HOW DO WE STUDY THE SEAFLOOR?

1. Line-sounding – starting around 85 B.C. lead weighted ropes were dropped over the side of the boat and the depth was recorded, typically in fathoms (1 fathom = 6 feet or 1.8 m), when the bottom was reached.

• A problem with line-sounding is that the boat has to be stationary and the line has to be perfectly vertical – difficult with ocean currents.

• Also, if the sediment is very soft (like mud), the lead weight may pass through the bottom and a deeper value will be given.

2. Underwater Photography – important because it provides the most detailed and accurate images of the ocean’s bottom.

• Camera’s can be mounted, on Submersibles (manned), on ROVs (Remotely Operated Vehicles), or towed along the bottom of the ship

• Problems with cameras include the need for artificial lighting, which limits the size of the area that can be photographed. Plus, the ocean is vast – there is too much seafloor for the current number of submersibles and ROVs to photograph.

3. Satellite Altimetry – a technique using radar pulses from satellites in Earth’s orbit to record variations in the ocean’s surface. They do NOT measure the seafloor directly. A surface difference in a broad sense, often translates to the seafloor topography.

• A limitation with this technique is that features that are less than 10 km (6 miles) in size are not detectable.

• Another problem is that near continental margins, sometimes the density of the underlying rock causes variations in sea height, not the topography.

4. Echo-sounding (SONAR – Sound Navigation and Ranging) – started in the 1920’s. A pulse of sound energy is transmitted

electronically toward the bottom. • The time it takes the

pulse to travel to the bottom and be reflected back up to the surface is measured. From this time interval, the depth of the water can be calculated.

DEPTH (m) = TIME of sonar sounding (sec) x 1500 m/sec 2

• When pulses are sent out and received in quick succession, an almost continuous recording of the ocean depth called a bottom profile can be obtained.

Today swath mapping sonar techniques are used to precisely map the seafloor because of the level of detail that can be obtained is far greater than single-beam sonar profiles.

• Multibeam sonar - measures and records the time for the acoustic signal to travel from the transmitter (transducer) to the seafloor (or object) and back to the receiver. – Multibeam sonars are generally attached to a

vessel, rather than being towed like a side scan. Therefore, the coverage area on the seafloor is dependent on the depth of the water, typically two to four times the water depth.

• Side scan sonar is a specialized sonar system for searching and detecting objects on the seafloor. Like other sonars, a side scan transmits sound energy and analyzes the return signal (echo) that has bounced off the seafloor or other objects.

– In a side scan the transmitted energy is formed into the shape of a fan that sweeps the seafloor from directly under the towfish to either side, typically to a distance of 100 meters.

– The strength of the return echo is continuously recorded creating a "picture" of the ocean bottom where objects that protrude from the bottom create a dark image (strong return) and shadows from these objects are light areas (little or no return).

– While the shape of the seafloor and objects on it can be well depicted, most side scan systems cannot provide any depth information.

• Multibeam sonar data undergo computer processing to produce images such as that shown at the left. Colors are assigned to specific ranges of water depths, shading can be added, and contour lines (lines of equal water depth in black -- like "bathtub rings") are drawn with computer programs.

• On the other hand, side scan sonar (black-gray-white image below the colored image) is a specialized sonar system to produce "photo-like" images of the seafloor and is often used for searching and detecting objects, in addition to seafloor mapping.

• While the "texture" of the seafloor can be well-depicted by side-scan techniques, most of these systems do not directly provide information on the water depth, like the multibeam systems.