To determine the thickness of an unconsolidated sediment layer, it 

 is commonly assumed that the velocity in these sediments is nearly equal 

 to that of water (1.5 km/sec). This assumption is accurate, as seen from 

 the data below: 



Velocity 

 Material km/sec 



unconsolidated deep-sea 1.6-2.2 

 sediments 



semi-consolidated deep- 1.7-2.9 

 sea sediments 



fine sand 1.7 



very fine sand 1.6-1.7 



silty fine sand 1.5-1.6 



medium silt 1.5 



clayey fine silt 1.5 



clays 1.43-1.70 



arenites 1. 6-1. 7 



Lcarenites 1.65-2.77 



Source 

 Houtz et al. , 1968 



Houtz et al. , 1968 



Hamilton et al., 1956 

 Hamilton et al. , 1956 

 Hamilton et al. , 1956 

 Hamilton et al. , 1956 

 Hamilton et al., 1956 

 Sutton et al. , 1957 

 Sutton et al. , 1957 



Sutton et al. 



1957 



However, as the sediments become increasingly more consolidated, 

 acoustic velocities rapidly increase. Thus, the use of velocity in water 

 for these consolidated sediments may lead to a misinterpretation of layer 

 thickness . 



Several methods of determining the true layer thicknesses have been 

 used. An interval velocity for the layer studied may be assumed or it may 

 be calculated from wide-angle reflection (Le Pichon et al. , 1968; Clay and 

 Rona, 1965) or seismic refraction methods. (The determination of in-situ 

 sediment velocity will be discussed in a subsequent section.) Also, the 

 interval velocity may be calculated from core-determinfixLvalues which have_ 

 been corrected to in-situ conditions (HamlTton, 1963). 



Another method of determining layer thickness is to assume or calcu- 

 late a sound velocity gradient for the material (Hamilton, 1967). Although 

 the sound velocity gradient curve may be exponential in form, a linear 

 curve is commonly assumed. Houtz and Ewing (1963) have used the following 

 equation when the sediment-surface sound speed, average vertical sound 

 velocity gradient, and time of sound travel within the layer are known. 



11 



