The Senate unanimously approved ratification of two U.S.-Soviet treaties limiting the explosive force of underground nuclear explosions yesterday as President Bush sent the Senate the recently negotiated pact authorizing the reunification of Germany.

Majority Leader George J. Mitchell (D-Maine) has said the Senate will act on the new treaty, which relinquishes rights and responsibilities of the four victorious World War II powers over Germany and Berlin, before the 101st Congress adjourns next month.

The Threshold Test Ban Treaty of 1974 limits nuclear weapons tests to no more than 150 kilotons, equivalent to the explosive force of 150,000 tons of TNT. The companion Peaceful Nuclear Explosions Treaty of 1975 puts the same limit on tests for peaceful purposes such as excavation or seismic exploration.

Although both countries say they currently observe the 150-kiloton limit, the two pacts languished for years because of concern by the United States about its ability to verify Soviet compliance.

But with the thaw in U.S.-Soviet relations, new verification protocols, providing on-site inspections as well as hydrodynamic measurements and seismic monitoring, were negotiated and signed June 1 by Bush and Soviet President Mikhail Gorbachev during the Washington summit.

In arguing for approval of the two treaties, Foreign Relations Committee Chairman Claiborne Pell (D-R.I.) said the United States and Soviet Union must now move toward further limits on testing, including a comprehensive test ban, as envisioned by language in the threshold test treaty.

But in a cautionary note, intelligence committee Vice Chairman William S. Cohen (R-Maine) said the U.S. must beef up counterintelligence capabilities and warned the White House not to allow changes in verification procedures without Senate consent.

The Senate vote, which amounts to the final step before formal ratification of the pacts by the two countries, was 98 to 0, with Sens. Pete Wilson (R-Calif.) and Warren B. Rudman (R-N.H.) not voting.

At a brief ceremony in the Rose Garden yesterday, Bush called on the Senate to act quickly on the Germany treaty, saying that "four decades of Western resolve and determination" had put the world on the brink of "bright new horizons." The treaty, which ends the divisions between East and West Germany and calls for complete withdrawal of Soviet forces by the end of 1994, was signed in Moscow Sept. 12.