Page 549 echo sounding 571 



connects with the trace of the echo from the bottom, yet the latter is plainly distinguishable, and 

 there is no doubt about the character of the shoaler trace. 



There is more doubt about a case such as is illustrated at (30) in figure 119. The trace of the 

 bottom is distinguishable through the shoaler trace at (29) but it cannot be distinguished at (30). 



Where the kelp is heavy enough to prevent any echo being received from the bottom, or where the 

 trace from the kelp obscures any trace there may be from the bottom, the fathograms are useless unless 

 supplemented by leadline soundings. In all such cases the fathogram should be carefully scanned 

 for evidence of the true bottom, and where it cannot be seen, the areas should be investigated with 

 the leadline. 



A careful study of such fathograms will generally, but not always, show a difference between 

 soundings on a true pinnacle and a reflection from kelp. Where the record appears as a A with open 

 bottom, as described above, it can be interpreted as a true shoal. However, should the trace show a 

 sudden shoaling with no indication of a A opening, it may or may not indicate an echo from kelp, and 

 further investigation should be made. 



g. Evidence of strays. — So-called strays may, at times, be difficult to distinguish from true shoals. 

 In A in figure 117, there is no doubt about the nature of the mark at (2) — it cannot possibly be a 

 shoal, although it can be, and probably is, an echo from some submerged debris, because close inspec- 

 tion of the record shows that the trace from the surface of the bottom is missing at this point. But if 

 such a mark as at (2) comes at a depth where it resembles a shoal protruding from the bottom, one 

 cannot always be certain of its true nature from the fathogram. For example, note that at (3) the 

 trace of the bottom can be followed through the stray, but this cannot be done at (4), which may be 

 a true shoal — at least there is no evidence that it is not. In / in figure 119 are similar examples. 

 The detached traces to the right of (32) and the fainter mark to the right of (31) are without doubt 

 strays. It is probable that (31) is a stray, from its characteristic shape, although the trace of the 

 bottom cannot be followed through it. The other sharp uplifts look more like true shoals, although, 

 the level nature of the bottom seems to make this unlikely. 



With reference to e, f, and g above, it cannot be assumed with assurance that a 

 continuous record of the bottom through the base of a A -shaped shoal trace proves the 

 nonexistence of a danger or obstruction. A fathogram record has been made over a 

 known wreck and on this fathogram the continuous trace of the bottom is clearly- 

 evident, in addition to the shoaler record of depths on the wreck. In this case the 

 recorded depths below the wreck are deeper than adjoining depths, which may be 

 evidence of echoes from an angle from the periphery of the cone of sound. Theoreti- 

 cally, of course, it is possible, assuming a sufficient intensity of sound and gain control, 

 to obtain a continuous record from adjoining depths while passing over a shoal, if the 

 area of the shoal is less than the area of the cone of transmitted sound, where it meets 

 the bottom. 



Hydrographers should not expect the fathograms to be always self-explanatory. 

 It is true that they contain a permanent record of much evidence that is lost when 

 soundings are read from a visual instrument. But hydrographers must still investigate 

 with vertical casts a sufficient number of representative places which cannot be surely 

 interpreted from the fathograms alone. If the investigation of representative cases 

 ensures a correct interpretation of all others of a similar nature, that should sujQEice; 

 otherwise all doubtful places on the fathograms must be investigated with vertical casts. 



It is important that the fathogi'am record be clear, that the fix marks and other 

 events be legibly and neatly identified, and that a minimum of notations be made on 

 the fathogram to ensure its correct interpretation. Figure 120 illustrates a fathogram 

 with the notations properly made. 



