Page 543 echo sounding 571 



(6) The motor speed must be kept at the correct calibration value (see 555). For instruments 

 with reed tachometers this is when the middle reed is vibrating at maximum amplitude. For graphic- 

 recording instruments the motor speed must be kept continuously at the correct value. For visual 

 instruments the fathometer attendant must see that it is correct at the time each sounding is read. 



(c) On a visual instrument where the depth is indicated by a line of appreciable width, the shoal 

 edge of the line must be read (see 5543). On a graphic-recording instrument where the depth is 

 indicated by a jog or offset from a continuous line the depth is indicated by the very beginning of the 

 offset mark. 



(d) Regardless of the sounding interval, the fathometer attendant must observe continuously 

 the depths registered on a visual instrument in shoal water or wherever there is the slightest possibility 

 of the existence of dangers or obstructions, in order to make certain that none of these is missed 

 (see 3415). In one test that was made with the 312 Fathometer on a vessel proceeding at standard 

 speed over a known shoal with depths of 7]4 fathoms rising from general depths of about 40 fathoms, 

 the shoalest depth was registered by only one flash of the neon tube, preceded and followed by only 

 one or two faint flashes from intermediate depths. These latter might easily have been mistaken for 

 strays. 



(e) Using a visual instrument the fathometer attendant must be certain that he is not reading a 

 multiple echo (that is, a second or third echo instead of the first). This is particularly important 

 where sounding is begun in comparatively deep water where even the approximate depth is not known. 



(f) Where rapid fluctuations in registered depths are known with certainty to be caused by the 

 vertical motion of the vessel in rough weather, a mean depth shall be recorded (or scaled from a fatho- 

 gram) which is as near as possible to what the depth would have been had the sea not been rough. 

 But where rapid fluctuations in registered depths are caused by bottom irregularities the shoalest of 

 these occurring near the time for a sounding shall be recorded or scaled. 



{g) Regardless of the sounding interval an intermediate depth must be recorded, or scaled, wherever 

 the intermediate depth differs from the normal slope of the bottom by more than 5 percent of the 

 depth (see 3433). 



(h) The fathometer attendant must learn to distinguish between an echo from the bottom and 

 a stray. Usually, but not always, the true echo on a visual instrument is slightly brighter in intensity 

 than the strays. Generally, strays appear on the dial at random and thus can be distinguished from 

 echoes, but under certain conditions strays may be repeated at the same place on the depth scale, 

 thus resembling soundings (see 5147). 



(i) In sounding over irregular bottom where the slopes are steep, and particularly in compara- 

 tively deep water, several echoes from different parts of the slope are often registered almost simultane- 

 ously on a visual instrument. These appear on the dial as a splattering of the light from the neon 

 tube. Theoretically at least, the shoalest depth registered on a sonic instrument should be from that 

 part of the bottom nearest to the vessel, regardless of verticality. This obviously is not the true depth. 

 Under certain circumstances, however, the true vertical depth may be registered as a deeper value. 

 In such cases a notation should always.be made in the Sounding Record, describing the appearance of 

 the registration on the instrument and stating which value i^as recorded. (See also 5145 and 563.) 



(j) If an allowance for settlement and squat has been made when the index was adjusted, any 

 soundings taken from the vessel stopped or at slow speed must be corrected arithmetically for this 

 allowance (see 553). The fathometer attendant must note in the "Remarks" column of the Sounding 

 Record all such soundings for which this correction is required. 



(k) Rubber Stamp No. 31, Graphic Record, must be impressed at the beginning and end of each 

 fathogram roll, and at the beginning and end of each day's work on the fathograms; entries must be 

 made in all applicable spaces. Figure 116 is a facsimile of this stamp with entries properly made. 



571. Fathogram Interpretation 



With a graphic-recording echo-sounding instrument a continuous and permanent 

 profile of the bottom is obtained. No trouble is experienced in scaling depths with the 

 required accuracy from the majority of fathograms. But certain precautions in 

 operation during sounding are required, certain recorded evidence should be explained 

 on the fathogram by the hydrographer, and certain skill in interpretation is required, 

 in order that the greatest value and the most accurate results are obtained from the 

 records. 



