M. J. Tucker and A. R. Stubbs 307 
— TRANSMISSION 
— DIRECT PULSE 
—SEA FLOOR 
70 FMS. 
-0-2 
-0O3 
— SECOND ECHO 
FROM SEA FLOOR 
—0'5 SECS. 
Se i 
Fig. 16.6. Record taken using the "Thumper." 
States using a more powerful Thumper. A typical record is shown in Fig. 16.6; 
this was obtained using the original Thumper and a nondirectional hydrophone. 
16.2.4. Seismic Refraction Shooting 
When the very deep rocks below the sea bed are to be examined, a system 
known as seismic refraction shooting is used. The principle of this method is 
shown in Fig. 16.7. An explosive charge, which may vary from a ‘)-Ib charge 
to a depth charge, is set off nearthe ship, A. The sound travels by various paths 
to a number of sono-radio buoys which transmit the received sound back to the 
ship by radio. The travel time of the first sound to arrive at the receiver is 
plotted against the horizontal distance given by the travel time of the direct 
water wave to give a diagram as in Fig. 16.8. At close ranges, the first sound 
to arrive has traveled through the direct water path, but at longer ranges the 
higher velocity of sound in the sediment layer means that sound through this 
path arrives first. At even longer ranges, the shortest path is through the deeper 
but even higher velocity layers. Thus, the diagram consists of a series of straight 
lines, each representing a stratum, whose slopes give the velocity of sound in 
that stratum. From the intercepts with the axis, the depth of the stratum can be 
determined. Thus, not only the true depth, but also some clue as to the compo- 
sition of the stratum can be obtained, by comparing the observed velocities with 
those of known rocks. This technique can penetrate several miles below the 
sea bed. 
16.2.5. Sea-Bed Survey Using Asdic 
Chesterman, Clynick, and Stride [8] provided the first published account 
relating certain bottom reverberation patterns on an asdic record to the sea-bed 
geology. The equipment in use at N.I.O. was designed primarily for fish-detection 
