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FUTURE DIRECTIONS IN OCEAN SCIENCES 93 



will continue to yield information about the seafloor, a region of 

 Earth that still remains largely unknown. Rock and sediment 

 cores obtained through the DSDP/ODP have provided glimpses of 

 the structure, composition, and the processes that formed these 

 materials. Continued systematic drilling will be required to ob- 

 tain a complete picture of the structure of the ocean crust and 

 particularly the chemical composition and hydrothermal alteration 

 processes. Drilling also allows geophysical and geochemical in- 

 struments to be placed within the drill holes to measure tempera- 

 ture, chemical fluxes, crustal strain, and other variables impor- 

 tant for understanding geological, geochemical, and geophysical 

 processes. 



The technique of seismic tomography began when it was learned 

 that by studying the propagation of seismic waves, both in terms 

 of speed and path, through the Earth, features of Earth's structure 

 could be discerned. Later, explosives and noise generated by "air 

 guns" were used to generate sound that can be transmitted some 

 distance into the seafloor and reflected back to acoustic receivers. 

 These techniques have been used to gain a more detailed picture 

 of the upper seafloor, particularly the sediment layer overlying 

 the crust. The newest seismic technique has been to drill holes 

 deep into the seafloor, placing acoustic sources in some holes and 

 receivers in others, to produce a horizontal seismic tomograph of 

 the intervening sediments and crust. 



These methods provide a snapshot of structure and composi- 

 tion from which processes and fluxes may be inferred. As with 

 other oceanographic disciplines, the importance of time-series ob- 

 servations for observing dynamic processes is critical. Scientists 

 that study marine geology and geophysics will increasingly use 

 time-series measurements of changing features, through repeat cruises, 

 rapid-response measurement techniques, and particularly sensors 

 moored on the seafloor. The area of fluxes is one in which chemi- 

 cal oceanography and marine geology and geophysics interact, be- 

 cause measurement of benthic chemical fluxes is important for 

 both fields. Finally, the concept of "seafloor observatories" is 

 being implemented through the RIDGE program and through re- 

 search sponsored by the Office of Naval Research. These "labora- 

 tories" are actually areas of the seafloor where repeated intensive 

 observations are made. For example, ONR has designated sites on 

 the Mid-Atlantic Ridge (a slow-spreading ridge) and the East Pa- 

 cific Rise (a fast-spreading ridge) as natural laboratories for com- 

 parative studies. 



