70 HERSEY [chap. 4 



B. Geological Interpretation 



The strata and other more complex structures dehneated by the reflection 

 method must be identified ; this is usually best accomplished by associating 

 them with known rocks or sediments, as by bottom cores. Inshore studies with 

 small survey boats often permit one the luxury of tracing a sub-bottom reflector 

 to an outcrop on shore. As mentioned previously there have been successful 

 comjiarisons of profiles with core samples and with ease of penetration measured 

 by the number of blows per foot required to ])enetrate bottom materials. Such 

 a correlation is reassuring, but core samples are more useful for geologic 

 inter})retation. Control by core samples is now mostly limited to a few tens of 

 feet in non-commercial research. The limitation is largely economic in shallow 

 water ; it is to be hoped that in the future the coring of deej^er holes will be 

 financed in areas of critical interest in shallow water as well as through such 

 deep-water projects such as the Mohole. 



Velocity and acoustic impedance (velocity x density) by themselves are not 

 reliable means of identifying rocks or sediments. Thus it is well known that 

 calcareous rock can have velocities ranging from close to 1,5 to over 7 km/sec. 

 Nevertheless, considerable progress is being made in appreciating the charac- 

 teristics of rocks and soft sediments affecting velocity. Some of these, such as 

 the relationship between velocity and density, velocity and water content, and 

 velocity and hydrostatic pressure, are discussed elsewhere in this work. We can 

 hope that more and more reliable geological deductions can be based on these 

 relationships. 



Where regional or nearby local geology is well understood the probable 

 geological structure can be deduced, as in the several examples cited under 

 "Results". 



6. Geophysical Measurements 



Exactly as this recording method helps to correlate echo sequences for 

 interpreting structure, so it can help to identify specific travel-paths in the 

 water and in the bottom, thus permitting the scientist to measure processes 

 which have previously been only vaguely identified in most work. Often the 

 oblique reflection method described by Officer (1955) has seemed inadequate 

 when applied to shot sequences because of small uncertainties of correlation. 

 This difficulty virtually disappears in continuous profiling. Continuous refrac- 

 tion profiling is just coming into use, but early indications are: (1) we will find 

 first arrival identification more certain, including ability to pick the first 

 motion, (2) "second" arrivals can be picked with much more confidence, and 

 (3) the effect of irregular topography on travel times will be displayed in much 

 greater detail. 



So far as the writer has been able to discover no measurements of acoustic 

 impedance have been attempted during continuous profiling researches. The 

 graphic records of multiple reflections, as in Figs. 18 and 19, suggest that such 

 quantitative studies would be fruitful. Furthermore, it should be possible to 

 use continuous profiling techniques to measure effective absorption coefficients. 



