function z=phiu_clay(zr,Lin,zz)
if Lin<0
   zet=(1-15*zr*Lin)^.25;
   zet0=(1-15*zr/zz*Lin)^.25;
   a1=zet0*zet0+1;
   a2=(zet0+1)^2;
   b1=zet*zet+1;
   b2=(zet+1)^2;
   z=log((a1*a2)/(b1*b2))+2*(atan(zet)-atan(zet0));
else
   z=7*zr*Lin
end;

