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Carl Wunsch
and
Spahr Webb

Abstract

The search for regions of the deep ocean where the canonical Garrett-Munk spectrum does not apply is continued here in an effort to obtain a zero-order wave climatology and some insight into wave dynamics. A number of such regions have been found 1) within canyons, 2) in immediate proximity to topographic features, 3) in regions of high mean shear and 4) on the equator. Near topographic features energy levels are higher (especially within a canyon), and there is a pronounced anisotropy, but the shape of the frequency spectrum changes little. The absence of the inertial peak on the equator seems to make little difference to the frequency spectrum there. Elsewhere the level and shape of the internal wave frequency spectrum are remarkably constant. Apparent deviations in the wavenumber spectrum do occur in proximity to the equator and in the Florida Current. Horizontal polarization changes are associated with topography and large mean shears.

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Spahr C. Webb
and
Scott L. Nooner

Abstract

Vibrating quartz force transducers are the critical component of most deep-sea pressure and depth gauges in use in oceanography, producing a frequency output that varies with pressure. Accurate and low drift pressure measurements can be obtained by precisely measuring this frequency. In most implementations, the frequency is determined by counting the number of cycles of a high-frequency standard oscillator occurring during a large number of cycles of the lower-frequency quartz force oscillator. Resolution is limited by the sampling interval (length of counting) and the frequency of the frequency standard. Alternative counting methods can provide significant (20–40 dB) improvements in resolution at sampling rates above 1 Hz. Each counting method can be described as a different filter applied to the output of a counter of the frequency standard gated at each transition of the transducer quartz oscillator. Improvements in resolution can be understood as the result of minimizing the aliasing of higher-frequency counting noise into the spectrum below the Nyquist frequency. A simple multipole infinite impulse response (IIR) filter designed to limit spectral leakage of high-frequency noise minimizes the noise spectrum and thereby optimizes the resolution of the pressure output. The resultant noise spectrum rises as frequency squared above 1 Hz, independent of the sampling rate. At frequencies below 1 Hz, it is limited by noise in the electronics driving the force transducer quartz oscillator. Resolution increases with frequency of the frequency standard up to about 200 MHz, plateauing for higher frequencies due to other noise sources (likely electronic).

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Charles Cox
,
Thomas Deaton
, and
Spahr Webb

Abstract

A pressure gauge configured to respond to the difference between the ocean pressure and the pressure within a confined volume of compressible oil is found to be especially useful for detecting pressure fluctuations in the frequency range from a few millihertz to a few hertz. In the middle of the range its noise level is lower than any other known gauge. The limitation of the gauge at the lower frequency limit is caused by unpredictable thermal expansion in the confined oil and at the upper limit by thermal agitation noise in the resistance of the strain gauge transducer. The gauge is insensitive to acceleration and tilting. Measurements with this gauge on the deep seafloor show two principal features in the spectrum of pressure fluctuations. At frequencies below 0.03 Hz there is evidence of pressures generated directly by long surface gravity waves. Above 0.11 Hz the pressures associated with microseisms are predominant. Between 0.03 and 0.11 Hz there is a spectral gap where the pressure level drops below 0.1 Pa2 Hz−1.

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