function [value] = extract(filename,hw)%EXTRACT: Extract a header word from a SU-SEGY file %%   example:    cdp = extract('data','cdp') will provide%               the cdp numbers of each trace in 'data' %               See documentation to see the rest of the header%               words.%%   example:    of = extract('data','offset') will extract offset%               of each trace.%%   example:    D = extract('data','trace') will extract the traces%               into a matrix called D.%%%  SeismicLab%  Version 1%%  written by M.D.Sacchi, last modified March 4, 1998.%  sacchi@phys.ualberta.ca%%  Copyright (C) 1999 Seismic Processing and Imaging Group%                     Department of Physics%                     The University of Alberta%   FID = fopen(filename,'r','n');      % n if for native   segy=segy_struct;                   % load the definitions of                                       % the header words   count=count_struct;                 % load the position of                                          % each word in the header (in bytes)   status = fseek(FID,count.ns,'bof'); % go to begging of file   ns = fread(FID,1,segy.ns);          % read ns from first trace   if strcmp(hw,'ns') == 0;            % get only ns and return    else     value = ns;      return       end   total = 60+ns;                      % total num of 4bytes words                                        % in the header   if strcmp(hw,'trace') == 0;                  elements = 1;     word=zeros(1,1);      else      elements = ns;       word=zeros(ns,1);        end   hc=eval(strcat('count.',hw));   hp=eval(strcat('segy.',hw));   max_traces = 999999;   for j=1:max_traces;     position = total*(j-1)*4+hc;      status = fseek(FID,position,'bof');        if status == 0                  % check status and read         word = fread(FID,elements,hp);        j = j + 1          value(:,j-1)  = word(:,1);      else       return                          % return if end of file reached       end    end  [message,errnum] = ferror(FID)