WASHINGTON--President Reagan yesterday announced he had sent Soviet leader Mikhail Gorbachev a "new, very specific and far-reaching proposal" on nuclear testing that could open the way to U.S. ratification of two longdormant test ban treaties.

The proposal, however, makes clear the United States still intends to detonate a nuclear device underground next month.

The Soviets have put international pressure on the Reagan administration to refrain from testing by announcing their own moratorium, which Gorbachev said Thursday would remain in effect until the United States explodes another nuclear device.

In a written statement issued by the White House, Reagan said he gave Gorbachev a technical description of a new method of detecting and measuring underground nuclear tests.

Reagan said the method, which he called CORRTEX, would enable both superpowers "to improve verification and ensure compliance" with the never-ratified 1974 Threshold Test Ban Treaty and the Peaceful Nuclear Explosion Treaty that was signed two years later.

The two pacts--which both sides have said they would observe, but which both have suggested the other has violated--limit underground nuclear tests to 150 kilotons. Nuclear explosions in the atmosphere were banned by a 1963 test ban treaty.

"I provided to Mr. Gorbachev a technical description of CORRTEX designed to demonstrate how this method will enhance verification procedures," Reagan said.

The president added that he invited the Soviet leader to send his own experts to the United States' nuclear test site in Nevada the third week in April to observe an American test and watch the new detection system in operation.

"I would hope this would provide an opportunity for our experts to discuss verification methods and thus pave the way for resolving the serious concerns which have arisen in this area," Reagan said.

"In making this offer, I made clear to General Secretary Gorbachev that if we could reach agreement on the use of an effective verification system incorporating such a method to verify the [Theshhold Test Ban Treaty], I would be prepared to move foward on ratification of both" the threshold treaty and its companion measure limiting peaceful nuclear explosions.

Several nonaligned nations, as well as the U.S. House of Representatives, have been pressing the Reagan administration to ratify the existing treaties and proceed to negotiate a comprehensive treaty that would ban all nuclear tests, not just those larger than 150 kilotons.

But Reagan has contended that the United States needs to continue testing to catch up with Soviet weapons technology.

Reagan previously had invited the Soviets to observe a U.S. weapons test, but he said yesterday the new proposal's inclusion of a CORRTEX demonstration and the details provided about the new technology made it unique in its "specificity and concreteness."

However, David McKillop, director of the comprehensive test ban task force of the proarms control Committee for National Security, called the new tactic an attempt to resume testing "without the monkey on their back" put there by Gorbachev's moratorium.

Skeptical

"I doubt very much the Soviets will fall for this," McKillop said. "The last time we made this offer, the Soviets said they were interested in ending explosions without coming and watching new ones. I expect they will react even more emphatically" to the new proposal.

The outside specialist acknowledged he had never heard of the new detection method described by Reagan.

The president called CORRTEX "a hydrodynamic yield measurement technique that measure the propagation of the underground shock wave of a nuclear explosion."

Reagan has contended that existing verification techniques are not good enough to determine whether the Soviets have violated various arms control treaties, including those limiting nuclear weapons as well as the test ban agreements.

He has refused to seek Senate ratification of the test ban agreements, insisting that the two superpowers renegotiate their verification provisions.