A collection of blogs and musings from the people that work at the St. Augustine Lighthouse & Museum - Florida's Finest Lightstation.

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Sonar Reveals Urca de Lima

On a volunteer dive training trip to the Urca de Lima site recently, LAMP archaeologist Brendan Burke was busy operating our Klein 3900 sidescan sonar to 'see' the site. This is the first time that the Urca de Lima wreck site, a 1715 Spanish plate fleet ship, has been viewed using this technology and we have presented the findings from this survey here on the Lighthouse Blog!

Earlier, we posted a blog entry about the Urca de Lima trip, explaining the central purpose of the trip and what was performed by the dive teams. However, we did not have the sonar images posted of the site.

Before getting into the images, let me explain a bit about what a sidescan sonar is and how it can be very useful. First, sidescan sonar is simply a device which emits an acoustic wave. This wave emanates from the towfish, which is the big torpedo-like device that gets lowered into the water (see the blog entry "Sonar Survey on the Total Blarney" dated May 15th). As the wave travels through the water it encounters items within the water as well as eventually running into the bottom. When this happens, the wave bounces back and it retrieved by the sonar. Each wave, or 'ping', is then recorded using its duration (how long it took to be sent and recieved), the time of its departure, and spatial location information to then turn this data into an image. What the archaeologist sees on the computer screen is a 'watefall', or scrolling, image of what is passing underneath the boat/sonar. This is all very similar to the fish finders used on recreational vessels but with a much higher level of sensitivity.

The term sidescan comes from the fact that any sidescan sonar will emit one or two beams that travel out through the water on a downward plane. So, if you were a fish looking at the sonar head-on and could actually see the invisible sonar waves, you would see an upside-down 'V' in the water column. The reason for this is so the oblique, or angled, wave can 'see' things that have relief off of the bottom and then interpret this to the sonar user. When we pass the sonar by a crab trap, we see the bottom with the trap sitting on it as well as the shadow created by the height of the crab trap. This process is very similar to going into a dark room and using a flashlight to look at small items on the floor. You can tell how tall they are by the length of the shadow as long as the height of your light is constant. The same thing with the sonar.

So here we were in Fort Pierce with our sonar. Why? Well, for one, despite the fact that divers were investigating the site and recording it using tapes and lines, we wanted to produce an image of the Urca de Lima site that can be used by the state for monitoring uses. If someone were to steal a replica cannon, the anchor, ballast, or hull timbers from the site, we would have a very accurate accounting of what shape the site was in and exactly where the artifacts were located. Secondarily, we were working with e Florida Department of Environmental Protection to demonstrate our sonar for them. They have area of artificial reef currently undergoing mapping by divers and have an interest in how sonar technology can speed up their mapping process.

So, Tuesday morning we met at the Smithsonian Marine Institute boat launch in order to load gear and begin the day. Cpt. Don was our skipper and welcomed me on board and his crew, Eric and Melissa of the DEP, helped to load and set up. Kiera Kaufmann of FPAN was also on board to witness the sonar setup and useage. We worked out way out the St. Lucie inlet and turned north to travel the 1/2 mile or so to get to the site. Once there, we could see the beach beginning to crowd up with divers and support personnel.

First, I set up the gear in order to get the 'fish', or sonar unit, in the water and operational. To prevent any accidents from occuring or injuring divers, we performed the sidescan survey before swimmers, kayakers, snorkelers, and divers entered the water. Once all of the wires were in place, all appropriate switched turned on and the fish was in the water and cleated on to the boat we began.

The site is not that big and to begin I had the captain run a basic ranging pass while I adjusted the range of the sonar beam. From the control computer, a laptop, I could tell the sonar what frequency to use, how far to broadcast the beam, and how fast or slow to ping. It seemed like 30-35 meters was about the right range for the terrain. Keep in mind that this was a half-tide on the way out so there was only 9-13 feet of water under us. The sonar had to be at least a couple feet down just to clear the bottom of the boat hull and so there wasn't a lot of water to work with. Sending the sonar bean out great distances in these circumstances meant that it begin to break up and bounce back to the surface of the water, washing it out.

All equipment adjusted, we began out passes over the site. For this operation I decided to just use line or sight navigation and not set up tracklines to follow on the boat. One disadvantage to the sonar is that the area between the its two sonar waves, and directly under the towfish, is called the nadir zone. Nothing can be seen here since the waves are parted. In order to see the image of this area, you have to make a second pass over the area so the fish can fill in the nadir zone from its previous pass. Usually, survey lines called tracklines are set up to make sure no nadir zones go unfilled. However, for this small survey and with an unknown potential for coral heads getting in the way of our boat, we made multiple passes over the site that I knew would provide adequate coverage.

On our first pass we were able to pick up the cannons from the Urca de Lima on the bottom. We had a copy of the site map on board as well as being able to navigate based on the site bouy placed there the day before. Soon, we were able to see the ship's anchor, ballast, site mooring blocks, and hull remains. This allowed us to figure out where we were on the wreck and then make subsequent passes to record the whole site. A total of 8 passes were made over the area. Two of these produced minimal data and were redundant since I was testing out different sonar settings.

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