Foucault-Wheatstone device in the deep-sea

Abstract

A pressure sensor, located for 4 months in the middle of a 1275-m long taut deep-sea mooring in 2380 m water-depth above a seamount with sub-surface top-buoys and seafloor anchor-weight, demonstrates deterministic spectral peaks at sub-harmonics of the local near-inertial frequency. None of these frequencies can be associated with oceanographic motions. No corresponding peaks are found in spectra of other observables like water-flow (differences), temperature, and pressure in the top-buoys of the mooring. The mid-cable pressure sensor was mounted on a nearly 1-kN weighing non-swiveled frame. Its data seem to reflect a resonant mechanical oscillation of the tensioned mooring cable under repeated short-scale Strouhal vibrations induced by water-flow. Equivalent vertical motions of 0.5-m amplitudes are found dominant at frequency f*/4, with monotonically decreasing peaks at f*/2, 3f*/4 and f* = 1.083f, where f denotes the local inertial frequency of Earth rotation. The observations provide, physically, proof of the Earth rotation and, oceanographically, of the relative vortex-rotation dzeta = 0.083f induced by (sub-)mesoscale eddies associated with a nearby seamount, so that effective near-inertial frequency f* = f + dzeta. As instrumentation only shows mid-cable resonant signals it is not considered as a Foucault pendulum, but more likely an Earth-rotation equivalent of the Wheatstone device with the mooring cable acting as elastic springs. While the f*/2 motion reflects the Foucault principle of linear orbits fixed in an inertial frame of reference, the dominant sub-harmonic f*/4 suggests resemblance with the precession of a spherical oscillator at half the angular speed of rotation.