Data_Sheet_1_High Resolution Attenuation Images From Active Seismic Data: The Case Study of Solfatara Volcano (Southern Italy).docx

The quality factor Q is highly sensitive to the medium properties related to the fluid presence, like porosity, permeability, and level of rock saturation. Therefore, it is very useful to image the subsoil of volcanoes in terms of anelastic images, in addition to more common elastic tomographic models. In this setting, indeed, fluids play a key role in controlling and governing the evolution of magmatic processes, so their accurate identification and tracking is crucial. Here, we describe a methodology that allows to obtain accurate 1-D and 3-D attenuation models from seismic active data. This methodology requires three steps: 1/the measurement of the attenuation parameter t∗, first by cross-correlating the signal with the known source function and then by using the spectral decay method; 2/the determination of a reference 1-D attenuation model through a grid model search procedure; 3/the reconstruction of the 3-D attenuation model through an iterative inversion of t∗ data. The map of t∗ residuals can be analyzed with respect to the reference 1-D model in order to validate the t∗ catalog. The methodology has been tested on a small-scale volcanic volume of the shallow Solfatara crater, in southern Italy, as for this area several multi-parametric geophysical surveys have been carried on. The shallowest subsoil of Solfatara is characterized as a volume with a very low P-wave quality factor (QP 5–40 in the near-surface layer 30 m thick), which globally increases with depth in the explored volume and shows a strong lateral heterogeneity. Within the well resolved central portion of the explored volume the QP model shows features consistent with the hydrothermal fluid distribution within Solfatara, inferred by the velocity images. The presented methodology can be therefore considered as a suitable tool to obtain attenuation models in volcanic areas, which, interpreted jointly with the velocity ones, provide a comprehensive image of complex hydrothermal systems.

品质因数(Quality Factor, Q)对与流体赋存相关的介质物性（如孔隙度、渗透率、岩石饱和程度）具有极高的敏感性。因此，相较于更为常用的弹性层析成像模型，采用非弹性成像手段对火山地下区域进行成像具备重要应用价值。在此类场景中，流体在管控岩浆活动演化过程中发挥着关键作用，因此精准识别与追踪流体分布至关重要。本文提出一种可基于主动地震数据构建高精度一维与三维衰减模型的方法。该方法包含三个步骤：1. 衰减参数t∗的测量：首先将观测信号与已知震源函数进行互相关处理，随后采用频谱衰减法完成参数提取；2. 参考一维衰减模型的确定：通过网格模型搜索流程完成建模；3. 三维衰减模型的重建：基于t∗数据开展迭代反演。可通过将t∗残差分布图与参考一维模型进行对比分析，完成t∗数据集的有效性验证。本方法已在意大利南部浅部索尔法塔拉(Solfatara)火山口的小型火山岩体中开展测试，针对该区域已完成多项多参数地球物理勘探工作。索尔法塔拉的浅部地下介质呈现出极低的P波品质因数(P-wave Quality Factor, QP)：在厚度30米的近地表层中，QP值为5~40，该值在探测范围内整体随深度增加而升高，且呈现显著的横向非均质性。在探测范围内分辨率良好的中央区域，QP模型所呈现的特征与索尔法塔拉热液流体的分布特征一致，该分布特征可通过速度成像结果推断得出。综上，本文提出的方法可作为火山区域衰减模型构建的有效工具，结合速度成像结果共同分析，能够全面刻画复杂热液系统的空间分布特征。