U.S. and Soviet negotiators have reached virtual agreement on ways to measure some underground nuclear tests, but a final agreement still eludes the two sides, the State Department said Friday.

The result is further postponement of possible Senate ratification of two nuclear test treaties completed more than a decade ago. The treaties would limit underground blasts to 150 kilotons.President Reagan has said he would submit the 1974 accord on weapons tests and the 1976 agreement on tests for civilian purposes only if agreement were reached on air-tight procedures to guard against cheating.

Reagan has accused the Soviets of carrying out tests that probably exceeded the permissible ceiling. Some American experts doubted the Soviets had violated the treaties, which both sides are supposed to be obseving even though the Senate has not approved the pacts.