InfraMatics, 2013, 2, 39-55
Published Online December 2013 (
Open Access InfraMatics
Application of Propagation Modeling to Verify and
Discriminate Ground-Truth Infrasound Signals
at Regional Distances
Christoph Pilger1, Florian Streicher2, Lars Ceranna1, Karl Koch1
1Federal Institute for Geosciences and Natural Resources (BGR), Hannover, Germany
2German Aerospace Center (DLR), Oberpfaffenhofen, Germany
Received November 5, 2013; revised December 11, 2013; accepted December 18, 2013
Copyright © 2013 Christoph Pilger et al. This is an open access article distributed under the Creative Commons Attribution License,
which permits unrestricted use, distribution, and reproduction in any medium, provided the original work is properly cited. In accor-
dance of the Creative Commons Attribution License all Copyrights © 2013 are reserved for SCIRP and the owner of the intellectual
property Christoph Pilger et al. All Copyright © 2013 are guarded by law and by SCIRP as a guardian.
An infrasound field campaign was performed in 2011/2012 utilizing single infrasound sensors along the great circle
path between a known ground-truth source (Ariane 5 engine test facility, Lampoldshausen, Germany) and a regional
receiver (German infrasound array IS26, Bavarian Forest) covering a distance of rough 320 km in total. The gathered
recordings provide new insights in the infrasonic wave propagation at regional and near-source distances by comparing
measured signals with modeling results within this study. Ray-tracing and parabolic equation approaches are utilized to
model infrasound propagation from the ground-truth source to the line profile sensors and explain the obtained detec-
tions and non-detections. Modeling and observation results are compared by estimating their amplitude, quantifying
amplitude deviations and also considering observed and calculated travel times and celerities. Modeling results show a
significant influence of small-scale atmospheric variations in effective sound speed profiles on the propagation pattern,
which results in varying tropospheric and stratospheric ducting behavior. A large number of gravity wave profiles are
tested to investigate the influences of atmospheric dynamics on the infrasound wave field and improve the modeling
results. The modeling is furthermore applied to a case of two potential, contemporaneous and closely spaced infrasound
sources. Propagation modeling is used here to resolve the source ambiguity between a ground-based and a higher alti-
tude source giving a strong preference to the latter with respect to the observed infrasonic signatures. The good agree-
ment between modeling and observation results within this study successfully shows the benefit of applying infrasound
propagation modeling to the validation of infrasound measurements, verification of ducting behavior and discrimination
of infrasound sources.
Keywords: Infrasound; Propagation Modeling; Ground-Truth
1. Introduction
Infrasound is generated by a variety of different natural
and artificial sources such as volcanic eruptions or mete-
orite entries [1-3] and anthropogenic events such as air-
craft/spacecraft signatures or explosions [4-6], respec-
tively. Infrasonic signals from different sources are de-
tected by microbarometers at nearby up to very remote
distances (e.g. see [7-9]). Usually, when an infrasonic
detection is made, the corresponding source is unknown
and can be identified by quantifying arrival time, signal
frequency, amplitude, velocity and back-azimuth [10].
However, difficulties in the proper source identification
are due to uncertainties introduced by atmospheric varia-
tions on the source-to-receiver propagation path, signal-
to-noise issues and ambiguities in active infrasound
sources in the source direction.
In contrast, ground-truth sources are events with well-
known source parameters such as time and exact location,
or clearly observed by complementary techniques, so
they have well-known back-azimuths and distances from
source to receiver. In this case, travel-time and celerity
for an observed signal can be quantified and thus addi-
tional information is available on how the signal was
ducted from the source to a receiver. Alternatively, in-
formation about the atmospheric conditions on the prop-
agation path can be acquired or verified by inverting ob-
served waveforms and their parameters from ground-
truth sources (e.g., [11-13]).
Infrasound propagation modeling (e.g., [14,15]) is
used to describe the propagation pattern of infrasonic
signatures from a surface or elevated source to a receiv-
ing instrument. Aspects of ducting through different alti-
tudes of the atmosphere (troposphere, stratosphere or ther-
mosphere), the extent of shadow zones as well as estima-
tion of amplitudes and sound attenuation can be covered
[16-18]. Propagation modeling is applied in this study to
verify ducting from a known ground-truth source to a
receiver and to discriminate between different potential
sources of a signature, also taking into consideration at-
mospheric conditions and small-scale changes on the
propagation path. Certain detections and their observed
travel-time and celerity values can only be explained by
including unusual propagation conditions, near-field tro-
pospheric ducting and small-scale atmospheric perturba-
tions such as gravity waves [19-22].
In Section 2 of this study, the observational setting of
Ariane 5 rocket engine tests as a ground-truth source [23]
and infrasound measurements on a line profile of micro-
barometers towards an infrasound array as measurement
setting are described. Two different propagation models,
a ray-tracing [24] and a parabolic equation approach [25],
described in Section 3, are applied to study infrasonic
source-to-receiver paths. Atmospheric background mod-
els using ECMWF analysis data, climatologies and fine-
scale gravity wave structures are included for a realistic
estimation of wave ducting. In Section 4, results are de-
scribed and discussed concerning tropospheric and stra-
tospheric wave ducting, estimation of source amplitudes
and consideration of atmospheric dynamics. Finally, in
Section 5, all results are summarized and conclusions are
2. Measurements
In a previous study [23], infrasound signals were analyz-
ed from Ariane 5 engine tests in the years 2000-2004 that
could be identified at station IS26 of the International
Monitoring System (IMS) network of the Comprehensive
Nuclear-Test-Ban Treaty Organization (CTBTO). In
spring 2011, information was obtained that a new series
of engine tests was planned between fall 2011 and spring
2012 at the rocket propulsion testing facility of the Ger-
man Aerospace Center (DLR) near Heilbronn, Southern
Germany (49.287˚N, 9.378˚E). Since this setting was
favorable for recording these tests along the path to IS26
based on the prevailing winter wind conditions, as had
been found earlier [23,26], a field campaign was de-
signed to cover local propagation distances as close as 20
km to regional distances of IS26, at a range of 320 km.
The geographical setting for this field experiment is
shown in Figure 1 and a summary of the ground truth
information for the different tests is provided in Table 1.
Figure 1. Layout of the field campaign in two different setups (stations DLR1a to DLR6a, yellow triangles, from 20 to 120 km
and stations DLR1b to DLR6b, orange triangles, from 120 to 240 km). Also shown is the Ariane 5 engine test source (black
star) near Heilbronn, nearby seismometer and sound pressure level microphone (green triangle, DLR-P4), complementary
sources near Bitburg, Germany and in Luxemburg (black stars, BIT, LUX), seismometer stations near the sources (blue tri-
angles, SIND, RUP, ABH) and the IS26 infrasound array (red hexagon) near Passau.
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Table 1. Details of ground-truth information (in black) and observations (in green/red) for nine Ariane 5 rocket engine tests
and two additional potential sources (BIT, LUX, see text and Figure 1). Durations for the additional sources are not applica-
ble, since signals are assumed to be impulsive. Positive detections of the source by instruments at a given distance are shown
in green, non-detections in red. Instruments set-up but not in operation (and thus not capable of signal detections) are not
Test Nr. Date Start Time DurationSeismo-AcousticLine Profile Microbarometers IS26 station
1 18-11-2011 15:50 UTC 600 s 20 km - 320 km
2 08-12-2011 15:30 UTC 668 s 20 km - 320 km
3 19-12-2011 15:17 UTC 646 s 20 km 40 km, 60 km, 80 km, 100 km,120 km 320 km
4 16-01-2012 16:43 UTC 285 s 20 km 20 km, 40 km, 60 km, 80 km, 120 km 320 km
5 16-02-2012 13:28 UTC 720 s 20 km 20 km, 40 km, 60 km, 80 km, 120 km 320 km
6 07-03-2012 15:53 UTC 700 s 20 km 120 km, 160 km, 180 km, 210 km, 240 km 320 km
7 22-03-2012 15:31 UTC 696 s 20 km 120 km, 160 km, 180 km, 210 km, 240 km 320 km
8 27-04-2012 14:39 UTC 653 s 20 km 120 km, 140 km, 180 km 320 km
9 14-05-2012 15:21 UTC 530 s 20 km 120 km, 140 km, 160 km, 180 km 320 km
BIT 16-02-2012 11:19 UTC N/A 50 km, 60 km 240 km, 260 km, 280 km, 300 km, 340 km 530 km
LUX 16-02-2012 11:17 UTC N/A 70 km, 100 km 270 km, 290 km, 310 km, 330 km, 370 km 560 km
As only a limited number of six mobile infrasound re-
cording systems was available, a deployment plan was
devised to set up a linear station profile with inter-station
spacing of about 20 km in two distinct stages: for ap-
proximately half of the initially announced seven engine
tests in the distance range from 20 to 120 km and for the
remaining tests to cover, as much as possible, the gap of
ranges to IS26. In addition to the mobile infrasound sta-
tions we were able to obtain seismo-acoustic waveforms
from a seismometer station (SIND) operated by the State
Seismological Survey of Baden-Württemberg, Germany
(pers.comm.) which was located at a similar range as the
closest infrasound station at 20 km, and only at a slightly
different azimuth to the source. The specific distances
covered in every test can be found in Table 1. The given
listing shows that for the first two tests only recordings
from the seismometer station (SIND) and IS26 could be
obtained, as it was not possible to respond to short-term
announcements of these tests with field deployments.
However, since DLR announced two additional tests by
the time the original tests were concluded, the shorter
and the longer range station profiles were covered by
three and four engine tests, respectively.
The mobile infrasound equipment consisted of Reftek
digitizers and single MB-2000/2005 microbarometers per
site. For reducing wind noise for our installations in
harsh fall and winter conditions we connected porous
hoses of 5 - 8 m length to the microbarometers. Addi-
tional details on the deployed equipment and the wind
noise reduction filters used at the mobile stations are
given in [27]. Furthermore, we also put a seismometer
about 50 m from the engine testing facility to monitor the
ground-truth source and its temporal stability. For the
final test, the sound pressure level was also recorded by
the German Aerospace Center, Heilbronn (pers.comm.)
at about 24 m distance to the source. Concerning the
infrasound observations from the various tests (see Tabl e
1) and the different profile configurations we found in-
termittent instances for rather short distances (20 or 40
km), lack of signals for more than 40 km and less than
120 km, and fairly consistent signals at or beyond 120
km (except in one case) as well as at IS26 where all nine
tests were identified, as expected from a testing cam-
paign within winter months [23]. Only the April and May
2012 tests are somewhat extraordinary, because there had
not been any observations at IS26 such late in spring for
past engine tests. For the quantitative modeling attempt-
ed in this study the data were analyzed to determine tra-
vel times and pressure amplitudes. Spectral analysis of
line profile and IS26 data showed a dominant frequency
of 4 Hz, on which we based the modeling described be-
low. For the IS26 infrasound array we also carried out
frequency-wavenumber (FK) analyses to confirm the
proper azimuth of the signals.
During the field campaign an incidental observation of
yet another example of possible ground-truth sources
occurred. On 16 February 2012, a few hours prior to one
of the Ariane engine tests, rather significant infrasound
signals were recorded along the profile of sensors. In the
following days, there were media reports about an explo-
sion event in the Grand-Duchy of Luxemburg. The re-
ported site of this explosion was some 240 km further to
the west-northwest of the rocket testing facility and al-
most on the same great circle path of the recording pro-
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file towards IS26. At the same time there were also re-
ports about a potential sonic boom from a fighter aircraft
in the Western parts of Germany, in the Eifel region. The
German Air Force confirmed a supersonic flight in this
region and eventually provided corresponding ground-
truth information about the flight times and path of the
relevant aircraft (pers.comm.). The Luxemburg explosion
site and an estimated source location on the supersonic
aircraft’s flight path are also marked in Figure 1 (LUX,
BIT). The event was registered by two nearby seis-
mometer stations (ABH, RUP) of the State Seismological
Survey of Baden-Württemberg, Germany (pers.comm.),
on which the source location estimate of the aircraft was
based, along the line profile stations and at IS26 in a dis-
tance of more than 550 km. The corresponding distances
of the stations to these sources are given in Table 1. The
clear observations obtained from the source, either in
Luxemburg or over the Eifel region, certainly confirmed
the proper operation of the line profile stations that were
mostly deployed in the acoustic shadow zone for the
early propulsion tests.
3. Modeling
Infrasound propagation modeling is carried out within
this study to verify detections and non-detections ob-
tained along the line profile infrasound measurements
towards the IS26 array. It is also performed to identify
and discriminate complementary detections observed
during the field campaign.
Propagation modeling approaches utilized within this
study are the parabolic equation (PE) algorithm of the
software package InfraMAP [25] and the ray-tracing (RT)
method HARPA/DLR [24], an enhanced version of the
original HARPA ray-tracing program [28]. Realistic at-
mospheric background conditions are included in the
modeling by the use of ECMWF model analysis data
( combined with climatologies for hori-
zontal wind and temperatures above 60 km (HWM07, see
[29]; MSISE00, see [30]). Fine-scale atmospheric struc-
tures are introduced in the modeling by using gravity-
wave disturbances of the effective sound speed following
Figure 2 shows two-dimensional representations of
the PE and the RT methods for atmospheric infrasound
propagation paths between source and receiver(s). The
two methods are shown and described separately in this
section/figure, but in the course of the study both are
combined in a joint representation for most cases. This
should mainly highlight commonalities of the two meth-
ods in explaining observations and atmospheric states.
Nevertheless, slight differences evident in the compari-
son of both methods are due to their different approaches
(parabolic vs. Hamilton equations), different dimension-
alities of modeling space (two-dimensional PE vs. three
dimensional RT, of which only two-dimensional projec-
tions are shown within this study) and background rep-
resentations (one-dimensional temperature and wind pro-
file for PE vs. four-dimensional temperature and wind
fields for RT). One particular difference is the presence
of a weak PE infrasound field that scatters outside the RT
ray tubes into shadow zones (e.g. the range between 30
km and 120 km in Figure 2). No ray paths are predicted
in this region, but a PE amplitude of about 60 dB is
modeled. This is due to small amounts of wave energy
leaking from wave ducts and scattering into the shadow-
zone (e.g., [19,21]); a process which can be represented
by the used PE but not RT methods. Another difference
is a slight horizontal shift of rays returning from the stra-
tosphere in some of the test cases. These differences are
inevitable due to the available model versions, but also
allow consideration and discussion of infrasound propa-
gation and atmospheric effects on different scales (see
Section 4). The main outputs by the two methods are
range-height-fields of infrasound wave amplitude for PE
and range-height-views of ray patterns for RT, both
modeled for a frequency of 4 Hz identified as the domi-
nant signal frequency (see Section 2). Further model
outputs of surface pressure amplitudes, travel times and
celerities are introduced in Section 4.
Atmospheric background parameters, which are essen-
tial for the propagation of infrasound, are temperature,
zonal and meridional winds from the ground to thermos-
pheric altitudes above 120 km. The effective sound speed,
which combines temperature (by temperature-dependent
sound-speed) and wind (in the direction of infrasound
propagation) into one parameter, controls ducting of in-
frasound between surface and certain altitudes as e.g. the
troposphere, stratosphere and thermosphere. Additional
fine-scale structure, e.g. by atmospheric dynamics like
gravity waves (see [32]), can furthermore change the
effective sound speed and thus modify ducting behavior.
Figure 3 shows an exemplary profile of temperature and
winds derived from ECMWF, HWM and MSISE as well
as the resulting effective sound speed with and without
an arbitrary gravity-wave perturbation.
4. Results and Discussion
The propagation of infrasound can be subdivided in tro-
pospheric, stratospheric and thermospheric ducts as well
as rays escaping the atmosphere [14]. Due to the high
signal frequencies of the observations in this study (about
4 Hz) and correspondingly increased attenuation in the
upper atmosphere, no thermospheric returns are expected
or observed.
Detections of Ariane 5 rocket engine signals in the
infrasound frequency range have been found during the
field campaign at distances from 20 to 320 km. Tropo-
spheric arrivals have been observed in the near field be-
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(a) (b)
Figure 2. Infrasound propagation using the parabolic equation (a) and ray-tracing (b) methods. Displayed are the vertical
profile from Ariane 5 engine source (origin) to the IS26 array (at 320 km distance) and altitudes up to 120 km. Propagation
conditions are shown for February 16th, 2012 (test case #5). Sound pressure amplitude in dB is color-coded in (a), ray paths
shown in (b); ray paths are terminated at the -120 dB level (reached at altitudes of approximately 110 km), red colored rays
indicate eigenrays between the source and the sensors (here only IS26), operational line profile sensors (here between 20 and
120 km) are shown by blue triangles.
Figure 3. Atmospheric background profiles for (a) Temperature (black), zonal wind (red) and meridional wind (blue), (b)
Derived effective sound speed, (c) Effective sound speed perturbation by an arbitrary gravity wave profile, (d) Effective
sound speed with the perturbation included. Analysis data and climatologies show the February 16th, 2012 conditions (test
case #5).
low 50 km distance in December 2011 and February
2012 and even at distances above 100 km in March and
April 2012 (see Section 4.1). Detections at longer dis-
tances, e.g. at the IS26 infrasound array in 320 km dis-
tance, are of stratospheric origin (see Section 4.2). Fur-
thermore, gravity wave influences changing the infra-
sound ducting characteristics are considered for selected
test cases (see Section 4.3). Finally, additional signals
obtained during the campaign, are discussed in the con-
text of source discrimination (see Section 4.4).
4.1. Tropospheric Ducting
Detections of tropospheric arrivals were obtained in the
near-source shadow zone, where no stratospheric arrivals
are predicted, or in the case of April 2012 during a sea-
son, where only weak or more likely no stratospheric
ducting from west to east takes place. These detections
and their association to tropospheric ducting are in good
agreement with infrasound propagation modeling. Fur-
ther seismo-acoustic detections at 20 km distance (sta-
tion SIND) for test cases #1, #2, #3, #5, #6, and #8 and
non-detections for cases #4, #7, and #9 (see Table 1)
confirm the near-field infrasound measurements as well
as the modeling. In the following, a few specific exam-
ples of tropospheric ducting at regional distances are pre-
Figure 4(a) shows the results of propagation modeling
for the 19 December 2011 test case (#3). Infrasound de-
tections from this case were obtained at 40 and 320 km,
while sensors at 60, 80, 100, and 120 km had no detec-
tions. Figure 4(b) shows an enlarged view of the propa-
gation modeling at short distances, highlighting a tropo-
spheric duct. HARPA/DLR ray-trace modeling is su-
perimposed on the PE amplitude field in the figure, both
showing a tropospheric duct between the surface and an
altitude of a few 100 meters. In the ray-tracing, a number
of paths are observed that are each terminated when a limit
of ten surface reflections is reached. From this test case
and based on the observations and the modeling results,
it can be expected that a duct of only a few hundred me-
ters altitude cannot be observed beyond a maximum dis-
tance of 10 - 50 km distance, i.e. a maximum of 5 - 10
reflections. This is due to leaking of the sound energy
from the duct, especially when the infrasonic wavelength
is in the order of the vertical dimension of the duct and
when defocusing by topographic effects near the surface
[33] takes place. Unfortunately our PE modeling cannot
be used to study this scenario in detail, because the 1-D
version used here does not allow for inclusion of hori-
zontal variations of background conditions, orography or
near-surface attenuation. Therefore the tropospheric duct
is removed from the PE model cases in the following
considerations of stratospheric arrivals within Sections
4.2 and 4.3 of this paper. The line profile observations as
well as a seismo-acoustic detection at 20 km distance
(see Tab l e 1) are in good agreement with the model re-
sults based on the meteorological conditions at hand: a
layer of higher effective sound speed in approximately
400 m altitude above the surface, generating a tropo-
spheric duct. At distances larger than 40 km, no signals
from the tropospheric duct were found. The detection at
320 km is of stratospheric origin (see section 4.2). The
16 February 2012 test case (#5, see Figure 2) is very
similar to this case (with a seismo-acoustic and acoustic
detection at 20 km, no detections at 40, 60, and 80 km
and stratospheric detections at 120 km and 320 km), and
thus not further discussed here.
Figure 5(a) shows modeling results of the 16 January
2012 test case (#4). The same station-distance-setup is
used as for test case #5, but no detections were made
from 20 to 80 km distance. Detections at 120 and 320 km
are again from stratospheric paths and are discussed in
section 4.2. No tropospheric detections were identified
and modeling results hence do not show a tropospheric
duct. Figure 5(b) shows the same range-height-detail as
Figure 4(b) (blue triangles for stations at 20 and 40 km),
but the PE sound amplitude profile and HARPA/DLR
rays are strongly bending upwards. Meteorological con-
ditions cause a decreasing effective sound speed with
altitude in the first 10 kilometers and thus inhibit a tro-
pospheric duct, such that the lack of detections and the
modeling are again in good agreement.
A case of long distance tropospheric ducting is evi-
denced by the 27 April 2012 test case (#8). PE- and RT-
(a) (b)
Figure 4. Combined parabolic equation and ray-tracing profiles for the 19 December 2011 test case #3. (a) Complete propa-
gation path from Ariane 5 engine source to IS26 infr asound array . (b) Zoom of the first 50 km horizontal distance and 10 km
altitude highlighting a tropospheric duct. See Figure 2 for further details.
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(a) (b)
Figure 5. As Figure 4, but for the 16 January 2012 test case #4, with atmospheric conditions for which no tropospheric duct
was generated.
modeling shown in Figure 6(a) include a tropospheric
duct over the whole distance range which is persistent
between the surface and an altitude of 3 - 4 km. Since
this altitude is much higher than the few hundred meters
in cases #3 and #5, tropospheric ducting over large dis-
tances is theoretically possible at this altitude [14], as
disturbances due to orography and boundary layer turbu-
lence are strongly reduced. Signal measurements by sen-
sors at 120, 140, and 180 km were obtained and could be
attributed to tropospheric ducting: Figure 6(b) shows a
range versus travel-time plot for the ray-trace model,
where incidence points of rays at the surface are com-
pared to travel-times observed by the line profile sensors.
Observations between 120 and 180 km correspond to the
modeled tropospheric duct. Their celerity (range over
ground divided by travel-time) is about 350 m/s, which is
a fairly high value and thus indicates tropospheric duct-
ing (see [17,34]). However, the travel-time estimate for
the observation at 320 km distance is higher than ex-
pected for the tropospheric duct. The celerity of this de-
tection is about 300 m/s, which would be in a typical
range for stratospheric ducting [17,34], even though no
such duct is predicted by the modeling. Potential expla-
nations for this disagreement between modeling and
measurement will be given in Section 4.3. The detection
and corresponding modeling of a tropospheric duct
reaching out to 180 km from the source is a rare finding
which gives new insights into infrasound propagation at
regional distances.
4.2. Stratospheric Ducting & Amplitude
Classical infrasound propagation from a surface source to
a sensor in some hundred kilometers distance is strongly
influenced by the stratospheric effective sound speed.
During atmospheric conditions, when the effective sound
speed in the stratosphere is higher than the effective
sound speed near the surface, stratospheric ducting takes
place and sound is (repeatedly) refracted from the strato-
sphere back to the surface. For infrasound propagation
from west to east, these conditions normally occur in
wintertime in the Northern Hemisphere (e.g. [26]). Since
most tests within this study took place during the winter
months, stratospheric arrivals at sensors installed at dis-
tances of more than 150 km were expected and regularly
observed. For certain test cases (e.g. #4, #5 and #7) with
extremely high vertical gradients of the effective sound
speed between 30 and 50 km altitude, stratospheric arri-
vals were already observed in 120 km distance from the
source. The zonal wind speed in test case #5 (see Figure
3) increases 70 - 80 m/s between altitudes of 30 - 40 km,
which can be associated, for example, with a strong jet
stream. Conditions in test cases #4 and #7 lead to similar
signal arrivals. Observations of signals in 120 km dis-
tance of the source and their arrival via stratosphere or
troposphere are discussed in the course of this section.
Comparisons between modeling results and measure-
ments were also performed with respect to infrasound
amplitudes. Amplitudes from modeling were estimated
using the surface level output from PE modeling as
sound pressure level in decibel (dB) at a reference range
of 1 km. For the comparison of these modeling ampli-
tudes with measurements in Pascal (Pa), a reference val-
ue for the source amplitude (in Pa at 1 km distance) to
equalize with 0 dB is needed.
Figure 7 shows a sound pressure level recording of the
last Ariane 5 engine test (#9) measured at 24 m distance
to the source. For a reference distance of 1 km, this sig-
nal of approximately 137 dB (corresponding to 142 Pa) is
reduced to a value between 120 dB (20 Pa) and 100 dB
(2 Pa) depending on the geometry assumptions for the
acoustic loss calculations [35]. Since different source and
propagation conditions may occur and thus different
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(a) (b)
Figure 6. (a) As Figure 4(a), but for the 27 April 2012 test case #8 and a tropospheric duct extending to an altitude of 3 - 4 km.
(b) Travel time estimation for all ray-tracing surface returns in the tropospheric duct (black dots) and onset-times derived
from observations (red crosses) in sensor distances (blue lines).
Figure 7. Sound pressure level measurement performed in
the audible range during the 14 May 2012 test #9 at a dis-
tance of 24 m to the Ariane 5 engine. The sound pressure
level is increased for the test duration of 530 s from 70 dB
ambient noise (which is already an increased value due to
background noise before and after the test, compared to
usually 30 - 50 dB) to about 137 dB during the engine test.
sound pressure levels for different test cases may result,
an intermediate value of 10 Pa source amplitude (at 1 km)
is chosen corresponding then to 0 dB modeling ampli-
Figure 8 shows comparisons of modeled amplitude
values derived using the surface level values from the PE
method and observed signal amplitudes for the test cases
#3, #4 and #5 (already discussed for tropospheric ducting
in section 4.1; propagation diagrams for these cases are
shown in Figures 2, 4 and 5). Amplitudes derived from
modeling and observations are in good agreement and
show stratospheric detections at IS26 (at 320 km). Fur-
thermore, stratospheric ducting beginning at 120 km dis-
tance explains detections in cases #4 and #5 (January and
February) but not in case #3 (December). A beginning
stratospheric duct at 120 +/ 5 km in the model explains
the January and February detections, whereas in Decem-
ber, due to the atmospheric conditions, stratospheric
ducting sets in at 150 +/ 5 km, which is in good agree-
ment with the lack of a stratospheric arrival at 120 km in
that case.
For cases #3 and #5 (and #6 in Figure 9), tropospheric
(dotted curves) and stratospheric ducting (dashed curves)
were modeled separately. Since the used PE method only
allowed one-dimensional background conditions, a tro-
pospheric duct does not fade over the modeling distance.
However, both observations and ray-tracing show that
the tropospheric ducts vanishes beyond the first observa-
tion point and thus the two ducting regimes were com-
bined in these cases by merging the profiles (solid lines)
and applying a linear transition from tropospheric to
stratospheric ducting.
The root mean square (RMS) deviation was calculated
as a measure to quantify the difference/agreement be-
tween observations and modeling. For the abovemen-
tioned cases, results are 3.26 dB (case #3), 8.27 dB (case
#4) and 25.07 dB (case #5), which corresponds to devia-
tions of about one order of pressure magnitude. However,
only two to three detections were available for these cas-
es and conclusions on the estimation of amplitudes from
this should be drawn with caution.
More specific examples with observations by six sen-
sors are the March 7 and March 22, 2012 Ariane test
cases #6 and #7 shown in Figures 9 and 10, describing
stratospheric duct modeling and corresponding observa-
tions in greater detail.
In test case #6, stratospheric ducting of the engine test
signal is observed at distances of 160, 180, 210, and 240
km as well as IS26 in 320 km. Modeling of the first and
second stratospheric return to the surface (see Figure
9(a)) explains most of these observations. Only the ob-
servation at the first sensor in 120 km distance cannot be
explained by stratospheric ducting and is (similar to case
#8, see Figure 6) of tropospheric origin. Infrasonic travel
Open Access InfraMatics
(a) (b) (c)
Figure 8. Amplitude e stimations from parabolic equation modeling (red line, left axis) and measured signals (black dots, right
axis) for test cases #3 (a), #4 (b) and #5 (c). Dotted red lines (in (a) and (c), at about 20 - 30 dB) indicate amplitudes for a tro-
pospheric duct, dashed red lines (in (a) and (c), from 0 to 20 - 40 km) indicate amplitudes after removal of the tropospheric
duct, solid red lines are a transition from tropospheric to stratospheric ducting (only in (a) and (c), while in (b) no tropo-
spheric duct is present). Green vertical lines indicate a signal detection, while red ver tical lines indicate a sensor not dete cting
the engine test signal.
(a) (b)
Figure 9. As Figures 4(a) and 8(a) & (c), but for the 7 March 2012 test case #6. The transition region from tropospheric to
stratospheric ducting is situated between 120 and 160 km for this case. The root mean square deviation between modeling
amplitude (solid red curve) and observations (at black dots) is 4.70 dB.
(a) (b)
Figure 10. As Figures 4(a) and 9(b), but for the 22 March 2012 test case #7. No tropospheric duct is present. The root mean
square deviation betw ee n mode ling amplitude (solid red curve) and observations (at black dots) is 6.69 dB.
times with a celerity of approximately 335 m/s (see later
Figure 11(b)) and atmospheric conditions where the ef-
fective sound speed at 2.5 km altitude is higher than the
effective sound speed at the surface support this conclu-
sion. Figure 9(b) thus combines influences of a tropo-
spheric duct over the first 120 km distance and removal
Open Access InfraMatics
of this duct beyond 160 km with the same approach as
used in Figures 8(a) and (c). The modeling results for
pressure amplitudes between 160 and 320 km are in good
agreement with observed signal amplitudes (with a root
mean square deviation of the 6 infrasound sensor obser-
vations to modeling amplitudes of 4.70 dB) and thus ex-
plain the infrasound propagation well for this test case.
Figure 10 describes test case #7, a similar case to
Figure 9, with stations (and detections) at the same loca-
tions between 120 and 320 km. All detections are now of
stratospheric origin, no tropospheric duct is present due
to atmospheric conditions, well in agreement with a sha-
dow zone of roughly 100 km width near the source. First
stratospheric returns are modeled and observed at 120
km distance from the source. For the amplitude changes
with distance and especially the reduced detectability at
210 km, between first and second stratospheric reflection,
modeling results and observations (with a RMS deviation
of 6.69 dB) compare favorably.
4.3. Gravity Wave Dynamics
The detailed and generally good agreement between
modeling results and measurements in different test cases
has demonstrated that infrasound propagation modeling
is extremely helpful for verification purposes of infra-
sound measurements from a well-known ground truth
source. Estimation of the overall ducting behavior, travel
times, and expected amplitudes can be accomplished by a
combination of parabolic equation and ray-tracing mod-
eling. However, subtle differences are still occurring
between observations and modeling. An example is a
non-modeled but obvious stratospheric arrival in test case
#8 (see Figure 6), which might be simulated by consid-
ering additional variations of atmospheric conditions
missing in the propagation models used so far. Therefore
gravity waves are introduced as a main source of atmos-
pheric dynamics and small-scale variations of the effec-
tive sound speed, which might suffice for the generation
or degradation of atmospheric wave guides or modifica-
tion of their intensity and range.
Gravity wave dynamics can influence the atmospheric
effective sound speed by variations of a few m/s near the
surface up to variations of some 10 m/s in stratosphere
and thermosphere. Such modifications of atmospheric
background conditions can also change infrasound duct-
ing behavior (e.g., [36,37]), which strongly depends on
effective sound speed. An increase of effective sound
speed can generate stratospheric ducting or even shorten
the distances of stratospheric returns to the surface, i.e.
the shadow zone. The consideration of gravity waves by
a statistical approach and a high number of random grav-
ity wave profiles in infrasound propagation modeling
thus allows completing the picture, where detections of
remote infrasound can be expected or where shadow-
zones and absent ducts make detections highly unlikely
Figure 11 shows the Ariane test case #6 again, but
with ray paths and travel times for 40 different instances
of background conditions including gravity wave effects.
Due to a higher number and variation of propagation
profiles the ray paths and travel time branches cover a
more extended area in range and time and thus even bet-
ter explain the association of detections to a tropospheric
or stratospheric duct. Figure 11(a) shows all potential
rays resulting from different gravity wave simulations. It
can be clearly seen, that no stratospheric duct, even in-
cluding gravity waves, would return to the surface before
135 km distance, so there is no explanation for the 120
km distance detection other than a tropospheric duct.
Figure 11(b) shows travel times for all ray tracing sur-
face returns with different gravity wave realizations. Tro-
pospheric ducting up to 120 km (with a celerity of ap-
proximately 335 m/s) and stratospheric ducting in two
different branches (celerities of approximately 300 m/s
and 280 m/s) can be found and are in good agreement
with observed travel time measurements along the line
profile and at IS26. Figure 12 shows the same ray trac-
ing and travel time modeling for Ariane test case #7 (also
see Figure 10). All detections including the one at 120
km can be explained by stratospheric ducting, no tropo-
spheric duct or corresponding celerity above 300 m/s is
The existence of corresponding ducts in modeling and
observation (shown in Figures 9(a) and 10(a)) as well as
a comparatively low RMS deviation between modeling
amplitudes and observed pressure amplitudes (see Fig-
ures 8, 9(b) and 10(b)) already show good agreement for
most test cases. Nevertheless, consideration of gravity
wave disturbances, as implemented in Figures 11 and 12,
can still increase this agreement and also further reduce
the deviation between modeling and measurements.
For test case #6 (Figure 9b), the RMS deviation be-
tween modeling and observation amplitudes was 4.70 dB.
Taking into consideration gravity wave modifications of
the effective sound speed and thus of the ducting behav-
ior and the model amplitudes derived thereof, this value
can be reduced to 3.65 dB for the best fitting out of 40
gravity wave realizations. For test case #7 (Figure 10(b)),
the RMS deviation of 6.69 dB can be reduced to 3.96 dB
respectively. These reductions already show the room for
improvement of up to 20% - 40% by introducing gravity
Instead of applying one fixed gravity wave profile to
the complete range of 320 km and all six observations,
one can also apply different gravity wave representations
for each point of observation. Since these measurement
sites are 20 to 80 km apart from each other, the atmos-
pheric variability and horizontal changes in gravity wave
amplitudes realistically lead to different perturbations
Open Access InfraMatics
Open Access InfraMatics
(a) (b)
Figure 11. As Figures 2(b) and 6(b), but for the 7 March 2012 test case #6 and multiple propagation and travel time repre-
sentations including 40 different gravity wave profiles (colored). In (a), the original propagation profile (see also Figure 9) is
superimposed in black. Travel time branches in (b) for the tropospheric duct and for the stratospheric duct with offsets of
100 and 200 s and thus decreased celerity are conspicuous.
(a) (b)
Figure 12. As Figure 11, but for the 22 March 2012 test case #7; only stratospheric arrivals are present.
along the observation profile [15,22,31]. However, only a
one-dimensional approach for the PE atmospheric back-
ground conditions was available within this study, so the
described compensation by choosing different gravity
wave profiles for the six different measurement sites was
applied to show that even more improvement can be
achieved. For test case #6 and the choice of different
best-fit gravity wave profiles for each measurement site,
the RMS deviation could be further reduced to 2.51 dB,
for test case #7 the value could be reduced to 0.73 dB.
These reductions show maximum improvements of 45%
to 90% by the modeling to observation comparisons con-
sidering gravity waves. Table 2 summarizes all RMS
values and improvement percentages. Calculation results
for the test cases #3, #4 and #5 (Figure 8) are also shown,
but the small number of available measurements should
be kept in mind, particularly in terms of the achieved
improvements (from 27% to 78%).
The impact of gravity wave perturbations can also be
an explanation for the generation of stratospheric ducting,
where due to seasonal wind direction no such duct is
expected in modeling. In such cases the effective sound
speed ratio between stratosphere and surface is near to 1,
so that small variations in effective sound speed are suf-
ficient for significantly changing the ducting behavior
(see [19,20]).
Figure 13 shows gravity wave effects considered in
Ariane test case #8, occurring end of April 2012. The
stratospheric wind already changed after the spring
equinox from winterly eastward to summerly westward
direction and no infrasound propagation related to a stra-
tospheric duct is expected for this test (see Figure 6).
(a) (b)
Figure 13. As Figures 4(a) and 11(b) but for the 27 April 2012 test case #8. Modeling in (a) shows the propagation with one
selected gravity wave profile included (generating a stratospheric duct in contrast to Figure 6(a)). Modeling in (b) shows tra-
vel time branches with 40 gravity wave representations (generating a stratospheric branch in contrast to Figure 6(b)).
Table 2. Table showing the root mean square deviations (RMSD) and percentage of improvement for test cases #3 to #7; only
cases #6 and #7, shown in green, have detections at all six measurement sites. The minimum RMSDs and maximum im-
provements are calculated for realizations with no gravity wave profiles (column 3), one common gravity wave profile (col-
umns 4 and 5) and different individual gravity wave profiles (columns 6 and 7).
Test Nr. Number of
measurement sites
profile [dB]
RMSD, common
GW profile [dB]
Improvement to
“no GW” case [%]
RMSD, individual
GW profiles [dB]
Improvement to
“no GW” case [%]
3 2 3.26 2.37 27 1.47 55
4 2 8.27 5.59 33 4.73 43
5 3 25.07 6.00 76 5.59 78
6 6 4.70 3.65 24 2.51 46
7 6 6.69 3.96 40 0.73 89
Nevertheless, detections between 120 and 180 km are
obtained due to an elevated tropospheric duct, whereas
the detection at IS26 in 320 km distance has a celerity of
300 m/s indicative of a stratospheric duct although no
such duct appeared in section 4.1 modeling. Figure 13(a)
shows a representation of varying the atmospheric back-
ground profile for this test case with a gravity wave per-
turbation thereby generating a stratospheric duct. Only a
few of the 40 gravity wave perturbations considered here
generated this duct, so a stratospheric branch in Figure
13(b) is rarely filled with ray trace returns. However, its
overall celerity fits well to the observations and gives the
best explanation for the detections made at this distance.
Thus the consideration of gravity wave dynamics is an
important tool for identification and verification of ob-
served infrasound signals and gravity wave representa-
tions should be included in infrasound propagation mod-
Figure 14 shows the same propagation and travel time
modeling for the last Ariane test case #9, which took
place in mid-May 2012. Considering normal propagation
models and neglecting the effects of atmospheric dy-
namics, no stratospheric ducting is expected after the end
of the winter season, when the stratospheric winds al-
ready reversed. However, by including selected gravity
wave disturbances (one example shown in Figure 14(a)),
stratospheric returns can be modeled in approximate ac-
cordance to observations at IS26. The non-detections at
the line profile stations in 120, 140, 160, and 180 km are
also quite well explained by Figure 14(b). Thus, the rare
detections of stratospheric returns for two test cases in
late spring (celerity in both cases approximately 300 m/s)
are consistent with a modeling taking gravity wave ef-
fects into account.
4.4. Source Discrimination
Beside the ground truth source detections of the Ariane
test cases described above, complementary detections of
other potential infrasound sources were found during the
field campaign. Especially one prominent observation
[27] of a transient signal with several phases at all five
line profile stations and IS26 is investigated in greater
Open Access InfraMatics
(a) (b)
Figure 14. As Figure 13 but for the 14 May 2012 test case #9; a stratospheric duct is generated only by including certain
gravity wave representations.
detail, because its potential origin is ambiguous. Consid-
ering back-azimuth and travel time, both a surface explo-
sion and a supersonic boom at higher altitude are possi-
ble sources.
Infrasound propagation modeling from both potential
sources to the line profile stations and IS26 is shown in
Figure 15. Modeling was performed first for a ground-
based source near Flebour, Luxemburg (a) and second
for an elevated source at 10.8 km altitude near Bitburg,
Germany (b). Differences in the propagation pattern oc-
cur due to different ray paths: For case (a), classical stra-
tospheric ducting takes place by ray paths starting from
the surface upwards and reflecting in the stratosphere; for
case (b), sound from an elevated source can move up-
wards, downwards or nearly horizontal, leading to an
increased number of different ray paths, smaller shadow
zones and ducting of rays in approximately 10 km alti-
tude. Both modeling approaches correspondingly show
these differences and thus indicate characteristics for
source discrimination.
Accordingly, Figure 16 shows travel times for all rays
impinging with the surface for both modeling cases.
While for the Ariane 5 engine test source only one
branch of a signal with a duration similar to the test dura-
tion arrives at the observing stations, for the observed
signal on 16 February 2012 more than one signal phase is
observed. An infrasonic signal propagates along different
propagation paths to reach the same receiver (e.g., [6]). If
the source is very impulsive, different signal onsets (tra-
vel time branches) can be distinguished in one observa-
tion [27], even if pulse separation is in the order of only
tens of seconds.
Differences in travel time branches shown in Figure
16 are caused by the different source altitudes and prop-
agation patterns. For a surface source, only one branch of
stratospheric travel times is modeled to arrive at the line
profile stations and two branches of stratospheric returns
arriving at IS26. For an elevated source it is two arrivals
at the line profile stations and four arrivals at IS26.
Comparing modeling results with observations, it can be
seen, that at least three to four phase arrivals are ob-
served at the line profile stations and up to six arrivals at
IS26. This observation favors an elevated infrasound
source as origin of the signatures due to a higher number
of different travel time branches and phase arrivals.
A comparison of the measured amplitudes with mod-
eling results for the potential ground-based and elevated
sources is shown in Figure 17. As the PE method does
not yield time-dependent amplitude values, all observa-
tions and modeling results are summarized in one range
vs. amplitude representation. For the comparison, a 1 Pa
observed signal amplitude was arbitrarily set to corre-
spond to a 30 dB modeling amplitude as a best-fit
choice since the source amplitude is unknown. The PE
amplitudes are the surface level estimates as outlined
before (see section 4.2). From the ray-tracing additional
estimates of minimum, maximum and average ampli-
tudes were deduced when considering geometric spread-
ing loss for three different intensities (i.e. 10 dB, 15 dB
and 20 dB for cylindrical, intermediate and spherical
The ground-based source (a) is characterized by larger
shadow zones before and between the multiple strato-
spheric returns to the surface, and thereby the ground-
based source modeling shows a high variation with dis-
tance in the absolute amplitudes (e.g. a jump between
25 and 60 dB at 200 km distance), but a weaker
spreading (variance) of neighboring values (i.e. about 5 -
10 dB variation from point-to-point). The elevated source
(b) generated smaller and weaker shadow zones, a
smoother variation of absolute amplitudes (e.g. only a 10
- 20 dB jump at 200 km), but a higher spreading (i.e. 10 -
Open Access InfraMatics
(a) (b)
Figure 15. As Figure 4(a), but for two different scenarios as potential explanations for the 16 February 2012 transient detec-
tion. Propagation from two potential sources toward IS26, passing the line profile stations nearby, is shown. (a) Source is a
farm explosion in Flebour, Luxemburg, located at 49.914˚N, 6.104˚E. (b) Source is a supersonic flight above Bitburg, Ger-
many, located at 50.03˚N, 6.5˚E and in 10.8 km altitude.
(a) (b)
Figure 16. Travel time modeling and observations (as Figure 6(b)) in two different scenarios, as described in Figure 15. (a)
Luxemburg farm explosion, (b) Bitburg supersonic flight. Multiple phase detections with different travel times at the line
profile stations and at IS26 are represented by multiple red crosses.
(a) (b)
Figure 17. Amplitude estimations (as Figure 8) for the two different scenarios, as described in Figure 15. Modeling was per-
formed by parabolic equation (red curve, left axis) and ray tracing methods for three different spreading loss estimates (of 10
dB, 15 dB and 20 dB, black curves, left axis), also shown are the measured amplitudes for different phase arri vals at the dif-
ferent sensors (blue boxes, right axis). (a) Luxemburg farm explosion, (b) Bitburg supersonic flight.
20 dB from point-to-point) due to the increased number
of possible ray paths. The spreading of the actual obser-
vations (at the same range, but for different observed
phases) is about one order of pressure magnitude, corre-
Open Access InfraMatics
sponding to 20 dB and thus favoring the elevated source
as the most likely source.
The ray tracing amplitudes shown can fairly well de-
scribe the general attenuation of amplitudes with distance,
however, their variance with respect to one particular
geometric spreading loss is extremely low. While ray-
tracing was very successful in the kinematic description
of the observed arrivals, it seems inadequate for resolv-
ing their dynamic properties. The bounds given by the
minimum and maximum spreading loss considered, how-
ever do to some extent capture the variability of the ob-
served arrival amplitudes and may hint at wave theoreti-
cal effects being the reason for the amplitude variability.
Overall, the accomplishments of this section outline a
promising approach of using infrasound propagation
modeling for the discrimination and identification of
competing infrasound source hypotheses.
5. Summary & Conclusions
Infrasound propagation modeling within this study was
applied to a number of ground-truth field campaign mea-
surements: a total of nine rocket engine test cases and
two additional source scenarios. The field campaign con-
sisted in the deployment of six microbarometers along a
line profile between an infrasonic ground-truth source
(Ariane 5 engine tests) and a CTBT-IMS infrasound ar-
ray (IS26) covering the regional distance range up to 320
km and including the acoustic shadow zone [27]. The
engine tests were proved to be an ideal infrasound
ground-truth source with a continuous, stable and ergodic
signal emission with a clearly defined duration and pre-
cise localization. The different tests during different
months and seasons provided a valuable data basis for
field campaign observations and corresponding model
efforts focusing on dynamic effects in infrasound propa-
gation. The additional source scenarios were based on the
investigation of the signatures of a surface explosion or a
supersonic flight both reported to have occurred at the
same time and in the same general region.
Detailed propagation modeling from near source to re-
gional distances was applied by using ray-tracing and
parabolic equation methods. It was thereby possible to pre-
cisely estimate the infrasound ducting behavior through
the troposphere and stratosphere and also to take into
account small-scale atmospheric disturbances like gravity
waves. Modeling results were found to be in good ac-
cordance with the observations.
The application of propagation modeling successfully
verified the observation of tropospheric ducting in four
different test cases. For two of these cases, a duct of a
few hundred meters altitude entailed observation dis-
tances of 30 - 50 km, for the other two cases, a duct of 3 -
4 km altitude implicated unusually large distances of 120
- 180 km. The observed celerities and amplitudes at these
distances are too large to be of stratospheric origin and
propagation modeling indeed confirms this. Propagation
modeling was furthermore applied to estimate sound pres-
sure amplitudes along the propagation path and also to com-
pare modeling results with the observations quantitative-
ly, i.e. explaining the observed amplitude characteristics.
The inclusion of fine-scale structures as an expression
of atmospheric dynamics [19,22] in the modeling of
infrasound propagation by considering effective sound
speed variations due to gravity waves allowed a more
realistic modeling of the atmospheric conditions and the
resulting infrasound ducting behavior. The comparison of
modeling and observed amplitudes in the different test
cases as well as the size estimation and appearance of
infrasonic ducting regions and shadow zones was im-
proved by this approach. Root mean square deviations
were calculated between observed and modeled ampli-
tudes and showed improvements of 20% - 40% when
considering one-dimensional gravity wave disturbances
of the effective sound speed background. Improvements
of up to 90% were estimated when using multiple gravity
wave profiles, one for each sensor, as a proxy for the
effects of higher-dimensional variability of the atmos-
pheric background that exceeded the capabilities of the
propagation models used in this study. The good agree-
ment of measurements and modeling was also underlined
by a complete and correct explanation of all detections at
the line profile stations and at IS26. In two cases after the
end of the winter season, in April and in May 2012, stra-
tospheric arrivals observed at IS26 could only be ex-
plained in modeling by including gravity wave perturba-
tions in the atmospheric background model. Using an
ensemble of gravity wave models to define the extent of
ducting regions and shadow zones resulted in a higher
confidence in our ability of explaining observations with
propagation modeling, thus validating and verifying the
infrasound measurements.
Infrasound source discrimination using propagation
modeling was furthermore described in this study. By
investigating the propagation behavior for two different
source scenarios, a transient signal at the same day as one
of the engine tests was successfully associated with a
supersonic flight infrasound source in contrast to a com-
peting ground-based explosion. The elevated infrasound
source was identified by ducting behavior, amplitude and
travel time estimations from the propagation models as
preferential explanation for the observed signals.
Propagation modeling within this study was applied to
the validation of infrasound measurements, examination
of ducting behavior and discrimination of sources at local
and regional distances and through the acoustic shadow
zone. It demonstrated its benefit in these fields by
achieving a good agreement with observations from well-
Open Access InfraMatics
constrained ground-truth sources. By describing atmos-
pheric background conditions with high resolution, in-
cluding fine-scale variations by an ensemble of gravity
waves, even observations not expected from the standard
model could be well explained and the realistic state of
the atmosphere could be well represented by the ap-
proach followed within this study.
Further investigations combining propagation model-
ing with infrasound observations should include the rep-
resentation of multi-dimensional background conditions
in state of the art propagation models at high spatial and
temporal resolution, consideration of small-scale influ-
ences due to e.g. orography and near surface irregulari-
ties as well as an increased representation and under-
standing of gravity waves and their influence on atmos-
pheric dynamics. Results of such studies are invaluable
for improving infrasound detection capabilities, atmos-
pheric models and the understanding of infrasound proc-
esses on different scales.
6. Acknowledgements
We thank the counterpart from German Aerospace Center
Lampoldshausen (DLR-RA), the colleagues from the State
Seismological Survey of Baden-Württemberg (LEDBW),
and the German Airforce (Bundeswehr, Luftwaffenamt,
Flugbetriebs-und Informationszentrale, FLIZ) for provid-
ing information, data and support to this study.
We are grateful to Edgar Wetzig and Uwe Stelling for
their support in installing and maintaining the mobile
infrasound stations during the field campaign.
This work was partly performed in the course of the
ARISE collaborative project within the Seventh Frame-
work Programme funded by the European Union
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