Concurrent measurements of the directional spectra of microseismic energy and surface gravity waves

Abstract

This paper presents concurrent measurements of surface wave directional spectra and double‐frequency, long‐wave microseisms. Long‐wave energy rapidly develops during periods of shifting winds which create bidirectional sea states. Theoretically, nonlinear sum interactions of opposing wavenumber vectors of approximately the same frequency create long‐wave energy at twice the frequency, which is only slightly attenuated in shallow water. Bidirectional sea states have been found using a buried ocean bottom seismometer measuring system from which the long wave energy has been measured at double frequencies. This system was incorporated into the Office of Naval Research sponsored Sources of Ambient Microseismic Ocean Noise (SAMSON) experiment for 3 months off the Army Corps of Engineers' Field Research Facility 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 km offshore of the FRF under 12–13 m of water and approximately 1 m of sediment. 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 that confirmed Longuet‐Higgins' theory, which proposes that the propagation direction of double‐frequency microseisms occurs in the direction of the vector sum of the opposing single‐frequency seas.