3. Submarine and surface ship gravimeters 



Although ocean gravity meters were designed initially only 

 for submarine use, recent improvements have made the meters 

 operable on surface ships. It is, therefore, preferable to designate 

 these instruments as ocean meters for use aboard either submarines 

 or surface ships. The Hydrographic Office owns two LaCoste and 

 Romberg meters and has technical control of three others. In addition, 

 this Office has been evaluating three Askania (Graf) gravimeters 

 constructed in West Germany. Several other commercial organizations 

 currently are developing meters for use at sea. The LaCoste and Rom- 

 berg meter costs about $150,000 and includes a computing system and a 

 means of keeping itself level. This meter is electrically operated and 

 has an attainable accuracy of 0.1 milligal with precise navigation in 

 average sea conditions. The Graf meter costs about $27,000 but 

 requires a stabilized platform for operation. 



4. Absolute gravity instruments (land) 



Gravity instruments for use on land are of two categories, 

 absolute and relative- reading. Although most of the requirements 

 for gravity surveys can be accomplished with relative- reading instru- 

 ments, absolute measurements of gravity are required for basic con- 

 trol of all surveys. No truly absolute gravity instrument is available 



XIII- 3 



