Identify features in side scan sonar images
Last time we learned what side scan sonar is and about how we will use the DSL-120 sonar instrument to survey the Puna Ridge. The DSL-120 collects both side scan sonar imagery and bathymetry at the same time. They complement each other by providing two methods for determining whether a seafloor feature is a locally-high spot, or a valley. In addition, we saw in Lesson 1 that the sonar data are very important for mapping very small features, such as fissures or lava flows that are less than several meters high, because these low-relief features do not show up in the bathymetric data.
In this lesson, we will look at more side scan data and show you the different types of volcanic and tectonic features we have come across. We'll use data that were collected along the Mid-Atlantic Ridge, the East Pacific Rise, and just the other day along the Puna Ridge. You will see that there are similar features on all images. We infer from these observations that the products of seafloor construction (seamounts, pillow lava ridges, fissures, faults) are common in all oceans.
|Image from the Mid-Atlantic Ridge axis showing hummocky pillow ridge and faults with small scarps. The sonar vehicle sent out energy from the left towards the right, casting a shadow behind (to the right) of the ridge. Based on the intensity of the returning sonar energy (white or black line), we can tell whether these faults face inward toward the rift axis, or outward away from the rift axis.|
First, let's look at tectonic features. By tectonic, we mean those features which cause deformation to the seafloor. Deformation describes processes such as faulting, shearing, folding, and compression or extension between rocks. The features that result include fissures, cracks or breaks in the seafloor which show no vertical offset between the two sides, and faults, in which one side is displaced relative to the other to form a fault scarp or cliff face. Based on sonar data from the rift axes, fault traces show up as either black (if a shadow is cast) or white (as a reflector) lines. The lines do not appear to be straight, but are jagged and sometimes curvilinear in map view (view from the top looking down) and following the shapes of topographic features
|Image from the Puna Ridge showing fissures and small graben features which cast a sonar shadows. The sonar vehicle sent energy from the top towards the bottom of the picture, so shadows are also observed behind the volcano mounds.|
|Image from the Puna Ridge showing a seamount with a summit crater. The sonar vehicle sent out energy from the top of the picture downward to the bottom, so we see a shadow closest to the vehicle and a reflector corresponding to the far wall of the crater is "seen" by the vehicle. A hill, which cast a bright crescent-shaped reflector, is also observed; the hummocky texture at its top suggests it is a mound of pillows.|
Now, let's look at volcanic features. On the seafloor, the most common types of volcanism that we see are pillow lavas (4), and only occasionally, we observe sheet flows, which have very low relief (less than 12 inches). The pillow lavas show up as scalloped sonar textures which together give the appearance of a hummocky or lumpy seafloor relief. Pillow lavas not only cover the seafloor, but they also build mounds, hills, and ridges. So far during our Puna Ridge rift axis survey, we have also noticed many flat-topped seamounts which have craters on their summits. This same type of seamounts are seen everywhere on the Mid-Atlantic Ridge rift axis. We can tell from the sonar records when there is a hill or seamount, because there is an acoustic shadow behind the feature (since no sound energy can get behind the hill and return back to the DSL-120 vehicle).
|Side scan image from the East Pacific Rise showing dark elongate areas corresponding to sheet lava flows. These were "ground-truthed" by collecting seafloor photos of the area. The black color resulted when no energy was returned from the sheet flows; instead the energy was reflected away from the sonar receiver because the bottom was very smooth and had very little relief.|
We've now seen a lot of different sonar images of the seafloor. Next time we will. learn how to place these features in a geological framework. We'll use the locations of the different features to infer something about when they were constructed.
Go to lesson 3.