Atocha Site Electro-Magnetic Sled Survey
In 2007 Mel Fisher’s Treasures began work with Aquasurvey, Inc. to design and build a sled mounted electro-magnetic detector. This detector can pinpoint targets deeper and more accurately than any other detector on the market today. This summer we have been testing this new system and recently deployed it to the Atocha site to survey the area between the Atocha “Motherlode” and an area 4.25nm up the trail where the 9 bronze cannons were recovered. This area has great potential for having more silver coin chests; silver and gold bars and bronze cannons that have been buried deep in the hard packed mud and lay undiscovered. The bottom topography along this section of the trail is perfect for our new EM sled due to its very flat terrain and lack of coral heads or any other obstructions.

The sled is 13’ long x 13’ wide x 2’ tall and weighs about 1,200lbs. It has one large transmit coil and three individual receive coils that give it the ability to detect objects at great depths. We tow the sled with the “Huntress” at a speed of approximately two knots and use an acoustic transceiver/transponder to accurately log the position of the sled in relation to the vessels differential GPS system. All of the incoming data or “hits” are logged on computers on board the “Huntress” and processed once we return to shore. We then take the “hit” file and overlay it on our site charts to see how the targets relate to other mag hits and artifacts located in the past. If we have a new EM target that does not have a corresponding magnetometer hit, that means we have a non-ferrous target. This could be gold, silver, bronze or copper and these will be the “high priority” targets to be checked first.

Here are some images showing how the first EM targets plotted out. The EM targets are the yellow cross-hair icons with the target intensity in millivolts. Initial test show a silver bar should read about 200mv at 5 feet. As always the mag hits are the red circles with gamma intensity inside. The small yellow dots show our mag runs all overlaid on depth contours. We did a dive on the 1023mv target not too far down the trail from the 9 cannons. (EM Hits-11.jpg) It had no corresponding mag hit which should make it a non-ferrous target and is a very large EM target, which could theoretically be a bronze cannon. Our buoy drop hit the sled tracks in the mud so we feel very good about the coordinates. After searching with a 10” loop detector we only found a few bomb frags to the east, nothing large enough to be our target. This makes me think the target is more than 24” down in the mud. We’re going to go back with a 15” loop and see if we can hit it.

The other target we checked was a 210mv hit with no corresponding mag hit. It was very close to where the Dauntless found the silver bar in 1999. (EM-hits-8.jpg) We had the same results as the other target. It’s apparently too deep to detect with a 10” loop.

This was just a quick list of a few targets on the first two runs. The data is going through full processing now and we will soon have many more targets to plot. If it’s too rough to tow but calm enough to dive we’re going to go out and try again with the larger coil on Monday. The next day calm enough to tow, we’re going to put the silver bar down and confirm our land based tests then continue on with the survey runs.
Sled Pic 1 Sled Pic 2
Loading the Huntress Mission Control
Towing the sled Hit 1
Hit 2 Hit 3
Hit 4 Hit 5
Hit 6 Hit 7
Hit 8 Hit 9
Hit 10 Hit 11
Hit 12 Hit 13
Hit 14 Hit 15

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