PANAMA CITY BEACH, Fla. (AP) _ Finding lost objects on the ocean floor is becoming almost commonplace. Witness the discovery in recent years of the Titanic and the recovery of a Mercury space capsule and wreckage from plane crashes.

Now the Navy has developed a new kind of sonar to take the next step: finding objects buried beneath the sea bottom. Of primary interest are explosive mines hidden under sediment.

Known as synthetic aperture sonar, the experimental device also could be used to look under the ocean floor for bombs, artillery shells and other unexploded ordnance as well as pipelines, shipwrecks and artifacts of interest to archaeologists, or even treasure.

``It's in operation in this experimental mode now and we've gone out and demonstrated it in a number of exercises,'' said Kerry Commander, head of acoustical sensing research at the Navy's Coastal Systems Station near Panama City Beach.

But there are some problems and further research is being conducted before the new sonar, designed to find objects in up to 3 feet of sediment, will becomes operational.

Sonar, which stands for Sound Navigation and Ranging, sends out sound signals, or pings, and analyzes the echo that bounces back. Synthetic aperture sonar, or SAS, combines multiple signals while being towed through the water by a ship or, in the future, aboard a small robot submarine.

``Through signal processing it knows exactly how far it's moved each time and it can basically take the information from each one of those pings and add it up,'' Commander said. The result is a single very high resolution, video-like image of the ocean floor.

The same thing can be accomplished by having a large sonar array about 50 feet long. Signal processing, however, shrinks the length to a more practical 3 feet.

Ordinary sonar was sufficient to find wreckage from TWA Flight 800 in 1996 and this year the small plane that carried John F. Kennedy Jr., his wife and her sister to their death, but it cannot penetrate the sediment.

The experimental SAS was dispatched last year to the scene of the Swissair crash in the Atlantic Ocean off Nova Scotia. The water, however, was clear enough to successfully use another experimental Navy mine-hunting device, a laser camera, to find the wreckage.

Synthetic aperture sonar and the laser camera also have been combined with other sonar, magnetic and chemical sensors in another system being developed at the Coastal Systems Station to find unexploded ordnance and debris.

The Mobile Underwater Debris Survey System, or MUDSS, is designed to help clear bombing ranges and other underwater sites the military has abandoned or closed.

A future use for SAS may be in environmental studies to find animals, such as certain types of shrimp, that burrow into the sediment, said University of Texas research engineering associate Nicholas Chotiros.

``The more of those creatures there are around, the healthier the environment,'' Chotiros said. ``If they happen to disappear all of a sudden, you might have a pollution problem on your hands you hadn't noticed before.''

Chotiros is among more than 100 scientists, engineers and support personnel from Navy, university and commercial organizations across the nation working on an ocean sediment study intended to improve the performance of the new sonar.

While SAS provides a clear image of the ocean bottom, buried objects often appear too fuzzy to determine what they are.

``We don't understand precisely what happens to the image when it gets into the sediment,'' Commander said. ``It distorts the signals coming back.''

To get some answers, the Navy commissioned the five-year, $8 million sediment study that will continue through 2002. Its first sea test began in early October and will continue through mid-November off the Florida Panhandle at Destin and Panama City Beach.

Three research vessels have been operating out of the Coastal Systems Station, placing test equipment on the bottom of the Gulf of Mexico in about 60 feet of water.

A key issue is determining whether one of two theories about sound movement through sediment, or perhaps a combination of the two, is correct, said the study's chief scientist, Eric Thorsos of the Applied Physics Laboratory at the University of Washington.

The older and simpler theory is that porous sandy sediment is just a fluid that is heavier than water. The newer theory is that ripples and other irregularities in the sediment surface change sound penetration.

That may explain why the SAS has detected objects in certain situations when, according to the old theory, it should not have, Commander said.

``We, the technology builders and the sensor builders, will benefit enormously from that information,'' Commander said. ``It will help us eventually to create better images and we'll become a lot smarter about how to go about imaging things that are in the sediment.''