
  n = [101, 101];
  d = [20, 20];
  o = [0, 0];

  vel    = 2*ones(n);
  velini = vel;
  for i = 1:n(1)
    for j = 1:n(2)
      if norm([i-51,j-51])<18
          vel(i,j) = 2.25;
      end
    end
  end
%
  model.zsrc = d(1):d(1)*2:d(1)*(n(1)-2);
  model.xsrc = d(1)*2;
  model.zrec = d(1):d(1):d(1)*(n(1)-2);
  model.xrec = d(1)*(n(1)-2);

  [zz xx] = odn2grid(o,d,n);

% The true model and initial model are
  figure;subplot(1,2,1)
  imagesc(zz,xx,vel);xlabel('X [m]');ylabel('Z [m]');a =caxis;colorbar
  tit