Geological Interpretation of Side Scan Sonar Data

Lesson 1

GOAL

Learn how geological maps are made, and how to make and interpret a geological map from side scan sonar and bathymetric data.

BACKGROUND

PROWL research success depends on the construction of accurate geological maps of the data we collect, and then on the intrepretation of these maps. Our geological map will show us variations in the amounts and types of volcanism and tectonism on the seafloor, and from these clues we can infer the time sequence of geological events. In the lessons we post here over the next week or so, we will learn how sidescan sonar images are collected, practice what we learn on sample data from research cruises to the Mid-Atlantic Ridge and East Pacific Rise, and finally, we’ll leap forward and apply these skills to interpret Puna Ridge data as we collect them.

LESSON

To learn how side-scan sonar works and what kinds of features it can image, first go to our Side Scan Sonar Science Factoid. We will be using WHOI’s Deep Submergence Lab’s side scan sonar instrument (DSL-120) to map the rift axis of the Puna Ridge. The sonar instrument, which "flies" over the seafloor at an altitude of 100 m, sends out sound signals at 120 kHz, and the data it collects will allow us to identify features on the seafloor that are only a few meters high. You can find out more about how we are planning the DSL-120 survey by going to our Survey Area Science Factoid.

Now that you’ve learned how sidescan sonar images are collected, let’s look at an area of the Mid-Atlantic Ridge that was imaged by both sidescan sonar and bathymetric mapping instruments.

Contour map and side-scan comparison

The two images shown are of the same area of the seafloor; the image on the left is a bathymetric contour map with a 20 meter (66 ft.) interval between contour lines, and the image on the right is a sidescan sonar image.

What do we notice from comparing these images?

First, and perhaps most important to a geologist, we notice that the image on the left is not precise enough to record some very important features. There are lots of textures in the side scan images, but there are no contours that mimic the shapes of the images. For example, we don’t see any "scallop-shaped" contours that might resemble the hummocky sonar texture, and we don’t see any linear, northwest-trending contours that mimic the trends of the white lines. These features are smaller than the 20 meter interval between contours.

What features can you pick out on the right-hand picture that you don’t see on the left?

The features we found most striking on the sidescan image, but that are missing from the bathymetric map are inward-facing faults, a clearly defined volcanic crater, and small volcanic mounds. Textures that are "bright", or white in color, mean that a strong acoustic signal was returned from the seafloor. The white lines represent sonar returns from small faults or fissures which have nearly vertical, steep walls. We know that the volcanic vent has a enlongated hole in its middle because it casts a black shadow; no acoustic signals are returned from the crater because the sound energy cannot get into it. Finally, we know that the top of this small seamount is relatively flat, because the sonar texture is smooth and the bathymetry map shows that water depth varies by less than 20 meters.

Next time we’ll look at more sonar images to show you other volcanic and tectonic features that we can identify in sonar data.