652 



[chap, is 



Fig. 5 shows simultaneous spectra at two stations, 100 m and 1500 m oif- 

 shore respectively, from the sheltered (eastern) shore of Guadalupe Island off 

 the coast of Mexico (Munk, Snodgrass and Tucker, 1959). The off-shore recorder 

 shows three broad peaks with reproducible fine structure! The on-shore record 

 shows the identical fine structure. The peaks (here included under shelf waves) 

 actually cover the frequency range previously allotted to surf beat. There are 

 two reasons for this : (i) the extremely narrow steep shelf off Guadalupe can be 

 expected to lead to shelf resonance at relatively high frequencies (as we shall 



10 20 



Cycles per kilosecond 



Fig. 5. The spectrum at Guadalupe Island, Mexico. Top : spectra at an on-shore instrument 

 (depth 21 ft, 100 m from shore) and an off-shore instrument (depth 372 ft, 1500 m 

 from shore). Center : Coherence between on-shore and off-shore records. Bottom : Phase 

 lead of the shore-based record relative to the off-shore record. (After Munk, Snodgrass 

 and Tucker, 1959.) 



demonstrate) ; (ii) on the sheltered side of the island, waves (and hence surf 

 beat) were virtually absent, thus permitting the relatively low-energy shelf 

 peaks to rise above the surf-beat background. 



The two most obvious features to be discussed are : the frequency of 

 the peaks and their sharpness. The latter are conveniently portrayed by the 

 dimensionless parameter Q, defined as the ratio of the central frequency to the 

 width at the half-power points ; alternately the energy amplification at resonance 

 is Q^. Parameters referring to the fundamental peaks are summarized in Table 1. 



