BOREHOLE TECHNIQUES IDENTIFYING SUBSURFACE CHIMNEY HEIGHTS IN LOOSE GROUND - SOME EXPERIENCES ABOVE UNDERGROUND NUCLEAR-EXPLOSIONS

The location of the subsurface top of the chimney formed by the collap se of the cavity resulting from an underground nuclear explosion is ex amined at five sites at the Nevada Test Site. The chimneys were invest igated by drilling, coring, geophysical logging (density, gamma-ray, c aliper), and seismic velocity surveys. The identification of the top o f the chimney can be complicated by chimney termination in friable vol canic rock of relatively high porosity. The presence of an apical void in three of the five cases is confirmed as the chimney horizon by coi ncidence with anomalies observed in coring, caliper and gamma-ray logg ing (two cases), seismic velocity, and drilling. In the two cases wher e an apical void is not present, several of these techniques yield ano malies at identical horizons, however, the exact depth of chimney pene tration is subject to some degree of uncertainty. This is due chiefly to the extent to which core recovery and seismic velocity may be affec ted by perturbations in the tuff above the chimney due to the explosio n and collapse. The data suggest, however, that the depth uncertainty may be only of the order of 10 m if several indicators are available. Of all indicators, core recovery and seismic velocity indicate anomalo us horizons in every case. Because radiation products associated with the explosion are contained within the immediate vicinity of the cavit y, gamma-ray logs are generally not diagnostic of chimney penetration. In no case is the density log indicative of the presence of the chimn ey.