How Does HPWREN Help the Optical Fiber Infrasound Sensor (OFIS) Development Team?

By Kris Walker, SIO IGPP

Atmospheric infrasound is acoustic energy below the frequency threshold of human hearing at ~18 Hz. There are a variety of interesting phenomena that produce infrasound: meteors, aurora, volcanoes, earthquakes, avalanches, wildfires, storm systems, tornadoes, ocean waves, and a variety of human activities. The physics of infrasound propagation is also very interesting due to the ducting influence of the stratospheric horizontal winds. The bane of recording atmospheric infrasound is noise generated by wind. We have been one of the leading institutions in the development of new infrasound sensors that are designed to be more resilient to wind noise, which can potentially further all basic research in infrasound. Traditionally, our outdoor testing facility has been Piñon Flat Observatory in the Southern California high desert (Fig. 1). During the last two years, we have expanded our testing to a new facility called Camp Elliott, located in San Diego county (Fig. 1). A near real-time web display is available at http://sail.ucsd.edu/~ofis/CEL/RT/ (Fig. 2). These sensors are broadband sensors with flat instrument responses from ~50 s to the kHz range, and they are sampled at 200 Hz, with about 10-30 channels at each site. Because they are interferometric sensors, they are sampled in double precision. At present, there is no commercial ISP at PFO that offers broadband connectivity, which our research requires. At Camp Elliott, the cost to IGPP for broadband service was prohibitive five years ago when it was first connected to HPWREN. More details on our past research can be found at http://sail.ucsd.edu/~ofis/sensors.html. In summary, our infrasound sensor development research at PFO and Camp Elliott would be greatly disrupted, indefinitely, if HPWREN was dismantled.

Fig. 1. Optical Fiber Infrasound Sensors (OFIS) at PFO (left) and Camp Elliott (right). There are up to six of these experimental sensors at each site. Each OFIS sensor has a length from 18 to 90 m. Burial in gravel at Camp Elliott provides thermal stability and additional protection from the wind. Each of these six sensors yields five 200 Hz channels (~250 kbps total) and therefore requires a broadband connection to transfer the data to our labs at IGPP.

Fig. 2. Near-real time display of the performance of the Optical Fiber Infrasound Sensor array at Camp Elliott ( http://sail.ucsd.edu/~ofis/CEL/RT/). The six OFIS sensors are separated by several tens of meters to form an array ( http://sail.ucsd.edu/~ofis/CEL/elliott_layout.pdf) that is capable of determining the direction from which the infrasound signal originated in the presence of wind. The time window is 15 minutes long. A high amplitude signal can be observed in the time series at t = -5 min. The ellipses on the left characterize the quality of the interferometer, which varies as a function of time. The graphs on the right show the meteorological data. For example, the average wind at this time was 1.4 m/s from the west-northwest.

HPWREN has benefited tremendously the research presented in the following papers:

Peer-reviewed: