Estimating the detection capability of the International Monitoring System infrasound network

A 59 station infrasound network is being established as part of the verification regime for the Comprehensive Nuclear-Test-Ban Treaty. We present a probabilistic model for predicting the network detection threshold (defined as the chemical explosive yield at which two stations will detect signals with 90% probability). We incorporate the influence of stratospheric wind and detection bandwidth while accounting for estimates of noise amplitude and propagation uncertainties. Inclusion of the stratospheric wind, using a state-of-the-art climatological model, tends to reduce the detection threshold. For 66%/88% of Earth's surface area the detection threshold is lower than that expected for the windless case for all/>= 90% of the time. However, preferential downwind detection makes location of explosions with yields close to the threshold more difficult due to the restricted azimuthal coverage. Completing the IMS network is also important: the detection threshold for the 39 station network operating in October 2008 is approximately 45% higher than that predicted for the full 59 station network. Moreover, the absolute detection threshold values are highly sensitive to the frequency band in which the noise is evaluated. A detector simulation comprising overlapping two-octave frequency bands indicates that, when accounting for low noise at frequencies above 0.2 Hz, detection capability at low yields (<0.5 kt) may be considerably better than previously estimated. Our simulations, using a two-octave bandwidth detector and incorporating stratospheric winds from a climatological model, predict that explosions of >210 t chemical explosive (>420 t nuclear equivalent yield) will be detected over >= 95% of Earth's surface at any time of year (with a 90% probability).