Noise Observatory for Imaging the Subsurface Beneath Yellowstone (NOISY)

技术摘要：我们正在研究如何从周围的地震场生成黄石地区稳定的延时层析成像图像。我们每年夏天(6-9月)在黄石公园周围部署和重新部署宽带地震仪，以捕获环境噪声场的延时快照。黄石地区具有独特的活动性，具有时变的地震活动、无地震形变、热流和间歇泉活动，使其成为成像次表层时变特性的理想地点。通过对每对宽带地震台站测量的环境噪声持续时间长达一小时的许多相关性进行平均，我们生成噪声关联函数(NCF)。这些NCF是两个(或多个)站之间格林？S函数的经验表示。通过对周围地震场进行重复采样，我们生成了不同时间段的台间格林函数的独立测量值。当地下属性(如地震速度或衰减)随时间变化时，NCFs也发生变化，本研究的主要目的是提高NCFs的信噪比和NCFs的时间导数。我们通过生成与整个网络估计的声源场相对应的合成NCF来消除时变环境噪声声源场的时间效应。用观测到的和均匀的源场产生的合成NCF之间的传递函数与观测到的NCF卷积在一起，产生源场校正的NCF。我们评估这种修正的NCF的时变相位和幅度，以获得对四维层析有用的一致图案。非技术摘要：确定黄石等主要火山的时变特性对于了解岩浆室是如何膨胀和收缩至关重要的。这项研究提供了对北美大陆地壳如何实时演化的更好理解。我们正在测试快速算法，以分析实时估计随时间变化的地下属性的潜力，这最终可能有助于实时危险评估。我们还在测试廉价的加速计如何连接到家庭和学校的联网电脑上，以帮助更好地理解火山的4D成像。通过该项目产生的新的地震数据为4D大陆过程提供了新的见解，并产生了对地球上最常去的地球旅游者之一-S-的新理解。

Technical Abstract:We are investigating how to generate stable time-lapsed tomography images of the Yellowstone region from the ambient seismic field. We deploy and re-deploy broadband seismometers each summer (June-September) around Yellowstone to capture time-lapsed snapshots of the ambient noise field. The Yellowstone region is uniquely active with time varying seismicity, aseismic deformation, heat flow and geyser activity, making it an ideal location to image time varying properties of the subsurface.By averaging many correlations of hour-long durations of ambient noise measured at each pair of broadband seismic stations, we generate noise correlation functions (NCFs). These NCFs are empirical representations of the Green?s function between two (or more) stations. By repetitively resampling the ambient seismic field, we generate independent measurements of the inter-station Green's function for separate time periods. When the subsurface properties (such as seismic velocity or attenuation) vary with time, the NCFs change as well.The major effort of this study is aimed at improving the signal-to-noise ratio of both the NCFs and time derivative of the NCFs. We remove the temporal effect of a time-varying ambient noise source field by generating synthetic NCFs corresponding to the source field estimated for the entire network. The transfer function between the synthetic NCFs generated with observed and uniform source-fields convolved with the observed NCF yields a source-field-corrected NCF. We evaluate time varying phase and amplitude of this corrected NCF for consistent patterns useful for four-dimensional tomography.Nontechnical Abstract:Determining the time-varying properties of a major volcano such as Yellowstone is essential to understanding how magma chambers inflate and deflate. This study is providing better understanding of how the North American continental crust is evolving in real time. We are testing fast algorithms to analyze the potential of real-time estimation of time-varying subsurface properties, which may eventually contribute to real-time hazard assessment. We are also testing how inexpensive accelerometers connected to internet-connected computers in homes and schools can contribute to greater understanding of 4D imaging of volcanoes. The new seismic data generated through this project are providing new insights into 4D continental processes and generating new understanding of one of Earth?s most visited geotourist locations.