Improving Magnitude Detection Thresholds Using Multistation, Multievent, and Multiphase Methods
Abstract
Research was conducted on a correlation detector (multievent and multiphase) for semiempirical synthetic tests and a regional case study in Xiuyan, China. The semiempirical runs took a 50 s window on an Lg-wave recorded at 750 km distance filtered from 1 to 3 Hz and embedded it 300,000 times in real, continuous, background seismic noise. The noise was selected for 36 days spread throughout the year to capture diurnal and seasonal variations. No screening for random, unknown signals in the noise was performed. A correlation detector has a 50% probability of detection with 1.5 false alarms per day for a signal-to-noise ratio (SNR) of 0.32, which corresponds to a full magnitude unit reduction in detection threshold over a standard STA/LTA technique. A scaled cross correlation coefficient performs slightly better with 1 false alarm per day and has fewer false triggers on unknown, random signals. Summing the cross correlation traces together for all three components enhances the detection signal similar to beamforming. A correlation detector summing the three components together has a 96% probability of detection with zero false alarms in 36 days for a SNR of 0.32. The case study looked at 90 events. Clear detection spikes are observed on all three components agreeing to the nearest sample allowing for constructive interference on the summation of the correlation traces. In contrast, detection maximums for unknown, random signals do not align to the nearest sample on the three components and destructively interfere with the summation. This is probably the single best indication of a true detection, the fact that three independent components all show clear spikes to the nearest sample. It is also observed that semisimilar events can provide useful detections (events that are not exactly colocated or don't have identical mechanisms). A scaled cross correlation detector is able to detect 90 out of 90 or 100% using Pg, Pn, and Lg phases.
Document Details
- Document Type
- Technical Report
- Publication Date
- Sep 01, 2006
- Accession Number
- ADA462264
Entities
People
- D. P. Schaff
- Felix Waldhauser
Organizations
- Columbia University