Washington — Little-noticed data in a new Senate report cast doubt on the Reagan administration's charge that the Soviet Union cheats on a treaty that limits the size of underground nuclear tests. The report, produced by the Senate Foreign Relations Committee, quotes the head of the Lawrence Livermore government laboratory as saying, ``The Soviets appear to have obeyed a testing limit.''
In the report, however, Livermore director Roger Batzel says he cannot rule out the possibility that a few Soviet tests could have exceeded the yield ceiling. (Soviets hold second test, Page 2.)
The pact in question is the Threshold Test Ban Treaty (TTB). Signed in Moscow in 1974, it caps allowable nuclear tests at 150 kilotons (kt) of force. The treaty was never ratified by the United States Senate, but both superpowers have pledged to observe it during the last 10 years.
For US scientists, verifying this treaty is a difficult proposition. Because they have no means of measuring the tests at their sites in the Soviet Union, they must make do with seismic listening posts in Norway and other allied countries that record the tremors in the earth produced by nuclear explosions.
Reagan administration officials have long interpreted the information gleaned this way as revealing that many Soviet tests are unacceptably large.
The President's annual report on Soviet noncompliance with arms treaties, released March 10, repeats the assertion that ``Soviet nuclear testing activities for a number of tests constitute a likely violation of legal obligations.''
Indeed, an unclassified Pentagon chart reproduced in the Senate report shows 24 Soviet tests larger than 150 kt since 1978.
The seismic data on which this chart is based are far from exact, however, and scientists at the Livermore lab interpret them differently.
The range of uncertainty for measuring tests is so large, they say, that the Soviet tests that appear to be too large could have been 150 kt or smaller.
Dr. Batzel backs up this less stringent view by pointing to the pattern of measurements of American nuclear tests.
According to exact readings taken on site, US underground explosions have never exceeded the allowable limit.
But measurements taken with the less reliable seismic method give the misleading impression that the US is cheating on the TTB. A chart of seismic information in the Senate report shows 12 US tests since 1976 that appear to have broken the treaty cap.
Chart patterns of US and Soviet test yields measured via seismic listening posts are ``very similar,'' according to Batzel. Since the US is not cheating, Livermore scientists say they do not feel the seismic data show the Soviets are breaking TTB limits, either.
On-site measurements would greatly reduce the uncertainties involved in the estimate process and give both nations more confidence in their estimation of what the other superpower is doing, concludes Batzel in the report, which is a summary of testimony on the testing issue before the Foreign Relations Committee.
Reagan officials agree with the assertion that on-site testing would help. Early this year President Reagan resubmitted the treaty to the Senate for its approval.
But the administration had one reservation: namely, that final ratification not take place until better verification procedures had been negotiated with the Soviets.
The Foreign Relations Committee agreed to this limit. Accordingly, it has recommended that the Senate approve the TTB, with the proviso that the President will reopen negotiations with the Soviets for better verification.
Once a better verification regime, presumably on-site measurement, is settled, the Senate would have to vote yet again on the final treaty before it took full legal effect.
A few members of the committee object to this somewhat peculiar recommendation. ``Under this procedure, the first Senate advice and consent would be a hollow exercise,'' says a report appendix subscribed to by Republican Sens. Nancy Landon Kassebaum of Kansas, Daniel Evans of Washington, and Jesse Helms of North Carolina.
Senator Helms, in another appendix, outlines vociferous opposition to the treaty as a whole. Soviet nuclear tests, he says, often vent radioactive debris in violation of the treaty.
In the face of such callousness, he says, the assumption that the Soviet Union is not breaking the 150-kt limit is ``ridiculous.''