Underwater exploration
Less than 10% of the ocean has been mapped in any detail, less has been visually observed, and the total diversity of life and distribution of populations is similarly obscure.
Underwater exploration is largely a recent development, as it relies heavily on fairly advanced technology over almost all of the relevant territory.
Deep-sea exploration is the investigation of physical, chemical, and biological conditions on the sea bed, and water column beyond the continental shelf for scientific, commercial or other purposes.
In general, modern scientific deep-sea exploration can be said to have begun when French scientist Pierre-Simon de Laplace investigated the average depth of the Atlantic Ocean by observing tidal motions registered on Brazilian and African coasts.
The first deep-sea life forms were discovered in 1864 when Norwegian Michael Sars obtained a sample of a stalked crinoid at a depth of 3,109 m (10,200 ft).
The equipment used varies depending on the circumstances, and ranges from breath hold to surface supplied, but almost all cave-diving is done using scuba equipment which gives the diver greater range and autonomy, but with a limited breathing gas supply, often in specialised configurations with redundancies such as sidemount or backmounted twinset.
[9] Surface coordinates can be collected via GPS and remote sensing, with varying degrees of precision and accuracy depending on the type of entrance.
In some caves the water surface is in view of GPS satellites, in others it is a considerable distance along a complex route from the nearest open air.
[9] Where the depth or other constraints prevent divers from exploring in person, tethered and untethered remotely operated underwater vehicles (ROUVs) have been used effectively, using sonar technology to scan and map the surroundings, and video to record the appearance.
[9] Unknown or poorly known parts of the underwater environment may be explored directly by human observers, or measured and recorded by instruments.
Crewed submersibles have a much larger depth and lateral range than divers, but are less dexterous at precision manipulation and handling delicate materials and organisms.
The presence of an operator with a direct view of the environment makes them logistically flexible, and plans can be changed on the fly to take advantage of serendipitous discoveries.
Crewed submersibles allow personal exploration of otherwise inaccessible ocean depths, and can perform a variety of observation, sampling and measurement tasks.
The hydraulic system is then used for propulsion and to power equipment such as torque tools and manipulator arms where electric motors would be too difficult to implement underwater.
The remote sensing platform is often a surface vessel, but may also be a crewed submersible, ROV, AUV, aircraft of satellite.
[10] Blue-green light of 532 nm produced by frequency doubled solid-state IR laser output is the standard for airborne bathymetry.
Turbidity causes scattering and has a signifincant role in determining the maximum depth that can be resolved in most situations, and dissolved pigments can increase absorption depending on wavelength.
[12] The ICESat-2 satellite has a laser altimeter intended for measuring the height of ice, but it was found that underwater reflections were also being recorded along shallow coastal zones.
Sonar is the most effective technology for underwater surveying, as sound propagates through water with less loss than electromagnetic energy, reflects well at a phase interface, can be produced at a wide range of frequencies with varying applications, and can be directionally focused with some precision, but the transducers must be in the water – sound does not propagate well through a gas-liquid or liquid-solid interface.
Three main applications are used: single beam echo sounding is generally used for depth measurement below the vessel, side-scan sonar produces images showing the shape of underwater objects well, but is not very accurate for depth measurement, and multibeam echosounders provide fairly accurate three dimensional positions for a swath of points spread across the track of the transducer array.
[2] A seaborne surface platform may be used as a base to deploy divers, crewed submersibles ROUVs and AUVs, or may be directly equipped with remote sensors.
[14] The 1872–76 Challenger expedition was the first major multidisciplinary undersea survey, which had the primary goal of discovering deep-sea life using dredging and nets, and also made physical, oceanographic, and chemical measurements of the oceanic environment.
[2] Work on mapping the ocean bed accelerated after World War II, when sonar technology made faster depth measurement possible.
[15] The first comprehensive map of the world ocean bottom was published in 1977 by geologists Marie Tharp and Bruce Heezen of Lamont Geological Laboratory at Columbia University in New York, in a collaboration that lasted from the 1950s into the 1970s.
[17] The mass distribution of the seabed topography affects the local gravity sufficiently for satellite radar altimetry to record variations of sea surface height, which can be used to calculate the approximate underwater geomorphology.
