REGIONAL GRAVITY 



bencli, or terrace thut slopes to 8 to 15 fathoms, 

 as well as the living wave-breaking face of the 

 reef. The terrace is an eroded surface or is 

 veneered witli organic growth. The living reef 

 front above the terrace in some places is smooth 

 and steep ; in other places it is cut up by grooves 

 separated by ridges that together have been 

 called groove and spur systems, forming comb- 

 tooth patterns. (56) (See figure for atoll.) 



reef patch — A term for all coral growths that 

 have grown up independently in lagoons of bar- 

 riers and atolls. They vary in extent from 

 expanses measuring several kilometers across to 

 coral pillars or even mushroom-sliaped growths 

 consisting of a single large colony. The smaller 

 representatives are called coral knolls or coral 

 heads. (2) (See figui-e for atoll.) 



reef segment — That portion of an organic reef 

 lying between passes, gaps, or channels. (2) 



reef talus — (or reef conglomerate, reef flank de- 

 posit, off -reef fades). Massive inclined beds 

 of debris derived jorincipally from a reef and 

 deposited along the seaward margin of a living 

 reef. 



reference level — 1. In underwater sound, the 

 standard to which other sound levels can be 

 related. Two reference levels commonly used 

 are : 1 dyne per square centimeter and 0.0002 

 dyne per square centimeter. 

 2. See chart datum. 



reference plane — See chart datum. 



reference point — A specified location (in plan 

 and/or elevation) to which measurements are 

 referred. (61) 



reference station — (or standard station, standard 

 po7't) . A place where tide or tidal current con- 

 stants have been determined from observations, 

 and which is used as a standard for the com- 

 l^arison of simultaneous observations at a sub- 

 ordinate station. It is also a place for which 

 independent daily predictions are given in the 

 tide or tidal current tables, from which corre- 

 sponding predictions are obtained for other loca- 

 tions by means of differences or factors. 



reflectance — The ratio of light given off by an 

 object to the amount of light striking the object, 

 expressed as percentage. 



reflectance function — The ratio of the up welling 

 irradiance at a depth to the downwelling ir- 

 radiance at the same depth. 



reflected wave — The wave that is returned sea- 

 ward when a wave impinges upon a very steei^ 

 beach, barrier, or other reflecting surface. (61) 



reflection — The process whereby a surface of dis- 

 continuity turns back a portion of the incident 

 radiation into the medium through which the 

 radiation approached. (5) 



reflection loss — The reflection loss at the junction 

 between an energy source and an energy load is 

 the transmission loss measured by the ratio of 

 (1) the load power which would be measured if 

 source and load were connected by a hypothetical 



transducer having an input impedance equal to 

 the source impedance of the source, an output 

 impedance equal to the load terminals as are 

 developed at its source terminals, to (2) the 

 actual load power when source and load are con- 

 nected directly to each other. (28) 



reflection of sound — The process whereby a sur- 

 face of discontinuity turns back a portion of the 

 incident sound into the medium through which 

 the sound ajoproached. (5) 



reflectivity — A measure of the fraction of radia- 

 tion reflected by a given surface ; defined as the 

 ratio of the radiant energy reflected to the total 

 that is incident upon that surface. 



refraction — The process in which the direction of 

 energy propagation is changed as the result of a 

 change in density within the propagating 

 medium, or as the energy passes through the 

 interface representing a density discontinuity 

 between two media. (5) 



refraction coefficient — In wave hydrodynamics, 

 the square root of the ratio of the spacing be- 

 tween adjacent orthogonals in deep water to 

 that at a selected point in shallow water. When 

 multiplied by the shoaling coefficient, this 

 becomes the wave height coefficient or the ratio 

 of the refracted wave height at any point to the 

 deepwater wave height. Also the square root of 

 the energy coefficient. See wave refraction, re- 

 fraction of water waves, index of refraction 

 (senses). 



refraction diagram — A drawing showing posi- 

 tions of wave crests and/or orthogonals in a 

 given area for a specific deepwater wave period 

 and direction. (61) See figure on next page. 



refraction index — See index of refraction. 



refraction loss — That part of the transmission 

 loss due to refraction resulting from nonuni- 

 formity of the medium. (6) 



refraction of water waves — 1. The process by 

 which the direction of a wave moving in shallow 

 water at an angle to the contours is changed. 

 That part of the wave advancing in shallower 

 water moves more slowly than the other part still 

 advancing in deepei- water, causing the wave 

 crest to bend toward alignment with the under- 

 water contours. 



2. The bending of wave crests by currents. 

 (61) 



refractive index — See index of refraction. 



refractivity — Rare. See index of refraction 

 (sense 1). 



regelation — The melting of ice by the application 

 of pressure and the refreezing of melt water 

 upon release of that pressure. (59) 



regional clearing — See polynya. 



regional gradient — AVv regional gravity, re- 

 gional magnetism. 



regional gravity — (or regional gradient). In 

 gravity prospecting, contributions to the ob- 

 served anomalies due to density irregularities 

 at much greater depths than those of the possible 



133 



