interactions with the sea floor. Consequently, measurements and studies of sound attenua- 

 tion have been a long-term, continuing project in the geology -geophysics group. 



Hamilton (1972) reported the results of in situ measurements of sound velocity and 

 attenuation in various sediments off San Diego. These measurements and others from the 

 literature allowed analyses of the relationships between attenuation and frequency and other 

 physical properties. It was concluded that attenuation in dB/unit length is approximately 

 dependent on the first power of frequency and that velocity dispersion is neghgible or ab- 

 sent in water-saturated sediments. The report also discussed the causes of attenuation, its 

 prediction (given grain size or porosity), and appropriate viscoelastic models which can be 

 applied to sediments. 



Studies From 1974 to 1977 



In 1975 and 1976 two reports were issued which revised data and two illustrations 

 in the 1972 report. The first was a Naval Undersea Center report, NUC TP 482 (Hamilton, 

 1975c), followed by its pubhcation in the Journal of the Acoustical Society of America 

 (Hamilton, 1976b). 



Figure 7 is reproduced from Hamilton (1976b, Figure 1). This figure illustrates the 

 relations between attenuation in dB/m and frequency in kHz. The new data in this revised 

 figure were given different symbols from the original (1972) figure. The new data comple- 

 ment and supplement the original data and strongly support an approximate first-power 

 dependence of attenuation on frequency. The data in Figure 7 include sands, silt-clays 

 and mixed-grained materials. The experimental evidence does not support any theory call- 

 ing for a dependence of attenuation on F^ or f- in either sands or silt-clays. 



If attenuation is dependent on the first power of frequency, as indicated by the 

 evidence in Figure 7, then in the equation a. = kf" (where the exponent n is one, a is attenu- 

 ation in dB/m, f is frequency in kHz and k is a constant), the only variable is the constant k. 

 This allows k to be related to common sediment properties such as mean grain size or poros- 

 ity (Figure 8). Figure 8 (reproduced from Figure 2, Hamilton, 1976b) was revised from a 

 similar figure in the 1972 report, with the addition of four new sets of measurements. 

 These measurements did not alter the original conclusions. An important conclusion is that 

 prediction of sound attenuation in the sediment surface can be based on mean grain size or 

 porosity. To predict attenuation, we simply determine the constant k from its relations 

 with porosity (or mean grain size in the 1972 report) and insert k into the above equation, 

 which should be good at any frequency. 



The main purpose of the 1975 and 1976 reports was to discuss the variations of 

 attenuation with depth in the sea floor. The sparse data were collected on attenuation and 

 depth at various frequencies. These data were hsted in a table and illustrated. Figure 3 of 

 Hamilton (1976b) is reproduced here as Figure 9; these data illustrate sound attenuation 

 (represented by the constant k) as a function of depth in the sea floor. It was concluded 

 that attenuation decreases with about the -1/6 power of depth in sands. As a silt-clay sedi- 

 ment (mud), or turbidite, is placed under increasing overburden pressure, there may be a 

 progressive increase in attenuation due to reduction in sediment porosity and a progressive 

 decrease in attenuation due to increasing pressure on the sediment mineral frame. At some 

 null point in the sediment (sparse evidence indicates about 200 meters), pressure becomes 



15 



