3. EQUIPMENT AND INSTRUMENTS 



101 



a graph, Figure 38, or by use of tables in 

 Special Publication No. 298, Sea Water Tem- 

 perature and Density Reduction Tables. 



3—127 Bottom samples. — A device to ob- 

 tain samples of bottom materials is required 

 so that bottom characteristics may be shown 

 on the nautical chart (see 5-76). The 

 mariner, and consequently the hydrographer, 

 is interested solely in the material on the 

 surface of the bottom. Minimum require- 

 ments for bottom sampling are stated in 1-42. 

 An armed lead or snapper type sampler will 

 provide the necessary data for charting pur- 

 poses. Sampling for oceanographic studies 

 requires the use of other instruments such 

 as dredges or corers. Bottom sampling in- 

 struments used by the Coast and Geodetic 

 Survey are briefly described in the following 

 paragraphs. 



3-128 Clamshell snappers. — Nearly all 

 bottom samples in hydrographic surveys are 

 obtained with a snapper- type device (Fig. 

 39). The armed sounding lead is used in- 

 frequently in shallow water. The snapper 

 type of sampler has two clamshell jaws that 

 are held open by an interior trigger, held 

 in place by the pressure of a compressed 

 helical spring around the shank of the in- 



FlGURE 39. — Snapper type bottom sampling devices. 



strument. The trigger is formed of two 

 metal pieces hinged on the inside of each 

 jaw. When the open jaws strike the bot- 

 tom, the trigger is released and the tension 

 on the spring closes the jaws around a por- 

 tion of the material on the surface of the 

 bottom. If the bottom material is rocky, no 

 sample will be obtained but the cup will 

 usually be scratched or dented slightly. In 

 gravel bottom the jaws may close on small 

 pebbles and the sample will be washed out 

 as the cup is retrieved. 



The correct adjustment of the tension of 

 the spring is important and must be reached 

 by trial. When the tension is correct, the 

 trigger will always be released when the 

 jaws strike the bottom and the jaws will be 

 held firmly closed. The tension may be in- 

 creased or decreased by moving a nut, which 

 compresses the spring, along a screw thread 

 on the shank. 



The shank of the sampler extends above 

 the spring so that it can be inserted and 

 riveted in a sounding lead. Snappers are 

 available from the Washington OflJice in 

 three sizes with capacities of approximately 

 15, 90, and 500 cubic centimeters. The var- 

 ious sizes should be used with sounding 

 leads of appropriate weights. A lead weigh- 

 ing 35 pounds or more should be used in 

 deep water. The small samplers are adequate 

 for hydrographic surveys, and the large one 

 should be used only when samples are to be 

 retained for analysis. 



3-129 Scoopfish sampler. — The scoopfish 

 bottom sampler, (Fig. 40) is designed to ob- 

 tain bottom samples in depths less than 100 

 fathoms while the ship is underway. The 

 sampler weighs 11 pounds, is 15 inches long, 

 and is lowered from the BT winch. It should 

 be lowered slowly until close to the surface 

 of the water and then dropped. Unless this 

 is done the nose lid may be released as the 

 scoopfish enters the water. When the sampler 

 strikes the bottom, the cup is pushed back, 

 releasing the nose lid which closes over the 

 sample, and the towing arm is rotated 

 forward. 



After retrieving the scoopfish, the cup and 

 sample are removed, another cup is inserted 



