Seismic crosshole tomography as contribution to understand sedimentation processes in glacially-overdeepened valleys

冰川过深的山谷和盆地构成了主要的沉积物圈闭。它们的构造和沉积顺序使我们能够对阿尔卑斯山更新世的演化作出结论。icdp批准的项目“钻探超深阿尔卑斯山谷- DOVE”旨在通过钻探分布在阿尔卑斯地区的七个地点来研究冰川的时空演变，从而实现这些目标。应用问题涉及水文、地质灾害、建筑和隧道掘进。过深山谷之谜的一个部分是揭示在水槽重新填充期间的沉积过程。在这些环境中，沉积过程可能形成各向异性的沉积环境。我们打算揭示沉积物的地震各向异性，并推导沉积过程，作为更新世演化的一部分。我们假设了导致沉积物地震各向异性的不同机制：(1)细层状，(2)河流沉积物的碎屑线理，(3)冰川构造叠加，(4)冰川负荷的过固结。为了将稀缺的、准时的钻孔信息转换为空间分布的信息，地球物理方法是可取的。由于地震近地表方法迄今缺乏地震各向异性的分辨率，我们将利用地震井间实验对沉积物进行研究。本次拟建项目的目标研究区域为德国baden - w<e:1> - rttemberg过深Tannwald盆地的DOVE地球物理试验场。它为地震井间实验提供了一个独特的机会和理想的环境，三个钻孔（约140米深）在优化的距离（约30-50米）。在实验中，我们将采用能够产生p波以及不同极化s波的不同源系统。偏振分析提供了双折射的证据和对各向异性的直接了解，使我们能够识别不同波型的到达。输入各向同性走时层析成像的每个波型的第一个到达。各向同性全波形反演将进一步提高地震速度的分辨率和精度。在第一步中，纵波和水平极化的横波将生成速度模型，然后进行完整的P-SV波形反演。利用钻孔数据、岩心分析和其他DOVE研究小组提供的沉积学解释，我们将综合沉积学和地震各向异性来推导沉积过程。研究结果将包括在各向异性VSP的系统评估和地面数据分析中，这些数据可以用于其他只有一个井眼的DOVE地点。

Glacially-overdeepened valleys and basins constitute major sediment traps. Their structure and their sedimentary succession allow conclusions to be made about the Pleistocene evolution of the Alps. The granted ICDP-project „Drilling Overdeepened Alpine Valleys – DOVE“ aims at these goals by drilling seven locations, distributed throughout the Alpine region, to investigate the spatio-temporal evolution of the glaciations. Applied questions refer to hydrology, geohazards, construction, and tunneling. One piece of the puzzle of overdeepened valleys is to reveal the sedimentation processes during the refill of the troughs. Within these environments, sedimentation processes may generate an anisotropic sedimentological setting. We intend to reveal seismic anisotropy of the sediments and derive the sedimentation processes, as part of the Pleistocene evolution. We hypothesize different mechanisms that lead to seismic anisotropy of the sediments: (1) fine-layering, (2) clast lineation of fluvial sediments, (3) glacio-tectonic overprint, and (4) overconsolidation by glacial loading.To convert the scarce, punctual borehole information into spatially-distributed information, geophysical methods are desirable. Since seismic near-surface methods lack resolution of seismic anisotropy so far, we will investigate the sediments using a seismic crosshole experiment. The targeted study area of this proposed project is the geophysical test site of DOVE in the overdeepened Tannwald Basin (Baden-Württemberg, Germany). It provides a unique opportunity and the ideal setting of three boreholes (ca. 140 m deep) at optimized distances (ca. 30-50 m apart) for a seismic crosshole experiment.For the experiment, we will employ different source systems that are able to generate P-waves as well as differently polarized S-waves. Polarization analysis give evidence of birefringence and direct access to anisotropy and let us identify the arrivals of different wavetypes. The first arrivals of each wavetype are input for isotropic traveltime tomography. Isotropic full waveform inversion will further increase the resolution and accuracy of seismic velocities. In a first step, P-wave and horizontally polarized S-waves will generate velocity models followed by a subsequent full P-SV waveform inversion.Using borehole data, core analysis, and sedimentological interpretation contributed by other DOVE research groups, we will integrate sedimentological and seismic anisotropy to derive sedimentation processes. The findings will be included in a methodical assessment of anisotropic VSP and surface data analysis that can be used at other DOVE locations, where only one borehole is present.