Multi-level continuous active source seismic monitoring (ML-CASSM): Mapping shallow hydrofracture evolution at a TCE contaminated site

We present results from the first deployment of a fully automated multi-source seismic tomography system designed to monitor hydrofracture initiation and propagation in near-surface environments. We utilized this system to track an induced fracture utilized as part of an enhanced bioremediation in a tight clay-rich formation at a TCE contaminated site. Several hundred full tomographic datasets were acquired with a temporal resolution of 3-4 minutes; this fine sampling in time allowed us to successfully capture localized P-wave velocity reductions associated with fracture emplacement. In addition to velocity changes, alterations in amplitude were observed as well as diffracted events and secondary scattered arrivals. This unique dataset suggests that real-time active source imaging strategies has a potential role in mapping induced fractures both in the shallow subsurface and in deeper environments such as enhanced geothermal and shale gas deposits.