Geophysical engineer and adviser to foreign governments, 1930–1953
Hoover did not stay at Western Air Express, however. During the 1930 election, there were rumors that Western Air Express had only won certain government contracts because of Hoover's status as the president's son, and saying that Hoover's advancement owed more to his famous name than his talent. Hoover submitted a letter of resignation in response to the allegations. Shortly thereafter, he was diagnosed with tuberculosis, and he spent 1931 convalescing, first at Rapidan Camp, then at Asheville, North Carolina. After his convalescence, he briefly returned to the airline, then taught business economics to aeronautical engineering students at the California Institute of Technology. He and his brother Allan also purchased the Herbert Hoover Birthplace at this time.
Hoover's interest in radio next turned him to the field of exploration geophysics, and the use of radio to prospect for oil. He founded United Geophysical, headquartered in Pasadena, California, in 1935 and by 1939 he had 200 employees working in five labs perfecting the art of exploring for oil by seismological means. In 1937, he founded a related company, Consolidated Engineering Corporation, which focused on instrument manufacturing.
Hoover's hearing impairment made him ineligible to serve in the United States armed forces during World War II.
In 1943, President of Venezuela Isaías Medina Angarita invited Hoover to advise the Venezuelan government in the negotiation of oil contracts with foreign governments. While there, Hoover oversaw a substantial rewriting of Venezuela's oil laws, which would provide a model for other countries in the years to come.
In 1944, the new Shah of Iran, Mohammad Reza Pahlavi, hired Hoover's company to advise the government of Iran in the negotiation of new oil concessions. At the time, the only oil company operating in Iran was the Anglo-Iranian Oil Company, operating under the terms of the D'Arcy Concession, as renegotiated in 1933. After the Anglo-Soviet invasion of Iran, Standard Oil, the Socony-Vacuum Oil Company, the Sinclair Oil Corporation, Shell, and the Soviet Union all sought access to the Iranian oil fields. Hoover provided the Iranian government with technical advice about the size of their oil reserves to allow the Iranian government to negotiate a fair deal.
United Geophysical was later bought by Union Oil, though Hoover stayed on as president of the company. Consolidated Engineering went public in 1945 and Hoover sold all of his stock.