M. J. Tucker and A. R. Stubbs 305 
TRANSMISSION ——> 
100 FM — 
200 FM —— 
300FM —— _ 
TILT CHANGED TO 20° 
1O MIN 
Fig. 16.4. Record from an echo-sounder employing multiple beams, From the record it 
can be shown that the bed of the canyon slopes at 2.5° in a direction of 110°T. 
16.2.3. Echo-Sounding for Subbottom Strata 
Ordinary echo-sounders do not usually show reflections from strata below 
the sea bed. This is probably due mostly to the high attenuation of these fre- 
quencies in the sea-bed material, but is probably also due in part to the high 
reverberation level from irregularities in this material. Under very favorable 
conditions, 10-kcps sounders will penetrate at least 100 ft of mud to show the 
bedrock underneath (Fig. 16.5), and where they will do so they are most useful 
because they have higher resolution than the devices described below. In order 
to get deeper penetration, and in less favorable conditions, lower frequencies 
are necessary, since these show much less attenuation and reverberation. 
The use of these low frequencies presents major design problems. Firstly, 
the ambient noise level in the sea rises rapidly at low frequencies, as does also 
ship's noise, and high pulse powers of the order of 1 Mw are therefore required 
from the transmitter. Owing to the long wavelength, the number of waves in the 
transmitted pulse must be kept small if useful resolution is to be obtained. A 
half wave would be ideal, but this requires wide-band receivers, which again 
increase the noise level. Thetypes oftransducers used at supersonic frequencies 
are thus not suitable. 
