380 UNDERWATER WEATHER STATION 



Currents at both stations showed a marked tendency for flow directions to parallel the 

 axis of Scripps Submarine Canyon rather than to cross the axis. This permitted the currents 

 to be plotted as two-dimensional currents using the notations "onshore" and "offshore," where 

 these notations indicate flows in the directions of 060° True and 240° True respectively. On 

 Sept. 25, 1965 (Fig. 142), maximum currents in excess of one-half knot were measured at the 

 underwater weather station, while those in the canyon head were in excess of one knot. Cur- 

 rents at both stations were irregular in speed and showed frequent reversals in direction. The 

 fluctuations at both stations had periods ranging from about five minutes to over one hour. It 

 is impossible to determine from these analog records whether there is coherence in the fluc- 

 tuations between two stations. It does appear that the fluctuation frequencies are similar at 

 the two stations, and it is obvious in this case that the stronger currents occur at the canyon 

 head. There is some indication, especially at the Sealab station, that the net current is off- 

 shore ebb tide and onshore during flood tide. 



Similar comparisons between the Sealab site and the 150-ft station are shown for Sept. 22 

 (Fig. 143). These differ from those in Fig. 142 in that the currents were stronger at the Sealab 

 site than at the canyon head. They are similar in that both records show fluctuations of cur- 

 rent with periods of a few minutes to over an hour and in that the reversals in current were 

 somewhat more frequent at the canyon head. This data differs from Fig. 142 in that the net 

 current appears to be onshore during most of the day. 



High waves were observed on Oct. 6 (Figs. 140 and 144). Inspection of the analog record 

 from the wave staff on Scripps Pier, where the water is 20 ft deep, showed that the waves were 

 as high as 6 ft (200 cm) and had periods ranging from less than 8 sec to over 16 sec. The wa- 

 ter was too rough for scuba divers to place the current meter in the head of Scripps Canyon. 

 However, a two-hour record of wave height from the end of Scripps Pier and current and pres- 

 sure records from Sealab II were made on the high-speed data-acquisition system. During this 

 run, each sensor was sampled every two seconds, and the data was processed through the CDC- 

 3600 computer to obtain the spectra and cross-spectral analysis for all channels. These data 

 are shown in Fig. 144, together with the phase and coherence between the Sealab pressure sen- 

 sor (7) and the current (5). The spectrum from the wave staff (sensor 13) shows the surface 

 wave energy to be concentrated over a broad band of waves varying in period from about 8 to 

 16 sec. It also shows a pronounced long-period spectral peak with a period of about 105 sec 

 which appears to represent the "surf beat" associated with these waves. Both the Sealab cur- 

 rent and pressure also show broad spectral peaks with periods in the range of 10 to 16 sec, 

 which are undoubtedly associated with the surface waves. The pressure record shows a series 

 of spectral peaks, some (periods of 50 and 25 sec) having frequencies that are multiples of the 

 surf beat frequency. Others, with periods of about 7 and 8 sec, are likely artifacts due to para- 

 sitic disturbances in the data sensing and/or transmission facilities. 



The energy density for the signal variations from the wave staff and the Sealab pressure 

 sensor is expressed in units of cm^ per unit of band width, Af. The corresponding spectral 

 estimate for the current is in units of velocity^ per Af. The proper scale in cm- /sec per Af 

 for the current spectra is obtained by multiplying the printed scale by a factor of 0.41. The 

 product of the spectral estimate (energy density) and the band width gives the mean square ve- 

 locity associated with any particular frequency band. It will be observed that the root-mean- 

 square velocity under the broad spectral peak of the current is approximately 5 cm per second. 



The orbital velocity associated with the passage of a simple wave of frequency f in still 

 water would show a maximum onshore velocity under the wave crest and a maximum offshore 

 velocity under the wave trough. The spectrum for this orbital velocity would show a single 

 spectral peak having a frequency twice that of the wave, because the square of the onshore 

 (positive) and the offshore (negative) orbital velocities has the same sign in the analysis proce- 

 dure. However, the spectrum for the current meter shows good agreement in frequency with 

 that of the surface waves (center of diagram) and shows little energy at twice this frequency 

 (right of diagram). This can only be interpreted as an orbital velocity superimposed upon a net 

 current of nearly the same speed or greater. Inspection of the analog record of current during 

 this period shows that the current had a speed varying between zero and one-quarter knot, and 

 the direction of flow was southwesterly or offshore. 



