INTRODUCTION 



The continuous reflection profiling technique (Figure 1) was developed 

 to provide a flexible system to map continuously and accurately subbottom 

 strata in water-covered areas. The sound pulses periodically generated 

 and reflected from the bottom and subbottom acoustic discontinuities are 

 recorded. 



The reflected signals are detected by a towed hydrophone array (or 

 transducer), amplified, filtered, and displayed on a precision variable 

 density recorder. Magnetic tape recording and sophisticated digital data 

 processing techniques are sometimes used to optimize the signal-to-noise 

 ratio in the recorded display. 



The principal differences between the continuous reflection profil- 

 ing systems and those systems used for echo-sounding of water depth are 

 the requirements of high acoustic power output and, generally, lower 

 frequency. Although penetrations to 90 feet into the seafloor have been 

 obtained with frequencies as high as 14 kHz (Murray, 1947), lower fre- 

 quencies are more commonly used. A lower limit to the frequency used is 

 set by the resolution desired — layers thinner than one half the wavelength 

 generally cannot be resolved clearly. Pulse duration is another important 

 factor and must be short for good resolution. In general: 



2d = vAt (1) 



where d = distance between reflecting layers (m) 



v = velocity of sound in water (1500 m/sec) 



At = difference between arrival times of the reflected 

 wave fronts 



Thus, when the pulse length becomes comparable with the time of 2d/v sec, 

 the two layers may not be resolved. 



Each type of reflection equipment which differs principally in the 

 type of energy source utilized has its own uses. For a comprehensive 

 discussion of subbottom systems, see Schlank (1968). Briefly, a trans- 

 ducer system operating at frequencies of 1.5 to 12 kHz can show, under 

 favorable circumstances (shallow water, silt and clay sediment), a sub- 

 bottom structure to depths of 50 m, although penetration of 20 m is more 

 common. A sparker system, operating with high power, will penetrate 1 to 



