Seismology Research
Bringing together seismology and atmospheric sciences
8. Modeling of the observed data by raytracing
January 2004
Läslo Evers
8.2 Stratosphere and mesosphere: far field sonic boom
In figure 8.2.1, the recording of coherent infrasonic energy, as recorded by
the 16 microbarometers of DIA, is shown. The amplitude of the event increases
with time, while the frequency content decreases.
Figure 8.2.1: Coherency as a function of time and frequency is plotted in
the lower frame. The best beam for the resolved source characteristics
is shown in the top frame. The event is identified as a sonic boom occurring
above the North Sea at a distance of 205 km from DIA (from cross bearing analysis).
The low frequent energy appears more coherent as it traveled over the array (i.e.
highest coherency is found after 275 seconds). We identify this event as a sonic
boom which occurred above the North Sea (from cross bearing) at a distance
of 205 km from DIA.
In order to explain the recorded data, we model the ray trajectories through a
velocity model obtained from ECMWF atmospheric data. Wind and temperature
information is available up to a height of 62 km. In figure 8.2.2, the results
of this modeling are shown. Infrasonic energy has traveled up to the high stratosphere
and low mesosphere, because of the considerable source receiver distance.
Figure 8.2.2: Results of modeling through raytracing. The velocity model is shown
in the lower frame. The effective sound speed (wind and temperature effects) in
brown, and the temperature depend sound speed, in green, are shown to the right of
the lower frame. In white are the various rays plotted. The rays depart from an
estimated source height of 10 km, at intervals of 5 degrees from the vertical. The
top frame shows the travel times for rays reaching the surface, in red, also
the best beam for DIA is plotted in this frame.
Comparison of the effective sound speed and temperature depend sound speed, resp. in brown
and green, right of the lower frame in fig. 8.2.2, shows two velocity gradients almost
totally controlled by the wind. The first is located at the tropopause, the top of the troposphere
around 10 km, the second can be identified at the stratopause, the top of the stratosphere around
50 km. These gradients make they rays turn back to earth due to refraction, as can be seen
by looking at the white ray trajectories.
The tropohops, earth-tropopause refractions, are the first to arrive, as follows from the
travel time curves in the top frame of figure 8.2.2. Their low amplitude is caused by the
multiple refractions and reflections, before arriving at DIA. On the contrary, less energy is
lost while traveling via the higher atmospheric paths. High stratospheric and low mesospheric
returns, arriving later than the tropohops, show the highest amplitudes in the recording.
The travel times of the high atmospheric arrivals are well modeled. The travel times of the
tropohops are underestimated by the model, they appear later in the recording. The unstable
troposphere is not well modeled by raytracing through actual atmospheric models.
