The Tournemire Underground Research Laboratory (Aveyron, France), developed by the French Institute for Radiological Protection and Nuclear Safety, is composed of a tunnel excavated in an argillaceous geological layer belonging to a limestone–argillite–limestone sub-horizontal sedimentary sequence. Sub-vertical fault zones are intercepted by drifts and boreholes excavated from the tunnel in the argillaceous layer at a depth of about 250 m. These fault zones are characterized by velocities lower than those of the embedding rocks. In this study, we assess the capacity of first arrival traveltime tomography to detect and characterize such fault zones in a surface-to-depth transmission configuration with a limited angle aperture. The source at the surface is a sledgehammer and turns out to be very repetitive. The source trigger is transmitted to the acquisition bay via a VHF transmitter. Receivers are located at depth in underground works. First arrivals are clear and traveltime picking is straightforward. Before inverting real data, we conduct synthetic tests with the real acquisition geometry to assess limits and validity of first arrival traveltime tomography to detect a lower velocity fault zone using different initial configurations and inversion constraints during the inversion. We show that introducing a priori information on velocities in the shallow subsurface, limestones and argillaceous layers and constraints on maximum velocities greatly helps improving results and subsequent interpretation. Velocities obtained from the real data display a main lower velocity zone that extends from the galleries in the argillaceous layer right up to the surface in limestones. The location and extension of this lower velocity zone are consistent with fault and fracture zones mapped from geological observations.

中文翻译：

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地隧道地震层析成像检测垂直断层的潜力：在法国 Tournemire 地下研究实验室的应用

Tournemire 地下研究实验室（法国阿韦龙省）由法国放射防护和核安全研究所开发，由在属于石灰岩-泥质-石灰岩亚水平沉积序列的泥质地质层中开挖的隧道组成。亚垂直断层带被泥质层中隧道开挖的巷道和钻孔截获，深度约 250 m。这些断层带的特征是速度低于嵌入岩的速度。在这项研究中，我们评估了初至走时断层扫描在有限角度孔径的表面到深度传输配置中检测和表征此类断层带的能力。表面的来源是一把大锤，结果非常重复。源触发信号通过 VHF 发射机传输到采集舱。接收器位于地下工程深处。先到者明确，旅行时间选择也很简单。在反演真实数据之前，我们使用真实采集几何进行综合测试，以评估初至走时断层扫描的限制和有效性，以在反演期间使用不同的初始配置和反演约束来检测较低速度的断层带。我们表明，引入浅层地下、石灰岩和泥质层中速度的先验信息以及对最大速度的限制极大地有助于改进结果和后续解释。从实际数据中获得的速度显示了一个主要的较低速度区域，该区域从泥质层中的通道一直延伸到石灰岩中的地表。这个低速带的位置和延伸与地质观测绘制的断层和断裂带一致。