134 • Marine Minerals: Exploring Our New Ocean Frontier 



Figure 4-9.— Seismic Reflection and Refraction Principles 



In the seismic reflection technique, sound waves fronn a source at a ship bounce directly back to the ship from 

 sediment and rock layers. In the seismic refraction technique, the sound waves from a "shooting" ship travel 

 along the sediment and rock layers before propagating back to a "receiving" ship. 



SOURCE: P. A. Rona, Exploration Methods for fhe Cor)tmer)tal Shelf: Geology, Geophysics, Geochemistry, NOAA Technical Report ERL 238-AOML 8 

 {Boulder, CO; National Oceanic and Atmospheric Administration, 1972), p, 15. 



single-channel systems. Because of cost, however, 

 a multi-channel system is usually not used for 

 reconnaissance work. 



High-resolution seismic reflection techniques are 

 able to detect the presence of sediment layers or 

 sand lenses as little as 1 meter thick. In addition, 

 information about the specific type of material de- 

 tected sometimes may be obtained by evaluating 

 the acoustic velocity and frequency characteristics 

 of the material. Seismic techniques may provide 

 clues for locating thin, surficial deposits of man- 

 ganese nodules or cobalt crusts, but side-looking 



sonar is a better tool to use for this purpose. Ryan 

 reports that a 1 to 5-kilohertz sub-bottom profiler 

 was very effective in reconnaissance of sediment- 

 hosted sulfides of the Juan de Fuca Ridge.*" WhUe 

 seismic methods provide a cross-sectional view of 

 stratigraphic and structural geologic framework, ge- 

 ologists prefer to supplement these methods with 

 coring, sampling, and drilling (i.e., direct meth- 

 ods), with photography and submersible observa- 



"W.B.F. Ryan, Lamont-Doherty Geological Observatory, OTA 

 Workshop op Technologies for Surveying and Exploring the Exclu- 

 sive Economic Zone, Washington, DC, June 10, 1986. 



