Construction of drilled shafts for transportation infrastructure presents quality assurance challenges related to structural anomalies, including voids and degraded concrete. Efforts to evaluate anomalies often use stress-wave non-destructive testing (NDT) techniques such as the crosshole sonic logging (CSL) method. CSL relies on the propagation of stress waves between access tubes inserted alongside the reinforcement cage. Differences in travel time and amplitudes can be used to estimate concrete quality within the source-receiver ray paths. Crosshole tomography (CT) advances this approach further and models ray paths to solve an inverse problem based on first arrival times. However, both stress-wave methods cannot typically evaluate anomalies smaller than 10% to 15% of the shaft cross sectional area. Recent advances in wave propagation modeling have led to the development of full waveform inversion (FWI) approaches. FWI attempts to generate a more detailed tomographic model that matches the entire waveform recordings. This paper presents findings from a numerical study where full waveform tomography was performed on a portion of a drilled shaft with a structural anomaly. Crosshole stress-wave propagation was simulated using a spectral element method (SEM) forward modeler. The resulting waveforms were then used as inputs into a CT travel-time inversion algorithm using ray-path modeling and into a FWI algorithm using the entire recorded signals. The results highlighted that FWI estimated concrete velocities and the geometry of the anomaly more accurately than CT in the idealized numerical model. This was the case even though information from more sources and receiver pairs was used for the CT inversion.

用于运输基础设施的钻孔轴的施工提出了与结构异常有关的质量保证挑战，包括空隙和退化的混凝土。评估异常的努力通常使用应力波非破坏性测试（NDT）技术，例如井间声波测井（CSL）方法。CSL依靠应力波在钢筋笼旁边插入的检修管之间的传播。传播时间和幅度的差异可用于估算源接收器射线路径内的混凝土质量。交叉孔层析成像（CT）进一步改进了该方法，并为射线路径建模以基于首次到达时间来解决逆问题。但是，两种应力波方法通常不能评估小于轴横截面面积10％到15％的异常。波传播建模的最新进展已导致全波形反演（FWI）方法的发展。FWI尝试生成与整个波形记录匹配的更详细的断层扫描模型。本文介绍了一项数值研究的发现，该研究对具有结构异常的钻探轴的一部分进行了全波形层析成像。井眼应力波传播是使用谱元方法（SEM）正向建模器模拟的。然后将得到的波形用作使用射线路径建模的CT行程时间反演算法的输入，以及用作使用整个记录信号的FWI算法的输入。结果表明，在理想的数值模型中，FWI比CT更准确地估计了混凝土的速度和异常的几何形状。