SACST_dc_psv_nmodel.m

function [REDsu,m1,Esu1,m0,Esu0] = SACST_dc_psv_nmodel(...
    sacst,...                      % sac data
    t0,t1,fs,...                   % time window for downward continuation
    nlyr,nmod,vp,vs,rho,thik,...   % earth models 
    sachd_rayp)                    % sac head field for ray parameter
%downward continuate surface plane wavefield through earth models
%output wave vectors in the first/last layer(m0,m1), and 
%Su energy reduction ratio 1-Esu1/Esu0;
%size(vp) = [nlyr,nmod]; %same for vs,rho,thik
%size(m1) = [4*nevt,nt,nmod]; %same for m0

%% parameters

%sac
% sacdir = 'sac/';
% saclst = [sacdir,'evt.lst'];
% cmpnm = {'.BHR','.BHZ'};

%time samples
% t0 = -5;
% t1 = 10;
% fs = 20;

%Earth model (ice/bedrock)
% vp = [3.87 5.8]; % km/s
% vs = [1.95 3.46]; % km/s
% rho = [0.917 2.72]; % g/cm^3
% thik = [35 0];  % km
% nlyr = 2;

%thickness of ice sheet
% z = 1:0.1:5;

% sachd_rayp = 'user0';

%% load data

%load sac files
% sacst = SACST_fread('list',saclst,'prefix',sacdir,'suffix',cmpnm);

%time window cut
dt = 1/fs;
t = t0:dt:t1;
sacst = SACST_interp(sacst,t,'ref','0','rmnan',1);

%get ray parameters
str_cmd = sprintf('[sacst(:,1).%s]',sachd_rayp);
rayp = eval(str_cmd);

%% downward continuate the surface wavefield

nevt = size(sacst,1);
nt = length(t);
v0 = zeros(4,nt);
m0 = zeros(4*nevt,nt,nmod);
m1 = m0;

for ievt = 1:nevt
    
    %state vector on the surface
    v0(1,:) = sacst(ievt,1).data;
    v0(2,:) = -1*sacst(ievt,2).data; %downward positive
    
    %mode vector in the ice/bedrock
    idx4_evt = 4*(ievt-1)+(1:4);
    for imod = 1:nmod
        [mm1,mm0] = dc_psv(...
            nlyr,vp(:,imod),vs(:,imod),rho(:,imod),thik(:,imod),... % earth model
            nt,v0,fs,...            % time samples of velocity-stress vector
            rayp(ievt));            % ray parameter
        m1(idx4_evt,:,imod) = mm1;
        m0(idx4_evt,:,imod) = mm0;
    end
end

%% calculate Su energy reduction ratio

Esu0 = zeros(nevt,nmod);
Esu1 = Esu0;
for ievt = 1:nevt
    
    %coef. from wave vector to energy
    %first layer
    qs0 = sqrt(vs(1)^-2-rayp(ievt)^2);
    coef0 = rho(1)*vs(1)^2*qs0;
    %last layer
    qs1 = sqrt(vs(nlyr)^-2-rayp(ievt)^2);
    coef1 = rho(nlyr)*vs(nlyr)^2*qs1;
    
    idx0 = 4*(ievt-1);
    for imod = 1:nmod
        Su0 = m0(idx0+4,:,imod);
        Esu0(ievt,imod) = coef0*sum(Su0.^2);
        Su1 = m1(idx0+4,:,imod);
        Esu1(ievt,imod) = coef1*sum(Su1.^2);
    end
end

%Su energy reduction ratio
REDsu = 1-Esu1./Esu0;

end