: Tannins shouldn't bother sonar, even a fish finder can find things
: on the bottom, just takes a fairly involved search pattern.

: Bill H.

Oh, we've got sonar, lidar, and magnetometer traces galore. We beat the nose flat on a very expensive sonar fish one day.

This is a man-made lake for power generation. The original lake bed is the deep part where we're concentrating the search, but most of it is about 20 feet deep or so. The are rock formations that come out of nowhere in the middle of a flat plain, and go vertical to within inches of the surface. Bad towed sonar conditions. The lake is a 'T' shape, about 2 miles by 3 miles. It's big. It used to be used for logging, so there's lots of old log boom chains on the bottom. I've got a map with over 300 mag hits on it.

The military and the oceanographic guys had a (top secret at the time) lidar system there for a while, and an ROV. The ROV's operator, used to salt water searches, was confident he'd be successful in no time flat. We told him the conditions we'd been diving in. Very low vis due to tannins, three feet of loose silt causing complete silt-outs if any contact is made with the bottom, etc. The first ROV splash, they thought their camera had malfunctioned. They brought it up, attached a third camera and tested the other two. Down it goes, absolute soup on the screen. They asked us to look, and we said, "Yeah! That's exactly what we see down there!"

One summer, they had a GPS transmitter for accuracy within inches, computers linking the sonars and GPS, and ran patterns for a week. We got some interesting targets, but the computer's CPU speeds were not up to the task, and the sonar target 'marks' and GPS coordinates weren't syncing right. The had a target one day they thought was 20 feet high off a 35 foot bottom. Hugh mound. My partner and I were the divers for the day, so we dropped off the zodiac by the pontoon boat, straight down to a flat bottom. I ran out a 100 foot reel and did a 360 sweep to see if the line snagged anything. Nothing. Thats the way it goes out there on this monster damn lake.

There's been hundreds of man-hours, and hundreds of bottom time hours by my group, military divers and others, but the Hudson is still hiding.

We'll find it one day.

Kirk