Small-Aperture Seismo-Acoustic Arrays: Design, Implementation, and Utilization

Abstract

A four-element, 1-km-aperture seismo-acoustic array has been designed and installed northeast of Seoul, Korea. Each element of the array consists of a GS-13 vertical seismometer (1 Hz) in a shallow borehole (∼10 m) and a low-frequency acoustic gauge connected to an 11-element hose array (7.6-m hoses) at the surface. The array is being used to assess the importance of colocated seismic and acoustic sensors for the purposes of (1) quantifying wind as a source of seismic and acoustic noise, (2) constraining propagation path effects in the atmosphere and solid earth, (3) locating the sources of the waves, and (4) characterizing the source type. Seismic noise estimates illustrate a level that is only slightly above the low-noise model on average. Acoustic noise levels resolve the microbaroms during low-noise times but document a nearly 50-dB increase in noise during the windiest periods. Infrasonic noise in the 0.01- to 5-Hz band increases rapidly with wind velocity. The seismic noise shows little or no dependence on wind velocity. Analysis of the data from a 2-month time period suggests that there are many more acoustic signals than seismic (4–10 times as many). Approximately 1/4 of all seismic signals are associated with an acoustic arrival. The vast majority of seismo-acoustic observations come from sources in the 30- to 200-km range and occur during working hours, local time. The 30- to 200-km observation distance is surprising in that average atmospheric velocity models predict no acoustic returns in this range. Average atmospheric models modified by meteorological data for the troposphere indicate the possibility of ducting in the troposphere as an explanation for these arrivals. Event location is based upon regional seismic phase identification ( P n, P g, P m P, L g, R g) using the array and backazimuth estimates from both the seismic and acoustic data. Many of the infrasound signals have good signal-to-noise ratios from 1 to beyond 4 Hz. Despite the small size of the array, event clusters are identified at regional distances. Events associated with acoustic signals are presumed to be from mining regions. The existence of Rg arrivals and dominance of P energy at high frequency are consistent with this interpretation. Ground truth in the form of in-mine observations has validated that two of the clusters come from construction and mine blasts.