263
refraction
warm, clean, shallow waters along many tropical coastlines and are built up
principally of coral, coralline algae, and debris of Mollusca and Foraminifera. They
disappear (a) where water temperatures are below about 14°C; (b) at depths
where the sunlight does not penetrate; (c) where there is freshwater run-off,
and (d) where the water is muddy. A common classification lists reefs as fringing reefs (reefs directly lining shores), barrier reefs (developed parallel to the
shore but separated from it, e.g., the Great Barrier Reef in Australia) and atolls
(circular reefs surrounding deep lagoons, ~Blake Expeditions). Fringing reefs
occur where a hard, stable substrate is present offshore (e.g., an island or a
rocky headland). Barrier reefs grow on the continental shelf at some distance
from the coast. Submarine reefs have a reef-front or fore-reef where corals
grow luxuriously and fish and many other organisms inhabit crevices and
caves. The reef is usually topped by a reef crest which is exposed during low
tide and usually contains a large (calcareous) algae population. The channels
between reefs (interreef channels, or off-reef floor) have a substrate of
unconsolidated, usually rippled sediment. The reef crest can merge into an
intertidal reef flat with often deep pools, flourishing coral and coarse sediment. Reef flats often (partly) enclose a shallow lagoon, covered with calcareous mud and patchy reefs. Beaches (cays) are formed from gravel- or
sand-size coral fragments; beach rock is often present. On intertidal muds
mangroves may develop. Coral islands (raised reefs) consist of pre-Recent reef
rock. Noncoral reefs. Several other groups of organisms build reef-like structures in intertidal and shallow sub-tidal environments: vermetid Gastropoda in
combination with coralline algae, oysters, polychaete worms (serpulids) and
Sabellaria. They form carbonate structures except the Sabellaria, which construct tubes of cemented sand grains and shell debris. Oyster reefs are
formed at salinities of 15 to 25 S in tropical and temperate lagoons, bays and
estuaries. The others are formed at approximately seawater salinities; serpulid reefs may occur at salinities between 4 and 55 S.
reflection shooting (seismic) a seismic survey technique based on measurement of
travel times of seismic waves that are reflected at near-vertical incidence from
boundaries separating media of different elastic wave velocities and densities. At sea, a cross sectional display of reflection time versus distance along
a traverse (or profile) is obtained by towing an acoustic source behind a ship
(seismic profiling). The source emits pulses at regular intervals and the subsequent echoes from the ocean floor and subsurface reflectors are detected by
a nearby line of hydrophones towed behind the source. ~refraction, ~seismic
instruments, and ~refraction shooting, (seismic). (Figure see p. 264)
refraction (phys.) the changing of the direction of propagating waves due to
transversal differences in wave velocity. Refraction occurs in optics, acoustics
and in the propagation of gravity waves. Refraction is described by Snell's law.
(Figure see p. 264)
refraction
warm, clean, shallow waters along many tropical coastlines and are built up
principally of coral, coralline algae, and debris of Mollusca and Foraminifera. They
disappear (a) where water temperatures are below about 14°C; (b) at depths
where the sunlight does not penetrate; (c) where there is freshwater run-off,
and (d) where the water is muddy. A common classification lists reefs as fringing reefs (reefs directly lining shores), barrier reefs (developed parallel to the
shore but separated from it, e.g., the Great Barrier Reef in Australia) and atolls
(circular reefs surrounding deep lagoons, ~Blake Expeditions). Fringing reefs
occur where a hard, stable substrate is present offshore (e.g., an island or a
rocky headland). Barrier reefs grow on the continental shelf at some distance
from the coast. Submarine reefs have a reef-front or fore-reef where corals
grow luxuriously and fish and many other organisms inhabit crevices and
caves. The reef is usually topped by a reef crest which is exposed during low
tide and usually contains a large (calcareous) algae population. The channels
between reefs (interreef channels, or off-reef floor) have a substrate of
unconsolidated, usually rippled sediment. The reef crest can merge into an
intertidal reef flat with often deep pools, flourishing coral and coarse sediment. Reef flats often (partly) enclose a shallow lagoon, covered with calcareous mud and patchy reefs. Beaches (cays) are formed from gravel- or
sand-size coral fragments; beach rock is often present. On intertidal muds
mangroves may develop. Coral islands (raised reefs) consist of pre-Recent reef
rock. Noncoral reefs. Several other groups of organisms build reef-like structures in intertidal and shallow sub-tidal environments: vermetid Gastropoda in
combination with coralline algae, oysters, polychaete worms (serpulids) and
Sabellaria. They form carbonate structures except the Sabellaria, which construct tubes of cemented sand grains and shell debris. Oyster reefs are
formed at salinities of 15 to 25 S in tropical and temperate lagoons, bays and
estuaries. The others are formed at approximately seawater salinities; serpulid reefs may occur at salinities between 4 and 55 S.
reflection shooting (seismic) a seismic survey technique based on measurement of
travel times of seismic waves that are reflected at near-vertical incidence from
boundaries separating media of different elastic wave velocities and densities. At sea, a cross sectional display of reflection time versus distance along
a traverse (or profile) is obtained by towing an acoustic source behind a ship
(seismic profiling). The source emits pulses at regular intervals and the subsequent echoes from the ocean floor and subsurface reflectors are detected by
a nearby line of hydrophones towed behind the source. ~refraction, ~seismic
instruments, and ~refraction shooting, (seismic). (Figure see p. 264)
refraction (phys.) the changing of the direction of propagating waves due to
transversal differences in wave velocity. Refraction occurs in optics, acoustics
and in the propagation of gravity waves. Refraction is described by Snell's law.
(Figure see p. 264)