This show was on OPB (Channel 10) last night. It's about the Florida aquifers. Interesting for a couple of reasons:
1) they've got people scuba diving in caves, which is always a hazardous business.
2) they are communicating underwater and through rock.
The show is about a group of people who are trying to map at least some of the Florida aquifers. They have two teams. One team is underwater, underground, wearing scuba gear and carrying a special tracking device. The second team is walking around above ground carrying a GPS (to keep track of their location) and a special radio receiver to track the scuba team.
As you may know, sending radio signals through water is a tricky business. The US Navy can send messages to their submarines, but their data rate is so low it makes GOES look fast. You may remember a scene from the movie "Crimson Tide" where they are aboard the submarine. There is a message coming in , character .... by .... character ..... That was not just a dramatic effect, that's about how fast it goes. A few years ago the Navy wanted to bury an antennae under most of Michigan to improve their communications with submarines. Last I heard the residents of Michigan had put a stop to it. Good thing, too, I think. They were going to be pumping megawatts of very low frequency radio waves into the ground so they could talk to their submarines.
Back to the Florida operation. The scuba divers have some kind device that let's them communicate with each other. A little research on the web leads me to believe it is probably an ultrasonic system. See: http://www.divelink.net The divers voice is picked up by a microphone and converted to an ultrasonic signal that is broadcast into the water. The second diver has an ultrasonic transducer that picks up the sound waves from the water. The receiver converts the electrical signal from the transducer, and sends it to the second diver's earphones.
Tracking is done with radio devices. The transmitter and receiver were specially made for this project by (take a big guess) an ex-Navy radio man. These devices were not extremely high-powered, as the divers and the tracking team were able to carry the devices on their persons without having to resort to carts or sleds.
I saw a couple of other interesting things in this show.
- The divers appear to be swimming very easily, not having to work very hard, which led me to think they were swimming downstream,which made me a little nervous. What happens if you come to a narrow spot? The flow of water could trap you there. But later on I saw them trying to get through a small spot, and you could see from the sand blowing by them that they were swimming upstream. All of which makes me think the current in the main stream is not very strong.
- At one point they came across the a homeowners well pump on the end of a pipe stuck right through the middle of natural tunnel.
- At some points in their travels the scuba divers were almost 200 feet below the surface.