Cavitation‐induced seismic events caused by small explosions in boreholes

Abstract

An electrolytically recharged sleeve exploder was fired at depths between 15.2 m and 51.8 m in a 200-mm diameter, steel‐cased, water‐filled shallow borehole in Hopkins County, Texas. Oxy‐hydrogen explosive gas charges were varied between 30 and 170 kJ. Seismic traces revealed three distinct, delayed, anomalous arrivals. Two of these moved out nonlinearly with increasing charge. The fact that the anomalies were not peculiar to gaseous sleeve explosions was confirmed by repeating part of the experiment using small high explosive charges. The anomalies appear to stem from the collapse of cavities that are created by shock waves. A model is advanced, in which up‐ and downgoing shock fronts sweep away from the explosion at the tube wave velocity. They reflect from the free and plugged ends of the borehole. Reflection at the free end transforms an upgoing overpressure into a downgoing rarefaction. Momentum transfer surges the fluid. Surges accumulate. Two rarefaction shocks eventually collide at the shot point. The fluid there cavitates if the net pressure drops to zero. Fluid moving above the shot rises. The column splits into two parts. Column separation of itself creates rarefaction shocks that enter, surge, and then break fluid slugs from the ends of the (rising) upper and (stationary) lower columns. The borehole fluid breaks into four parts. Three fly up and descend as weight drops to create a train of seismic events. Model predictions correlate with seismic observations. The model provides a means for explosive source calibration. An oxy‐hydrogen charge of 100 kJ in a minisleeve exploder appears to develop an overpressure of ≈8.8 atm. A 25-gm solid explosive charge yields similar results. A simple extension of the model suggests that the potentially destructive effects of implosion can be avoided by plugging the borehole mouth before shooting in an uncased borehole.