Pressure Fluctuations on the Open Ocean Floor Over a Broad Frequency Range: New Program and Early Results

Abstract

A two-month ocean-floor pressure record obtained 330 km to the east of the main island of Hawaii by means of a Bourdon tube-type transducer with optical readout is discussed in detail. An approach to subtraction of the drift component associated with plastic flow of the heavily strained transducer is assessed. In spite of a 40 m progressively accumulated error, it is shown that fluctuations with periods as long as a few cycles per record length are resolved with a remarkable precision. The lunar fortnightly tide, with its period around 14 days, for instance, appears to be in error by no more than 0.3 cm, on the assumption that the transfer function between gravitational driving corrected for earth tides and sea floor pressure has a modulus of 0.7 and a negligible phase shift. Eleven tidal constituents in each of the diurnal and semidiurnal bands, as well as constituents M3 and Mf are tabulated. These estimated tidal constants come within a few percent of those published for Hilo, revealing a relative uniformity of tidal behavior for this area. The use of tide constants from Hilo to cheek or to constrain mathematical models of the Pacific tides thus appears acceptable. Because of its inherent high-frequency response and its high accuracy (response up to 1 cycle per second; resolution 0.0206 cm in present data) the instrumentation used in the experiment described here can contribute to the investigation of a variety of problems of ocean geophysics. For instance, the low-frequency end of the surface wind-generated wave spectrum is clearly resolved and is shown to vary slowly from day to day with considerable variations over weekly or longer time spans. The close approach on 20 July 1978 of Tropical Cyclone Fico to the area of our sea-floor station provided an opportunity to investigate its effect on sea-floor pressure fluctuations. Although somewhat disappointing, this attempt does stress the great advantage to be gained by using an array of stations rather than individual ones to identify and to sort out the many processes at play.