SOUNDINGS TAKEN DURING THE DISCOVERY INVESTIGATIONS, 1932-1939 53 



In general the angle of a slope can be determined approximately by plotting the series of echo 

 distances obtained, at right angles to the contour lines, and measuring the angle of slope with 

 a protractor. 



In practice the echo distance can usually be assumed to be the actual sounding since, from the 

 navigational point of view, the primary object is to establish the ship's position when making a landfall. 

 Normally this implies fixing the ship's position with regard to the continental shelf, or charting her 

 approach to shallow water from deep and vice versa, and there are few slopes where the correction 

 for slope is sufficiently large to be shown on charts constructed on the scales usual for navigational 

 purposes. In shallow water the correction becomes negligible. It should also be remembered that if 

 soundwaves are reflected from the nearest point of the bottom, a ship approaching shoal water will 

 obtain soundings from a position fairly well ahead if the slope is steep or moderately so. This is of 

 distinct advantage to the navigator. 



In the ' Discovery II ' soundings were normally taken at intervals of one hour (or approximately 

 9 miles) and in these circumstances an accurate correction for slope is not possible. In the open ocean, 

 however, the bottom is comparatively level and, even with soundings closely spaced, correction for 

 slope is negligible unless the rise is greater than i in 10. When a ridge or bank was being crossed, 

 however, we endeavoured to get as many soundings as possible, and, if the weather was favourable, 

 to obtain a continuous record with the ' Acadia ' recorder. Correction for slope then became possible, 

 but it must be remembered that considerable errors may occur in fixing a ship's position in mid-ocean; 

 in the most favourable circumstances it is doubtful whether accuracy can be greater than ± i mile, 

 and it is probable that the error is of the order ± i to i mile. This fact, together with the necessity for 

 plotting oceanic soundings on charts of a relatively small scale, makes it doubtful whether a correction 

 for slope, either by displacement of position or by an addition to the echo distance, is practicable. It 

 should be realized that a four-figure sounding, plotted on an oceanic chart of the usual scale and on 

 Mercator's Projection, may cover an area of as much as 40-50 sq. miles in the latitude of 60° S and 

 that this area increases to 100 120 sq. miles at the Equator. Thus it may not be possible to plot more 

 than a very small fraction of the soundings from a bank or ridge, and from the navigational point of 

 view correction for slope in oceanic waters is therefore of no value. To the geologist and geophysicist, 

 however, the correct outlines of a ridge may be of definite use, but it remains doubtful whether 

 correction for slope would greatly affect hydrological calculations on upwelling and the movements 

 of the water masses. 



The form of the beam of sound projected by the transmitter of a sonic echo-sounding set is of 

 considerable importance in the interpretation of the soundings. It must be assumed that it is in the 

 form of a cone in which the energy is at a maximum along the axis and fades out at the periphery. 

 The amount of 'spread' or dispersal should vary inversely with the frequency of the soundwaves. 



It is clear that an allowance must be made for a certain amount of spread. Hayes (1933, p. 154) 

 states that with a relatively high-frequency sonic transmitter (presumably at 2000 cyc./sec.) the 

 diameter of the beam at 2000 fm. (3758 m.) will be i mile. This is equivalent to an angle of spread of 

 14° and, presumably, is the limit of transmission of soundwaves of significant strength. Furthermore, 

 the multiple echoes picked up on the recorder (see p. 56) must be attributed to echoes received from 

 diflFerent directions. On the other hand, echoes were obtained from depths of more than 4000 fm. 

 (7515 m.), and re-echoes from lesser depths; and this could hardly be expected were it not that the 

 maximum sound energy is transmitted along the axis of the cone. 



Some indication of the limit of dispersal of relatively high-frequency sonic transmissions is perhaps 

 afforded by a sounding record obtained on one occasion when the anchor was being hove-up from 

 40 fm. (73 m.). The record showed a strong echo from the bottom and, as is natural with a small 



