Introduction

For most people, the submarine world is inaccessible both because we are not naturally sea creatures and because the science is often difficult to understand. However, technology today allows us to visualise this largely unseen world. This requires several stages including seabed survey, post-survey processing of large datasets, constructing an integrated picture (mosaicing), classifying the features, constructing a 3-dimensional model (visualising) and interpreting the story which the images tell.

Seeing the seabed

Early charts were for coastal navigation. The oceans were known to be deep, but the ocean floor was often regarded as a flat plain. Submarines can only operate at depth if navigators know the shape of the seabed over which they sail. If the ship’s position, the depth and the shape of the seabed are known, the ship can be “flown” over the submarine landscape like an aircraft navigating safely through a mountain range. To “see” the submarine landscape we use sound.

Sound in the sea

Sounds occur naturally through mechanical disturbances of water such as waves and cetacean sonic pulses. Sound helps identify objects in the sea, including prey and the seabed. Sound travels very efficiently through water, although water density, temperature, pressure and chemical composition affect its speed.

Surveying the seabed

Sound is used to survey the seabed. Echo-sounding uses repeated predictable signals to determine the depth of the seabed. Knowing the speed of sound through water and the time taken for the echo to return allows the depth to be estimated. The echo pattern also provides information about the nature of the seabed.

Sidescan sonar uses sound emitted by transducers as fan-shaped sound pulses. As each pulse is emitted and reflected back to the ship, an image of the seabed grows line-by-line along predetermined overlapping tracks. The intensity of the echo depends on the topography and material. Surfaces angled towards the transducer produce a strong echo whilst surfaces hidden from its ‘line of sight’ result in a shadow or area of weak backscatter.
We see seabed features by using sound to produce patterns of “lit” surfaces and shadow. The seabed is ensonified rather than photographed. Unlike true pictures which rely on reflected and retransmitted light, these “pictures” rely on sound reflections from the sea bed.

Post-survey Processing

Each individual track is merged, after corrections for potential errors, into a single mosaic, generated as geo-referenced seabed images. Features can be measured and located on the image. Seabed forms similar to landforms such as ripples, rock ledges, cliffs and boulders can be classified and mapped. 3-dimensional models of the seabed can be constructed which allow visualization of the seabed landforms. Combined with the terrestrial landscape, this provides a continuous image of the coastal landscape.
Interpreting

In Weymouth Bay, the seabed is cut across a wide range of different strata and structures. It is a very flat landscape compared to the surrounding land with many small ledges, accumulations of boulders and areas of sand ripples typifying of this submarine landscape.