the dominant effect and attenuation decreases smoothly thereafter with depth and over- 

 burden pressure. It was concluded that Figure 9 can be used to aid prediction of sound 

 attenuation in sediment and rock layers in the sea floor. 



The study of sound attenuation in marine sediments and rocks is a continuing pro- 

 ject. Since the 1976 report new measurements have indicated the vaHdity of the above 

 approach and conclusions. The most important of these measurements were by Tyce (per- 

 sonal communication), using the Marine Physical Laboratory deep-tow equipment in both 

 the Atlantic and Pacific Oceans. 



VARIATIONS OF DENSITY AND POROSITY WITH DEPTH IN DEEP-SEA 

 SEDIMENTS 



Introduction 



The values and variations of density and porosity with depth in marine sediments 

 and rocks are of importance in both basic and applied studies of the earth. Specifically (in 

 the field of sound interactions with the sea floor), density of various layers of the oceanic 

 crust are important in the propagation of shear and compressional waves and other elastic 

 waves. Values of density are required in all mathematical models of sound interacting with 

 the sea floor. However, work at the Applied Research Laboratories of the University of 

 Texas has indicated that, in many cases, the gradient of density may have only a small 

 effect on bottom losses. At high grazing angles (above the shear wave critical angle), the 

 effects amount to about 1 to 2 dB change in bottom loss. At low angles, very Httle effect 

 is observed except in the vicinity of the low angle shear anomaly (discussed in a following 

 section) where it can amount to as much as 2 to 8 dB (Hawker et al, 1976, p. 65). 



Studies From 1974 to 1977 



Three reports were issued during the three-year period concerning variations of den- 

 sity and porosity with depth in the sea floor. The first was issued by the Naval Undersea 

 Center as TP 459 (Hamilton, 1975a); this report was later published in the Journal of 

 Sedimentary Petrology (Hamilton, 1976c). A resume with selected figures are noted below. 



Bachman and Hamilton (1976) obtained a suite of samples from the Deep Sea Drill- 

 ing Project Site 222 (Leg 23) in the northern Arabian Sea. At this site there was an unusu- 

 ally thick section of homogeneous, terrigenous sediment which was drilled to about 1300 

 meters. Density, porosity, and grain density were measured in the laboratory. These data 

 were also included in the Hamilton and Bachman and Hamilton reports. 



The critical question in relating laboratory measurements of sediment density and 

 porosity to in situ measurements in a deep borehole is: how much has the sample expanded 

 elastically as a result of removal from overburden pressure in the borehole to atmospheric 

 pressure in the laboratory? This was the problem addressed in Hamilton (1976c). The 

 abstract of this report follows. 



Reduction of sediment porosity and increase in density under overburden pressure 

 in the sea floor are important subjects in earth sciences. Data and samples from the Deep 

 Sea Drilling Project allow a new look at these subjects, and are used to estabhsh profiles of 



16 



