HIGH-FREQUENCY ANALYSIS OF SEISMIC BACKGROUND-NOISE AS A FUNCTION OF WIND-SPEED AND SHALLOW DEPTH

We used a deep (1500 m) cased borehole near the town of Datil in west- central New Mexico to study high-frequency (>1 Hz) seismic noise chara cteristics. The remote site had very low levels of cultural noise, but strong winds (winter and spring) made the site an excellent candidate to study the effects of wind noise on seismograms. Along with a three -component set of surface sensors (Teledyne Geotech GS-13), a vertical borehole seismometer (GS-28) was deployed at a variety of depths (5, 43, and 85 m) to investigate signal and noise variations. Wind speed w as measured with an anemometer. Event-triggered and time-triggered dat a streams were recorded on a RefTek 72-02 data acquisition system loca ted at the site. Our data show little cultural noise and a strong corr elation between wind speed and seismic background noise. The minimum w ind speed at which the seismic background noise appears to be influenc ed varies with depth: 3 m/sec at the surface, 3.5 m/sec at 43 m in dep th, and 4 m/sec at 85 m in depth. For wind speed below 3 to 4 m/sec, w e observe omni-directional background noise that is coherent at freque ncies below 15 Hz. This coherence is destroyed when wind speeds exceed 3 to 4 m/sec. We use a test event (M(d) similar to 1.6) and superimpo sed noise to investigate signal-to-noise ratio (SNR) improvement with sensor depth. For the low Q valley fill of the Datil borehole (DBH) si te, we have found that SNR can be improved by as much as 20 to 40 dB b etween 23 and 55 Hz and 10 to 20 dB between 10 and 20 Hz, by deploying at a 43-m depth rather than at the surface. At the surface, there is little signal above noise in the 23- to 55-Hz frequency band for wind speeds greater than 8 m/sec. Thus, high-frequency signal information t hat is lost at the surface can be recorded by deploying at the relativ ely shallow depth of 40 m. Because we observe only minor further reduc tions in seismic background noise (SBN) at deeper depths, 40 m is like ly to be a reasonable deployment depth for other high-frequency-monito ring sites in similar environmental and geologic conditions.