DEHAVEN Special Operations off TsingtaO (DESOTO) patrols were patrols conducted by U.S. Navy destroyers equipped with a mobile “van” of signals intelligence (SIGINT) equipment used for intelligence collection in hostile waters. 
The USS De Haven (DD 727) is the namesake for these patrols.  The USS DE HAVEN conducted the first patrol off the coast of China in April 1962, and in December the USS AGERHOLM  (DD 826) conducted the first SIGINT patrol to target North Vietnam in the Gulf of Tonkin.

uss-agerholm-dd-826
USS Agerholm (DD 826)

These patrols were initially a response to the Chinese Communists’ unexpected re-definition of their territorial waters to include all waters shoreward from lines drawn tangentially to, and between, twelve mile circles drawn around their offshore islands. Such a declaration represented a huge expansion of their claims. This inhibited the lawful navigation of international waters and increased the likelihood and frequency of formal diplomatic “serious warnings” issued by Beijing when any Seventh Fleet ship navigated through these areas. This became a situation to which Commander Seventh Fleet felt compelled to respond.

These types of patrols had previously been conducted off the coasts of the Soviet Union, China, and North Korea, but are widely recognized for their role in the Vietnam War. There were three components to the purpose of these patrols. First, they would establish and maintain the presence of the U.S. Seventh Fleet in the international waters off the China coast and later the Vietnamese coast. Second, they would serve as a minor Cold War irritant to the Chinese Communists. Third, they would collect as much intelligence as possible during the patrols.

08-02-1964-tonkin1
Gulf of Tonkin

The tactical purpose of the patrols in Vietnam was to intercept North Vietnamese Army intelligence and relay it to South Vietnamese Army forces. With the intercepted communications, the South Vietnamese were able to more effectively coordinate their raids. The USS Maddox (DD 731) and USS Turner Joy (DD 951), the destroyers taking part in the DESOTO patrols in Vietnam were accompanied by air support provided by the aircraft carrier USS Ticonderoga (CV 14) (August 4 and 5 1964).  The events that surrounded these patrols resulted in the Gulf Of Tonkin Resolution, giving President Johnson authority to conduct offensive operations in Southeast Asia.

Sources:
en.wikipedia.org/wiki/DESOTO_patrol
James W. Montgomery, “The First DESOTO Patrol”
National Security Agency, “Gulf of Tonkin”
National Security Agency, “The Gulf of Tonkin Incident”