By WILLIAM J. BROAD Though the Senate voted two years ago to reject a treaty that bans nuclear testing, one of its provisions is alive and thriving: the global network of sensors meant to listen for clandestine nuclear blasts. Though still under construction, the International Monitoring System is already yielding a wealth of science spinoffs, detecting violent winds, volcanic eruptions and the crash of meteoroids from outer space. "It's a vast new tool," said Hank Bass, director of the National Center for Physical Acoustics, based at the University of Mississippi. "For the first time, we'll have a global system of microphones listening to the atmosphere of the planet." The Comprehensive Test Ban Treaty calls for 90 countries to be host to a network of 321 stations whose sensors monitor the land, sea and air for faint vibrations and other telltale signs of nuclear blasts. More than 100 stations are now relaying data by satellite and cable to Vienna, where 220 people work at the system's headquarters. Despite the Senate rebuff in 1999, the United States is a major backer of the monitoring system. It pays about a quarter of the total costs, and United States technical and scientific support is regarded as crucial to the network's success. Earlier this year, some treaty opponents tried to halt the financial aid, saying the ban's goals were illusory or contrary to American interests. But its backers fought back vigorously, led in part by Senator James M. Jeffords of Vermont, whose defection from the Republican Party put Democrats in control of the Senate earlier this month. Battles over the monitoring system continue in Washington, and it is unclear if American support will continue. Experts on both sides say the existence of an effective monitoring system, which its proponents see as central to treaty policing, would increase the chances that the accord might one day be revived. In all, the surveillance system is to have 170 stations that detect underground shock waves, 11 that track undersea explosions, 80 that sniff the air for telltale radioactivity and 60 that listen for revealing sounds in the atmosphere, including winds and shock waves. Dr. Gerardo Suarez, a geophysicist from Mexico who directs the International Monitoring System in Vienna, said the emerging network was starting to excite experts far beyond the world of arms control. "The scientific community is awakening to the enormous possibilities," he said in an interview. Interested groups, he said, include the World Meteorological Organization, which wants wind data for global weather forecasting, and the World Health Organization, which wants to track radioactivity in the atmosphere. "It's a tremendous challenge," Dr. Suarez said of building the global network. "There's never been anything like it. We have stations from the Arctic to Antarctica." New additions to the surveillance system include ground-based microphones that listen to the air for low- frequency sounds far below the range of human hearing. Dr. Douglas Christy, head of the acoustic group in Vienna, said that by the end of the year some 20 of the 60 sound stations will be operational. "Things are moving along very rapidly," he said. "It's hectic. But we're happy with it." On April 23, the fledgling system detected a speeding meteoroid that crashed into the atmosphere over the Pacific, where it produced a blast nearly as powerful as the atomic bomb dropped on Hiroshima. In the past, such explosions often escaped notice because they usually occur over the sea or uninhabited lands. The new information will help scientists calculate how often these strikes occur and the odds of "doomsday rocks" hitting the planet. Today, the International Monitoring System and its member states are keeping the data private among themselves until global agreements can be made for its wider release, Dr. Suarez said. A few nations, he said, fear that improper analysis of the data might confuse small explosions in the mining or construction industries with clandestine nuclear blasts. Preliminary work on the monitoring system began in late 1996 after the treaty was opened for signature and has been accelerating ever since. In the United States, the Defense Department does much of the work. Treaty opponents have argued that small blasts can elude the monitoring system and that America might one day need to test its old nuclear arms or design new ones. When the Senate in 1999 rejected the treaty, conservative Republicans tried, but failed, to cut the monitoring funds as well. Early this year, just after President Bush took office, they launched a new drive. On March 12, Senator Jesse Helms, the North Carolina Republican who then was chairman of the Foreign Relations Committee, wrote the State Department to urge that the United States remove its signature from the test-ban treaty and "terminate funding" for its organizations, including the network of sensors. On April 4, 10 Senate Republicans, including Mr. Helms and Trent Lott of Mississippi, then majority leader, made the same argument to Donald H. Rumsfeld, the defense secretary. "We urge you," they wrote, "to terminate Defense Department efforts to implement the treaty." Treaty opponents call support for the system — or any provision or organization called for in the treaty — a surrogate for backing the treaty itself, which is why they want the monitoring effort halted. Frank J. Gaffney Jr., a former Pentagon official who opposes the pact, said in an interview that the monitoring is "a backdoor way to get us" into the treaty. Mr. Gaffney, who directs the Center for Security Policy, a private group in Washington, said establishing the monitoring system "creates a rubric in which a future administration might endorse the treaty." Senator Jeffords, a longtime treaty supporter, fought back on April 6, urging Secretary of State Colin L. Powell to persevere. "We must avoid any weakening of our commitment to international nuclear test monitoring," he wrote in a letter with Senator Lincoln Chafee, a moderate Republican from Rhode Island. A few weeks later, on May 10, Secretary Powell told Congress that the Bush administration would seek $20 million for the test-ban work next year. That figure is what the program office in Vienna had requested. Secretary Powell is one of the few officials in the Bush administration to have supported the Senate's approval of the treaty, which he did in January 1998 along with three other former chairmen of the Joint Chiefs of Staff. Mr. Jeffords, in announcing his departure from Republican ranks on May 24, made no mention of the test ban or its monitoring. But aides said the topic was one of many where he foresaw growing disagreements with the Bush administration and Senate leaders. Daryl Kimball, executive director of the Coalition to Reduce Nuclear Dangers, a private group in Washington, said the Senate's shift into Democratic hands will aid the monitoring and "make life far more difficult for the Dr. Strangelove caucus." If the United States and the 159 other nations of the treaty organization maintain their contributions, construction of the monitoring system could be completed by late 2005, Dr. Suarez said. That is somewhat behind the schedule envisioned a few years ago. By late this year, he said, his team will have finished surveying 90 percent of the proposed station sites around the world, many of which lie in remote or inhospitable regions. In the United States, despite the political clash over monitoring, 26 of 37 planned stations have already been built, a Bush administration official said. The White House might want to pull out of the monitoring program after it finishes its reviews of nuclear policy, the official added. But the president and his aides, though largely treaty opponents, will probably choose to avoid that step and the likely uproar. "The politics are really hairy," the official said. "They may want to let it limp along because of its high political profile." Copyright 2001 The New York Times Company