function[k,AlphaW,AlphaS,vel,rnear]=alphaws(T,S,D,frq,sedrad,tranrad,densed)
%
%  function[k,AlphaW,AlphaS,vel,rnear]=Alphaws(T,S,D,frq,sedrad,tranrad,densed)
% 
%  Output: k      = acoustic wavenumber
%          AlphaW = attenuation of sound of frequency frq due to water
%          AlphaS = attenuation of sound of frequency frq due to sediment
%                   particles (db m^-1)
%          vel    = velocity of sound (m/s)
%          Rnear  = nearfield range (m)
%
%  Input :  T      =  water temperature (degC); 
%           S      =  salinity (ppt); 
%           D      =  depth (m);
%           frq    =  frequency of transducer (Hz)
%           sedrad = sediment radius (m)
%           tranrad= ABS transducer radius (m)
%           densed   = density of sediment (kg/m3)
%
%  Values defined internally in the function 
%  tranrad  = ABS transducer radius (m)
%  denfresh = density of fresh water (kg/m3)
%  densalt  = density of salt water (kg/m3)
%
% The required parameters are...
% k,AlphaW, AlphaS, alphaV, vel
%
% Chris Vincent, UEA
% 
%  Change from Alpha to Alphaws to avoid conflict with
%  a different MATLAB function
%
% Modified by csherwood@usgs.gov as follows:
%  2006-05-31 - corrected comments in help section
%  2004-06-07 - make densed an input argument and add default values for
%               densed and tranrad


if(exist('tranrad')~=1),
   tranrad=0.006;      % ABS transducer radius (m) - 
		       %  Bow this is Given by
                       %  (ABS_Transducer_diameter_mm/1000)/2
end
if(exist('densed')~=1),
  densed=2670;        % density of sediment (kg/m3)
end
  denfresh=1000;      % density of fresh water (kg/m3)
  densalt=1025;       % density of salt water (kg/m3)

  mus=1.17e-3;              %   coefficient of shear viscosity (N.s/m2)
  muv=2.81*mus;             %   coefficient of bulk viscosity (N.s/m2)
  power=9.-1520./(T+273.);   
  Mg=21.9*10^(power);        % relaxation frequency for MgSO4 (Hz)

  patm=densalt*9.8*D*1e-5;   % pressure due to water column in atmospheres
  omega=2*3.14159*frq;       % angular frequency (rad/s)
  vel=1449.2+4.6*T-5.5e-2*T^2+2.9e-4*T^3+(1.34-1e-2*T)*(S-35)+1.6e-2*D;

% vel                'speed of sound in SALT water (m/s)

  velfresh=1449.2+4.6*T-5.5e-2*T^2+2.9e-4*T^3-35*(1.34-1e-2*T)+1.6e-2*D;
% velfresh           'speed of sound in FRESH water (m/s)

  k=omega/vel;               % wavenumber (1/m)
  ka=k*sedrad;               % dimensionless constant - k*sediment size
  rnear=omega*tranrad^2/vel; % range of the near-field

  AlphaW=0.5*(omega^2)*((4*mus/3)+muv)/(denfresh*velfresh^3)+(S*2.34e-9*Mg*frq^2/(frq^2+Mg^2))*(1-1.23e-3*patm);
  AlphaS=0.18*k*(ka^3)/(densed*(1+ka^2+0.24*ka^4));

  beta=(omega/(2*mus))^0.5; tor=0.5+(9/(4*beta*sedrad));
  s0=(9/(4*beta*sedrad))*(1+1/(beta*sedrad)); densig=densed/densalt;
  
  AlphaV=(k*(densig-1)/(2*densed))*(s0/(s0^2+(densig+tor)^2));
  
  AlphaS=AlphaS+AlphaV;  % combined attenuation due to sediment