Analysis of Signals from an Unique Ground-Truth Infrasound Source Observed at IMS Station IS26 in Southern Germany

Abstract

Quantitative modeling of infrasound signals and development and verification of the corresponding atmospheric propagation models requires the use of well-calibrated sources. Numerous sources have been detected by the currently installed network of about 40 of the final 60 IMS infrasound stations. Besides non-nuclear explosions such as mining and quarry blasts and atmospheric phenomena like auroras, these sources include meteorites, volcanic eruptions and supersonic aircraft including reentering spacecraft and rocket launches. All these sources of infrasound have one feature in common, in that their source parameters are not precisely known and the quantitative interpretation of the corresponding signals is therefore somewhat ambiguous. A source considered well-calibrated has been identified producing repeated infrasound signals at the IMS infrasound station IS26 in the Bavarian forest. The source results from propulsion tests of the ARIANE-5 rocket's main engine at a testing facility near Heilbronn, southern Germany. The test facility is at a range of 320 km and a backazimuth of *280°from IS26. Ground-truth information was obtained for nearly 100 tests conducted in a 5-year period. Review of the available data for IS26 revealed that at least 28 of these tests show signals above the background noise level. These signals are verified based on the consistency of various signal parameters, e.g., arrival times, durations, and estimates of propagation characteristics (backazimuth, apparent velocity). Signal levels observed are a factor of 2-8 above the noise and reach values of up to 250 mPa for peak amplitudes, and a factor of 2-3 less for RMS measurements. Furthermore, only tests conducted during the months from October to April produce observable signals, indicating a significant change in infrasound propagation conditions between summer and winter months.

Application of Propagation Modeling to Verify and Discriminate Ground-Truth Infrasound Signals at Regional Distances

“…at about 24 m distance to the source. Concerning the infrasound observations from the various tests (see Table 1) and the different profile configurations we found intermittent 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 campaign 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.…”

“…In a previous study [23], infrasound signals were analyzed 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 German Aerospace Center (DLR) near Heilbronn, Southern Germany (49.287˚N, 9.378˚E).…”

“…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 German 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 designed 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.…”

“…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 microbarometers 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.…”

Assessment of infrasound signals recorded on seismic stations and infrasound arrays in the western United States using ground truth sources

“…Several studies Nippress et al 2014;Blom et al 2015) use infrasonic signals from ground truth events in the western United States (US) to improve propagation modelling and to improve source location accuracy. Ground truth sources from rocket engine tests recorded at an International Monitoring System station in southern Germany were used to characterize infrasound signals (Koch 2010) as well as to verify propagation models (Pilger et al 2013). Green et al (2010) document the use of ground truth events recorded on seismometers and infrasound arrays at global and regional distances; for example, ground truth information from the Buncefield explosion in Hertfordshire, England, was used in the analysis of infrasound propagation.…”