An automated system for detecting and classifying in-water explosions and T-phases

Abstract

In support of the proposed Comprehensive Nuclear Test Ban Treaty, a large database of hydrophone recordings including T-phases, explosions, and noise has been compiled and cross referenced with known seismic events at the Center for Monitoring Research. Using this database, an automated hydroacoustic arrival detection and classification system has been developed. Detection is accomplished with a long-term-average/short-term-average power detector operating in several passbands. Station specific tuning of SNR thresholds and passband bounds allows the detector to trigger reliably on T-phases and explosions while passing over the majority of noise events such as whale calls. For each detected arrival, features such as duration, energy moments, spectral ratio, and order statistics are measured in multiple passbands from 2–85 Hz. A neural network uses these features to classify each arrival as signal or noise. Declared signals are passed to a second-stage network which classifies them as T-phases, explosions, or unknown events. T-phases arriving within a 4-min window around the time predicted from a seismic location are associated with that seismic event. These associations reveal relationships among event parameters such as location, magnitude, depth, duration, and coupling region.

Operational Processing of Hydroacoustics at the Prototype International Data Center

“…At the PIDC, hydroacoustic signals are classi®ed into three categories: T, H and N. The traditional phase designation in seismology for a phase whose path includes propagation through the ocean is T for tertiary because the signal arrives after the primary (P) and secondary (S) phases (LAY and WALLACE, 1995). The commonly observed T phase is a long duration (minutes), band-limited (2±40 Hz) wavetrain and is commonly generated by earthquakes near ocean boundaries.…”

“…It is short in duration (< 45 seconds), and broad in frequency content (2±100 + Hz) (e.g., LAVERGNE, 1970;CHAPMAN, 1985;URICK, 1983). The hydroacoustic signals from in-water explosions are very useful in locating events because the ocean sound speed is well known and the impulsive source function is small in both duration and spatial extent.…”

“…2). An automatic system processes waveform data and produces a series of standard event lists (LANEY et al, 1996). These are used as a starting point for interactive processing in which analysts review all of the events and detections made in the automatic system.…”

“…Initially data were received from one hydrophone (WK30), but in February 1998 data from a second hydrophone (WK31) became available. The hydrophones are part of the U.S. Air Force Missile Impact Locating System (MILS) that were installed in the 1950s and 1960s to aid in the location of missile splash downs (MCCREERY and WALKER, 1988). The signals from the hydrophones are transmitted through underwater cables and digitized on the island (320 Hz sample rate).…”