The frequency of the sound wave is chosen based on how far the wave has to travel. Click image for larger view and image credit.
In the color coded image, warm colors ( red, orange, yellow) correspond to shallower water depths, while cooler colors ( green, blue, purple) indicate deeper depths. A multibeam image of a submarine landslide off the coast of Bermuda.
Click image for larger view and image credit.įigure 4. Use of multibeam sonar to map the seafloor. This approach produced the very dense point cloud data set that revealed the King George wreck in such high detail ( figure 6).įigure 3. For this reason, during the Bermuda cave mapping project, when we needed the highest possible resolution achievable for these greater water depths, we narrowed down the swath angle squeezing all 500 beams into a narrower swath.ĭoing so not only focused more beams on the bottom per unit area across the swath, but also reduce the maximum slant range the ping had to travel at the outer edges of the beam swath, thereby reducing the maximum ping travel time and enabling us to increase the ping rate over what we would have used at a wider swath angle (i.e., higher resolution but narrow coverage). This operational constraint sets an upper limit on the ping rate for a given water depth and swath width combination. The ping rate is also limited by the speed of sound in water, because with most multibeam systems the transducer must wait to receive the last ping sent before generating the next one (i.e., there can only be one ping in the water at a time). Also, if the area being covered is very deep, then the boat must move slower over that area, or the unit must use a higher ping rate, in order to achieve an equal quality image as to images at shallow depths because it takes longer for the sound pulse to return to the transducer. If the boat is moving very fast and the rate that the sonar unit is pinging is slow, then the number of times sound is bounced off the seafloor in the area covered is fewer than if the boat was moving slowly while the unit was set to a high ping rate. The sonar pings several times per second which, with the speed of the boat, determines the horizontal resolution of the images created. The farther away the object the more area there is for the sound to echo off. The ping is emitted in a fan shape outward from the transmitter. The swath width is determined by the depth of the seafloor being surveyed. Multibeam sonar has several transducers that allow a large swath of area to be surveyed at once making surveying much faster and more accurate ( figure 3). Assigning a color range or grey scale for depth can create a top-side view of the seafloor. The more time it takes for the pulse to return, the farther away the object is. A computer determines how long it takes to receive the returning pulse which, when the transducers are pointed toward the seafloor, translates to depth.
The transmitter converts an electrical signal into an acoustical pulse and the receiver converts an acoustical pulse into an electrical signal. The transmitter and receiver are called transducers because they convert energy into another form ( Figure 1). These systems work by transmitting a sound pulse, called a ping, through a transmitter at a specific frequency, and then receiving that same pulse through a receiver placed very close to the transmitter. Multibeam sonar sensors - sometimes called multibeam acoustic sensors or echo-sounders - are a type of sound transmitting and receiving system.
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