流动地震观测背景噪声的台基响应

大规模流动地震台阵技术发展为高分辨率深部结构成像提供了重要基础,背景噪声是影响流动地震观测质量的关键因素. 为掌握流动地震观测噪声规律,发展流动地震观测降噪技术, 编制流动地震观测技术规范, 我们开展了针对不同台基流动地震观测背景噪声的观测实验与分析. 其中,山西省临汾市五个地点架设了共22个对比观测台站, 进行了超过一年半的连续观测. 通过计算不同频段范围内背景噪声记录的加速度功率谱密度, 研究了不同场地条件和环境噪声下流动地震观测台站的噪声特征及其台基响应,分析了不同台基处理方式对噪声的抑制效果. 结果表明:(1)高频人为噪声和长周期自然噪声是影响流动地震观测质量的主要噪声, 可以通过增加台基深度和改善台基处理方式等方法降低其影响; (2)增加台基深度能有效地降低长周期噪声和高频噪声, 2 m深坑能使高人为噪声台站各分量的高频频段和长周期频段分别降低5 dB和10 dB; (3)由于其不稳定性, 沙子台基的水平分量在长周期频段一般要高于摆墩台基5 dB, 流动地震观测中推荐使用摆墩台基; (4) 台站位置、台站内部温度和空气流动都是影响台站噪声的重要因素. 在此基础上提出了不同场地条件和噪声环境下的台基处理建议和适合国情的移动地震台阵台站建设参考方案, 有助于流动地震观测野外工作的标准化和规范化.

Transportable seismometer response to seismic noise in vault

Dense transportable seismic array is now an essential tool for subsurface structure imaging. Ambient seismic noise is one of the critical factors that influence seismic recording quality especially in sedimentary cover and noisy condition. With the advent of ever larger campaign deployment of seismic array, it is increasingly imperative to standardize and optimize the seismic station construction. We have conducted a comparative observation experiment in Linfen city of Shanxi Province. 22 stations were built in 5 localities of different geologic settings (sedimentary deposit and bed rock) and different background noise levels (quiet and high culture noise). The focus of the experiment is to determine the ambient noise characterization and site response under various noise levels or different vault construction, and evaluate the denoising effect for improving the recording quality. Ambient noise power spectral densities (PSDs) were computed as a function of frequency for stations using continuous data recorded from Jun 2010 to Jun 2011, and all PSDs are smoothed in full-octave averages at 1/8 octave intervals and then a statistical analysis is applied to yield probability density functions (PDFs). The analysis results are then compared with NLNM and NHNM to identify noise levels. The result shows that: (1) High-frequency culture noise and long-period natural environmental noise are the main source of transportable seismic noise, which can be significantly reduced with specific vault construction; (2) Deep vault is a remarkably effective method for seismic background noise reduction, and a 2 m deep vault can lower high-frequency and long-period noise approximately by 5 dB and 10 dB respectively. (3) Rather than sand-slate vault, pier vault should be adopted for transportable deployment, attributed to its stability and sustainability. (4) The station site, the temperature and air flow in the vault are also important factors that should be treated carefully during station deployment. Based on the above analysis, we present vault construction advices for different geologic settings and noise levels, also proposed is reference station scheme for transportable array. The research will facilitate field work standardizing of transportable seismic station deployment.