activity, consisting of xenon and krypton, had evidently been forced into the interstitial spaces between the conductors and subsequently migrated up the cable. About 1-1/2 hr later all cables coming from GB-E were cut at the well head and sealed off with a cap. It is estimated that during the time of cable cutting, about one curie of gaseous activity escaped to the atmosphere. A maximum dose rate of about 600 mR/hr was read at the cap shortly after the cables were cut. This reading had decreased to 48 mR/hr 8 hr later, at which time all but a small area immediately adjacent to the well head was opened to normal activity. GROUND MOTION MEASUREMENTS AND RESULTS GEOPHONE DATA In order to ascertain the approximate time after the detonation at which the cavity collapsed and the chimney formed, as well as to establish a safe time for area reentry, three geophones were placed just below the ground surface at distances of 2600, 4200, and 8400 ft from surface ground zero. Only the geophone at 8400 ft continued to operate through the immediate postshot interval. Its records show very large initial excursions (due to direct ground shock), gradually decreasing to background at about 1 min after the explosion. With the exception of a few short, relatively low-amplitude signals at around 10 min, no further excursions were detected in the next 18 hr. The conclusion is that the cavity collapsed very soon after its formation, most likely within the first 30 sec. CLOSE-IN SUBSURFACE SHOCK AND FRACTURE A number of instruments to measure peak shock pressure, shock velocity, cavity collapse, and fracture propagation were installed in emplacement hole GB-E and in instrument hole GB-1, about 190 ft away. Unfortunately, a subsurface pump overheated during the stemming of the emplacement hole, which caused all instrument cables in this hole to short out and resulted in complete loss of peak pressure and chimney formation information. Hole GB-1 was instrumented about two months prior to the detonation; Fig. 3 shows one of the instruments in the process of being lowered. On the day before the shot, the resonance cable of the slifer instrument2 shorted at the oscillator (presumably from corrosion or leakage). Nevertheless, a few time-of-arrival points were obtained and are compared in Fig. 4 with curves calculated on the SOC code." 3 Of considerable interest are the data from the fracture cable system installed in GB-1. In this measurement, discontinuities in a 1/4-in. diam, stiff electrical cable are recorded as they progress up from the shot horizon. Figure 4 shows that the cable is severed at discrete points up to the 3800-ft level, at a time 5 Acceleration and velocity gages were emplaced at four depths in instrument hole GB-D, located 1470 ft from the emplacement hole. These measurements, carried out by Sandia Laboratories, were made primarily to study the generation 4 of the seismic wave. The detailed results are presented by W. Perret. These data also permit a correlation of ground motion with the ability of conventional gas wells to withstand such motions. Results of a preliminary data reduction are given in Table I, where the station designations correspond to gage depths below the surface in hundreds of feet. All acceleration gages were saturated; this was partly due to accelerations higher than expected, and partly due to the influence of the downhole temperature on instrument damping. The velocities are also somewhat higher than expected, with displacements about as predicted. These facts are consistent with a stress wave of relatively sharp rise time and short duration. Fig. 3. Instrument being lowered in hole GB-1. Grout was pumped through the fiberglass pipe to which the instrument is attached to provide coupling with the formation. SURFACE ACCELERATIONS AND VELOCITIES Surface accelerations and velocities were measured over distances spanning Fig. 4. Dynamic shock and fracture data. The fracture data show that the cable breaks progressively from the bottom; it remains intact above the 3800-ft level. to the objectives of Gasbuggy. The preliminary data are compared with preshot predictions in Figs. 5 and 6. A considerable amount of data, primarily velocity information in the 20- to 70-mile range, remains to be reduced and analyzed; the agreement of data with predictions is seen to be quite good. Out to a distance of at least two depths of burial, surface spall (the separation of a surface layer from the ground) appears to have taken place. This is shown by the values of peak surface motion listed in Table II. Two distinct periods of maximum motion are observed, the first occurring when the stress wave is first incident at the surface, and the second when the separated layer returns to contact the unspalled surface. As observed in other underground explosions, this second peak (due to closing of the surface spall) is up to three times higher than the first. Part of the seismic array consisted of five stations on or near the Navajo Dam, about 38 km west of the site, and two stations on and near the El Vado Dam, about 42 km east of the site. A preliminary reduction of the data shows that surface velocities recorded at the crests near the dam centers were about twice 10-1 Hard rock |