The Navy, after sifting through piles of documents and studying interviews conducted with a wide range of present and former acquaintances of four members of the alleged Walker spy ring, has tentatively concluded that it has suffered a "serious" but "not disastrous" loss of its secrets to the Soviets, according to a top Pentagon official who has been briefed on the case.

Other high-ranking Pentagon officials said yesterday they shared that assessment.

Although the concern of Defense Secretary Caspar W. Weinberger has increased as he learns from his frequent damage assessment briefings about what might have been lost, the worst fears of Navy leaders -- that the Soviets would learn the Navy's innermost secrets about submarine warfare -- have not yet been realized, officials said.

Near the top of this Navy list are the advanced techniques for monitoring and, in wartime, destroying Soviet submarines, and for making U.S. submarines invisible during the silent combat that could take place under the sea someday.

No evidence yet is in hand, officials said, to suggest that any of the accused in the Walker spy organization managed to break through the several rings of secrecy around such "black" programs.

A U.S. intelligence official said another prime area of concern is what the Soviets may have learned about top secret communications equipment, encryption techniques and daily code cards from two of the accused who had access to that information while in the Navy.

Under the worst case scenario, the official said, the Soviets could have received manuals on the coding machines themselves, together with "key cards" used to transmit secret messages on cryptographic gear.

With this combination, the Soviets may have been able to detect patterns that could compromise U.S. military codes. There is no evidence to date, officials added, that this has happened.

The FBI and the Naval Investigative Service, officials said, have cast a broad net in hopes of learning what information the Soviets did receive. The FBI, as part of this intensive damage assessment effort, has been giving lie detector tests to present and former acquaintances of the suspects in the Walker spy ring.

"All we've got now are the papers the Russians didn't get," said one Pentagon official.

John A. Walker Jr., 47, a retired Navy chief warrant officer, was arrested May 20 on an espionage charge after allegedly leaving a bag of classified documents for a Soviet diplomat in a rural section of Montgomery County. Three other former and current Navy personnel also have been charged with espionage: Jerry Alfred Whitworth, 45, a retired communications specialist; Walker's son, Michael Lance Walker, 22, a Navy seaman; and John Walker's brother, retired lieutenant commander Arthur James Walker, 50.

The fears of civilian and military officials are offset somewhat because the United States has new ways to use sound waves to find Soviet submarines if the present-day listening systems have been irreparably compromised by secret papers sent to the Russians.

The Soviets have had years to gather information about the Sosus (sound surveillance system), the network of underwater submarine-detecting microphones strung along the Atlantic and Pacific coasts decades ago. Given that, some submarine specialists doubt that the spy ring could add much to the Soviets' knowledge.

Aides said Weinberger is worried about what the Soviets might have learned about the whole range of U.S. military operations and capabilities that uniformed specialists might regard as close to routine.

Asked yesterday if Weinberger's concern has increased because of what he has learned in his most recent briefings, Pentagon spokesman Michael I. Burch said the defense secretary's concern has "gone up" since last week, when he termed the loss "serious. If you want to say even more serious, that's fair enough." Burch added in an interview that it would be premature to characterize the loss of military secrets as the biggest the Navy has yet suffered.

Sen. David F. Durenberger (R-Minn.), chairman of the Senate Intelligence Committee, downplayed the possible damage.

"I'm not that worried about the information" the suspects had access to, Durenberger said. "It certainly wasn't helpful for the information to end up in Soviet hands, but it wasn't of such significance that there's any kind of alarm. I think a lot of information may corroborate stuff that is stolen outright" or intercepted electronically by the Soviets. "But it isn't damaging in the larger sense that, for example, the theft of some plans for some supersecret intelligence collector like a spy satellite might be."

Durenberger added, "I'm not minimizing this. I'm saying it is a good reason for the American public to pressure the bureaucracy of the government to change the way we handle national security information."

From the professional military viewpoint, the investigation to date points to John A. Walker Jr. as the biggest risk because of access to "top secret crpyto" information, service on a nuclear powered missile submarine and work as a Navy radioman at the Navy's Atlantic Fleet submarine headquarters at Norfolk.

"Everything flows through that," retired vice admiral Bobby Ray Inman, formerly head of the National Security Agency and deputy director of the Central Intelligence Agency, said of the headquarters. "If you had to pick out a sensitive facility in the fleet that would rank in one of the top 20 or so, that would be it." Walker served there from 1967 to 1969.

Copies of radio messages between the headquarters and the submarine fleet at sea, Inman said, would be extremely valuable to the Soviets because "there are no other regular sources of submarine information, no constant flow of information about what they do and how they do it."

Inman characterized John Walker's service from 1965 to 1967 as senior chief radioman on the nuclear missile submarine Simon Bolivar as his second most sensitive assignment.

Walker received his "top secret crypto" clearance, allowing access to highly sensitive material, in 1965 and held it until he retired from the Navy in 1976 with the rank of chief warrant officer. Federal court documents say the spy ring may have been in operation as early as 1965.

The communications Walker might have seen, if presented to the Soviets, might appear to have no value today. But submarine specialists said they might be damaging because of what they might reveal about the general pattern and area of missile submarine operations.

Missile submarines must know in advance the features on the bottom of the ocean so they know exactly where they are at every second of their slow patrol. Otherwise, they could not achieve accuracy with their missiles. Also, certain conditions are needed for maximum stealth and reliable communications.

Analysts said that even if John Walker provided the Soviets with sensitive information about the United States submarine force as early as the 1960s, the Soviets are unlikely to change their submarine tactics in a way that would reveal their knowledge to the United States.

Compared with the information John Walker had, the access enjoyed by his older brother Arthur originally seemed to be minor. But some officials recently have become more concerned about the risk he may have posed. Arthur Walker, who joined the Navy as a seaman in 1953, received submarine training and served on a number of submarines in the 1950s and 1960s. He specialized during his career in antisubmarine warfare, and may have told the Soviets about U.S. tactics, Pentagon sources said.

In the late 1960s and early 1970s, Arthur Walker, then a lieutenant commander, was an instructor in antisubmarine warfare tactics at the Atlantic Fleet Tactical School.

Arthur Walker's work at VSE Corp., a Chesapeake, Va., defense contractor where he worked on maintenance schedules for ships, was "almost penny ante" compared to the intelligence potentially compromised by brother John, said former CIA director Stansfield Turner.

Military experts have varying views about the potential damage done by Whitworth, a 21-year Navy veteran who was a communications specialist assigned to duty in the Pacific Ocean. His most sensitive assignment was in 1982 and 1983, when he was communications watch officer aboard the USS Enterprise, an aircraft carrier.

During most of his career he held sensitive jobs handling communications and codes.

Whitworth had access to many manuals on building and operating communications gear. Military officials fear that Whitworth might have given away both the content of messages he read, as well as the detailed plans for the machinery. However, Whitworth's access to the most sensitive material would have been limited.

"A radioman is terribly helpful to you," said George A. Carver Jr., a former CIA deputy director. "It's not the compromise of any single message. It's the compromise of techniques, signatures, call signs, frequencies . . . . In the intelligence trade, there are no secrets more secret, none you want to protect more than those dealing with communications."

Military officials agree that the least informed of the four alleged spies is Michael Walker. Working in the operations department of the aircraft carrier Nimitz, he would have known about the daily workings of the carrier and nearby ships. He had access to materials bound for the "burn bag," a device used in destroying documents, but he saw nothing more sensitive than material available under his relatively lowly "secret" clearance.