function rho=gardner(v,a,m,rho)

% rho=gardner(v,a,m,rho)
% rho=gardner(v,a,m)
%
% GARDNER estimates a density vector given an instantaneous velocity vector
% and values for the empirical parameters a and m via the Gardner relation
%	rho=a*v.^m (a times v to the mth power)
% If a partial density vector is available, then it may be provided as a 4th
% argument which must be expanded with NaN's to be the same length as v. (Take 
% care that the existing densities are paired with the proper v's.) Densities
% will only be computed with Garnders relation where they are missing and any
% non-NaN input densities are simply passed to output.
% 	v= vector of instantaneous p-wave velocities
%	a= scalar multplier
%		*********** default is .31 **********
%	m= scalar exponent
%		*********** default is .25 **********
%	rho= on input, a vector of known densities which must be expanded with 
%		NaN's to be the same size as v
%	rho= on output, aa vector of densities the same size as v and computed
%		via Gardner's relation where densities were missing on input.
%
% G.F. Margrave, March 1994
%
% NOTE: It is illegal for you to use this software for a purpose other
% than non-profit education or research UNLESS you are employed by a CREWES
% Project sponsor. By using this software, you are agreeing to the terms
% detailed in this software's Matlab source file.
 
% BEGIN TERMS OF USE LICENSE
%
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% of Geology and Geophysics of the University of Calgary, Calgary,
% Alberta, Canada.  The copyright and ownership is jointly held by 
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if(nargin<4)
	rho=nan*ones(size(v));
end

if(nargin<3)
	m=.25;
end

if(nargin<2)
	a=.31;
end

ind=find(isnan(rho));

rho(ind)= a*v(ind).^m;