ALIA Expedition
Mapping the Ocean Floor

In order to make a detailed map of the ocean’s floor, oceanographers use sonar, in much the same way that topographers use radar to map the land above the ocean. On a modern vessel equipped to gain bathymetric (topography but underwater) data, there is an array of individual sonar transducers, which are like special speakers and microphones, on the ship.  By having many sonar elements, positioned at slightly different angles, it is possible to determine the depth of the ocean for many kilometers to either side of the ship, and by moving the ship at the same time, it is possible to map large areas of the ocean.

Sound travels at a more or less constant rate through water, knowing this, one can bounce a sound wave (which is actually like a beam) off the ground, and back to the ship. By multiplying the time it takes the sound to return by the rate at which sound travels (approx. 1500m/s), it is easy to find the distance the ground is away from the ship. Unfortunately, since the sonar transducers are pointed at angles, this is not the depth at the given point, but a simple trigonometric calculation reveals the depth.

While accurate to say that the distance away from the ground is in concept, found from the time of return, modern systems use a few different methods to calculate the distance. The most accurate method is to fire three beams at the same area, and look at how the frequency of the returning sound waves has changed, relative to the other two. Another, less accurate, method uses the alteration of the return wave’s amplitude to figure out how far the ground is. An easy way to think about the difference between these two methods is the quality difference between FM and AM radio. One relies on Frequency Modulation, and the other on Amplitude Modulation, just as the FM radio stations have better sound quality, so too does the FM sonar transducer; this provides a more accurate depth measurement.

Once the data is collected however, it is still a ways away from being made into a map. Since there are frequent holes in the sonar data, the computer creates a complete, interpolated (educated guess), data set of where the ground probably is. To confirm this however, a person must manually look through all the data and remove points that are clearly erroneous, for there are many environmental and other conditions which can lead to a confusing echo return for the computer. For instance, data at the outer reaches of the array is frequently incorrect because of the shallow angle that the transducers have in relation to the ground, this creates a return which is less than ideal. Contrary to how it might seem however, the oceanographers are not altering the data, they are merely removing data they are unsure of, and marking it as unmapped, so that people do not think that an area has been mapped when in actuality the location has been “mapped” by a computer or sonar generated artifact.

After this, a map can easily be made from the data, which is stored in a series of latitude/longitude and depth files, which can then be displayed in a variety of colorful and interesting charts and pictures, which can help with a variety of activities.

Ryan Delaney onboard the R/V Kilo Moana.
25 April, 2005