Changes in Fiber-Optic Distributed Acoustic Sensing Noise Amplitude due to Tropical Depression Cristobal

Tropical Storm Cristobal made landfall near New Orleans, LA, on June 7, 2020. The storm weakened into Tropical Depression Cristobal as it proceeded northward. The center of the storm passed 50 miles away from a fiber-optic array installed about 0.5 m (about 2 ft) deep in loess soil. The fiber optic array is connected to a distributed acoustic sensing (DAS) interrogator, which is sensitive to strains in the fiber-optic cable caused by seismic waves passing through the surrounding soil, including sound waves transmitted into the soil. The purpose of this research was to identify changes in the ambient noise amplitude collected via DAS during Tropical Depression Cristobal. The study area was an open field with a gravel access road to minimize potential impacts of tree cover. Data were collected via DAS continuously during the storm with a sampling rate of 2,500 Hz. A meteorological station is located onsite and recorded hourly average temperatures and hourly rainfall totals. Sustained winds of 37 kph (23 mph) and gusts of 60 kph (37 mph) were observed at a nearby weather station. Onsite 17.8 mm (0.70 in.) of rain fell from 2:00 a.m. to 3:00 a.m. Along three DAS channels, ambient noise levels at three time points—during the storm, at the same time of day once the storm had passed, and for typical activity noise levels along the study section—were compared. Results show a very significant increase in noise attributable to the storm.