Vice President M. Venkaiah Naidu on Tuesday called for greater investment to improve the country's infrastructure and said that the Delhi airport will soon be investing nearly Rs 9,000 crore.Naidu was speaking at an event here to unveil two publications - "The Economic Impact of Delhi Airport" and a coffee table book on the 10-year journey of the Indira Gandhi International Airport in its new avatar.The "Economic Impact Study" of the airport has been done by National Council of Applied Economic Research (NCAER).According to the report, operations of the airport have a significant economic and social impact in Delhi-NCR and surrounding regions.