Title

Measurements of the directional spectra of surface gravity waves and seafloor microseisms using a pressure sensor and two buried horizontal seismometers

Date of Award

1992

Availability

Article

Degree Name

Doctor of Philosophy (Ph.D.)

First Committee Member

Tokuo Yamamoto, Committee Chair

Abstract

This dissertation presents a new and unique method of measuring the directional spectra of ocean surface waves with a buried instrument containing horizontally oriented seismometers and a pressure sensor. Based upon the assumption that the seabed responds in an elastic, incompressible, and linear manner to ocean wave induced pressure fluctuations, the directional spectra of the sediment motion has been found using the Maximum Entropy Method. This motion is converted to a surface wave directional spectrum using linear wave theory and the differential pressure gauge's frequency spectrum. This system was incorporated into the ONR sponsored Sources of Ambient Micro-seismic Ocean Noise (SAMSON) experiment for 3 months off the Army Corps of Engineers' Field Research Facility (FRF) near Duck, North Carolina in the fall of 1990. Four working sensors produced directional spectra results from nearly 22 gigabytes of recorded data which was collected 2 kilometers offshore of the FRF under 12-13 meters of water and approximately 1 meter of sediment. Comparisons to the FRF's 12 point array of bottom mounted pressure sensors which was located 1 km inshore and in 8 meters water depth are made with favorable results.During the SAMSON experiment, concurrent measurements of surface wave directional spectra and double frequency, long wave microseisms were made. Long wave energy rapidly develops during periods of shifting winds which create bidirectional sea states. Theoretically, non-linear sum interactions of opposing wavenumber vectors of approximately the same frequency creates long wave energy at twice the frequency which is only slightly attenuated in shallow water. Bidirectional sea states have been found using the buried OBS measuring system during which the long wave energy has been measured at double frequencies. Because of the insignificant attenuation, the measured energy levels of the double frequency microseisms at the seafloor are of the same order of magnitude as the single frequency, surface wave energy induced seafloor motion. Various data sets were analyzed which confirmed Longuet-Higgins theory which proposes that the propagation direction of double frequency microseisms occur in the direction of the vector sum of the opposing single frequency seas.

Keywords

Physical Oceanography; Engineering, Mechanical

Link to Full Text

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