Survey Area Calculation

Mapping the seafloor with the DSL 120 kHz system at the Puna Ridge

The DSL 120 kHz side-scan sonar instrument maps a swath of seafloor as it is towed behind the ship as shown in the cartoon on the left.  The DSL-120 vehicle which houses the instrument is shown in the photograph on the rift as it is being put over the side of the ship.

How long will it take to map the seafloor in the survey box?

The DSL-120 vehicle is towed at 1 knot (which is 1 nautical mile per hour) at an altitude of 100 m (300 ft) above the seafloor.  The side-scan sonar collects a continuous swath of data that is 1 km (0.6 miles) wide. Since no sound is directed straight down, there is a data gap in the middle of the swath shown here as a white strip.

We are surveying the entire top of the Puna Ridge using the DSL-120 side-scan sonar instrument.  In the figure below, the area that we will survey is shown as a rectangle on a contour map of the Puna Ridge. The survey area is 60 km long and 8 km wide and has an area of 480 square km (60 km multiplied by 8 km).

In 24 hours, the ship steams 44 km (24 nautical miles). The survey box is 60 km long so it will take 1.4 days (60 km divided by 44 km/day) to steam from one end of the box to the other. To survey the full width of the box, we will have to go back and forth 8 times (1 km wide swath x 8 swaths = 8 km). Since it takes 1.4 days to run down or up the ridge it will take us 11.2 days (1.4 days/line multiplied by 8 lines) to complete the survey. The region that we will survey in one day is outlined and colored red on the figure below.

Surveying with the ARGO II camera system

The ARGO II vehicle contains electronic still cameras, video cameras, and a 675 kHz high-resolution scanning sonar. ARGO II is towed 10 m (30 ft) above the seafloor at a speed of about 0.5 kt. The ARGO II system is used to survey very small areas.  For example, the region photographed by an electronic still camera is about 3 m across as illustrated to the right. We will pick small areas of the Puna Ridge to survey with ARGO II. The  areas will be chosen based on what we discover during the DSL-120 side-scan sonar survey.

Test your knowledge

How long would it take to get ARGO II photographs of an area the size of a football field (100 m by 100 m)? The size of an ARGO II photograph is 3 m wide.  The ship steams at 925 m/hour when towing ARGO II

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Check your answer below to see if you are right.