A team of Navy divers heads into Narragansett Bay Thursday to demonstrate how the new system finds swimmers and divers.

MIDDLETOWN — The diver appeared on the sonar display and an alarm sounded.

“I have an underwater contact that’s coming toward me, and I don’t want it to,” Navy Reserve Capt. Keith Bruce said.

The diver was 380 yards offshore and quickly approaching the Navy pier, despite sub-surface alarms and a verbal command to surface.

“Arm the gun,” Bruce said. “I’m going to engage the contact.”

An underwater air gun fired four times, its movements displayed on a television screen, its explosions audible in a tent on the pier.

By the time it quieted, a diver was bobbing on the surface.

In the recent test of a new anti-diver detection system, the diver off the Middletown coast was a Navy volunteer, arriving on schedule and trailing a buoy.

But the adversaries the technology is designed to spot are less cooperative, aiming to deposit explosives by Navy ships and swim off before an explosion kills U.S. sailors and destroys millions of dollars in military equipment.

The United States has discovered al-Qaida diving manuals abroad, and recently the FBI asked dive-shop owners to be alert for terrorists seeking scuba training. Countries considered hostile to the United States, such as Iran and North Korea, have trained divers in their militaries.

“The threat is real,” Ron Carmichael, a Navy systems engineer in Washington, said last week as he observed the system test. “It’s about catching the guy before he gets close enough to do any damage.”

The Coast Guard already deploys similar systems at strategic locations, and Navy ships at foreign ports guard against underwater threats by using radar, sonar and infrared technologies.

But the system being tested in Middletown, by Naval Undersea Warfare Center researchers and 10 military contractors such as DRS Technologies Inc., is the first to employ software that incorporates data from multiple types of sensors.

The goal, Carmichael said, is to reduce false alarms caused by surf, sea mammals and small boats, easing the demands on port patrols while heightening protection against incidents similar to the 2000 attack on the American destroyer Cole, severely damaged while docked in Yemen.

Existing systems require constant monitoring, and suspicious objects must be inspected manually by sailors aboard patrol boats.

The new technologies, Carmichael said, do not need to be watched. When a suspicious object is identified, an alarm notifies the patrol to track its trajectory and, if necessary, to deploy the underwater alarms and air gun.

In the United States, the Navy says, a terrorist could lay mines in commercial shipping channels, place a bomb on a ship in port or damage a bridge’s foundation.

The prototype “data fusion” system being developed in Rhode Island, however, is designed to be operated by mobile teams that will accompany Navy expeditionary forces.

But for that mission, the Navy will have to find a lightweight sonar head. The one used in last week’s test — a bulky, blue egg made by the British firm QinetiQ — weighed 600 pounds. (Next month, several companies, including Sonardyne International Ltd. will come to Middletown to demonstrate deployable sonar heads.)

Navy researchers are also struggling to reduce false alarms.

During the test last week, for example, the “hostile diver” appeared to disappear from view several times after first triggering the alarm.

“We just lost all tracks,” Bruce said during the test. “Where are my tech guys?”

bgedan@projo.com