Accession Number:

ADP204497

Title:

Low-Frequency Noise Study and Surface Wave Enhancement Using Data from Large and Small Aperture Arrays,

Descriptive Note:

Corporate Author:

PHILLIPS LAB HANSCOM AFB MA

Report Date:

1995-08-14

Pagination or Media Count:

13.0

Abstract:

The detection and parameter estimation of surface waves originating from earthquakes and explosions are important procedures in nuclear test monitoring with seismic arrays, usually performed on a routine on-line basis. For small teleseismic and far regional events performing these procedures is often complicated because weak low-frequency surface waves from these events are as a rule registered against a background of temporally and spatially correlated seismic noise with a frequency spectrum overlapping that of the event surface waves. Low frequency LF seismic noise has two explicit components the first is transient and propagates as surface waves the second has the features of a scattered field. The statistical properties of a LF noise field are affected by meteorological factors and vary in time. In this work we investigated temporal and spatial features of the LF seismic noise below 0.5 Hz based on records of long period instruments of large and small aperture arrays 6 z-component sensors of the NORSAR array, 11 similar sensors of the Grafenberg array, and 12 3-component very broad band sensors of the Geyokcha PASSCAL small aperture array. For assessment of noise field spatial correlation we evaluated noise coherence functions for array sensor pairs and implemented high resolution wide-band F-K analysis. The latter allows us to estimate azimuths and apparent velocities of the transient noise component of frequency 0.04-0.07 Hz which was observed at all the arrays and to confirm its origin as surface waves generated by sea shore surf. As an example, for the Geyokcha array the noise coherency for different sensors was to be greater than 0.8 in the frequency band 0.-1. Hz.

Subject Categories:

  • Government and Political Science
  • Seismology
  • Computer Programming and Software
  • Seismic Detection and Detectors
  • Nuclear Weapons
  • Acoustics

Distribution Statement:

APPROVED FOR PUBLIC RELEASE