Overview
Seismic Veto
Overview
Many of the PAMGuard detectors set detection thresholds automatically as a function of background noise. Loud sounds from airguns can upset this automatic threshold setting. It is of course extremely unpleasant listening on headphones when airguns are firing nearby and airgun sounds may even pose a risk to the operators hearing.
During automatic threshold setting, a period of zero signal can upset threshold levels almost as much as a period of too much signal for some detector types
The seismic veto module detects the airgun pulse automatically using a simple in-band energy detector operating on spectrogram data. When the signal level exceeds some threshold, that period of sound is either taken out and set to zero, or replaced with randomly generated noise with the same spectral properties as measured in the signal prior to the airgun pulse. The time window of vetoed data can be set to continue after the signal has fallen below threshold to ensure the end of the pulse is discarded. It can also be configured to start the veto before the onset of the pulse (this is achieved by delaying the signal slightly in an internal buffer).
The seismic veto module has two output streams, the first containing vetoed FFT data and the second containing vetoed raw audio data. Other PAMGuard modules can subscribe to the appropriate output of the seismic veto module and operate as normal, but without the loud airgun sounds.
The figure above shows a simple PAMGuard configuration with the output of the seismic veto connected to a sound playback module, a click detector and a whistle detector. The figure below shows spectrograms and waveforms of data before and after the seismic veto module.
A veto can be inserted anywhere in the PAMGuard data model where there is FFT / Spectrogram data.