ALOHA Cabled Observatory bottom pressure spectra

Frequency spectra from deep-ocean near-bottom acoustic measurements obtained contemporaneously with wind, wave, and seismic data are described and used to determine the correlations among these data and to discuss possible causal relationships. Microseism energy appears to originate in four distinct regions relative to the hydrophone: wind waves above the sensors contribute microseism energy observed on the ocean floor; a fraction of this local wave energy propagates as seismic waves laterally, and provides a spatially integrated contribution to microseisms observed both in the ocean and on land; waves in storms generate microseism energy in deep water that travels as seismic waves to the sensor; and waves reflected from shorelines provide opposing waves that add to the microseism energy. Correlations of local wind speed with acoustic and seismic spectral time series suggest that the local Longuet-Higgins mechanism is visible in the acoustic spectrum from about 0.4 Hz to 80 Hz. Wind speed and acoustic levels at the hydrophone are poorly correlated below 0.4 Hz, implying that the microseism energy below 0.4 Hz is not typically generated by local winds. Correlation of ocean floor acoustic energy with seismic spectra from Oahu and with wave spectra near Oahu imply that wave reflections from Hawaiian coasts, wave interactions in the deep ocean near Hawaii, and storms far from Hawaii contribute energy to the seismic and acoustic spectra below 0.4 Hz. Wavefield directionality strongly influences the acoustic spectrum at frequencies below about 2 Hz, above which the acoustic levels imply near-isotropic surface wave directionality.

本研究对与风场、波浪场及地震数据同步采集的深海近底声学测量所得到的频谱进行了描述，并利用这些数据开展了变量间相关性分析，同时探讨了潜在的因果关联。相较于水听器（hydrophone），微震能量似乎源自四个不同区域：传感器上方的风浪会为海底观测到的微震提供能量；该局部波浪能量的一部分会以地震波的形式横向传播，为海洋及陆地上观测到的微震提供空间积分贡献；风暴中的波浪会在深水中产生微震能量，并以地震波的形式传输至传感器；经海岸线反射的波浪会形成相向传播的波，进一步增加微震能量。局部风速与声学及地震频谱时间序列的相关性表明，在约0.4 Hz至80 Hz的声学频谱中可观测到局部朗格特-希金斯机制（Longuet-Higgins mechanism）。在0.4 Hz以下频段，水听器处的风速与声学信号强度相关性较弱，这表明0.4 Hz以下的微震能量通常并非由局部风浪产生。海底声学能量与瓦胡岛（Oahu）的地震频谱以及瓦胡岛附近的波浪频谱之间的相关性表明，夏威夷海岸的波浪反射、夏威夷附近深海的波浪相互作用，以及远离夏威夷的风暴，均为0.4 Hz以下的地震及声学频谱提供了能量来源。波场方向性对约2 Hz以下频段的声学频谱具有显著影响；而在2 Hz以上频段，声学信号强度表明海面波浪的方向性接近各向同性。