Using probabilistic infrasound reflectivity modelling for single station event characterization Christoph Pilger and Lars Ceranna

BGR / B4.3, Hannover, Germany; christoph.pilger@bgr.de

Reflectivity Modelling Single Station Event Characterization

Infrasound reflectivity modelling is used as a method adapted from seismic wave propagation (using pure PP-reflectivity for explosion signatures, see Mueller, 1985). Synthetic barograms are calculated by directly solving the wave equation instead of using high-frequency approximations like ray-tracing. Signal refractions from effective sound speed maxima, e.g. in the stratosphere around 40 km altitude, lead to wave ducting and repetitive surface returns which can be observed as pressure variations in corresponding distance ranges:

Fig. 1: Idealized effective sound speed profile (left) and resulting reflectivity pattern (right) showing five iterations of a stratospheric duct as well as weak thermospheric and tropospheric signatures.

Atmospheric Gravity Wave Perturbations

Fig. 2: Gravity wave perturbed effective sound speed profile (left) and resulting reflectivity pattern (right) showing many additional intermediate surface returns due to partial reflections, scattering and waveguide interaction.

Discussion and Conclusions

Modifying the idealized background conditions with probabilistic atmospheric disturbances like gravity waves results in a more complex reflectivity pattern with large waveguide interaction between the distinct surface returns shown above:

T1.1-P21

The ability of a single infrasound station detecting and characterizing an explosive event with the reflectivity method is demonstrated for the Buncefield oil depot explosion from 2005 Dec 11 (e.g. see Ceranna et al., 2009). The event near London, UK and propagation paths to three infrasound stations are shown on the left, while effective sound speed profiles for the three paths are shown on the right (to Flers in black, to IGADE in blue and to I26DE in red). Reflectivity modelling for the three cases, resulting (color-coded) pressure variations and comparisons with station observations are shown below:

Fig. 3: Color-coded reflectivity pattern for the propagation from Buncefield explosion to infrasound stations Flers (left), IGADE (middle) and I26DE (right). A logarithmic (dB scale) color representation of the synthetic barogram pressure variations is chosen to highlight ducts and regions of waveform interaction. Recorded pressure time series are superimposed in grey in the corresponding station distance. Tropospheric (Iw), thermospheric (It) and multiple stratospheric (Is,(Is)2,...) refractions are modelled and identified in the observations. Various intermediate signatures are also present in the modelling, corresponding to areas of increased wave activity in the observed time series. Geometric spreading loss and 0.5 Hz attenuation are considered by the reflectivity method.

This study utilizes infrasound reflectivity modelling, considers probabilistic gravity wave perturbations and applies the method to characterize explosive event detections. Implementation and application of the method shows a number of advantages and large potential compared to other methods as e.g. ray-tracing, but also some deficiencies and limitations. Detailed pro (+) and contra (-) aspects are:

capability to represent direct, tropospheric, stratospheric and thermospheric propagation as well as intermediate phases due to partial reflections, scattering and waveguide interaction

possibility to include realistic background profiles and small-scale atmospheric disturbances like gravity waves

ability of the method (as shown for the Buncefield example) to explain observed phases, potential to explain additional early and late wave signatures and intermediate wave activity

efficiency to identify and characterize explosive event signatures at medium source-receiver ranges (~200-1000 km)

limitation of the implementation (currently) only allows frequency/attenuation of 0.1, 0.2 or 0.5 Hz, 1d backgrounds and a maximum distance of 1500 km (e.g. not applicable to Swedish stations shown in the Buncefield example map)

open question regarding deviations between model and observation in the onset time of the thermospheric phase (Buncefield to Flers case)

open questions in the interpretation of lower number phase arrivals using different models (Buncefield to IGADE case: (Is)2 to (Is)4 compared to Ceranna et al, 2009: (Is)3 to (Is)5 / Buncefield to I26DE case: (Is)4 to (Is)6 instead of (Is)5 to (Is)10)

The reflectivity method is (currently) limited by a number of boundary conditions and open questions, but it has proven usefulness for identifying and characterizing infrasonic wave signatures from explosive events.

Fig. 4: Gravity wave disturbed effective sound speed (left) and resulting reflectivity pattern (right) for the propagation from Buncefield to infrasound station I26DE. The same parameters and color scale are used as in figure 3.

To investigate the potential influences of gravity wave perturbations on the reflectivity pattern, the propagation from Buncefield to I26DE is modified by adding strong atmospheric disturbances (twice the gravity wave profile amplitude from figure 2) to the effective sound speed profile: The resulting reflectivity pattern shows complex interactions of the modelled wave signatures due to many partial reflections, refractions and scattering. This approach results in larger regions of sound pressure variations near the station observations and indicates gravity waves being a likely explanation of early and late observations in the time series (before (Is)4 and after (Is)6).

References

Mueller, G. (1985) The reflectivity method: A tutorial. Journal of Geophysics 58:153-174. Ceranna, L., Le Pichon, A., Green, D.N. and Mialle, P. (2009) The Buncefield explosion: a benchmark for infrasound analysis across Central Europe. Geophysical Journal International 177: 491-508.