REVERSED TIDE 



The relaxation time of an instrument is 

 commonly called its time constant or lag 

 coefficient. (5) 



relief — The inequalities (elevations and depres- 

 sions) of the sea bottom. (2) 



remanent magnetism — Permanent magnetism in- 

 cluded in a material by an applied magnetic 

 field. (2) 



Rennell's Current — A relatively strong (1.0 to 1.5 

 knots) nonpermanent current that sets north- 

 ward across the western approaches to the Eng- 

 lish Channel. The current appears to be inde- 

 pendent of the North Atlantic Drift or local 

 winds and occurs most frequently during winter. 



replacement — A nearly simultaneous capillary so- 

 lution and deposition by which a new mineral 

 may grow in the body of an old mineral or min- 

 eral aggregate. (2) 



residual deposits — Material formed by the decay 

 or disintegration of rock in place. 



residual gravity — In gravity prospecting, the 

 portion of a gravity effect remaining after re- 

 moval of some type of regional, usually the 

 relatively small or local anomaly components of 

 the total or observed gravity field. 



residual magnetic field — (also called anomalous 

 field) . That part of the observed magnetic field 

 that remains after removal of the regional mag- 

 netic field. 



resistivity — The electrical resistance per unit 

 length and jaer unit reciprocal cross section area 

 of a given material at a specified temperature. 

 It is also possible to define the resistivity of a 

 substance as the resistance of a cube of that sub- 

 stance having edges of unit lengths, with the 

 understanding that the current flows normal to 

 opposite faces and is distributed uniformly over 

 them. Resistivity is commonly expressed in 

 units of ohm centimeters. 



The reciprocal of resistivity is conductivity. 

 (5) _ 



resolution — 1. In gravity or magnetic pros- 

 pecting, the indication in some measured 

 quantity, such as the vertical component of 

 gravity, or the presence of two or more close but 

 separate disturbing bodies. 



2. In wave theory, the separation of spectral 

 components. 



resolution limit — In gravity and magnetic pros- 

 pecting, the separation of two disturbing bodies 

 at which some obvious indication in a measured 

 quantity of the presence of two separate bodies, 

 ceases to be visible. 



resonance — 1. The phenomenon of amplification 

 of a free wave or oscillation of a system by a 

 forced wave or oscillation of exactly equal 

 period. The forced wave may arise from an 

 impressed force upon the system or from a 

 boundary condition. The growth of the reso- 

 nant amplitude is characteristically linear in 

 time. (5) 



2. In tides, the water movement resulting f i-om 

 the natural period of oscillation of a body of 

 water which approximates the period of one of 

 the tide-producing forces. 



resonance angle — The angle at which the com- 

 ponents of the wind speed acting in the direction 

 of the waves is equal to the wave speed. It is 

 expressed as V cos 6=C, where V is tlie wind 

 speed, 6 the resonance angle, and C the wave 

 speed. 



respiration — An oxidation-reduction process by 

 which chemically bound energy in food is trans- 

 formed to other kinds of energy upon wliich cer- 

 tain processes in all living cells are dependent. 

 The measurement of carbon dioxide as a product 

 of respiratory activity in marine phytoplankton 

 is essential in determining the net productivity. 



(57) 

 response — The response of a device or system is 

 the motion or other output resulting from an 

 excitation or stimulus under special conditions. 



(3) 



response time — See time constant. 



resultant current — (or vector mean). The 

 vectorial average of all current observations 

 for a specified area, usually for a specified 

 period of time. 



resurgence — The continued rising and falling of 

 a bay or semienclosed water body many hours 

 after the passage of a severe storm. 



retardation — The amount of time by which cor- 

 responding tidal phases grow later day by day 

 (averages approximately 50 minutes) . (61) 



reticulated bars — Bars with a criss-cross pat- 

 tern, with both sets diagonal to the shoreline. 

 (Y3) 



retrogression of a beach — See recession. 



reverberation — Sound scattered towards the 

 source, principally from the ocean surface 

 (surface reverberation) or bottom (bottom re- 

 verberation), and from small scattering sources 

 in the medium such as bubbles of air and sus- 

 pended solid matter (volume reverberation). 



reverberation index — The measure of the ability 

 of an echo-ranging transducer to distinguish 

 the desired echo" from the reverberation. 

 Computed from the directivity patterns as ratio 

 in decibels of the bottom, surface, or volume re- 

 verberation response of a specific transducer to 

 the corresponding response of a nondirectional 

 transducer. 



reverberation strength — The difl'erence between 

 the level of a plane wave producing in a non- 

 directional transducer, a response equal to that 

 produced by the reverberation corresponding 

 to a range of one yard from the effective center 

 of the transducer and the index level of the pulse 

 transmitted, on any bearing, by the same 

 nondirectional transducer. (28) 



reversed tide — A gravitational tide which is 

 completely out of phase with the apparent 



135 



