Broadband Seismic Noise Attenuation versus Depth at the Albuquerque Seismological Laboratory

Seismic noise induced by atmospheric processes such as wind and pressure changes can be a major contributor to the background noise observed in many seismograph stations, especially those installed at or near the surface. Cultural noise such as vehicle traffic or nearby buildings with air handling equipment also contributes to seismic background noise. Such noise sources fundamentally limit our ability to resolve earthquake-generated signals. Many previous seismic noise versus depth studies focused separately on either high-frequency (>1 Hz) or low-frequency (<0.05 Hz) bands. In this study, we use modern high-quality broadband (BB) and very broadband (VBB) seis-mometers installed at depths ranging from 1.5 to 188 m at the Albuquerque Seismological Laboratory to evaluate noise attenuation as a function of depth over a broad range of frequencies (0.002-50 Hz). Many modern seismometer deployments use BB or VBB seismometers installed at various depths, depending on the application. These depths range from one-half meter or less in aftershock study deployments, to one or two meters in the Incorporated Research Institutions for Seismology Transportable Array (TA), to a few meters (shallow surface vaults) up to 100 m or more (boreholes) in the permanent observatories of the Global Seismographic Network (GSN). It is important for managers and planners of these and similar arrays and networks of seismograph stations to understand the attenuation of surface-generated noise versus depth so that they can achieve desired performance goals within their budgets as well as their frequency band of focus. The results of this study will assist in decisions regarding BB and VBB seismometer installation depths. In general, we find that greater installation depths are better and seismometer emplacement in hard rock is better than in soil. Attenuation for any given depth varies with frequency. More specifically, we find that the dependence of depth will be application dependent based on the frequency band and sensitive axes of interest. For quick deployments (like aftershock studies), 1 m may be deep enough to produce good data, especially when the focus is on vertical data where temperature stability fundamentally limits the low-frequency noise levels and little low-frequency data will be used. For temporary (medium-term) deployments (e.g., TA) where low cost can be very important, 2-3 m should be sufficient, but such shallow installations will limit the ability to resolve low-frequency signals, especially on horizontal components. Of course, one should try for maximum burial depth within the budget when there is interest in using the data for low-frequency applications. For long-term deployments like the permanent observatories of the GSN and similar networks, 100-200 m depth in hard rock is desirable to achieve lowest noise, although 30-60 m may be acceptable.