Sec. 13. The bonds and the interest on the bonds may be secured by pledge or mortgage of:
(1) any property, real or personal, used or acquired or to be acquired and used for the property and the improvements made or to be made on the property;
(2) the net income from the property;
(3) the property and the net income from the property; or
(4) any unobligated net income of any property financed under this chapter;
as the board of trustees may determine.