Many of the approximately 150 chemical plants and refineries lining the Houston Ship Channel are veritable sitting ducks for terrorist attacks, even though security at some has been increased as the result of overseas violence, authorities say.

Industry officials publicly insist their security systems are adequate.

But Michael Guidry, a Houston security expert, said 75 percent of the companies along the channel are inadequately protected.

And a high-ranking law enforcement official, speaking only with the promise of anonymity, said "anyone could walk right into several of the companies that claim to have tight security."

Guidry, a former hostage negotiator for the state of Texas, is studying security problems in Houston-area industrial complexes for a report scheduled for release this summer.

"It's not a time to get scared about terrorism," he said. "It's a time to get educated about terrorism. It's time to form a plan and resolve any potential problems."

Guidry said Houston's position as the oil capital of the United States makes it a likely target of terrorism, either in the form of bombings or kidnappings.

He said easy access to plants along the channel, nearby highways and ships could result in a tragedy 50 times greater than the April 1947 Texas City disaster that left 576 people dead and 5,000 injured.

The explosion of a French freighter, filled with ammonia nitrate, in the Texas City harbor was measured on seismographic equipment in Colorado.

Lt. Cmdr. Pedro Filipowski of the U.S. Coast Guard in Galveston said: "We can't ignore the possibility of a terrorist strike at a U.S. waterfront. To be honest, I am bewildered there has not been one. There are easy targets all over the place."

Filipowski was a featured speaker recently at an anti-terrorism seminar for oil company officials in Galveston.

The problems of lax security at some of the plants was demonstrated just last week when a visitor found two entry points into one petrochemical facility unprotected. One entrance led to oil storage tanks, each containing hundreds of thousands of gallons of explosive substances. The other led to a tanker being loaded with petroleum products.

Harris County Sheriff Johnny Klevenhagen said his department has been working with area companies to show them faults in their security systems.

"There is a lot of work going on above, below and around the seams," he said. "Virtually all the companies are taking drastic steps to increase security."

The threat of terrorism, Klevenhagen said, also has been a much-discussed subject in monthly meetings with chiefs of police in the county.

But the sheriff acknowledged that much of the recent discussion has centered on after-the-fact response instead of prevention.

The Sheriff's Department, for example would take the leadership role in a mass disaster situation. Police departments would be responsible for carrying out necessary measures within their jurisdictions. The Coast Guard would come to the fore in attacks on vessels, and the FBI would be in charge of negotiations.

Klevenhagen said one major problem for oil companies trying to improve security is the availability of funds in view critically low oil prices.

Union Carbide took the first step in guarding against terrorist attacks last month by tightening procedures for stocking ships at its dock. Foreign seamen must be escorted by plant personnel at all times when on company property.

The New York-based company has had its share of crisis as a result of the deadly chemical leak in Bhopal , India, and the bombing of its Tarrytown, N.Y., plant by demonstrators protesting the company's dealings with South Africa.

Port of Houston officials declined to discuss security measures in detail - a reluctance also expressed by spokesmen for some of the channel-side plants.

Warren Gafford at Ethyl Corp. said his company has not increased security since the U.S. strike on Libya, noting, "We feel we are covered."

But, he said, "We have discussed the potential for any kind of terrorist activity. There has been a great deal of interest in the subject and a great deal of meetings on the subject.

Shell's Bob Gibson said changes have been made at the Dutch company's Deer Park refinery. "If you feel you're secure enough, then you're probably letting down your guard," he said. "And, that would be dangerous. I wouldn't want to comment on any changes. We have made some changes, but it's basically a matter of an overall greater sense of awareness."

At the Port of Houston, with many of its 41 public docks and two bulk handling plants pointed out during public tours on board the "Sam Houston," there have been no changes in security measures.

"We do have controlled access," said spokeswoman Ann Bordelon. "The general public is only allowed in the Pier 8 area without permission or an escort. We have security patrols 24 hours a day."

Many security staff members do carry weapons, but they don't have the authority to make arrests, said Lee Vela, director of communications. "We do have the right to have a police force, but we don't have it now."