Visualizing the Seafloor
In order to understand the shape of the seafloor oceanographers go out on ships and collect sonar data. Sonar data are collected using echosounders and side-scan sonar systems. The digital data are then converted into maps and images. How does this work?
Sound travels from the ship to the seafloor and is reflected back. Knowing the velocity of sound through water and the travel time one can calculate the depth of the water.
Since World War II echosounders have been used to determine water depths of the oceans. Echosounders are usually attached to the hull of a ship. The echosounder sends an outgoing sound pulse into the water. The sound energy travels through the water to the ocean bottom where it is reflected back towards the source, received, and recorded.
The time that it takes for sound to make the round trip to the seafloor and back is accurately measured. Water depth is determined from the travel time and the speed of sound in water. Water depth can be estimated simply by using an average sound speed and the following relationship: Distance = speed multiplied by time/2
The time is divided by 2 to account for the two-way trip from the echosounder to the sea floor and back again.
How are water depths turned into a map?
As a ship steams forward through the water, multibeam echosounders provide water depths for a swath of the seafloor. The water depths are located in space using satellite navigation. From these data oceanographers can make maps of the seafloor that resemble topographic maps of land areas.
Cartoon from a NOAA web site showing the swath of seafloor insonified by the multibeam echosounder.
Early on, bathymetry maps were drawn by hand. Contours (lines) of equal water depth were drawn though a grid of numbers that had been plotted on a sheet of paper. Colors, put on by hand, indicated regions of equivalent water depth. Eventually computers took over and produced paper charts of the data, contoured and colored automatically. Now computer software allows individual scientists to process the data and display them on their own computer monitors. Maps can be imported into graphic software applications and annotations and other marks added.
Three-dimensional bathymetry of the submarine Puna Ridge. The southeast corner of the Big Island of Hawaii is shown in white and labeled. The Puna Ridge extends northeast from there and plunges to a water depth of 5000 m.
Side scan sonar
Similar to the multibeam echosounder, the sound transmitted by a side scan sonar instrument travels to the seafloor, bounces off of the seafloor, returns to the instrument, and is recorded.
The side scan sonar instrument is towed beneath the ship. Sound is transmitted into the water and images are made based on the strength of the return recorded.
In the case of the side scan sonar, it is the intensity or strength of the returning acoustic signal that is recorded. This is controlled primarily by the slope of the seafloor and what the seafloor is made of. A stronger return is received if the seafloor slopes toward the instrument. Also, the return is stronger if the seafloor is made of bare rock. The strength of the return is much lower if the seafloor is covered by mud or sand.
How is echo strength turned into an image?
The strength of the sound recorded by the side-scan sonar instrument is converted to shades of gray. A very strong return, say from bare rock, is white; a very weak return is black. The echo strengths that fall between these two extremes are converted to different shades of gray. Historically, side scan sonar data have been displayed on a hard copy, paper recorder. The paper chart used to be the most convenient method of displaying and storing these data (as well as bathymetry data). Since the 1980s or so software and hardware were developed to process side scan data using computers and display the data on computer screens.
The figure below shows the side scan sonar data of a seafloor volcano that has a large crater on its top. It is illuminated from the right. Notice that the volcano casts a shadow to the left, and the slope facing to the right is very bright. Smaller bumps also cast small shadows making the topography look lumpy. The image is 3 km (1.8 miles) wide.
Side scan sonar image of a cratered volcano in the deep ocean. The black circular region at the center of the image is a large hole (crater). It formed when hot rock (lava) drained away and the surface collapsed.
Combining two data sets
It is also possible to combine side scan sonar and bathymetry data if we know their location on Earth (i.e., their latitude and longitude). Below is a figure of the same volcano as shown above. The bathymetry data were obtained from a multibeam echosounder. As mentioned above the contours are lines of equal water depth; the color also represents water depth with reds being the shallowest and dark greens the deepest. The volcano stands about 150 meters (450 feet) above the surrounding seafloor. The dark and light areas seen through the color represent the side scan data shown above. Combining data sets produces an image with much more information then looking at the data sets individually.
Side scan sonar data (dark and light shading) from the figure above draped on to the bathymetry data, which are shown with contour lines and colored for depth. Reds/yellows are shallow, greens are deep.