HOW DO YOU MAP THE OCEAN FLOOR WITH THE HMR-1?

By Katy Myrick

The HMR-1 uses two types of images that they put side to side. One is called bathymetry and the other is echo sounding. The difference in the two is that bathymetry tells you the depth and the shape of the ocean floor, and echo sounding is very much like a photograph. Echo sounding gives you a good idea at what the formations look like and maybe what they are made of, but it is only two dimensional. You do not know what is going up and what is going down. When you combine this with the bathymetry map that shows the depth and shape you have a three dimensional picture.

Now, how does the HMR-1 take these pictures?

The HMR-1 is towed behind the ship, because of this it is referred to as the tow fish. It has two transducers on it. One on its port side (left) and one on its starboard side (right). These transducers send out a ping and measures the time it takes for the signal to go and come back. Since we know how fast sound travels in sea water we can calculate the distance that the sound has travelled. Thus we know the depth. One transducer is set at 11 megahertz and the other at 12. This way they do not hear the others signal. Each transducer is set at a 60 degree angle so that you have a total area covered of 120 degrees. The bathymetry is a measure of how long the signal takes to come back and the angle that it comes back in. The echo sounding is measuring how strong or loud the signal comes back. The signal will come back loud on hard objects like lava, and they appear black on the picture. The signal comes back soft on objects like mud or sediment and appear light on the picture.

The only problem is that sound travels faster in a denser substances. Since cold sea water is denser then warm sea water it would travel faster in the colder water. They have to consider this in their calculation. The way they find out where the cold water is to put an instrument down into the water called an XBT. This machine tells us the temperature of the water as it goes down to the ocean floor. They are looking for the thermocline, which is the area in the water column where the water starts to get cold very fast and then levels off. Then they place the tow fish at a depth below the thermocline.

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