ERT is an established method of remotely imaging the bulk electrical conductivity (BEC, the reciprocal of resistivity) of the subsurface. 4D, electrical resistivity tomography (ERT). In this paper we demonstrate an emerging capability for imaging rock damage patterns and gas flow paths from underground explosions using time-lapse 3D, i.e. Although advanced hydrodynamic modeling capabilities exist, data to validate or otherwise inform simulation codes are sparse, primarily due to sampling requirements imposed by subsurface heterogeneity and the general difficulty of accessing the near-source region. Of particular interest are how rock damage patterns, such as rock crushing, block motion, spallation, and fracturing, influence gas flow to the surface and far-field seismic characteristics such as the generation of S-waves (e.g. One key question is how the near field (~ 0 to 200 m from the working point) geologic response to the explosion influences far field signals. The importance of the first few hundred meters of signal propagation of both waveform and materials could be amplified when considering lower yield nuclear tests because the signatures are lower in magnitude and may suffer from severe distortion in the local to regional scales of the subsurface. Atmospheric radionuclide monitoring, which relies on the detection of radioactive gases and particulates that migrate from the UNE working point to the surface and are subsequently transported through the atmosphere, is currently the primary method of nuclear versus non-nuclear discrimination (e.g. Although seismic monitoring is the primary method for detecting and locating underground explosions, current understanding does not enable discrimination between nuclear and chemical explosions. ![]() In addition, seismic waveforms implicitly contain further information concerning the geologic structure along the travel paths, which is vitally important at local and regional distance scales. Denny & Johnson, 1991 Ekström & Richards, 1994 Ford & Walter, 2010). Seismic energy recorded at monitoring stations contains information concerning the timing, location, and magnitude of an underground explosion (e.g. Seismic monitoring and environmental radionuclide monitoring are two primary methods of detecting the occurrence of underground nuclear explosions (UNEs). These results point to the utility of ERT imaging for understanding rock damage and gas flow patterns under experimental conditions, and to the importance of understanding the effects of geologic heterogeneity on UNE detection signals, particularly gas surface breakthrough times. ![]() Time-lapse ERT imaging of heated air injected into the detonation borehole revealed the primary gas flow paths to be within the upper margin of the same primary damage zone. Post-explosion imaging revealed that the damage zone was non-symmetric and was focused primarily within the pre-existing fracture zone, located 10 m above the first explosion and 5 m above the second explosion. Pre-explosion ERT and crosshole seismic imaging revealed a natural fracture zone within the test bed. ![]() In this paper we demonstrate the emerging use of electrical resistivity tomography (ERT) for imaging rock damage and gas flow patterns resulting from two relatively small-scale underground chemical explosions. For example, the effects of in-situ mechanical heterogeneities on the explosively generated damage/fractures that provide gas flow pathways to the surface are not well understood, due largely to the difficulty in accessing and characterizing the near-source region. Although advanced numerical simulation capabilities exist to predict rock damage patterns and corresponding detection signals, those predictions are dependent on (generally) unknown properties of the host rock. Rock damage from underground nuclear explosions (UNEs) has a strong influence on sub-surface gas movement and on seismic waveform characteristics, both of which are used to detect UNEs.
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