The bottom and bedrock beneath the bottom produce the strongest reflections; 

 soft sediments allow the greatest penetration, and clean sands and gravels permit 

 very little signal penetration. The detection of subbottom sedimentary layers 

 depends upon the acoustic contrast of the horizon in question and the sediments 

 above. The differences in acoustic impedance, producing the contrast between 

 sediments, are mainly due to textural and density differences. 



The first problem encountered in interpreting subbottom records is to determine 

 real echoes from multiple echoes and side, or ghost, reflections. Multiple reflections 

 from the air-water Interface can generally be resolved .in as much as the multiple 

 will appear on the record at twice the depth of the true horizon,and any dip to the 

 horizon will be shown on the trace at twice the apparent dip of the real horizon. 

 Multiples due to reverberation or scattering of the sound in a very coarse sedimentary 

 deposit is also a possibility. Reflections such as those on run I (Figs. 4, 5, and 6) be- 

 tween marks G and N may be caused by a deposit of this type. This record shows 

 a dark irregular zone of reflections. 



Side reflections are due to reflections from horizons or point reflectors not 

 beneath the ship but in the insonified area. As an example, in TOO feet of water^ 

 the Sonoprobe will insonify an area beneath the ship to a distance of over 25 feet 

 from the normal to the transducer. A point reflector on the bottom within this radius 

 will be recorded as a reflection up to 3 feet below the water bottom. When steam- 

 ing or drifting these side reflections may be recorded as an arcuate (concave down- 

 ward) trace on the record. An example of these reflections may be seen in Figure 

 7; mark N of run II . Side reflections from a hummocky bottom are shown in Figure 

 15A at station 51 . 



Once the validity of a reflection Is established, the nature of the reflecting 

 surface can then be determined. The water bottom is the first reflecting surface and 

 in this area may be sediments, coarse glacial deposits such as till or outwash, or 

 bedrock. The nature of the bottom is determined from the Sonoprobe record by the 

 depth of signal penetration and the topography qf the bottom. A sedimentary bottom 

 will generally have a smoother profile and allow greater penetration than bedrock 

 or glacial till, depending upon the grain size of the sediment. A bedrock bottom will 

 show considerable relief and will reflect nearly all of the signal, giving a dark bottom 

 trace with little if any penetration . Coarse sediments over bedrock are more diffi- 

 cult to determine. The topography and character of the reflections are similar to 

 those obtained over bedrock. Bottom sampling in this case usually resolves the prob- 

 lem only to the extent that It tells whether or not some sediments are present. 



The above criteria also are used to determine bedrock reflections below the 

 bottom. Coarse glacial deposits again present a problem, but some penetration is 

 usually obtained. The penetrating signal may experience reverberation, and random 

 patterns of reflections may be recorded^dependlng upon the coarseness of the material . 



42 



