5. HYDROGRAPHY 



159 



may not correctly reflect the operating speed. 

 A speed count should be made and the paper 

 speed measured to see that it is correct. 

 This type of echo sounder is also sensitive 

 to changes in gain and the operator should 

 be cautioned against using a low gain. 



5-76 Character of the bottom. — In all 



hydrographic surveys, the character of the 

 sea bottom shall be determined at frequent 

 and regular intervals throughout the project 

 area to meet the needs of navigation and for 

 other purposes. This applies particularly to 

 harbors and anchorages and in all depths 

 where vessels may anchor. In addition to 

 furnishing data for selection of an anchor- 

 age, fishermen are assisted in selecting 

 places where fish may be found and in avoid- 

 ing places where their equipment may be 

 damaged. 



A sampling of the surface layer is usually 

 adequate to define bottom characteristics for 

 charting. An armed lead, a small snapper 

 cup, or a scoop fish, will furnish samples of 

 sufficient size. Since nearly all soundings are 

 obtained by echo sounders, it is necessary 

 for the hydrographer to make a special ef- 

 fort to obtain bottom samples. In depths 

 of 20 fathoms or less this is readily accom- 

 plished with a lead line, but in greater depths 

 a sounding machine should be used. When- 

 ever possible the samples in deep water 

 should be obtained by the ship or a ves- 

 sel equipped with a power driven sound- 

 ing machine. Launch hydrographers are in- 

 clined to defer this operation and frequently 

 fail to obtain as many samples as they should 

 to adequately define the character of the bot- 

 tom. The frequency of bottom sampling with 

 respect to depth of water is specified in 1-42. 



If a more detailed study of the ocean floor 

 is contemplated, the bottom samples shall be 

 obtained by a suitable corer or by dredging. 

 The project instructions will specify the den- 

 sity of sampling required and the type or 

 types of samplers to be used. A core sample 

 should be preserved intact in the sleeve of 

 the corer or be carefully extruded into a suit- 

 able container. Dredging samples should be 

 preserved in sturdy containers. All samples 

 shall be carefully labeled and cross referenced 



to detailed records of place and time of 

 sampling. 



5-77 Classification of bottom materials. 



— A complete description of a bottom sam- 

 ple consists of: one or more adjectives de- 

 scriptive as to size or consistency ; one or 

 more adjectives designating color; and one 

 or more nouns naming the class of bottom 

 material. The descriptions should follow a 

 standard classification and utilize standard 

 abbreviations as shown in part "S" of Chart 

 No. 1, Nautical Chart Symbols and Abbrevia- 

 tions, a part of which is shown in 5-88. 

 Descriptive terms needed, which are not 

 included in the chart, should be written in 

 full. The description shall be arranged in 

 the following order: size or consistency, 

 color, and noun. Bottom characteristics shall 

 be shown in black ink on the boat sheet, 

 below and to the right of the position. 



A precise classification of bottom mate- 

 rials in most cases would require a labora- 

 tory analysis, but this is ordinarily imprac- 

 ticable in most hydrographic surveying. An 

 inspection of a sample by sight and touch 

 will enable the hydrographer to give a rea- 

 sonably accurate description of the material. 



Close to shore and on the continental shelf 

 the ocean bottom will generally consist of 

 sands, gravels, muds, and remains of plant 

 and animal life. Ledge rock may be exposed 

 in a few areas close to shore where slopes 

 are steep. Sand is generally easily recog- 

 nized; even the finer grained sands feel 

 gritty when rubbed with the finger in the 

 palm of the hand and when dry will separate 

 into grains visible to the naked eye. 



Technically there are two classes of ma- 

 terial finer than sand. These are silt and 

 clay. For practical purposes these are often 

 classified under the general term of mud. 

 The particles are much finer than sand and 

 do not feel gritty to the fingers. Clay is a 

 finer grained deposit than silt and is usually 

 sticky to the touch. 



Sediments are variously typed according to 

 the size of the particles composing them. 

 The following tabulation will serve as a gen- 

 eral guide for classification of the sands and 

 coarser particles. It is not intended that the 



