Reconstructing bomb trajectories resulting from Strombolian activity can provide insights into near-surface dynamics of the conduit system. Typically, the high number of bombs involved represents a challenge for both automatic and manual bomb identification and tracking methods. Here, we present a method for the automated recognition of hundreds of bombs (100 to 400 depending on the explosion observed) and for the reconstruction of their trajectories in time and 3D space by stereophotogrammetry. The data involve video collected at 30 Hz with two synchronised cameras (Basler 1300-30), separated by 11°, targeting explosions at Stromboli (Aeolian Islands, Italy) in September–October 2012. In total, six data sets were collected for emissions lasting less than 15 s. The 3D reconstructions provided more accurate velocity estimations (error < 10%) than 2D analyses (errors up to 90–100% for bombs moving parallel to the line of sight of the camera). By coupling the measured trajectories with a numerical ballistic model, we show that the method can be used to estimate the directional distribution of bombs and their velocities at the vent (which in this case was 30–130 m s −1 ), the wind velocity (~ 3.5 m s −1 from the NW) and the drag coefficients (10 −3.5 − 10 −0.5 ) of the bombs. The 3D reconstructions also provide a quantification of the directions of explosions and show that explosions can be radial, oriented in a predominant direction of ejection or in several directions; these dispersion patterns can change during a few seconds in a single explosion. We relate the changing directions of ejections to rheological variations in the upper part of the magmatic system probably filled with a mixture of partially crystallised magma which can direct rising slugs along preferential paths.

中文翻译：

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一种火山弹弹道3D重建方法

重建由 Strombolian 活动引起的炸弹轨迹可以提供对管道系统近地表动力学的深入了解。通常，涉及的大量炸弹对自动和手动炸弹识别和跟踪方法都是一个挑战。在这里，我们提出了一种自动识别数百颗炸弹（100 到 400 颗，取决于观察到的爆炸）以及通过立体摄影测量法重建它们在时间和 3D 空间中的轨迹的方法。数据涉及使用两个同步相机 (Basler 1300-30) 以 30 Hz 收集的视频，间隔 11°，目标是 2012 年 9 月至 10 月在斯特龙博利（意大利伊奥利亚群岛）发生的爆炸。总共收集了六个排放数据集持续时间少于 15 秒。3D 重建提供了更准确的速度估计（误差 < 10%）比 2D 分析（对于平行于相机视线移动的炸弹，误差高达 90-100%）。通过将测量轨迹与数值弹道模型耦合，我们表明该方法可用于估计炸弹的方向分布及其在通风口处的速度（在这种情况下为 30-130 ms -1 ）、风速（ ~ 3.5 ms -1 距离 NW) 和炸弹的阻力系数 (10 -3.5 - 10 -0.5 )。3D 重建还提供了对爆炸方向的量化，并表明爆炸可以是径向的，在喷射的主要方向或多个方向上定向；这些分散模式可以在一次爆炸中的几秒钟内发生变化。