BL = 20 log R (7) 



where BL = bottom loss of a plane wave at normal Incidence 

 (decibels) 



The amplitude of the signal received at the hydrophone is dependent 

 on the amplitude of the transmitted signal, the path length of the signal 

 to and from the reflecting horizon, and the reflecting properties of the 

 seafloor. Thus, if it can be assumed that the source signal is constant 

 in at-sea operations — only a compensation due to losses in the water 

 column need to be made. These losses due to transmission through the 

 water column are of two types : spherical spreading losses proportional 

 to 1/L, where L is the signal path length, and attenuation losses propor- 

 tional to e~ aL where a is the attenuation coefficient of seawater. Thus, 

 two signals, A^ and A2, received by the hydrophones may differ in ampli- 

 tude due to differences in path length alone without there being a change 

 in bottom conditions (Smith and Li, 1966). That is, 



£-£.-<^ 



In a sufficiently flat area, however, the difference in the losses due to 

 differing path lengths is small and is commonly neglected. 



Since sediment porosity is related to bulk density and the reflec- 

 tion coefficient is a function of bulk density, it is possible to estab- 

 lish a relationship between bottom reflectivity (or bottom loss) and 

 porosity (Breslau, 1967). 



That is 



Psed = Pw n + Ps ( 1-n > < 9 > 



where Psed = saturated bulk density of sediment (gm/cnH) 

 p w = density of seawater (gm/cnw) 

 p s = density of sediment solid material (gm/cm^) 

 n = porosity of sediment 

 and (Wood's Equation) 



v = {[P w n + p s tt-nfl DV + 6 S (1-n)]} (10) 



where 8 W = compressibility of seawater (cm^/dyne) 



B s = compressibility of solid material (cm^/dyne) 



