Sub-Bottom Layering 



Sound penetration of the sea floor and subsequent re- 

 cording by echo sounder have been known for some decades, 

 but the successful production of instruments to record sub- 

 bottom reflecting layers is no more than 10 years old (ex- 

 cepting explosive seismology, in which travel times are 

 used to postulate models). An excellent resume of the tech- 

 niques and instruments available through 1962 is by Hersey. 



As higher power and lower frequency are used, the 

 sound penetrates deeper into the sea floor. Each type, or 

 class, of reflection equipment has its own uses, which may 

 overlap but do not, essentially, compete. The echo sounder 

 under very favorable bottom conditions (i.e. , a good reflector 

 at relatively shallow depths beneath porous silts and clays) 

 can show a sub-bottom to depths as great as 50 meters, 

 although depths to 20 meters are more common. 3 The 

 Sonoprobe type will operate from a surface vessel in water 

 depths to about 1200 feet and will penetrate 50 to 100 feet 

 (or sometimes more) of sand and softer silts and clays, 

 and will, in some cases, penetrate soft rock and show 

 bedding surfaces. 4 The spark-source type , operating with 

 high power, will penetrate 1 to 3 kilometers, and explosive - 

 source seismic equipment is used to depths of 5 kilometers 

 or more within the earth's crust. 3 It is important to re- 

 member that the spark-source equipment may record at the 

 water-sediment interface only a black line which includes 

 several tens of meters of surficial sediment, of interest to 

 only some aspects of underwater sound and geophysics. In 

 other words, the correct instrument must be selected for 

 the data required. 



It has been noted that pingers, operating at 12 kc/s 

 on equipment lowered to the bottom (e_. g. , cameras, 

 corers), frequently record reflecting layers below the 

 water-sediment interface when the ship's echo sounder 

 does not. A promising, possible method of future sub- 

 bottom acoustic reflection surveying could be the attachment 

 of special-purpose pinger transducers to lowered equipment 

 in areas being studied for acoustic experiments. Some 



