One year of sound recorded by a mermaid float in the Pacific: hydroacoustic earthquake signals and infrasonic ambient noise
One year of sound recorded by a mermaid float in the Pacific: hydroacoustic earthquake signals and infrasonic ambient noise
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DOI：
10.1093/gji/ggab296
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发表时间：
2021-09-11
影响因子：
2.8
通讯作者：
Simons，Frederik J.
中科院分区：
文献类型：
--
作者：
Pipatprathanporn，Sirawich;Simons，Frederik J.
一群自动漂流的剖面浮标配备了水听器（其缩写为“美人鱼”），从海洋内部监测全球地震活动。这些仪器被编程为检测和传输来自远震P波到达的声压转换，用于地幔断层扫描。近乎实时地报告地震图，在记录后几小时或几天内，仪器通常不会被恢复，但如果恢复了，它们的内存缓冲区可以被读出。我们展示了由 amermaidfloat 在法属波利尼西亚周围的南太平洋以 0.1 至 20 Hz 频率记录的长达 1 年的独特声音数据集，该数据集实际上已被恢复。利用时域、频域和时频域技术对时间序列进行梳理，我们识别出了已发布目录中已知的213次全球地震的信号，震级为4.6-8.0级，震中距在24°至168°之间。观测到的信号包含穿过地壳、地幔和岩心的压缩波和剪切波的震声转换，包括P、S、Pdif、Sdif、PKIKP、SKIKS、表面波和水声T相。仪器仅自动报告了 10 条地震记录，其余的被机载处理算法视为低优先级。从记录中删除所有地震信号以及来自其他瞬态（主要是非地震）源的信号后，我们留下了在 1500 米深度记录的次声环境噪声场。我们将随时间变化的噪声谱密度与时间分辨海浪模型 WAVEWATCH III 联系起来。通过海洋表面重力波的相互作用，噪声记录在频谱形状和时间变化方面都得到了很好的解释。根据众所周知的倍频机制，它们会产生 0.1 至 1 Hz 声频的二次微震。
A fleet of autonomously drifting profiling floats equipped with hydrophones, known by their acronymmermaid, monitors worldwide seismic activity from inside the oceans. The instruments are programmed to detect and transmit acoustic pressure conversions from teleseismicPwave arrivals for use in mantle tomography. Reporting seismograms in near-real time, within hours or days after they were recorded, the instruments are not usually recovered, but if and when they are, their memory buffers can be read out. We present a unique 1-yr-long data set of sound recorded at frequencies between 0.1 and 20 Hz in the South Pacific around French Polynesia by amermaidfloat that was, in fact, recovered. Using time-domain, frequency-domain and time-frequency-domain techniques to comb through the time-series, we identified signals from 213 global earthquakes known to published catalogues, with magnitudes 4.6–8.0, and at epicentral distances between 24° and 168°. The observed signals contain seismoacoustic conversions of compressional and shear waves travelling through crust, mantle and core, includingP, S, Pdif, Sdif, PKIKP, SKIKS, surface waves and hydroacousticTphases. Only 10 earthquake records had been automatically reported by the instrument—the others were deemed low-priority by the onboard processing algorithm. After removing all seismic signals from the record, and also those from other transient, dominantly non-seismic, sources, we are left with the infrasonic ambient noise field recorded at 1500 m depth. We relate the temporally varying noise spectral density to a time-resolved ocean-wave model, WAVEWATCH III. The noise record is extremely well explained, both in spectral shape and in temporal variability, by the interaction of oceanic surface gravity waves. These produce secondary microseisms at acoustic frequencies between 0.1 and 1 Hz according to the well-known frequency-doubling mechanism.