U.S. officials shipped tons of equipment to the Soviet Union on Friday for an experiment that could speed Senate ratification of two nuclear threshold treaties and possibly lead to a ban on atomic testing, the State Department said.

A C-5B cargo airplane laden with gear was flying from Indian Springs air field, adjacent to the U.S. nuclear test site in Nevada, to the Soviet test site at Semipalatinsk, 2,000 miles southeast of Moscow, officials said.The plane carried 12 scientists and engineers plus equipment to measure shafts being dug for Joint Verification Experiments, said Roger Harrison, an assistant secretary of state, and Troy Wade, acting assistant secretary of energy.

The experiments are designed to demonstrate the reliability a technology the Reagan administration says is needed to verify the threshold treaties, which were reached in the 1970s but never ratified by the Senate.

Negotiations in Geneva are expected to produce an agreement, in time for the Moscow summit at the end of May, spelling out conditions for the experiments.

A more formal protocol might be signed after the tests are analyzed, and the administration would submit to the Senate the 1974 Threshold Test Ban Treaty and the 1976 Peaceful Nuclear Explosions Treaty, which limit the superpowers to nuclear tests with explosive yields of no more than 150 kilotons.

Both sides claim to have observed the pacts, although the Reagan administration has said the Soviets "likely" exceeded the ceiling on some tests.

The next step would be negotiations on limiting the size and number of nuclear tests. A complete ban on nuclear testing could come as the two sides near completion of negotiations on the elimination of nuclear weapons, Harrison said.

"As long as we have to rely on nuclear weapons for our security, we will test," Harrison told reporters.

The Soviets have been pushing for a ban on nuclear tests, and unilaterally suspended their tests for 18 months ending in February 1987. The Reagan administration declined to join the moratorium.

The equipment leaving Nevada Friday will be used for an experiment tentatively scheduled in July to test a U.S. technique, known as CORRTEX, which measures the size of a nuclear blast by means of a cable sunk down a hole within 15 yards of the test shaft. Instruments measure the speed at which the cable is crushed.

The Soviets long balked at using CORRTEX, which stands for Continuous Reflectometry for Radius Versus Time Experiment, saying the size of tests could be monitored using seismic devices.

A team of 40 to 50 Americans will travel to the Soviet test site to monitor the underground explosion of a Soviet nuclear charge near the 150 kiloton threshold.

A team of Soviets will set up their own equipment to measure a test to be conducted at the U.S. test site in Nevada, Wade said.

Both tests are expected to be completed by mid-July, according to arms control officials.

U.S. officials had hoped to conduct the experiments in April, but they were delayed due to negotiations over technical details, one official said.