571 HYDROGRAPHIC MANUAL PaGE 546 



Insufficient pressure of the stylus will sometimes cause the instrument to cease recording for very 

 short intervals, usually preceded by intervals during which the stylus marks only faintly. An ex- 

 ample of faulty stylus pressure is illustrated in F in figure 118, where the stylus did not record at (23) 

 near the right-hand side of the record. This is another example of excessive amplifier gain, probably 

 accompanied by regeneration of the amplifier. Note hov/ much wider the trace of the transmitted 

 signal is at (24) than at (25). 



A poor contact in one of the several mechanical contacts may be responsible for no signal being 

 recorded. An example of this is illustrated in E in figure 118. Note that at (20), (21), and (22), and 

 at several other places the record is momentarily lacking. That this instrument is faulty in operation 

 is evidenced by the ragged line made by the transmitted signal. 



Carbon nodules occasionally form on the platen over which the record paper travels and these 

 cause horizontal marks to be traced through the fathogram as at (33) in figure 119. Several such 

 marks are also evident in F in figure 118. The platen should be cleaned and polished daily in accord- 

 ance with 52376, or more of ten, if necessary to remove the trouble. 



d. Types of bottom. — Bottoms of different types record characteristic echoes; hard bottom may be 

 recognized by a shorter and blacker mark than soft bottom. The longer and fainter trace from the 

 latter is because of the low density of the soft mud which permits a certain penetration of the signal, 

 the echo being received from the true bottom and a multitude of depths immediately beneath. This 

 difference is vividly illustrated in B in figure 117 where the depressions are evidently filled with 

 material of varying density, and invariably the trace of the echo reflected from these is much longer than 

 from the rocky portions between them. The same thing is illustrated in A in figure 117 where at 

 (1) there is an intrusion of hard material through several sedimentary layers. 



In extremely steep and broken bottom, echoes from slopes may sometimes be recorded. Under 

 exceptional circumstances the echo from a slope may block out the true vertical echo, distorting the 

 profile. (See 563.) 



A slope may be so steep that no echo from it will be recorded, particularly if the Fathometer is 

 being operated on the fathom scale. Such a case is illustrated at (26) in figure 118 where no echo 

 has been received from the left-hand side of the slope. A critical examination of the trace will disclose 

 that there is an appreciable horizontal distance between the edge of the top of the slope where the echo 

 is lost and the bottom of the following deep from which the echo again registers. 



Because of the great exaggeration of the vertical scale at launch sounding speeds, slopes that 

 appear to be nearly vertical from the record will, after careful scaling and analysis, become rational. 

 (See 563.) 



Reflections from the sides of a depression can give a record which resembles a layer of silt over the 

 depression, somewhat similar to that illustrated at (6) in figure 117. It must be borne in mind that 

 such echoes can come from slopes to one side, as well as from ahead or astern, and that the dimensions 

 of the features in this direction are not evident from the profile. Where deposits of silt are rare, such 

 indications should be investigated by vertical leadline or wire soundings. 



e. Evidence of shoals. — Over submarine topography of a rugged nature, in areas of heavy sub- 

 merged, or floating, kelp or other marine growth, and when the Fathometer is not operating properly, 

 the hydrographer must make a careful study of the indicated shoals before accepting or rejecting 

 them. The study of similar records in which shoal indications have been checked with the leadline is 

 helpful but not always conclusive. 



True shoals in extremely irregular bottom can generally be distinguished by the fact that there is 

 no evidence of a record of the bottom through the base of the trace of the shoal. In other words, the 

 trace of a true shoal appears as a A with an opening in the trace at its base. This is illustrated in D 

 in figure 118, more especially at (14), (15), (17), (18), and (19). This record warrants careful study. 

 Also note that the trace at (IG) is from a true shoal, although the open A at its base is not so apparent, 

 because it is filled by the trace of the double echo. 



/. Reflections from kelp or other marine growth. — Probably the fathograms that are most difficult 

 to interpret properly are those in which echoes from kelp, or other marine growth, appear; particularly 

 where the kelp is well submerged. Where the main bulk of the kelp floats at some distance above the 

 bottom, the trace from the kelp appears to be detached or semidetached from the bottom with the 

 result that the bottom trace is partly obscured and reduced in intensity although it is still possible to 

 follow its outline. An exceflent example of kelp on shoals, the character of which there is no doubt, 

 is illustrated in C in figure 117. On every shoal near the surface, at (10), (11), (12), and (13), kelp is 

 growing and its trace is shown in the record, yet there is no doubt about the true bottom at some 

 depth below the kelp. At (32) in figure 119 the trace from the kelp, or other marine growth, almost 



