1. Introduction 



Topics chosen for presentation in a single lecture on the study of 

 the ocean using acoustic signals must be carefully selected and 

 treated briefly. The ocean is mostly opaque to light and radio waves, 

 so that acoustic and other mechanical probes become extremely 

 important in gaining information about the insides of the ocean. The 

 generalized view taken here of acoustic oceanography is one of many 

 possible approaches. 



The oceans cover almost 71 percent of the Earth's surface and have 

 a mean depth of 3,800 meters. Some ocean trenches are deeper than the 

 height of the highest mountain on earth. In this report submarine 

 geology is treated first. Natural processes occurring beneath the 

 ocean floor influence the shape and structure of the floor. Sedimen- 

 tary processes also modify it. The resulting morphology affects 

 acoustic propagation when the ocean floor is involved. Next, marine 

 life is studied in terms of interaction with the ocean environment. 

 Use of acoustic techniques has revealed some interesting habits of 

 sea life, of which one example is the diurnal migration of the deep 

 sound scattering layers. The section on chemical and physical 

 properties of seawater is slanted towards speed of sound and absorption 

 of sound. Although the ocean is a variable medium both temporally and 

 spatially, the next section describes mean state with regard to 

 temperature, salinity, and currents. Acoustic effects caused by 

 stratification of the ocean, as represented by the surface mixed layer, 

 thermocline, and nearly-isothermal cold deep ocean, are treated in 

 terms of the surface duct and deep sound channel. Examples are given 

 of the four chief paths of acoustic wave propagation using ray sche- 

 matics. The final topic concerns ocean surface waves and internal 

 waves and their statistical variability. Acoustic effects related to 

 the ocean surface, such as noise spectra and sea surface loss, are also 

 treated briefly. 



2. Submarine Geology 



Submarine geology concerns topography of the sea floor, composition 

 and physical character of sedimentary and igneous materials found on 

 the ocean bottom, and processes involved in the development and modifi- 

 cation of topographic relief. 



a. The Earth Beneath the Oceans 



Earth is a sphere of 6,371-km radius and is considered to be 



composed of four idealized concentric spherical layers: crust (0-33 km), 



mantle (33-2,900 km), liquid outer core (2,900-5,100 km), and inner core 



(5,100-6,371 km) (fig. 2-1). 



The boundary between Earth's crust and the denser mantle at an 

 average depth of 33 km is called the Mohorovicic discontinuity, or the 

 Moho for short. The Moho lies at a greater depth under land masses 



