By Edwin Schiele
October 6, 1998. One hundred and twenty-five floppy disks every 15 minutes. That is how much data the DSL 120 produces 24 hours a day as it maps a swath of Puna Ridge. In its rawest form, the data from the DSL 120 is a series of numbers. These data then undergo a series of complex transformations. The result is a series of colorful, accurate maps showing the contours and features of Puna Ridge. Lets follow this metamorphosis from numbers to maps.
|Learn more about how to interpret side-scan sonar images.|
The DSL 120 is equipped with two side-scan sonarsone that maps to the right, and one that maps to the left. Each sonar has one pinger that sends sound waves to the ocean floor once every 0.8 seconds, and two receivers that catch the echoes. The sonars gather two types of information. First, they measure the intensity of the returning signal. Just as a tennis ball bounces higher off of a driveway than off of the soft grass, the sound waves bounce harder off of hard surfaces than soft surfaces. Second, the sonars measure the bathymetry or contours of the surface it is mapping. Here is where the two receivers come into play. When the pinger bounces a signal off of a flat area, the echo should reach the two receivers at the exact same time. If, however, the pinger bounces the signal off of a slope, the echo returns at a different angle and reaches one of the receivers before the other. The time interval between when the echo reaches the first receiver and the second receiver is called the phase shift. The length of the phase shift depends on the slope.
These two numbers, the intensity and the phase shift, travel up the cable and into the control van. These raw numbers are recorded on tape cassettes that are stored away and used as backups. The raw numbers are also routed through a computer that converts the phase shifts into slopes and the intensities into different colors. The result is that beautiful colorful display on the monitor that I must stare at for four hours every morning during my watch. At the same time, a thermal printer to my left produces a black-and-white printout showing the contours of the ridge. From this printout, it is usually possible to pick out volcanic features such as cones, craters, and fissures. (To learn how to interpret side-scan sonar data, see Lesson 1 in the Geological Interpretations Module.)
At this stage, the maps are still far from complete. First, there are glitches in the data. Characters might be dropped. Time sequences may be misread. These mistakes in the data can distort the features on the printout and even create features that dont really exist. Second, the printouts dont take into account changes in speed, altitude (distance from the bottom), the heading (direction the fish is pointing as opposed to the direction it is moving), or the fact that the fish is pitching and rolling as it is being towed along. Lets look at an example of how these factors can distort the printout of the ridge. Suppose you wanted to photograph two people standing next to each other. When you photograph the first person, you stand five feet away and keep the camera upright. When you photograph the second person, you step back twenty feet and tilt the camera to one side. Now put these photographs together. It looks like the second person is much smaller than the first person and is lying on his/her side. Obviously that wasnt the case at all. The changing perspective of the camera has created this illusion. In the same way, changes in the position of the sonar in relation to the ridge it is mapping will create a distorted picture of the ridge.
Peter Lemmond and Steve Gegg must turn these imperfect pictures that stream in 24 hours a day into accurate, useful maps. To do so, they apply a series of filters to the endless streams of data that come in, whether it is the sonar bathymetry data, the altitude data, the heading data, or the navigation data. These filters weed out the noise and artifacts from all the data that are real. There is an art to this process. Peter and Steve must determine which filters to apply and how heavily to apply them. If they are too liberal with the filters, they may weed out some of the good data along with the bad.
For the final step, Peter and Steve must match the bathymetry and sonar data with the speed, altitude, and heading of the fish. Most importantly, they must match all this information with the exact location of the fish in relation to the transponders. As you know, maps are useless if they show features in the wrong places. Time is the unit that links all of these pieces of information together. So at any given moment of time, we know the speed, altitude, heading, and exact position of the fish in relation to the sonar images. If any one of these strings of information is just a few seconds off, the map will be incorrect. Once all these data are pieced together, Steve and Peter run programs that adjust the images to account for changes in altitude, heading, speed, and position.
The results of this hard work are beautiful, accurate maps that the scientists depend on for studying the processes that shaped Puna Ridge.