Identification of Acoustic Wave Signatures in the Ionosphere From Conventional Surface Explosions Using MF/HF Doppler Sounding

Abstract

We present an experiment to detect one ton TNT‐equivalent chemical explosions using pulsed Doppler radar observations of isodensity layers in the ionospheric E region during two campaigns. The first campaign, conducted on 15 October 2019, produced potential detections of all three shots. The detections closely resemble the temporal and spectral properties predicted using the InfraGA ray tracing and weakly nonlinear waveform propagation model. Here the model predicts that within 6.5–7.25 min of each shot a waveform peaking between 0.9 and 0.4 Hz will impact the ionosphere at 100 km. As the waves pass through this region, they will imprint their signal on an isodensity layer, which is detectable using a Doppler radar operating at the plasma frequency of the isodensity. Within the time windows of each of the three shots in the first campaign, we detect enhanced wave activity peaking near 0.5 Hz. These waves were imprinted on the Doppler signal probing an isodensity layer at 2.785 MHz near 100 km altitude. Despite these detections, the method appears to be unreliable as none of the six shots from the second campaign, conducted on 10 July 2020 were detected. The observations from this campaign were characterized by an increased acoustic noise environment in the microbarom band and persistent scintillation on the radar returns. These effects obscured any detectable signal from these shots and the baseline noise was well above the detection levels of the first campaign.

Remote Sensing Small Explosives With an Ionospheric Radar

“…During this time there was good ionospheric propagation at the transmit frequencies and recent ionograms combined with the group delay indicated the reflection height to be near 100 km altitude. Furthermore, unlike the experiments presented in Obenberger et al (2022), there was very little scintillation impact within an hour window of the first set. During the second set at 19:18:00.0 UTC, however, there was significant RF absorption due to a C9.8 Solar X-ray flare and subsequent enhanced D region (GOES-R Series Program, 2019).…”

“…Or in the case of a powerful over the horizon radar, Doppler variations of ground scatter could be used for detection over a large area. Finally, both this study and Obenberger et al (2022) indicate that even explosions taking place close in space and time are not always detected consistently. A comprehensive understanding of the prevalence and cause of such false negatives is urgently needed before this technique can be used in operational monitoring systems.…”

“…At LWA-SV we used 30 dB pre-amplifiers and the other two RX locations used 25 dB units. For more information on the RX processing see Obenberger et al (2022). Once range gated, the primary observable for each RX is the phase (ϕ) of the signal at each frequency.…”

“…Of the two experiments presented in Obenberger et al (2022), only one resulted in detections, and these were low SNR and somewhat perplexing. The arrival time and spectral content fit well to the modeled expectation, but the temporal characteristics were difficult to interpret.…”

“…Recently, in Obenberger et al (2022) we presented two experiments to detect small 1-ton TNT equivalent explosions using Doppler sounding of the lower ionospheric E region. Shots of this magnitude have spectral content peaking near a few Hz, with no measured content below 0.1 Hz (Obenberger et al, 2022). Modeling the waveform using the InfraGA/GeoAc package provided by Los Alamos National Laboratory, we found that the signal would have a peak frequency between 0.9 and 0.4 Hz in at the altitude range between 96 and 110 km.…”