Pn 



Hydrophone 

 Array 





/q n\ source 







" *- *"-.-"■■ 7 • ■ " ■ -^' •-. "- ^ " W '/ 



— ■ *~~ ■ ' ■ ~~ 1 ^^ i ^ \T4r 





~-- ^- 







Water 



Surface 



Bottom 



Horizon 1 



Horizon 2 



Horizon 3 



Figure 25. Principles of obtaining subbottom seismic data 



acoustic impedance between bounding units, minimal thickness of the units, or 

 masking by gas (Sheriff 1980). Because of this, seismic stratigraphy should 

 always be considered tentative until supported by direct lithologic evidence 

 from core samples. 



The two most important parameters of a subbottom seismic reflection 

 system are its vertical resolution, or the ability to differentiate closely spaced 

 reflectors, and penetration. As the dominant frequency of the output signal 

 increases, the resolution becomes finer. Unfortunately raising the frequency 

 of the acoustic pulses increases attenuation of the signal and consequently 

 decreases the effective penetration. Thus, it is a common practice to use two 

 seismic reflection systems simultaneously during a survey; one having high 

 resolution capabilities and the other capable of greater penetration. 



Side-scan sonar is used to distinguish topography of the seafloor. Acoustic 

 signals from a source towed below the water surface are directed at a low 

 angle to either or both sides of a track line, in contrast with the downward- 

 directed Fathometer and seismic reflection signals (Figure 26). The resulting 

 image of the bottom is similar to a continuous aerial photograph. Detailed 

 information such as spacing and orientation of bed forms and broad differ- 

 ences of seafloor sediments, as well as features such as rock outcrops, 

 boulders, bed forms, and man-made objects, can be distinguished on side- 

 scan. It is generally recommended that bathymetry be run in conjunction with 

 side-scan to aid in identifying objects with subtle vertical relief. The side-scan 



60 



Chapter 3 Field Data Collection and Observation 



